BOVINE PETECHIAL FEVER

(Ondiri disease)

Bovine petechial fever is an infectious disease of cattle characterized by hemorrhage and edema. It has been confirmed only in Kenya at altitudes >5,000 ft (1,500 m), although it is considered likely to occur in neighboring countries of similar topography. The causative agent, Ehrlichia (Cytoecetes) ondiri, can multiply after experimental infection in cattle, sheep, goats, bushbuck, impala, Thomson’s gazelle, and wildebeest, and hence, probably in most domestic and wild ruminants.

Etiology and Epidemiology: Ehrlichia ondiri can be seen in circulating granulocytes and monocytes while cattle are ill and in the spleen at necropsy. It is believed to multiply initially in the spleen, with subsequent spread to other organs. Latent infections occur after recovery in some animals. Immunity lasts several years.

The disease is restricted to scrub or forest edge areas that have heavy shade, a thick litter layer, and high relative humidity. It occurs sporadically throughout the year in imported breeds of cattle. An arthropod vector is suspected, but extensive attempts to incriminate ticks, biting insects, and mites have failed. Bushbuck (Tragelaphus scriptus ) are reservoirs of E ondiri in endemic areas, and other wild ruminants may be potential reservoirs.

Clinical Findings: The disease ranges from inapparent to fatal and is characterized by fever, apathy, and petechiation of mucous membranes. After an incubation period of 4-14 days, animals develop a high fever; 2-3 days later, most animals appear dull, and petechiae appear on mucous membranes, particularly the lower surface of the tongue and the vaginal mucosa. These hemorrhages enlarge over several days and then regress. Marked conjunctival edema and hemorrhage (“poached egg eye”) are characteristic in some severe cases. Typically, there is total eosinopenia and marked lymphopenia, followed by an equally pronounced neutropenia. Anemia is a characteristic sequela. Outbreak mortality in untreated cases can be as high as 50%. At necropsy, widespread hemorrhage and edema are accompanied by lymphoid hyperplasia. No characteristic histologic abnormalities have been described. The organism has been seen by electron microscopy in cytoplasmic vacuoles in capillary endothelium, damage to which may account for the characteristic hemorrhages and edema.

Diagnosis: In areas where the disease is endemic, a history of movement to rough

grazing areas, coupled with clinical and postmortem signs, allows a presumptive diagnosis. In other areas, demonstration of the causal organism is necessary, either in Giemsa-stained blood or spleen smears, or by inoculation of tissue suspensions into susceptible cattle or sheep. Blood smears from the recipient animal should be taken daily for 10 days and examined for E ondiri. The organism stains blue with Giemsa and, in natural and experimental infections, can be seen as small bodies (0.4 m), larger

bodies (1-2 m), groups of small and large bodies, and groups or morulae of small bodies. They occur in cytoplasmic vacuoles and are most commonly seen in neutrophils.

Treatment and Control: Dithiosemicarbazone and tetracyclines have been used successfully to treat early experimental cases. In endemic areas, avoidance of areas associated with previous cases is practiced where possible.

CAPRINE ARTHRITIS AND ENCEPHALITISCaprine arthritis and encephalitis (CAE) virus infection is manifest clinically as polyarthritis in adult goats and less commonly as progressive paresis (leukoencephalomyelitis) in kids. Subclinical or clinical interstitial pneumonia, indurative mastitis (“hard udder”), and chronic wasting have also been attributed to infection with this virus. Most CAE virus infections, however, are subclinical. Infection with the CAE virus decreases the lifetime productivity of dairy goats and is a barrier to exportation of goats from North America.

Under natural conditions, the CAE virus appears to be host-specific, but experimental infection of sheep with this virus is possible. Prolonged commingling of naive sheep with infected goats usually does not result in infection or seroconversion, but lambs allowed to suckle infected goats will seroconvert and develop persistent CAE virus infections. Experimental inoculation of the CAE virus into the joints of lambs produces arthritis, seroconversion, and virus-positive joints.

CAE virus infection is widespread among dairy goats in most industrialized countries but rare among indigenous goat breeds of developing countries unless they have been in contact with imported goats. In countries such as Canada, Norway, Switzerland, France, and the USA, seroprevalence of CAE virus is >65%.

Etiology, Epidemiology, and Pathogenesis: The CAE virus is an enveloped, single-stranded RNA lentivirus in the family Retroviridae. The several strains of the virus differ in virulence and antigenicity. Other lentiviruses include the progressive pneumonia virus of sheep, the equine infectious anemia virus, and the feline and human immunodeficiency viruses.

The infection is widespread in dairy goat breeds but uncommon in meat- and fiber-producing goats. This has been attributed to a heritable predisposition to infection in dairy breeds. Prevalence of infection increases with age but is not influenced by sex. Most goats are infected at an early age, remain virus positive for life, and develop disease months to years later.

The chief mode of spread of CAE is through ingestion of virus-infected goat colostrum or milk by kids. The feeding of pooled colostrum or milk to kids is a particularly risky practice, because a few infected does will spread the virus to a large number of kids. Horizontal transmission also contributes to disease spread within herds and may occur through direct contact, exposure to fomites at feed bunks and waterers, ingestion of

contaminated milk in milking parlors, or serial use of needles or equipment contaminated with blood. Unlikely methods of transmission, indicated by experimental studies, include in utero to the fetus, infection of the kid during parturition, breeding, or through embryo transfer.

The pathogenesis of CAE is not fully understood. Virus-infected macrophages present in colostrum and milk are taken up intact through the gut mucosa. Infection is subsequently spread throughout the body via infected mononuclear cells. Periodic virus replication and macrophage maturation induces the characteristic lymphoproliferative lesions in target tissues such as the lungs, synovium, choroid plexus, and udder. Persistence of the CAE virus in the host is facilitated by its ability to become sequestered as provirus in host cells. Infection induces a strong humoral and cell-mediated immune response, but neither is protective.

Clinical Findings: Arthritis is the syndrome exhibited by adult goats infected with the CAE virus. Clinical signs include joint capsule distention and varying degrees of lameness. The carpal joints are most frequently involved. The onset of arthritis may be sudden or insidious, but the clinical course is always progressive. Affected goats lose condition and usually have poor hair coats. Encephalomyelitis is generally seen in kids 2-4 mo old but has been described in older kids and adult goats. Affected kids initially exhibit lameness, ataxia, and hindlimb placing deficits. Picture Hypertonia and hyperreflexia are also common. Over time, signs progress to paraparesis or tetraparesis and paralysis. Depression, head tilt, circling, opisthotonus, torticollis, and paddling have also been seen in affected goats. The interstitial pneumonia component of CAE virus infection rarely produces clinical signs in kids. However, in adult goats with serologic evidence of CAE virus infection, chronic interstitial pneumonia that leads to progressive dyspnea has been documented. The “hard udder” syndrome attributed to CAE virus infection is characterized by a firm, swollen mammary gland and agalactia at the time of parturition. Milk quality is usually unaffected. Although the mammary gland may soften and produce close to normal amounts of milk, production remains low in many goats suffering from indurative mastitis.

Lesions: Pathologic lesions of CAE virus infection are generally described as lymphoproliferative with degenerative mononuclear cell infiltration. Lesions in joints are characterized by thickening of the joint capsule and marked proliferation of synovial villi. In chronic cases, soft-tissue calcification involving joint capsules, tendon sheaths, and bursae is not uncommon. Severe cartilaginous destruction, rupture of ligaments and tendons, and periarticular osteophyte formation have also been described in advanced cases. Microscopic features of articular lesions include synovial cell hyperplasia, subsynovial mononuclear cell infiltration, villous hypertrophy, synovial edema, and

synovial necrosis. Gross lesions associated with the neurologic form of CAE include asymmetric, brownish pink, swollen areas, most commonly in the cervical and lumbosacral spinal cord segments. Histopathologically, these lesions are characterized by multifocal, mononuclear cell inflammatory infiltrates and varying degrees of demyelination. On gross examination, lungs of affected goats are firm and gray-pink with multiple, small, white foci, and do not collapse. The bronchial lymph nodes are invariably enlarged. Histologic findings include chronic interstitial pneumonia with mononuclear cell infiltration in alveolar septae and in perivascular and peribronchial regions. In does with udder induration, mononuclear infiltration of periductular stroma obliterates normal mammary tissue.

Diagnosis: A presumptive diagnosis can be based on clinical signs and history. Traumatic arthritis, and infectious arthritis caused by Mycoplasma spp, are differential diagnoses for arthritis induced by CAE virus. Differential diagnoses for the progressive paresis and paralysis exhibited by young kids should include enzootic ataxia, spinal cord abscess, cerebrospinal nematodiasis, spinal cord trauma, and congenital anomalies of the spinal cord and vertebral column. If neurologic examination indicates brain involvement, polioencephalomalacia, listeriosis, and rabies should be considered as possible causes. The pulmonary form of caseous lymphadenitis may have a similar clinical presentation to the pulmonary form of CAE.

Serologic tests available for diagnosis are the agar gel immunodiffusion (AGID) and ELISA. In general, the ELISA is more sensitive than the AGID, but the latter test is more widely available in North America. A positive test result in an adult goat implies infection but does not confirm that clinical signs present are caused by the CAE virus. Kids infected at birth develop a measurable antibody response between 4 and 10 wk after infection. However, positive test results in kids <90 days old usually reflect colostral antibody transfer. Negative test results do not reliably rule out CAE virus infection, because the time for postinfection seroconversion is variable and occasional goats produce a very low titer that may not be detectable. Low antibody titer is common in late pregnancy. Because of the limitations of serologic testing, definitive diagnosis of clinical CAE requires demonstration of characteristic lesions in biopsy specimens or at necropsy. Virus isolation should be performed to further substantiate the diagnosis.

Treatment and Control: There are no specific treatments for any of the clinical syndromes associated with CAE virus infection. However, supportive treatments may benefit individual goats. The condition of goats with the arthritic form of CAE may be improved with regular foot trimming, use of additional bedding, and administration of nonsteroidal anti-inflammatory drugs such as phenylbutazone or aspirin. Goats with encephalomyelitis can be maintained for weeks with good nursing care. Antimicrobial therapy is indicated to treat secondary bacterial infections that may complicate the interstitial pneumonia or indurative mastitis components of CAE virus infection. Providing high-quality, readily digestible feed to goats positive for CAE virus may delay the onset of the wasting syndrome. In commercial herds, one or more of the following have been recommended for control of CAE: 1) permanent isolation of kids beginning at birth; 2) feeding of heat-treated colostrum (56°C for 60 min) and pasteurized milk; 3)

frequent serologic testing of the herd (semiannually), with identification and segregation of seronegative and seropositive goats; and 4) eventual culling of seropositive goats. If the control program includes segregation of herds into seropositive and seronegative groups, shared equipment should be disinfected using phenolic or quaternary ammonium compounds.

CRIMEAN-CONGO HEMORRHAGIC FEVER

Crimean-Congo hemorrhagic fever (CCHF) is a severe hemorrhagic viral disease of man acquired from infected ticks, tissues of infected wild or domestic animals, and from people with the disease. In experimental inoculations, sheep and cattle become infected but suffer only transient and mild increases of body temperature with little evidence of clinical disease.

The etiologic agent, CCHF virus (genus Nairovirus, family Bunyaviridae) is an enveloped negative-sense, single-stranded RNA virus. The virus has been described in a wide area from South Africa through southern Europe, Eurasia, and into parts of western China. The virus is principally associated with ticks of the genus Hyalomma, although it has also been isolated from other genera of ixodid ticks. The global distribution of the virus roughly approximates that of Hyalomma ticks. Recent analyses of the genome of the virus suggest that there is significant genetic diversity somewhat correlated with geographic origin of the virus. However, anomalies to this pattern suggest that dispersal of host ticks by migratory wildlife such as birds or the movement of livestock by man may act to perturb the “normal” geographic distribution of CCHF virus subpopulations.

The virus replicates in the host tick as it passes from larval through adult stages (trans-stadial transmission) and can also be transmitted from one generation to the next (transovarial transmission). Thus, the tick not only is a vector but also can be a reservoir of the virus via vertical transmission. Vertebrates ranging from small rodents, lagomorphs, and birds have all been incriminated in infection of immature stages of the tick, while most Hyalomma spp ticks are multihost and use larger vertebrates as the host for the adult stage of their life cycle.

In experimentally infected vertebrates, viremia levels and duration are relatively low and short, and antibodies are detectable shortly after cessation of viremia. Some tests (principally IgG ELISA) can detect antibodies for the remainder of the life of the animal, while other tests, such as complement fixation and indirect fluorescent antibody, can detect antibodies for shorter periods after infection. Antibody prevalence in adult livestock species in endemic regions can be >50%.

Control strategies for man include the avoidance of tick bites through use of repellents and appropriate protection when slaughtering or grooming animals. Medical personnel should use appropriate barrier nursing techniques and universal precautions when

handling suspect patients. The antiviral drug ribavirin is effective in in vitro sensitivity tests and, although not evaluated in controlled trials, seems effective in treatment of cases in South Africa.

HEARTWATER

(Cowdriosis)

Heartwater is an infectious, noncontagious, rickettsial disease of ruminants in areas infested by ticks of the genus Amblyomma. These include regions of Africa south of the Sahara, and the islands of Zanzibar, Madagascar, Reunion, and Mauritius, as well as Guadeloupe, Antigua, and Marie Galante in the Caribbean. Many ruminants, including some antelope species, are susceptible. Some animals may become subclinically infected and act as reservoirs. Indigenous African breeds of cattle (Bos indicus) appear more resistant than imported breeds.

Etiology and Transmission: The causative organism, Cowdria ruminantium, is an obligate intracellular parasite, closely related to a number of Ehrlichia spp. Under natural conditions, C ruminantium is transmitted by ticks of the genus Amblyomma. These three-host ticks become infected during either larval or nymphal stages and transmit the infection during one of the subsequent stages (transstadial transmission). The progeny of an infected female tick are most probably not infective (ie, there is no transovarial transmission). Therefore, the infection rate in tick populations tends to be low. Intrastadial transmission by male ticks may also occur, as well as some degree of vertical transmission from cow to calf (eg, by colostrum) in areas where the disease is endemic.

Cowdria ruminantium can be propagated experimentally by serial passage, either by inoculating infective blood into, or by feeding infected nymphal or adult stages of the tick on, susceptible animals. It can also be propagated in endothelial-cell tissue culture. At room temperature, infective material loses its infectivity within a few hours, but the organisms, together with suitable cryoprotectants, may be preserved in liquid nitrogen for years.

Immunity to heartwater appears to be chiefly, if not exclusively, cell mediated. There is no, or only partial, cross-protection between different strains (stocks) of C ruminantium. Most of them are infective for, but cannot be serially passaged in, mice; however, a few stocks are pathogenic to mice infected by the IV route. One of these, the Kümm stock, can even be passaged by the intraperitoneal route.

Clinical Findings and Pathogenesis: The signs are dramatic in the peracute and acute forms. In peracute cases, the animals develop fever, which is followed rapidly by hyperesthesia, lacrimation, and convulsions. In the acute form, the animals show anorexia and nervous signs such as depression, a high-stepping stiff gait, exaggerated blinking of eyes, and chewing movement. Both forms terminate in convulsions and

prostration. Diarrhea is occasionally seen. In subacute cases, the signs are less marked, and CNS involvement is inconsistent.

Cowdria ruminantium seems to initially reproduce in macrophages and then invades and multiplies in the vascular endothelium. During the febrile stages, and for a short while thereafter, the blood is infective to susceptible animals. Signs and lesions are associated with functional injury to the vascular endothelium, resulting in increased vascular permeability. The latter precipitates a fall in arterial pressure and general circulatory failure. The lesions in peracute and acute cases are hydrothorax, hydropericardium, edema and congestion of the lungs, splenomegaly, petechiae and ecchymoses on the mucosal and serosal surfaces, and occasionally hemorrhage into the GI tract, particularly the abomasum.

Diagnosis: In acute forms, diagnosis can be based on clinical signs, but demonstration of colonies of the organism in the cytoplasm of capillary endothelial cells is necessary for definitive diagnosis. Traditionally, this is done with “squash” smears of cerebral or cerebellar gray matter, stained with Romanowsky-type stains, of which Giemsa affords the best color differentiation. Organisms in autolyzed material lose their stainability, and diagnosis then becomes difficult. Using an immunoperoxidase staining method, a definite diagnosis can be made on formalinized tissue samples, even from autolyzed carcasses. Serodiagnosis of animals previously exposed to the disease, ie, recovered from subclinical or clinical infection, still poses problems. Several tests are currently in use, including several indirect fluorescent antibody tests and ELISA tests. All serologic tests, possibly excluding an ELISA that uses a recombinant antigen, are plagued by cross-reactions with sera from animals infected with one of several Ehrlichia spp (false positive) and the fact that immune cattle on repeated exposure may become seronegative (false negative). Although DNA-based diagnostic tests using polymerase chain reaction technology are being worked on, they are not yet routinely available.

Treatment and Control: Control of tick infestation is a useful preventive measure in some instances but may be difficult and expensive to maintain in others. Excessive reduction of tick numbers, however, interferes with the maintenance of adequate immunity through regular challenge and may result in heavy losses. For immunization, the “infection and treatment method” is still in use in southern Africa; infected sheep blood is used for infection, followed by monitoring rectal temperature and antibiotic therapy after fever develops. In certain conditions, the “controlled” infection is followed by preventive “block-treatment” (cattle on day 14 [susceptible breeds] or day 16 [resistant breeds], and small stock on day 11) or use of a subcutaneous doxycycline implant at the time of infection. Young calves (<6 wk old) and lambs and kids (<1 wk old) are fairly resistant and may recover spontaneously from natural and induced infections. Oxytetracycline at 10 mg/kg body wt, or doxycycline at 2 mg/kg body wt, usually effect a cure if administered early. In sheep, goats, and susceptible cattle

breeds, a higher dosage (10-20 mg/kg) may be required, particularly if treatment begins late during the febrile reaction or when clinical signs appear. In such cases, the first treatment should preferably be given IV. A second or third treatment may be necessary

before the fever abates. Corticosteroids have been used as supportive therapy (prednisolone, 1 mg/kg body wt), although there is debate as to their effectiveness.

HEARTWATER

(Cowdriosis)

Heartwater is an infectious, noncontagious, rickettsial disease of ruminants in areas infested by ticks of the genus Amblyomma. These include regions of Africa south of the Sahara, and the islands of Zanzibar, Madagascar, Reunion, and Mauritius, as well as Guadeloupe, Antigua, and Marie Galante in the Caribbean. Many ruminants, including some antelope species, are susceptible. Some animals may become subclinically infected and act as reservoirs. Indigenous African breeds of cattle (Bos indicus) appear more resistant than imported breeds.

Etiology and Transmission: The causative organism, Cowdria ruminantium, is an obligate intracellular parasite, closely related to a number of Ehrlichia spp. Under natural conditions, C ruminantium is transmitted by ticks of the genus Amblyomma. These three-host ticks become infected during either larval or nymphal stages and transmit the infection during one of the subsequent stages (transstadial transmission). The progeny of an infected female tick are most probably not infective (ie, there is no transovarial transmission). Therefore, the infection rate in tick populations tends to be low. Intrastadial transmission by male ticks may also occur, as well as some degree of vertical transmission from cow to calf (eg, by colostrum) in areas where the disease is endemic.

Cowdria ruminantium can be propagated experimentally by serial passage, either by inoculating infective blood into, or by feeding infected nymphal or adult stages of the tick on, susceptible animals. It can also be propagated in endothelial-cell tissue culture. At room temperature, infective material loses its infectivity within a few hours, but the organisms, together with suitable cryoprotectants, may be preserved in liquid nitrogen for years.

Immunity to heartwater appears to be chiefly, if not exclusively, cell mediated. There is no, or only partial, cross-protection between different strains (stocks) of C ruminantium. Most of them are infective for, but cannot be serially passaged in, mice; however, a few stocks are pathogenic to mice infected by the IV route. One of these, the Kümm stock, can even be passaged by the intraperitoneal route.

Clinical Findings and Pathogenesis: The signs are dramatic in the peracute and acute forms. In peracute cases, the animals develop fever, which is followed rapidly by hyperesthesia, lacrimation, and convulsions. In the acute form, the animals show

anorexia and nervous signs such as depression, a high-stepping stiff gait, exaggerated blinking of eyes, and chewing movement. Both forms terminate in convulsions and

prostration. Diarrhea is occasionally seen. In subacute cases, the signs are less marked, and CNS involvement is inconsistent.

Cowdria ruminantium seems to initially reproduce in macrophages and then invades and multiplies in the vascular endothelium. During the febrile stages, and for a short while thereafter, the blood is infective to susceptible animals. Signs and lesions are associated with functional injury to the vascular endothelium, resulting in increased vascular permeability. The latter precipitates a fall in arterial pressure and general circulatory failure. The lesions in peracute and acute cases are hydrothorax, hydropericardium, edema and congestion of the lungs, splenomegaly, petechiae and ecchymoses on the mucosal and serosal surfaces, and occasionally hemorrhage into the GI tract, particularly the abomasum.

Diagnosis: In acute forms, diagnosis can be based on clinical signs, but demonstration of colonies of the organism in the cytoplasm of capillary endothelial cells is necessary for definitive diagnosis. Traditionally, this is done with “squash” smears of cerebral or cerebellar gray matter, stained with Romanowsky-type stains, of which Giemsa affords the best color differentiation. Organisms in autolyzed material lose their stainability, and diagnosis then becomes difficult. Using an immunoperoxidase staining method, a definite diagnosis can be made on formalinized tissue samples, even from autolyzed carcasses. Serodiagnosis of animals previously exposed to the disease, ie, recovered from subclinical or clinical infection, still poses problems. Several tests are currently in use, including several indirect fluorescent antibody tests and ELISA tests. All serologic tests, possibly excluding an ELISA that uses a recombinant antigen, are plagued by cross-reactions with sera from animals infected with one of several Ehrlichia spp (false positive) and the fact that immune cattle on repeated exposure may become seronegative (false negative). Although DNA-based diagnostic tests using polymerase chain reaction technology are being worked on, they are not yet routinely available.

Treatment and Control: Control of tick infestation is a useful preventive measure in some instances but may be difficult and expensive to maintain in others. Excessive reduction of tick numbers, however, interferes with the maintenance of adequate immunity through regular challenge and may result in heavy losses. For immunization, the “infection and treatment method” is still in use in southern Africa; infected sheep blood is used for infection, followed by monitoring rectal temperature and antibiotic therapy after fever develops. In certain conditions, the “controlled” infection is followed by preventive “block-treatment” (cattle on day 14 [susceptible breeds] or day 16 [resistant breeds], and small stock on day 11) or use of a subcutaneous doxycycline implant at the time of infection. Young calves (<6 wk old) and lambs and kids (<1 wk old) are fairly resistant and may recover spontaneously from natural and induced infections. Oxytetracycline at 10 mg/kg body wt, or doxycycline at 2 mg/kg body wt, usually effect a cure if administered early. In sheep, goats, and susceptible cattle

breeds, a higher dosage (10-20 mg/kg) may be required, particularly if treatment begins late during the febrile reaction or when clinical signs appear. In such cases, the first treatment should preferably be given IV. A second or third treatment may be necessary before the fever abates. Corticosteroids have been used as supportive therapy (prednisolone, 1 mg/kg body wt), although there is debate as to their effectiveness.

HEMORRHAGIC SEPTICEMIA

Hemorrhagic septicemia (HS) is an acute pasteurellosis, caused by particular serotypes of Pasteurella multocida and manifest by an acute and highly fatal septicemia principally in cattle and water buffalo; the latter are thought to be more susceptible than cattle. HS occurs infrequently in swine and even less commonly in sheep and goats. It has been reported in bison, camels, elephants, horses, and donkeys, and there is evidence of its occurrence in yak. An acute pasteurellosis indistinguishable from HS occurs infrequently in deer, elk, and probably other feral ruminants. Laboratory rabbits and mice are highly susceptible to experimental infection.

The only true outbreaks in North America have occurred in bison in Yellowstone National Park. Occurrence in Central and South America has not been confirmed. HS is a major disease of cattle and water buffalo in Asia, Africa, and some countries of southern Europe and the Middle East. Although it may occur at any time of year, the worst epidemics occur during the rainy season. It is most common in the river valleys and deltas of southeast Asia among buffaloes used in rice cultivation.

The HS serotypes of P multocida have not been recovered from human infections. However, because many serotypes of P multocida have the potential to infect man, appropriate precautions should be taken.

Etiology: Epidemic HS is caused by one of two serotypes of P multocida, designated B:2 and E:2. Serotype E:2 has been recovered only in Africa; B:2 causes the disease elsewhere and also has been recovered from cases in Egypt and the Sudan. Serotypes closely related antigenically to serotype B:2 have been implicated in limited outbreaks of a disease indistinguishable from HS in deer and elk. Pasteurella multocida is an extracellular parasite, and immunity is primarily humoral.

Transmission, Epidemiology, and Pathogenesis: Infection occurs by direct or indirect contact. The source of infective bacteria is thought to be the nasopharynx of bovine or buffalo carriers. As many as 5% of cattle and water buffalo may be carriers in endemic regions.

It is hypothesized that animals become susceptible as a result of various stresses, eg, the inanition seen in cattle and water buffalo at the beginning of the rainy season. Natural infection occurs by ingestion or inhalation. The initial site of proliferation is thought to be the tonsillar region. In susceptible animals, a septicemia develops rapidly, and death due to endotoxemia ensues within 8-24 hr after the first signs are

seen. Exotoxins have not been demonstrated.

The motality rate is high when the agent is introduced to virgin or nonendemic regions. Losses vary widely in endemic areas. The heaviest losses occur during the monsoon rains in southeast Asia, and it is thought that the organisms, which can survive for hours and probably days in the moist soil and water, are transmitted widely at this time.

Clinical Findings: Most cases are acute or peracute, resulting in death within 8-24 hr after onset. Because the course is so short, clinical signs may easily be overlooked. Animals first evince dullness, then reluctance to move, fever, salivation, and serous nasal discharge. Edematous swelling is frequently seen, beginning in the throat region and spreading to the parotid region, neck, and brisket. Mucous membranes are congested. There is respiratory distress, and usually the animal goes down and dies within hours. Occasional cases linger for several days. Recovery is rare. There appears to be no chronic form.

Lesions: The most obvious changes in affected animals are the edema, widely distributed hemorrhages, and general hyperemia. In most cases, there is an edematous swelling of the head, neck, and brisket region. Incision of the swellings reveals a clear or straw-colored serous fluid. The edema is also found in the musculature, and the subserous petechial hemorrhages, which are found throughout the animal, are particularly characteristic. Blood-tinged fluid is often found in the pericardial sac and in the thoracic and abdominal cavities. Petechial hemorrhages are particularly prominent in the pharyngeal and cervical lymph nodes. Gastroenteritis is seen only occasionally and, unlike pneumonic pasteurellosis, pneumonia usually is not extensive.

Diagnosis: Some characteristic epidemiologic and clinical features aid in the recognition of HS. Of particular significance is a history of earlier outbreaks and a recent failure to vaccinate. Sporadic cases are more difficult to diagnose clinically. The season of the year, rapid course, and high herd incidence, with fever and edematous swellings indicate typical HS. Characteristic necropsy lesions support the clinical diagnosis. Although typical outbreaks are not difficult to recognize clinically, particularly in endemic regions, acute salmonellosis, anthrax, pneumonic pasteurellosis, and rinderpest should be considered.

A presumptive diagnosis is based on the isolation of P multocida from the blood and vital organs of an animal with typical signs. Definitive diagnosis depends on identifying the serotype as B:2 (or closely related serotypes) or E:2. Other serotypes cause various infections in cattle and buffalo but not typical HS. The passive mouse protection test using specific B:2 and E:2 immune rabbit sera is used in Asia and Africa to identify these serotypes. More precise tests, such as indirect hemagglutination, coagglutination, and counterimmunoelectrophoresis and immunodiffusion tests, are available in some laboratories.

If there is postmortem decomposition, the causative agent may be overgrown and obscured by extraneous bacteria. In such cases, the subcutaneous inoculation of mice or rabbits with small amounts of blood and tissue suspensions facilitates the recovery of the pasteurellae in pure or nearly pure culture.

Serologic tests are of no value in diagnosis. However, the indirect hemagglutination procedure and passive mouse protection test are of value in determining the immune status of animals.

Treatment and Prevention: Various sulfonamides, tetracyclines, penicillin, and chloramphenicol (where its use is permitted) are effective if administered early. Because of the rapid course of the disease and the frequent difficulty of access to animals, antimicrobial therapy often is not practicable. Although multiple antibiotic resistance has been reported for some strains of P multocida, it has not been described for the HS serotypes.

The principal means of prevention is by vaccination. Three kinds of vaccine are widely used: plain bacterin, alum-type precipitated bacterin, and oil-adjuvant bacterin. The most effective bacterin is the oil-adjuvant—one dose provides protection for 9-12 mo; it should be administered annually. The alum-precipitated-type bacterin is given at 6-mo intervals. Maternal antibody interferes with vaccine efficacy in calves. The oil-adjuvant vaccine has not been popular because of difficulty in syringing and occasional adverse tissue reactions. A live vaccine prepared from a B:3,4 serotype of deer origin is being used with reported success in southeast Asia.

MALIGNANT CATARRHAL FEVER

(Malignant head catarrh, Snotsiekte, Catarrhal fever, Gangrenous coryza)

Malignant catarrhal fever (MCF) is an acute, sporadic, infectious disease of cattle and some other Bovidae and Cervidae characterized by low morbidity and extremely high mortality, although on occasions, morbidity can be high, particularly in susceptible species such as Pere David’s deer and Bali cattle. MCF has become an important disease of farmed deer.

Etiology: While MCF is a single clinicopathologic entity, there are at least two distinct but related agents that can cause the disease naturally. One, alcelaphine herpesvirus 1 (AHV-1), which is carried inapparently by wildebeest (Connochaetes taurinus and C gnou), is prevalent in Africa and in zoological parks and is responsible for “wildebeest-derived” MCF (WD-MCF). The other principal cause is the “sheep-associated” agent of MCF. While the sheep-associated agent has not been isolated, molecular and serologic evidence indicate it is similar to AHV-1. It has been designated ovine herpesvirus 2 (OHV-2). It occurs worldwide in both wild and domestic sheep and goat species, usually without causing disease. Experimentally, both MCF agents can be transmitted to cattle,

deer, rabbits, and hamsters with blood or lymph node cells from affected animals. WD-MCF can also be transmitted to guinea pigs and rats with infected lymph node cells. In addition, viruses similar to AHV-1 isolated from two other species of alcelaphine antelope, topi, and hartebeest have caused MCF after experimental inoculation in cattle. Herpesviruses isolated from these two species are distinct from AHV-1 and have been designated AHV-2.

Transmission and Epidemiology: AHV-1 is transmitted vertically and horizontally in populations of wildebeest. Some wildebeest calves, infected in utero, later spread virus in nasal and ocular secretions and feces to others, most being infected by 6 mo of age. Fomites, particularly those contaminated at calving, are thought to be able to carry infection to susceptible ruminants. It seems likely that a similar scenario occurs with the sheep-associated agent at lambing time, but the evidence to support this hypothesis is circumstantial. Under farm conditions, MCF usually occurs in adults. In zoos, juveniles may also be affected. Deer farms can experience a 1% annual mortality. Susceptibility to MCF among deer varies: wapiti and red deer are much more susceptible than cattle; Pere David’s, sika, and white-tailed deer are highly susceptible. Disease has not been reported in fallow deer, which suggests they are resistant. Susceptible ruminants are “end hosts,” which develop clinical MCF after an incubation period of 3 wk to 6 mo. In such animals, the virus is cell-associated, and horizontal transmission is believed to be rare. Epidemiologic and serologic evidence suggest that cattle and farmed deer may become subclinically infected and may much later develop clinical disease after episodes of severe stress.

Pathogenesis: The most plausible current hypothesis is that development of the disease hinges on infection of immunoregulatory, large, granular lymphocytes, with “natural-killer” (NK) activity. The normal MCF reaction has the characteristics of a T-lymphocyte hyperplasia, a polyclonal response resulting from deregulation of the T lymphocytes. It is suggested that the necrotizing process of the terminal phase of the disease is an autoimmune phenomenon arising through the expression of NK-like activity of certain immune system cells.

Clinical Findings: MCF can take peracute, acute (“head and eye”), subacute, or chronic forms. Deer often have the peracute disease with sudden death, usually preceded by evidence of disseminated intravascular coagulation. Dyspnea also may be present. Acute disease is the most common in cattle and wild Bovidae. This form is characterized by fever, depression, enlarged lymph nodes, serous nasal and ocular discharges, erosions of the buccal papillae, ophthalmia, and diarrhea (sometimes hemorrhagic). Additional signs include inflammatory and erosive lesions in the mucosa of the upper respiratory tract that lead to profuse mucopurulent nasal discharge with encrustation of the muzzle, ulceration of the oral mucosa, salivation, and mucopurulent conjunctivitis with corneal opacity that begins at the corneoscleral junction and progresses centripetally. Hypopyon may be present. Patchy exanthema with matting of the hair and ulceration of the perineum, vulva, coronet, interdigital skin, and teats may occur. Some animals show CNS signs such as excitability, hyperesthesia, and muscular tremors. Occasionally, these may progress to convulsions or an aggressiveness

suggestive of rabies. The course of the “head and eye” form can last for up to 2 wk. In chronic MCF, inanition develops. Recovery is rare. The hemogram may show lymphocytosis followed by lymphopenia. Neutrophilia may occur if tissue damage is extensive.

Lesions: Lesions are widespread and usually affect all organs, but their severity and nature vary considerably. The principal changes that characterize MCF are epithelial necrosis (GI, respiratory, or urinary) associated with mucosal or dermal lymphoid inflammation, lymphoproliferation, interstitial infiltration of nonlymphoid tissues, and vasculitis. In deer, hemorrhage into the lumen of the ileum, cecum, and colon may be prominent, together with ecchymoses in the colonic serosa. Most lymph nodes are hyperplastic and the paracortex is prominent, but hemorrhage and necrosis may occur terminally. A prominent feature is interstitial infiltration of organs by lymphocytes, especially heart, liver, adrenal gland, meninges, CNS perivascular spaces, and kidney; in the kidney, it can be detected grossly as white, raised foci under the capsule. Vascular lesions can occur in most body systems and vary in intensity from a mild lymphocytic infiltration of the adventitia to transmural lesions resulting in fibrinoid necrosis, lymphoid infiltration, and occasionally thrombosis. In chronic cases, proliferative changes in the walls of affected vessels may lead to their enlargement and prominence. The rete mirabilis is a tissue of choice to demonstrate vascular lesions.

Diagnosis: MCF is a clinicopathologic entity and, while it may be suspected clinically, confirmation relies on histologic examination, especially the demonstration of multisystemic lymphoid infiltration, disseminated vasculitis, and degenerative epithelial lesions. When AHV-1 is the cause, virus isolation from the buffy coat of antemortem and postmortem blood samples can confirm a diagnosis. Antibody that reacts with this virus can be detected in serum from wildebeest, sheep, and some clinically affected animals. Virus neutralization, indirect immunofluorescence, immunoperoxidase, and conventional ELISA and competitive inhibition ELISA (CIE) may provide evidence of infection. MCF antibodies may cross-react with other bovine herpesviruses in the immunofluorescence, immunoperoxidase, and ELISA tests, but not by CIE or virus neutralization. A virus neutralization or CIE titer of 1:4 or greater is considered diagnostic of MCF. The polymerase chain reaction (PCR) can be used to detect and amplify MCF virus-specific DNA segments in tissues of animals clinically infected with MCF or in virus carriers. It is the most sensitive test for detecting asymptomatic virus carriers and, when properly applied, is highly specific. Differential diagnoses include rinderpest, bluetongue, vesicular diseases, East Coast cattle fever (Theileria parva), infectious bovine rhinotracheitis, bovine viral diarrhea and mucosal disease, shipping fever, and when there are nervous signs, rabies and the tick-borne encephalitides. When farmed red deer are affected, yersiniosis must be excluded.

Treatment and Control: Survival is rare. Antibiotics or sulfonamides to control secondary bacterial infection and supportive therapy (fluids) may be worthwhile in valuable animals, but if these survive, they will likely remain virus carriers. The separation of susceptible animals from the source of infection, ie, wildebeest, goats, and sheep, is essential to prevent the disease.

PARATUBERCULOSIS

(Johne’s disease)

Paratuberculosis is a chronic, contagious enteritis characterized by persistent and progressive diarrhea, weight loss, debilitation, and eventually death. It affects cattle, sheep, goats, llamas, camels, farmed deer, and other domestic, exotic, and wild ruminants. It has also been recognized in wild rabbits; horses and pigs can be infected experimentally. Distribution is worldwide.

There are conflicting data on the involvement of the causative organism in Crohn’s disease, a chronic enteritis in man. Animals with paratuberculosis should be considered as potential zoonotic risks until the situation is clarified.

Etiology and Pathogenesis: The causative organism is Mycobacterium avium in utero paratuberculosis, formerly known as M paratuberculosis or M johnei. Occasionally, other M avium subspecies are isolated from cases. The organism is quite resistant and can survive on pasture for >1 yr, but sunlight, alkaline soils, and drying reduce its survival rate. It is shed in large numbers in feces of infected animals, and infection is acquired by ingestion of contaminated feed and water. Introduction of the disesae into a clean herd is usually by subclinically infected carriers.

Infection is acquired early in life, but clinical signs rarely develop in cattle <2 yr old. Resistance increases with age, and cattle first exposed as adults are unlikely to become infected. Most calves are infected soon after birth either by nursing udders contaminated with feces from infected animals or by being housed in contaminated pens. The organism can also be present in colostrum and milk of infected cows, and intrauterine infections have also been described. After ingestion, the bacteria infect macrophages in the mucosa of the lower small intestine and in associated lymph nodes. Most animals will eliminate infection by an early cell-mediated immune response that encourages microbicidal activity in macrophages. In susceptible animals, the organisms multiply and provoke a chronic enteritis that leads to clinical disease. This may take months to years to develop and is usually paralleled by a decline in cell-mediated immunity and a rise in ineffective serum antibody. However, fecal shedding begins before clinical signs are apparent. Mycobacterium avium paratuberculosis can be isolated from feces, mesenteric and ileocecal lymph nodes, thickened intestinal wall, and less frequently the udder and the reproductive tracts of both sexes.

Clinical Findings: The disease is characterized by weight loss and diarrhea, but initial signs are variable and often vague. Diarrhea may be intermittent. It is typically thick; does not contain blood, mucus, or epithelial debris; and is passed without tenesmus. Over weeks or months, the diarrhea becomes more severe, there is further weight loss, coat color may fade, and ventral and intermandibular edema may develop. In dairy cattle, milk yield may drop or fail to reach expected levels. Animals are alert, and temperature and appetite are usually normal, although thirst may be increased. The disease is progressive and ultimately terminates in emaciation and death. Most cases

occur in cattle 2-6 yr old. The protein-losing enteropathy leads to low concentrations of total protein and albumin in plasma, although gamma globulin levels are normal. In infected groups, the mortality rate may be only ~1%, but up to 50% of animals may have subclinical infection with associated production losses. The disease in sheep and goats is similar, but diarrhea is not a common feature, and advanced cases may shed wool easily. In deer, the course of the disease can be more rapid.

Lesions: At necropsy, carcasses may be emaciated and edematous. Lesions can be mild, but typically the distal small intestinal wall is diffusely thickened with a nonulcerated mucosa thrown into prominent transverse folds. Some strains of the organism cause yellow-orange mucosal pigmentation. Lesions may extend proximally and distally to the jejunum and colon. Serosal lymphangitis and enlargement of mesenteric and other regional lymph nodes is usually apparent. Histologically, there is a granulomatous enteritis characterized by the progressive accumulation of epithelioid cells and giant cells in the mucosa and submucosa of the gut. Often, there is no correlation between clinical signs and the severity of lesions. Sheep, goats, and deer sometimes develop foci of caseation with calcification in the intestinal wall and lymph nodes.

Diagnosis: There is no single, good test for paratuberculosis, and a combination of tests is often used. Diagnosis for a group of animals is easier than in an individual. Fecal culture is highly specific but not highly sensitive because animals may shed organisms intermittently, and repeated testing may be necessary to confirm infection. Culture of M avium paratuberculosis requires mycobactin, a growth factor derived from mycobacteria, and is a slow process, taking up to 3 mo. Some strains in sheep are particularly difficult to culture. Microscopical examination of Ziehl-Neelsen-stained fecal samples for mycobacteria has an even lower sensitivity but may give a rapid result in a heavy shedder. Gene probes specific for M avium paratuberculosis DNA, such as IS900, can be used on feces in conjunction with polymerase chain reaction amplification techniques. This is a highly sensitive and rapid procedure that can detect infection when very few organisms are present, although fecal material may reduce the test sensitivity.

Serologic tests vary in sensitivity and specificity, although they are all generally poorer at identifying subclinical cases. The complement fixation test has low sensitivity and specificity. The agar gel immunodiffusion test is widely used but not very sensitive. ELISA has quite high sensitivity, and specificity is improved by preabsorbance of sera

with M phlei. Tests of cell-mediated immunity, such as the intradermal Johnin test, lymphocyte transformation test, and interferon stimulation test, are of unproven specificity and may be negative in advanced clinical cases.

Necropsy with histopathologic examination of intestine and lymph nodes is usually effective for diagnosis. Ziehl-Neelsen stains for acid-fast bacteria usually reveal abundant mycobacteria in lesions; however, in some cases, a careful search may be necessary to confirm their presence. In live animals, rectal punch biopsies or scrapings

have a very poor sensitivity. Although biopsies of intestinal and mesenteric lymph nodes are very sensitive, their use is limited in practice.

Control: No satisfactory treatment is known. Control requires good sanitation and management. Herds with confirmed cases should be tested to determine the extent of infection, and positive animals sent to slaughter. Retesting, at 6-mo to 1-yr intervals, should be continued until three or more consecutive negative tests are obtained. Calves should be removed from cows immediately after birth, bottle-fed colostrum that has been pasteurized or obtained from negative cows, and then reared completely segregated from adults until >1 yr old. Because intrauterine infection can occur, calves from dams that have or develop signs of the disease should be culled. Even if replacements are from herds believed to be free of the disease, they should be tested before and after purchase. More general procedures to minimize fecal contamination across the farm can also help, eg, raising food and water troughs, providing piped water in preference to ponds, harrowing frequently to disperse feces on pasture, etc.

In many countries, use of vaccines is subject to approval by regulatory agencies and restricted to infected herds. Vaccines, like the diagnostic tests, are limited in efficacy by the current lack of knowledge about the important antigens of the organism. Calfhood (<1 mo of age) vaccination can be effective in reducing disease incidence but does not eliminate infection. Cattle inoculated with inactivated whole-cell, mineral-oil vaccine develop granulomas, one to several inches in diameter, at the site of inoculation (brisket) and may react positively on subsequent tuberculin tests. Accidental self-inoculation can result in severe acute reactions with sloughing and chronic synovitis and tendinitis. Vaccination does not eliminate the need for good management and sanitation.

PESTE DES PETITS RUMINANTS

Peste des petits ruminants (PPR) is an acute or subacute viral disease of goats and sheep characterized by fever, necrotic stomatitis, gastroenteritis, and pneumonia. It is also known as pseudorinderpest of small ruminants, pest of small ruminants, goat plague, pest of sheep and goats, stomatitis-pneumoenteritis syndrome, contagious pustular stomatitis, and pneumoenteritis complex. Sheep are less susceptible than goats; cattle are only subclinically infected. Man is not at risk.

Etiology and Epidemiology: The causal virus, a Morbillivirus of the family Paramyxoviridae (see also RINDERPEST), QuickSearch has a particular affinity for lymphoid tissues and epithelial tissue of the GI and upper respiratory tracts, in which it produces characteristic lesions.

PPR is prevalent in West and Central Africa and the Middle East. Outbreaks that affect only a few animals frequently are not reported; epidemics occur when the population of susceptible animals increases. Such an epidemic may eliminate the goats and sheep in an area. PPR virus is transmitted by aerosol and invades the body through the epithelial tissue of the upper respiratory tract. The virus is disseminated to contact animals before

the onset of clinical signs. Secretions and excretions of sick animals are the sources of infection. As in rinderpest, it is generally accepted that there is no carrier state; however, subclinical cases may spread the infection during their incubation phase. White-tailed deer are fully susceptible; these and other wild ruminants may play a role in the epidemiology of the disease. Pigs with experimentally induced subclinical infections do not transmit the disease to susceptible pigs or goats; therefore, pigs may have no role in PPR epidemiology. Although cattle are susceptible to infection, they usually do not exhibit clinical signs or transmit the disease.

Clinical Findings: In the acute form of PPR, body temperature rises suddenly to 104-106°F (40-41°C). Affected animals appear ill and restless and have a dull coat, dry muzzle, congested mucous membranes, and depressed appetite. Early, the nasal discharge is serous; later, it becomes mucopurulent and gives a putrid odor to the breath. The incubation period is usually 4-5 days. Small areas of necrosis may be seen on the mucous membrane on the floor of the nasal cavity. The conjunctiva is frequently congested, and the medial canthus may exhibit a small degree of crusting. Some animals develop a profuse catarrhal conjunctivitis with matting of the eyelids. Necrotic stomatitis affects the lower lip and gum and around the insertions of the incisor teeth; in more severe cases, it may involve the dental pad, palate, cheeks and their papillae, and the tongue. Picture Diarrhea may be profuse and is accompanied

by dehydration and emaciation; hypothermia and death follow, usually after 5-10 days. Bronchopneumonia, characterized by coughing, may develop later. Pregnant animals may abort. Morbidity and mortality rates are higher in young animals than in adults. Latent infections may be activated and complicate the clinical picture. Sheep and, less frequently, goats develop a subacute disease, with a longer incubation period and a longer disease course, characterized by a slight fever, nasal catarrh, recurring erosions of the oral mucosa, and intermittent diarrhea, often followed by recovery.

Lesions: Emaciation, conjunctivitis, and stomatitis are seen; necrotic lesions are seen inside the lower lip and on the adjacent gum, the cheeks near the commissure, and on the ventral surface of the tongue. In severe cases, the lesions may extend to the hard palate and pharynx. The erosions are shallow, with a red, raw base and later become pinkish white; they are bounded by normal epithelium that provides a sharply demarcated margin. The rumen, reticulum, and omasum are rarely involved. The abomasum exhibits regularly outlined erosions that have a red, raw floor and ooze blood.

Severe lesions are less common in the small intestines than in the mouth, abomasum, or large intestines. Streaks of hemorrhages, and less frequently erosions, may be present in the first portion of the duodenum and terminal ileum. Peyer’s patches are severely affected; entire patches of lymphoid tissue may slough. The large intestines are usually more severely affected, with lesions occurring around the ileocecal valve and at the cecocolic junction and rectum. The latter exhibits streaks of congestion along the folds of the mucosa resulting in the characteristic “zebra-striped” appearance.

Petechiae may appear in the turbinates, larynx, and trachea. Bronchopneumonia may be present.

Diagnosis: A presumptive diagnosis, based on clinical, pathologic, and epidemiologic findings, may be confirmed by virus isolation and identification. The specimens required are unclotted blood, lymph nodes, tonsils, spleen, and lung. Detection of viral antigens by complement fixation or agar-gel precipitin tests does not differentiate the disease from rinderpest. Detection of virus-neutralizing antibodies with a rising titer in surviving animals is diagnostic. PPR and rinderpest can be differentiated by a cross-neutralization test or by ELISA, using purified antigens and specific monoclonal antibodies. PPR must be differentiated from other acute GI infections, respiratory infections (eg, contagious caprine pleuropneumonia), and such other diseases as contagious ecthyma, bluetongue, heartwater, coccidiosis, and mineral poisoning.

Control: When PPR is suspected, state and federal authorities should be notified. Eradication is recommended when the disease appears in countries previously free of PPR; rinderpest eradication methods are useful. There is no specific treatment; however, treatment for bacterial and parasitic complications decreases mortality in affected herds. An attenuated PPR viral vaccine has been prepared in embryonic caprine kidney cell culture; it affords protection from natural disease for ~1 yr.

Rinderpest cell culture vaccine also has been used successfully for immunization against PPR.

RIFT VALLEY FEVER

Rift Valley fever (RVF) is a peracute or acute zoonotic disease of domestic ruminants in Africa. Signs of the disease tend to be nonspecific, rendering it difficult to recognize individual cases. During epidemics, the occurrence of numerous abortions and deaths among young animals, together with an influenza-like disease in man, tends to be characteristic.

Etiology and Epidemiology: Rift Valley fever virus is a typical Bunya virus. It has a three-segmented, single-stranded, negative-sense RNA genome with a molecular weight of 4-6 106, and each of the segments, L (large), M (medium), and S (small), is contained in a separate nucleocapsid within the virion. No significant antigenic differences have been demonstrated between RVF isolates from many countries, but differences in pathogenicity are seen. The disease is endemic in tropical regions of Africa, and cyclic epidemics have occurred at 5- to 20-yr intervals in drier areas. The cycles are normally associated with periods of abnormally heavy rainfall. In the periods between epidemics, the virus is believed to be dormant in eggs of the mosquito Aedes lineatopennis in the dry soil of grassland depressions (dambos). With adequate rainfall, the infected mosquitoes develop and infect ruminants, which amplify the virus. The virus is spread by many species of mosquitoes characteristic of different regions. The incidence of RVF peaks in late summer. After the first frost, both the disease and

vectors may disappear. In warmer climates where insect vectors are present continuously, seasonality is usually not seen.

Man is also readily infected through aerosols from infected animals, their tissues, aborted fetuses, and laboratory procedures, and has the potential to introduce the disease (via mosquitoes) to animals in uninfected areas.

Clinical Findings: The incubation period is 12-36 hr in lambs. A biphasic fever of up to 41¡C may develop, and affected animals are listless and disinclined to move or feed and may show signs of abdominal pain. Lambs usually die within 2 days. Older animals may die acutely or develop an inapparent infection. Sick animals may regurgitate and develop a bad-smelling diarrhea and icterus. Sometimes, abortion may be the only sign of infection. In pregnant ewes, the mortality and abortion rates vary from 5 to almost 100% in different outbreaks and on different farms. The rates in cattle are usually <10%.

Lesions: The hepatic lesions are similar in all species and vary mainly with the age of the infected individual. The most severe lesions seen in aborted fetuses and newborn

lambs are moderately to greatly enlarged, soft, friable livers with irregular congested patches. Numerous grayish white necrotic foci are invariably present but may not be clearly visible. Hemorrhage and edema of the wall of the gallbladder and mucosa of the abomasum are common. Intestinal contents are dark chocolate-brown. In all animals, the spleen and peripheral lymph nodes are enlarged and edematous and may have petechiae. In man, RVF is usually inapparent or associated with moderate to severe, nonfatal influenza-like illness. A minority may develop severe disease with ocular lesions, encephalitis, and severe hepatic lesions.

Diagnosis: RVF should be suspected when abnormally heavy rains are followed by the widespread occurrence of abortions and mortality among newborn animals characterized by necrotic hepatitis, and when hemorrhages and influenza-like disease occur in people handling animals or their products. The virus can readily be isolated from tissues of aborted fetuses and the blood of infected animals. The titer of virus in these tissues is often high enough to use organ suspensions as antigen for a rapid diagnosis in neutralization, complement fixation, or agar gel diffusion tests; however, these tests should be supplemented by isolation in suckling mice or hamsters injected intracerebrally or in cell cultures such as baby hamster kidney (BHK21), monkey kidney (Vero), CER, or primary kidney and testis cell cultures of lambs. Demonstration of RVF antigen by polymerase chain reaction is not yet available for routine diagnostic purposes.

All conventional serologic tests can be used to detect antibody against RVF virus and are helpful in epidemiologic studies. In some areas, however, serologic surveys may be complicated by cross-reactions between RVF virus and other phleboviruses. The use of serologic tests for diagnosis is of limited value.

Control: Control of vectors, movement of stock to higher altitudes, and confinement of stock in insect-proof stables are usually not practical, instituted too late, and of little value. Immunization remains the only effective way to protect livestock. The mouse neuro-adapted Smithburn strain of RVF virus can readily be produced in large quantities, is inexpensive, and induces a durable immunity 6-7 days after inoculation. It should normally not be used for the protection of pregnant animals because it may cause abortion, congenital defects, and hydrops amnii of the fetus; however, its use in pregnant ewes may be contemplated in the face of an outbreak when its adverse effects may be outweighed by the dangers of a natural infection. Although not proved, it is theoretically possible for the attenuated virus to revert to full virulence. A plaque variant and a mutagen-induced strain have been investigated as potential vaccine strains but have not been accepted as replacements for the Smithburn strain. Outbreaks of RVF cannot be predicted and are usually of sudden onset. Therefore, it is advisable to immunize lambs on a regular basis at 6 mo of age, which should afford lifelong protection. The offspring of susceptible ewes can be immunized at any age.

Pregnant ewes and cattle should preferably be vaccinated with a formalin-inactivated vaccine. Revaccination after 3 mo is advisable to induce an immunity that will last ~1 yr and will confer colostral immunity to the offspring.

People involved in the livestock industry should be made aware of the potential dangers of exposure to RVF-infected animals and tissues. The use of protective clothing and treatment with immune plasma and the antiviral drug ribavirin should be considered when necessary.

RINDERPEST

(Cattle plague)

Rinderpest is a disease of cloven-hoofed animals characterized by fever, necrotic stomatitis, gastroenteritis, lymphoid necrosis, and high mortality.

Etiology and Epidemiology: The cause is a Morbillivirus closely related to the viruses of peste des petits ruminants, QuickSearch canine distemper, QuickSearch and measles. Strains of the virus vary markedly in host range and virulence. Sera from recovered or vaccinated cattle cross-react with all rinderpest viral strains in neutralization tests, but minor antigenic differences have been demonstrated. The virus is fragile and rapidly inactivated by heat and light but remains viable for long periods in chilled or frozen tissues.

In epidemic form, rinderpest is the most lethal plague known in cattle. Susceptibility varies among species: it is high in African buffalo, giraffes, wild Suidae, Tragelaphinae, and breeds of cattle such as Ankole, Channel Islands, and Japanese Black; moderate in wildebeest and East African zebus; and mild in gazelles and small domestic ruminants. Rinderpest in hippopotami and European pigs takes a subclinical form. It is endemic in

India and Africa. Lack of control in bordering countries has recently led to epidemics in west, east, and north Africa; the Near East; and parts of Asia.

In endemic areas, young cattle become infected after maternal immunity disappears and before vaccine immunity begins, with possible auxiliary cycles in sheep, goats, and wild ungulates. In epidemic areas, the virus infects most susceptible animals and tends to limit itself unless the population is large enough to support endemicity. In endemic areas, morbidity is low and clinical signs are often mild; in epidemics, morbidity is often 100% and mortality up to 90%.

Transmission and Pathogenesis: The virus is present in small amounts in nasal secretions of affected animals 1-2 days before fever; levels are high in secretions and excretions during the first week of clinical disease and decrease rapidly as animals develop antibody and begin to recover. Transmission requires direct or close indirect contact; infection is via the nasopharynx. There is no carrier state; the virus maintains itself by continuous transmission among susceptible animals.

After primary growth in lymph nodes associated with the nasopharynx, the virus

proliferates throughout the lymphoid tissue and spreads via the blood to the mucosae of the GI and upper respiratory tracts. Tissue damage is caused by viral cytopathology. Viral antigens induce a potent immune response that controls the infection and allows recovery if tissue damage is not too severe.

Clinical Findings: An incubation period of 3-15 days is followed by fever, anorexia, and depression. Oculonasal discharge develops 1-2 days later. Within 2-3 days, pinpoint necrotic lesions, which rapidly enlarge to form cheesy plaques, appear on the gums, buccal mucosa, and tongue. The hard and soft palates are often affected. Picture The oculonasal discharge becomes mucopurulent, and the muzzle becomes dry and cracked. Diarrhea, the final clinical sign, may be watery and contain blood, mucus, and mucous membrane. Animals show severe abdominal pain, thirst, and dyspnea and may die from dehydration. Convalescence is prolonged and may be complicated by concurrent infections due to immunosuppression.

Lesions: Gross pathologic changes are evident throughout the GI and upper respiratory tracts, either as areas of necrosis and erosion, or congestion and hemorrhage, the latter causing classic “zebra-striping” in the rectum. Lymph nodes may be enlarged and edematous, with white necrotic foci in the Peyer’s patches. Histologic examination reveals lymphoid and epithelial necrosis with viral syncytia and intracytoplasmic inclusions.

Diagnosis: Clinical and pathologic findings may be sufficient for diagnosis in endemic areas and after initial laboratory confirmation of an epidemic. In areas where the disease is uncommon or absent, laboratory tests must be used to differentiate rinderpest from bovine viral diarrhea in particular, as well as East Coast fever, foot-and-mouth disease, infectious bovine rhinotracheitis, and malignant catarrhal fever. Virus

isolation and detection of specific viral antigens in affected tissues are standard tests, and demonstration of rising antibody titers is useful. Simple, rapid tests for antigen detection (immunodiffusion or counterimmunoelectrophoresis) are valuable in the field. ELISA, for detecting viral antigens or serum antibodies, is also a valuable diagnostic test.

Specimens for the laboratory must be collected from several animals during the early stages of clinical disease, preferably before onset of diarrhea. Uncoagulated whole blood, lymphoid tissue, spleen, and gut lesions should be collected aseptically and transported frozen or refrigerated.

Control: Treatment usually is not attempted, but supportive nursing care with fluid and antibiotics may aid recovery of valuable animals. Active immunity is usually lifelong; maternal immunity lasts 6-11 mo. Control in endemic areas is by immunization of all cattle and domestic buffalo >1 yr old with attenuated cell culture vaccine. Vaccines produced from heat-stable attenuated strain are preferred in hot climates (ie, tropical and subtropical Africa) where maintenance of refrigeration is not always guaranteed. In these areas, outbreaks are controlled by quarantine and “ring vaccination” and

sometimes slaughter of affected animals. In epidemic areas, the disease is best controlled by slaughter and quarantine. Control of animal movement is paramount because most outbreaks are due to introduction of infected cattle. Countries free of the disease and that border endemic areas must be extremely vigilant or vaccinate as a precaution. There are commitments to eradicate rinderpest globally through mass vaccination; cooperative efforts of national, regional, and internal organizations in Africa and Asia in eradication campaigns are promising.

TICK PYEMIA

Tick pyemia affects lambs 2-12 wk old and is characterized by debility, crippling lameness, and paralysis. Pyemic abscesses are found commonly in joints but may be present in virtually any organ. The disease causes significant economic loss through debilitation and death of lambs. The disease is enzootic in many regions of the UK and Ireland where the tick Ixodes ricinus is common and is likely to be present in other parts of Europe where the same tick is found.

Etiology: Staphylococcus aureus is regarded as the main cause of the pyemic abscesses because it has been isolated consistently from superficial and deep-seated lesions and it is rare to find other bacteria. The bacteria are believed to gain entry into the bloodstream either by direct inoculation during tick feeding or from local superficial wounds or through the infected umbilicus. However, there is clinical and experimental evidence that I ricinus does not simply act as a vector directly injecting staphylococci into the bloodstream. The main role of I ricinus is as a vector of the rickettsial agent Ehrlichia (Cytoecetes) phagocytophila, which causes tick-borne fever (TBF) QuickSearch which in turn creates factors favorable to development of pyemia. Lambs affected with TBF suffer from severe leukopenia, and their peripheral blood neutrophils

are less capable of phagocytizing and killing S aureus. Experimental studies have shown that lambs with TBF were more susceptible to experimental infections with S aureus during the period of neutropenia and that up to 30% of lambs with TBF may develop staphylococcal infections.

The epidemiology of the disease is closely related to the biology of I ricinus. The disease is limited to areas populated by I ricinus and to seasons of the year climatically favoring high tick population and activity.

Clinical Findings: Abscesses form in various parts of the body, mainly in the joints, tendon sheaths, and muscles, resulting in lameness—hence the common use of the term “crippled lambs.” In some outbreaks, >30% of lambs may be affected, and they are usually dull and lame and often suffer from loss of body condition. Internal abscesses without joint lesions may result in no clinical signs other than the loss of condition, but when lesions are present in the CNS, there may be ataxia, paraplegia, or other nervous signs. The crippling disease lasts for days or weeks, but the disease may also appear as an acute septicemia. On occasion, there may be sudden deaths resulting from multiple internal abscesses without other visible signs. Up to 50% of affected lambs may die, and the survivors recover slowly.

Lesions: Apart from the joints and other superficial structures, abscesses are commonly found in the liver, lungs, and kidneys. They may also be present in the meninges of the spinal cord and in the pericardium and myocardium. The diaphragm, thymus, and adrenal glands are less commonly affected. Ticks are often found attached to an inflamed area.

Diagnosis: History and clinical signs are valuable indicators. The restriction of the disease to tick-infested areas, its occurrence during seasons of tick activity, and the demonstration of E phagocytophila in bloodsmears of affected lambs or other sheep in the flock are diagnostic features. Isolation of S aureus from lesions and the absence of other bacteria will help to confirm tick pyemia. The loss of condition and ill-thrift without lameness may be difficult to recognize as tick pyemia, and the acute condition can be confused with other septicemic diseases. Tick pyemia may also resemble other suppurative infections of the newborn, including navel ill and joint ill due to infections by other bacteria such as streptococci and Actinomyces pyogenes.

Control: Control of tick infestation is the most effective way of prevention. This can be achieved either by restricting lambs and ewes to low-ground, tick-free pastures for the first few weeks of life or by dipping ewes before lambing and administering acaricides as dips or smears on lambs. In young lambs, pour-on preparations of cypermethrin or smears applied before lambs are moved from lambing fields to hill pastures are reported to be effective in controlling ticks.

Administration of long-acting oxytetracycline at the time of risk can help prevent both TBF and tick pyemia during the first weeks of life. A single injection at double the standard dose at 3 wk of age can significantly reduce mortality and morbidity in young

hill lambs on tick-infested pasture and improve weight gains and condition in the remainder. Prophylactic treatment with a long-acting antibiotic may prevent development of TBF for up to 3 wk, without pyrexia and immunosuppression, so that the incidence of tick pyemia and other infections such as pasteurellosis and colibacillosis are reduced. Although treatment with oxytetracyline may inhibit the development of immunity to TBF, if the lambs eventually develop TBF, they are several weeks older and apparently less susceptible to tick pyemia. Deliberate exposure of lambs to TBF by injections, followed by treatment with oxytetracycline, could provide some immunity to TBF before the lambs enter tick-infested areas; however, strains specific to the area must be used because some strains of E phagocytophila have no cross-immunity.

Treatment of clinical cases of tick pyemia with penicillin or tetracycline can be effective, provided the lesions are not too advanced.

CANINE DISTEMPER

(Hardpad disease)

Canine distemper is a highly contagious, systemic, viral disease of dogs seen worldwide. It is characterized by a diphasic fever, leukopenia, GI and respiratory catarrh, and frequently pneumonic and neurologic complications. The disease occurs in Canidae (dogs, foxes, wolves), Mustelidae (eg, ferret, mink, skunk), most Procyonidae (eg, raccoon, coati mundi), and some Viverridae (binturong).

Etiology and Pathogenesis: Canine distemper is caused by a paramyxovirus closely related to the viruses of measles and rinderpest. The enveloped virus is sensitive to lipid solvents and most disinfectants and is relatively unstable outside the host. The main route of infection is via aerosol droplet secretions from infected animals. Some infected dogs may shed virus for several months.

Virus replication initially occurs in the lymphatic tissue of the respiratory tract. A cell-associated viremia results in infection of all lymphatic tissues, which is followed by infection of respiratory, GI, and urogenital epithelium, as well as the CNS. Disease follows virus replication in these tissues. The degree of viremia and extent of spread of virus to various tissues is moderated by the level of specific humoral immunity in the host during the viremic period.

Clinical Findings: A transient fever usually occurs 3-6 days after infection and there may be a leukopenia (especially lymphopenia) at this time, but these signs may go unnoticed. The fever subsides for several days before a second fever occurs, which lasts <1 wk. This may be accompanied by serous nasal discharge, mucopurulent ocular discharge, and anorexia. GI and respiratory signs may follow and are usually complicated by secondary bacterial infections. An acute encephalomyelitis may occur in association with or immediately after the systemic disease, or in the absence of systemic manifestations. Hyperkeratosis of the footpads (“hardpad” disease) and epithelium of the nasal plane may be seen. Neurologic signs are frequently seen in

those dogs with hyperkeratosis. CNS signs include the following: 1) localized involuntary twitching of a muscle or group of muscles (myoclonus, chorea, flexor spasm, hyperkinesia), such as in the leg or facial muscles; 2) paresis or paralysis, often beginning in the hindlimbs evident as ataxia, followed by ascending paresis and paralysis; and 3) convulsions characterized by salivation and chewing movements of the jaw (petit mal, “chewing-gum fits”). The seizures become more frequent and severe, and the dog may then fall on its side and paddle its legs; involuntary urination

and defecation (grand mal seizure, epileptiform convulsion) often occur. A dog may exhibit any or all of these neurologic signs in addition to others in the course of the disease. Infection may be mild and inapparent or lead to severe disease manifest by most of the above signs. The course of the systemic disease may be as short as 10 days, but the onset of neurologic signs may be delayed for several weeks or months.

Chronic distemper encephalitis (old dog encephalitis [ODE]), a condition often marked by ataxia, compulsive movements such as head pressing or continual pacing, and incoordinated hypermetria, may occur in adult dogs without a history of signs related to systemic canine distemper. Convulsions and neuromuscular twitching (chorea) do not seem to occur with ODE. Although canine distemper antigen has been detected in the brain of dogs with ODE by fluorescent antibody staining, dogs with ODE are not infectious and replication-competent virus has not been isolated. The disease is caused by an inflammatory reaction associated with persistent canine distemper virus infection in the CNS.

Lesions: Thymic atrophy is a consistent postmortem finding in young puppies. Hyperkeratosis of the nose and foot pads may be present. Depending on the degree of secondary bacterial infection, bronchopneumonia, enteritis, and skin pustules may also be present. Histologically, canine distemper virus produces necrosis of lymphatic tissues, interstitial pneumonia, and cytoplasmic and intranuclear inclusion bodies in respiratory, urinary, and GI epithelium. Lesions found in the brain of dogs with neurologic complications include neuronal degeneration, gliosis, demyelination, perivascular cuffing, nonsuppurative leptomeningitis, and intranuclear inclusion bodies predominantly within glial cells.

Diagnosis: Distemper should be considered in the diagnosis of any febrile condition in puppies with multisystemic manifestations. While the typical clinical case is not difficult to diagnose, the characteristic signs sometimes fail to appear until late in the disease. The clinical picture may be modified by concurrent toxoplasmosis, neosporosis, coccidiosis, parasitoses, and numerous viral and bacterial infections. Distemper is sometimes confused with other systemic infections such as leptospirosis, infectious canine hepatitis, or Rocky Mountain spotted fever. Intoxicants such as lead or organophosphates can cause simultaneous GI or neurologic sequelae. A febrile catarrhal illness with neurologic sequelae justifies a clinical diagnosis of distemper. At necropsy, diagnosis is usually confirmed by histologic lesions or immunofluorescent assay for viral antigen in tissues, or both. In dogs with multisystemic signs, conjunctival, tracheal, vaginal or other epithelium, or the buffy coat of the blood can be examined by

immunofluorescent assay. These samples are usually negative when the dog is showing only neurologic manifestations or when circulating antibody is present (or both). The diagnosis can then be made by serologic demonstration of virus-specific IgM or an increased ratio of CSF to serum virus-specific IgG.

Treatment: Treatments are directed at limiting secondary bacterial invasion, supporting the fluid balance and overall well-being of the dog, and controlling nervous

manifestations. Antibiotics, electrolyte solutions, protein hydrolysates, dietary supplements, antipyretics, nasal preparations, analgesics, and anticonvulsants are used. No one treatment is specific or uniformly successful. Dogs may recover completely from systemic manifestations, but good nursing care is essential. Despite intensive care, some dogs do not make a satisfactory recovery. Unfortunately, treatment for neurologic manifestations of distemper are unsuccessful. If the neurologic signs are progressive or severe, the owner should be appropriately advised.

Prevention: Successful immunization of pups with canine distemper modified live virus (MLV) vaccines depends on the lack of interference by maternal antibody. To overcome this barrier, pups are vaccinated with MLV vaccine when 6 wk old and at 2- to 4-wk intervals until 16 wk old. Measles virus induces immunity to canine distemper virus in the presence of relatively greater levels of maternal distemper antibody. An MLV measles vaccine and a combination of MLV measles and MLV canine distemper vaccine are available. These vaccines must be administered IM. Pups 6-7 wk old should receive the measles or combination vaccine and at least two more doses of MLV distemper vaccine when 12-16 wk old. Many varieties of attenuated distemper vaccine are available and should be used according to manufacturers’ directions. Annual revaccination is suggested because of the breaks in neurologic distemper that can occur in stressed, diseased, or immunosuppressed dogs.

CANINE HERPESVIRAL INFECTION

Canine herpesvirus is a fatal, viral infection of puppies worldwide. It also may be associated with upper respiratory infection or a vesicular vaginitis or posthitis in adult dogs. Only canids (dogs, wolves, coyotes) are known to be susceptible.

Etiology: The disease is caused by an enveloped DNA canine herpesvirus (CHV), which is sensitive to lipid solvents and most disinfectants. CHV is relatively unstable outside the host.

Transmission usually occurs by contact between susceptible puppies and the infected oral, nasal, or vaginal secretions of their dam or oral or nasal secretions of dogs allowed to commingle with puppies during the first 3 wk of life. In utero transmission may occur.

Infection of newborn susceptible puppies results in replication of CHV in the surface cells of the nasal mucosa, pharynx, and tonsils. If the pups become hypothermic, viremia and invasion of visceral organs occur.

Clinical Findings: Deaths due to CHV infection usually occur in puppies 1-3 wk old, occasionally in puppies up to 1 mo old, and rarely in pups as old as 6 mo. Typically, onset is sudden, and death occurs after an illness of 24 hr. Older dogs exposed to or experimentally inoculated with CHV may develop a mild rhinitis or a vesicular vaginitis or posthitis. In utero infections may be associated with abortions, stillbirths, and infertility.

Lesions: The characteristic gross lesions consist of disseminated focal necrosis and hemorrhages. The most pronounced lesions are seen in the lungs, cortical portion of the kidneys, adrenal glands, liver, and GI tract. All lymph nodes are enlarged and hyperemic, and the spleen is swollen. Lesions may also occur in the CNS. The basic histologic lesion is necrosis with hemorrhage in the adjacent parenchyma. Most often there is no inflammatory reaction. Single, small, basophilic, intranuclear inclusion bodies are most common in areas of necrosis in the lung, liver, and kidneys; occasionally, they occur as faintly acidophilic bodies located within the nuclear space.

Diagnosis: CHV infection may be confused with infectious canine hepatitis, QuickSearch but it is not accompanied by the thickened, edematous gallbladder often associated with the latter. The focal areas of necrosis and hemorrhage, especially those that occur in the kidneys, distinguish it from hepatitis and neosporosis.

QuickSearch CHV causes serious disease only in very young puppies. The rapid death and characteristic lesions distinguish it from canine distemper. QuickSearch The virus can be isolated from fresh lung, liver, kidney, and spleen by cell culture techniques. The tissues should be submitted to the laboratory refrigerated but not frozen.

Control: No vaccine is available. Infected bitches develop antibodies, and litters subsequent to the first infected litter receive maternal antibodies in the colostrum. Puppies that receive maternal antibodies may be infected with the virus, but disease does not result.

Removal of puppies from affected bitches by cesarean section and rearing them in isolation has prevented deaths under experimental conditions. However, infections have been noted even in puppies delivered by cesarean section. Deaths may be reduced when infected puppies are reared in incubators at increased temperatures (95°F [35°C], 50% relative humidity) and given adequate fluids and supportive therapy. The prognosis of puppies that survive neonatal infections of CHV is guarded because damage to lymphoid organs, brain, kidneys, and liver may be irreparable.

Anaplasmosis

Anaplasmosis, formerly known as gall sickness, is a disease of ruminants caused by obligate intraerythrocytic parasites of the order Rickettsiales, family Anaplasmataceae, genus Anaplasma. Cattle, sheep, goats, deer, antelope, giraffes, and buffalo may be infected. Bovine anaplasmosis is of economic significance in the cattle industry and is the focus of this discussion.

Anaplasmosis occurs in tropical and subtropical regions worldwide (~40° N to 32° S), including South and Central America, the USA, southern Europe, Africa, Asia, and Australia.

Etiology: Clinical bovine anaplasmosis is usually caused by A marginale. Cattle are also infected with A caudatum, which may result in severe disease, and A centrale, which generally results in mild disease. Anaplasma ovis may cause mild to severe disease in sheep, deer, and goats.

Transmission and Epidemiology: Anaplasmosis is not contagious. Most transmission occurs via numerous species of tick vectors. At least 16 species of ticks of 7 genera (Boophilus, Dermacentor, Rhipicephalus, Ixodes, Hyalomma, Argas, and Ornithodoros) have been shown experimentally to transmit A marginale. Not all of these are likely to be significant vectors in the field. Boophilus species are major vectors in Australia and Africa, and Dermacentor species have been incriminated as the main vectors in the USA. After feeding on an infected animal, intrastadial or trans-stadial transmission may occur. Transovarial transmission may also occur, although this is rare, even in the one-host Boophilus species. A replicative cycle occurs in the infected tick. Mechanical transmission via biting dipterans occurs in some regions. Transplacental transmission has been reported and is usually associated with acute infection of the dam in the second or third trimester of gestation. Anaplasmosis may also be spread through the use of contaminated needles or dehorning or other surgical instruments.

There is a strong correlation between age of cattle and severity of disease. Calves are much more resistant to disease (although not infection) than older cattle. This resistance is not due to colostral antibody from immune dams. In endemic areas where cattle first become infected with A marginale early in life, losses due to anaplasmosis are minimal. After recovery from the acute phase of infection, cattle remain chronically infected carriers of the parasite and immune to further clinical disease. However, these chronically infected cattle may relapse to anaplasmosis when immunosuppressed (eg, by corticosteroids), when infected with other parasites, or after splenectomy. Carriers serve as a reservoir for further transmission. Serious losses due to anaplasmosis occur when mature cattle with no previous exposure are moved into endemic areas or under endemically unstable situations when transmission rates are insufficient to ensure all cattle are infected before reaching the more susceptible adult age.

Clinical Findings: Anaplasmosis is characterized by progressive anemia due to extravascular destruction of infected and uninfected erythrocytes. The prepatent period of A marginale is directly related to the infective dose and typically ranges from 2 to 8 wk. Parasitemia approximately doubles every 24 hr during the exponential growth phase. Generally, 10-30% of erythrocytes are infected at peak parasitemia, although this figure may be as high as 65%. Erythrocyte count, PCV, and hemoglobin values are all severely reduced. Macrocytic anemia with circulating reticulocytes may be present late in the disease. There is moderate anisocytosis, slight polychromasia, and an increase in unconjugated bilirubin in the serum.

Acutely infected animals lose condition rapidly. Milk production falls. Inappetence, loss of coordination, breathlessness when exerted, and a rapid bounding pulse are usually evident in the late stages. The urine may be brown but, in contrast to babesiosis, hemoglobinuria does not occur. A transient febrile response, with the temperature rarely exceeding 106°F (41°C) occurs at about the time of peak parasitemia. Mucous membranes appear pale and then yellow. Pregnant cows may abort. Surviving cattle convalesce over several weeks, during which hematologic parameters gradually return to normal.

There is no clear evidence that Bos indicus cattle are any more resistant to infection than B taurus breeds.

Lesions: The carcasses of cattle that die from anaplasmosis are generally markedly anemic and jaundiced. Blood is thin and watery. The spleen is characteristically enlarged and soft, with prominent follicles. The liver may be mottled and yellow-orange. The gallbladder is often distended and contains thick brown or green bile. Hepatic and mediastinal lymph nodes appear brown. Epicardial and pericardial petechiae and ecchymoses are often present. Widespread phagocytosis of erythrocytes is evident on microscopical examination of the reticuloendothelial organs. A significant proportion of erythrocytes are usually found to be parasitized after death due to acute infection.

Diagnosis: Anaplasma marginale, together with the hemoprotozoa Babesia bovis and B bigemina, are the causative agents of tick fever in cattle. These three parasite species have similar geographic distributions, except that anaplasmosis occurs in the absence of babesiosis in the USA. Microscopical examination of Giemsa-stained thin and thick blood films is critical to distinguish anaplasmosis from babesiosis and other conditions that result in anemia and jaundice, such as leptospirosis and theileriosis. Blood in anticoagulant should also be obtained for hematologic testing. In Giemsa-stained thin blood films, Anaplasma spp appear as dense, homogeneously staining blue-purple inclusions 0.3-1.0 m in diameter. Picture Anaplasma marginale inclusions are usually located toward the margin of the infected erythrocyte, whereas A centrale inclusion bodies are located more centrally. Anaplasma caudatum cannot be distinguished from A marginale using Giemsa-stained blood films. Special staining techniques are used to identify this species based on observation of characteristic appendages associated with the parasite. Inclusion bodies contain between 1 and 8 initial bodies 0.3-0.4 m in diameter, which are the individual rickettsia.

Chronically infected carriers may be identified with a fair degree of accuracy by serologic testing using either complement fixation or card agglutination tests. Experimental DNA-based detection methods have been reported but are not yet in general use.

At necropsy, thin blood films of liver, kidney, spleen, lungs, and peripheral blood should be prepared for microscopical examination.

Treatment: The tetracyclines and imidocarb are currently used for treatment. Cattle may be sterilized by treatment with these drugs and remain immune to severe anaplasmosis subsequently for at least 8 mo.

Prompt administration of tetracycline, chlortetracycline, or oxytetracycline in the early stages of acute disease (eg, PCV >15%) usually ensures survival. A commonly used treatment consists of a single IM injection of long-acting oxytetracycline at 20 mg/kg. Blood transfusion to partially restore the PCV greatly improves the survival rate of more severely affected cattle. The carrier state may be eliminated by administration of a long-acting oxytetracycline preparation (20 mg/kg, IM), at least two injections with a 1-wk interval. Withholding periods for tetracyclines apply in most countries. Injection into the neck muscle rather than the rump is preferred.

Imidocarb is also highly efficacious against A marginale as a single injection (as the dihydrochloride salt at 1.5 mg/kg, SC, or as imidocarb dipropionate at 3.0 mg/kg). Elimination of the carrier state requires the use of higher repeated doses of imidocarb (eg, 5 mg/kg, IM or SC, two injections of the dihydrochloride salt 2 wk apart). Imidocarb is a suspected carcinogen with long withholding periods and is not approved for use in the USA or Europe.

Prevention: In some countries, infection with live A centrale is used as a vaccine to protect cattle against severe disease due to subsequent infection with the more pathogenic species A marginale. Anaplasma centrale vaccine produces severe reactions in a small proportion of cattle. In the USA, where A centrale vaccine cannot be used, a vaccine comprising nonliving A marginale purified from infected bovine erythrocytes is available. Immunity generated using this killed vaccine protects cattle from severe disease on subsequent infection. Instances of isoerythrolysis in suckling calves have occurred due to prior vaccination of dams with preparations of this vaccine

that contained bovine erythrocytic material. Long-lasting immunity against A marginale is conferred by preimmunization with the live parasite, combined with the use of chemotherapy to control severe reactions. The use of attenuated strains of A marginale as a live vaccine has been reported. In some areas, sustained stringent control or elimination of the arthropod vectors may be a viable control strategy.

RICKETTSIAL DISEASES IN DOGS

Canine Ehrlichiosis

There are a number of newly identified ehrlichial species that infect dogs. The classic disease is an acute to chronic disease that is caused by infection of mononuclear cells by Ehrlichia canis and is transmitted by the brown dog tick, Rhipicephalus sanguineus. It is endemic in many parts of the USA and occurs worldwide. Various tick vectors harbor the other recognized ehrlichial species. Ehrlichia ewingi is a granulocytic species that has been isolated from dogs in the southern, western, and midwestern USA. Human granulocytic ehrlichiosis is caused by an as yet unnamed ehrlichial species in

the northern midwest and northeastern USA. This species also causes clinical disease in dogs. Ehrlichia equi is closely related to the human granulocytic ehrlichiosis (HGE) agent and infects dogs and horses in the western USA, while E phagocytophila infects dogs and ruminant species in Europe. Ehrlichia chaffeensis, a cause of human monocytic ehrlichiosis, and E risticii, the cause of equine monocytic ehrlichiosis, can experimentally infect dogs but naturally occurring disease with these species is not well defined. Ehrlichia platys is the cause of infectious cyclic thrombocytopenia and infects only platelets; it results in minimal if any hemorrhagic tendencies in dogs. Ehrlichia chaffeensis and the HGE agent, which infect people, are acquired from infected ticks. There is no evidence of direct transmission from dogs to man. Acute, naturally occurring human and canine illness with ehrlichial species mimics Rocky Mountain spotted fever. The discussion below primarily describes infection in dogs caused by E canis.

Etiology: The causative agent is seen rarely, appearing as colonies of coccoid bodies in the cytoplasm of usually monocytes. The brown dog tick (Rhipicephalus sanguineus) is the primary vector and reservoir and may transmit the disease for up to 5 mo after engorgement. Blood transfusions, or other means by which infected WBC are transferred, also transmit the disease.

Clinical Findings: Signs arise from the involvement of the hemic and lymphoreticular systems and commonly progress from acute to chronic, depending on the strain of organism and immune status of the host. In acute cases, there is reticuloendothelial hyperplasia, fever, generalized lymphadenopathy, splenomegaly, and thrombocytopenia. Variable signs of anorexia, depression, loss of stamina, stiffness and reluctance to walk, edema of the limbs or scrotum, and coughing or dyspnea may occur. Most acute cases occur in the warmer months, coincident with the greatest activity of the tick vector.

In the acute phases, the hemogram is usually normal but may reflect a mild normocytic, normochromic anemia; leukopenia; or mild leukocytosis. Thrombocytopenia is common, but petechiae may not be evident, and platelets may not be obviously decreased on a blood smear. Vasculitis and immune-mediated mechanisms induce a thrombocytopenia and hemorrhagic tendencies. Lymph node aspiration reveals hyperplasia.

In the acute stage, death is rare; spontaneous recovery may occur, the dog may remain asymptomatic, or chronic disease may ensue. In chronic cases, the bone marrow becomes hypoplastic (a unique feature of E canis infection), and lymphocytes and plasmacytes infiltrate various organs. Depending on which organs are affected, and to what degree, signs are variable and appear without regard to season. Clinical findings may include marked splenomegaly, glomerulonephritis, renal failure, interstitial pneumonitis, anterior uveitis, and meningitis with associated cerebellar ataxia, depression, paresis, and hyperesthesia. Severe weight loss is a prominent finding.

The hemogram is usually markedly abnormal in the chronic state. Frequently, severe thrombocytopenia may cause hemorrhagic diathesis. In dolichocephalic breeds, epistaxis is common. Hematuria, melena, and petechiae and ecchymoses of the skin

and mucous membranes occur in all breeds. Variably severe pancytopenia (mature leukopenia, nonregenerative anemia, thrombocytopenia, or any combination thereof) may occur. Aspiration cytology reveals reactive lymph nodes and, usually, marked plasmacytosis. Frequently, polyclonal, or occasionally monoclonal, hypergammaglobulinemia occurs.

Lesions: During the acute stage, lesions generally are nonspecific, but splenomegaly and heavy, discolored lungs are common. Histologically, there is lymphoreticular hyperplasia, and lymphocytic and plasmacytic perivascular cuffing. In chronic cases, these lesions may be accompanied by widespread hemorrhage and increased mononuclear cell infiltration of organs.

Diagnosis: Because thrombocytopenia is a relatively consistent finding, a platelet count is an important screening test. Clinical diagnosis is confirmed by demonstrating the organisms within WBC, although this can be fortuitous. Low numbers of organisms make demonstration difficult, except in the acute phase before treatment. More commonly, a diagnosis is made by a combination of clinical signs, positive indirect serum fluorescent antibody (FA) titer, and response to treatment. The antibody response may be delayed up to 28 days; thus, serologic testing may not be a reliable diagnostic tool early in the course of the disease. Serologic cross-reactivity is strong between E canis, E chaffeensis, and E ewingi and between E equi, HGE agent, and E phagocytophila. These reactions should be considered in appropriate geographic areas.

During the acute stage, differential diagnoses include other causes of fever and lymphadenomegaly (eg, Rocky Mountain spotted fever, brucellosis, blastomycosis,

endocarditis); immune-mediated diseases, especially thrombocytopenia and systemic lupus erythematosus; and lymphosarcoma. During the chronic stage, differential diagnoses include estrogen toxicity, myelophthisis, immune-mediated pancytopenia, and other multisystemic diseases associated with specific organ dysfunction (eg, glomerulonephritis).

Treatment: The drug of choice for all forms of ehrlichiosis is tetracycline (22 mg/kg, PO, t.i.d.) for a minimum of 2 wk in acute cases, 1-2 mo in chronic cases. Because of better intracellular penetration, doxycycline (5-10 mg/kg, PO or IV, daily for 10-14 days) is effective in some cases in which tetracycline fails. Two doses of imidocarb dipropionate (5-7 mg/kg, IM), 2 wk apart, are variably effective against both ehrlichiosis and babesiosis; however, the drug is not approved for use in the USA. In acute cases, the temperature returns to normal within 24-48 hr after treatment, and the dog becomes more active and begins to eat. In chronic cases, the hematologic abnormalities may persist for 3-6 mo, although clinical response occurs much sooner. Supportive therapy may be necessary to combat wasting and specific organ dysfunction; platelet or whole-blood transfusions may be required if hemorrhage is extensive. The E canis antibody titer should be measured again within 6 mo of illness to confirm a seronegative status indicative of successful therapy. Serum titers that persist at lower but positive levels should be rechecked in another 6 mo to make certain that they are not increasing.

Prevention: Prevention is enhanced by controlling ticks and using indirect FA-negative blood donors. Tetracycline (6.6 mg/kg, PO, daily) is an effective preventive in kennels in which ehrlichiosis is endemic. Treatment must be extended for many months through at least one tick season to hopefully eliminate the endemic cycle.