Review Article Oorsigartikel

Canine distemper infections, with special reference to South Africa, with areview of the literature

A L Leisewitza, A Carterb, M van Vuurenc and L van Blerkd

INTRODUCTIONCanine distemper (CD) is a severe,

life-threatening disease with a worldwidedistribution. It was first reported inEurope in 17618. During the first half ofthe 19th century it was the most lethaldisease of dogs. It affects mainly domesticdogs but has become a serious problem ina wide range of hosts, threatening captiveand free-ranging wildlife populationsincluding several marine mammals such

as seals, dolphins and whales18,161. Out-breaks of disease with significantmortality continue to occur amongst do-mestic dogs even in developed communi-ties with high levels of vaccination26,51,82. Itis caused by a Morbillivirus (of the familyParamyxoviridae) similar to the humanmeasles virus, which has decimatedhuman populations for centuries andremains a significant cause of childhoodmortality in developing countries. Rin-derpest, caused by a related Morbillivirus,was responsible for great cattle plagues inEurope and Africa where it caused mas-sive die-offs around the turn of the 20thcentury. It remains a threat to animalhealth in some developing countries tothis day5. CD is well established in SouthAfrica and continues to threaten domesticdogs and wild animals2,5,70,102,130,132,151. It is

incumbent upon the veterinary profes-sion as guardians of animal health to beunrelenting in tackling this problem.

AETIOLOGYCanine distemper virus is an enveloped

virus with a single-stranded, linear, nega-tive polarity RNA genome. The latter isenclosed in a rod-shaped, helical nucleo-capsid. The morphology is pleomorphicand the size varies between 100250 nm,although many virions can be muchlarger and filamentous forms have beenobserved. It is a labile virus that is sensi-tive to heat, UV irradiation, lipid solvents,detergents and oxidising agents86. It cansurvive at room temperature in tissuesand exudates for between 20 minutes and3 hours. In environmental temperaturesbelow zero it will survive several days ifprotected by organic material62.

EPIDEMIOLOGYCanine distemper has a worldwide

distribution and affects a wide range ofmammalian hosts (domestic and wild,land and sea-living)13,23. The virus isreadily transmitted between susceptiblehosts and the dog remains the mostimportant reservoir for the infection. Be-cause morbilliviruses do not persist in aninfectious form following an acute infec-tion, and infection results in life-longimmunity in recovered hosts, the virusneeds a constant supply of susceptiblehosts for its maintenance17. Virus is shedin the stool, saliva, urine and conjunctivaland nasal exudates during the acutephase of the infection.

In a developing peri-urban communityof southern Africa, disease incidence washighest in the spring and early summermonths of August to November49. The ageat which the highest incidence of infectionoccurs depends on whether the diseaseis enzootic or epizootic. In a susceptiblepopulation, all ages are susceptible26. En-zootic disease, on the other hand, tends toaffect mainly young dogs96. Aerosol anddroplets are the most important means oftransmission59. Virus may be shed forbetween 60 and 90 days following infec-tion, although shorter periods are moretypical62. This has bearing on the decision

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aDepartment of Companion Animal Medicine, Faculty ofVeterinary Science, University of Pretoria, Private BagX04, Onderstepoort, 0110 South Africa.

bFourways Veterinary Hospital, PO Box 68159,Bryanston, 2021 South Africa.

cDepartment of Veterinary Tropical Diseases, Faculty ofVeterinary Science, University of Pretoria, Private BagX04, Onderstepoort, 0110 South Africa.

dKenilworth Veterinary Hospital, 47 Kenilworth Road,Cape Town, 7708 South Africa.

Received: October 2000. Accepted: August 2001.

ABSTRACTCanine distemper virus is a member of the genus Morbillivirus of the family Paramyxoviridaethat causes severe disease in dogs and a range of wild mammals. The clinical signs relateessentially to the respiratory, gastrointestinal and central nervous systems. In South Africa,infection with Ehrlichia canis and canine parvovirus may present similarly. Many dogs willinitially present with a wide range of central nervous system signs without any history ofsystemic disease. A recent South African study evaluating ante mortem diagnosis high-lighted the importance of recognising clinical signs, cerebrospinal fluid IgG titres, serumIgM titres and immunocytochemistry of epithelial tissue. A 2-year retrospective evaluationof cerebrospinal fluid samples collected from dogs presented to the Onderstepoort Veteri-nary Academic Hospital indicates that distemper infection is common, and this diseaseshould routinely be suspected in cases of diverse neurological disease in dogs. The SouthAfrican dog population is specifically at high risk for the disease because of the large pool ofunvaccinated, reproductively-active dogs that expose the wildlife resources of the countryto risk of fatal disease. Outbreaks of disease in dogs continue to occur in developed anddeveloping communities in both vaccinated and unvaccinated dogs worldwide, and havealso been described in a wide range of free-ranging wildlife, including seals, dolphins andlions, and in endangered zoo animals. Modified live virus vaccines have contributedmarkedly to disease control in the dog population but have caused mortality in some wildcarnivores. New recombinant vaccines are being developed that will be safe in wildanimals. The pathogenesis of CNS demyelination has been compared to various importantdemyelinating diseases in humans and, amongst other things, relates to down-regulationof the oligodendrocyte gene coding for myelin synthesis and non-immunocyte CNS cellexpression of type II major histocompatibility receptors. Early CNS lesions are character-ised by demyelination and later lesions by perivascular round cell cuffing. Treatment issupportive.

Key words: canine distemper virus, dog, South Africa, wildlife.Leisewitz A L, Carter A, Van Vuuren M, van Blerk L Canine distemper infections, withspecial reference to South Africa, with a review of the literature. Journal of the South AfricanVeterinary Association (2001) 72(3): 127136 (En.). Department of Companion Animal Medi-cine, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort,0110 South Africa.

clinicians need to make about the admis-sion of suspect cases to their hospitals thatcould expose the hospital population tothe CDV. As a rule, cases that present withacute catarrhal disease should not beadmitted to an in-patient facility unlessvery strict isolation and barrier nursingcan be applied. Specimens should becollected from these cases for diagnosis,and the dogs should then be managed asoutpatients. A scenario that becomes verydifficult to control is an isolation wardwhere puppies with Parvovirus infectionare kept. The 2 diseases have clinicalfeatures in common and CD can be devas-tating in a ward of Parvovirus-infectedpuppies. Chronic CD that manifests withneurological signs or nasodigital hyper-keratosis is far less likely to be a source ofinfection. Because CDV remains a differ-ential diagnosis in many cases showingCNS disease, it becomes impractical toseparate these cases from the rest of thehospital population. The risk in such situ-ations is so low that for practical purposesit can be regarded as insignificant. Dogsrecovering from the infection are usuallyimmune, are not persistently infected anddo not shed the virus. Transplacental in-fection has been reported93, although thegeneral consensus is that morbillivirusesare not transmitted vertically86.

The infection rate is significantly higherthan the disease rate, with between 25and 75 % of susceptible dogs clearing theinfection without showing any clinicaldisease62. A constant supply of puppiesor susceptible dogs is required to main-tain the infection in a population. Severaloutbreaks of CDV infection have beendescribed in both vaccinated and unvac-cinated dog populations in developedand developing environments26,51,82,130. In across-sectional study of the canine popu-lation in a rural town in southern Africa,5.5 % of the dogs examined were diag-nosed with active CDV infection130. In asecond study that compared the diseasestatus of canine hospital patients fromdeveloped communities with those fromdeveloping communities over a 3-yearperiod, a remarkable difference wasfound between the 2 groups. While 43.6 %of the dogs from the developing commu-nity had infectious disease, only 8.2 % ofdogs from the developed communitywere similarly diagnosed. CD was diag-nosed in 4.1 % of the infectious diseasecases in the developing community dogs,but infection with CDV was rarely diag-nosed in the dogs from the developedcommunity49. This highlights the epide-miological requirement for CDV to persistin dogs, and the risk to certain southernAfrican dog populations. The disease isconstantly active in developing commu-

nities, where outbreaks can be expected.Owners in these communities must beencouraged to be especially vigilant inkeeping to annual vaccination practices.The estimated dog population in SouthAfrica varies widely but conservativeestimates place it at around 4 million(1999/2000). Approximately 1 milliondogs will visit a veterinarian at least oncea year (Pfizer and Intervet companies,South Africa, pers. comm., 1999). This im-plies that the greater proportion of theSouth African dog population is unvacci-nated and probably also reproductivelyactive. The conditions for persistentlarge-scale CDV disease are thereforeideal, as it has been estimated that at least300 000 individuals are required to main-tain a Morbillivirus in circulation23. Thissituation poses a threat to the domesticdog population, but the threat it poses tothe wildlife resource of the country isequally serious.

PATHOPHYSIOLOGYDogs are exposed to the virus by aerosol

droplets contacting the upper respiratorytract epithelium11,62. The virus multipliesthere in tissue macrophages and spreadsvia the lymphatics to the tonsils andrespiratory tract lymph nodes within 24hours16,92. By d 2-4 other lymphoid tissuesare infected, and by day 6 the spleen, gas-trointestinal mucosa and hepatic Kupffercells are infected. By this time there is asystemic reaction to the infection charac-terised by pyrexia and lymphopaenia.Further haematogenous spread of virusoccurs to involve other epithelial tissuesand the central nervous system (CNS).Virus has been demonstrated to travelcell-free in plasma as well as associatedwith leukocytes and thrombocytes91,95. By2 weeks after infection, dogs with anadequate cellular and humoral responseclear the infection, and no clinical signs ofillness persist75. Intermediate immuneresponsiveness allows further spread ofvirus to epithelial tissues. Virus persists insome tissues (such as nervous tissue andepithelium, especially foot pads), result-ing in delayed clinical signs in thesetissues. A poor immune response at thispoint allows pan-systemic viral dissemi-nation that causes the gastrointestinaland respiratory signs seen with acuteinfection100,140. Serum antibody responsevaries inversely with the severity of thedisease75. The mortality in gnotobioticdogs is similar to that caused by naturalinfections, which discounts the impor-tance of secondary bacterial infections indisease severity despite their obviousinvolvement9698.

Canine distemper virus is a significantcause of immune suppression in host

animals100,124. Although the molecularbasis for immune suppression is knownfor only a few viral diseases, canine dis-temper is characterised by viral replica-tion or the presence of virus during theviraemic phase in T lymphocytes, Blymphocytes and macrophages. Necrosisand lymphoid depletion are the essentiallesions in the lymphoid tissues121. Infec-tion of cells involved in phagocytosis,antigen presentation and the non-specificeffector aspects of cell-mediated immu-nity in dogs with distemper leads to aninability to generate an effective in vivoimmune response134. Acutely-infecteddogs fail to respond to intradermal skintests with phytomitogens, and do notdevelop significant antibody responses.The latter suggests a direct viral effect onB cells or their precursor cells in the bonemarrow124.

Spread to the CNS depends on theefficacy of the initial immune responseand many dogs probably develop tran-sient CNS infections without clinicalsigns. Virus probably gains access to theCNS via infected white cells in the CSFand thrombocytes16,94. Antigen is firstdetected in CNS capillary and venularendothelium and perivascular astrocyticfoot processes21,157, but this occurs longbefore signs of CNS disease occur. Itseems plausible that viral access is via CSF,as the first lesions seen are in the peri-ventricular sub-pial tissue and in thechoroid plexus. Areas of predilection forinfection include the cerebral cortex, optictracts, cerebral peduncles, cerebellumand spinal cord21,131,135,153,157. Many factorsaffect the type of CNS lesions observed,including the efficacy of the immuneresponse and the neurotropic and im-mune-suppressive characteristics of theparticular virus strain144. Chronic lesionsare classically inflammatory and developin dogs unsuccessful at clearing the virusfrom the CNS27,120,157,165,166.

The neurobiology of demyelination inCDV infection is of particular interest andhas been the topic of several studies, as itis regarded as a model of human demye-linating disease36,37,42,142. The acute demye-linating lesions are non-inflammatory,develop during a period of severe im-mune suppression, and coincide withreplication of virus in glial cells of thewhite matter118,122,153,157. An obvious expla-nation for this would be infection of theoligodendrocytes, which are the myelin-producing cells. Viral protein is, however,only very rarely demonstrated in thesecells, and in fact it was initially believedthat oligodendrocytes could not becomeinfected with virus158. Astrocytes are themost commonly-infected cells. Althoughviral protein is not produced in oligoden-

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drocytes, viral mRNA corresponding toall viral genes has been demonstrated byin situ hybridisation in oligodendrocytes,and it has been shown that these cellswill support transcription of all CDVgenes167,168,168. Thus a restricted form ofoligodendrocyte infection occurs andmay well be the mechanism responsiblefor demyelination. Down-regulation ofmyelin gene transcription (the effectiveshutting-down of the oligodendroglialcell metabolic machinery) in the areas ofdemyelination in the CNS has beendemonstrated61.

The mechanisms of chronic demyeli-nation are varied, but appear to be relatedto the inflammation that characterises thisphase of the disease154,157. As the immunecompetence of the host recovers, lympho-cytes, plasma cells and macrophages ac-cumulate in the perivascular regions,leading to tissue damage and even necro-sis. This inflammation is characterised byintrathecal immunoglobulin production.Dogs that recover have decreasing levelsof immunoglobulin production in theCSF whereas increasing levels are associ-ated with progressive disease. Intrathecalproduction of antibodies to CDV is clearlyan important mechanism of immunity, asclearance of the virus from the CNS isassociated with recovery41,80,139,147,159,162. CSFanti-CDV titres may be higher than in theblood155. Intrathecally-produced antibod-ies to myelin are not a consistent featureof CNS CDV infection, and titres are alsonot related to disease severity162. A cell-mediated response to myelin basic pro-tein has been demonstrated in experi-mental disease but is poorly correlated tothe occurrence of demyelination145,155.Thus, although autoimmunity has beensuggested as a mechanism of progressiveneural injury in CDV infection, it remainsa topic of debate and is regarded by someto be an epiphenomenon not primarilyinvolved in the demyelinating process143,157.On the other hand, major histocompat-ibility complex class II (MHC II) expres-sion has been demonstrated in the CNS ofCDV-infected dogs, especially in themicroglia and astrocytes of the whitematter3,160. The brain is usually regardedas an immunologically-privileged site thatcannot express MHC II and thus can-not mount its own immune response.Aberrant antigen-presenting capacity bycells of the CNS has been reported inautoimmune-like diseases of the CNSsuch as multiple sclerosis, experimentalallergic encephalitis and Theiler s murineencephalomyelitis virus infection103.Expression of MHC II by microglia andastrocytes in CDV infections is probablya direct viral effect. The significance ofthis lies in the fact that microglia and

astrocytes expressing MHC II wouldallow presentation of self antigens thatcould lead to virus-independent demyeli-nation. In chronic lesions, MHC II wasfound to be highly expressed despite theabsence of viral antigen, suggesting thatdemyelination may be triggered and per-petuated by non-viral (self) antigens.

Antiviral immune responses havebeen blamed for bystander demyelina-tion28,65,66. Macrophages constitute an im-portant proportion of the cells thatrespond to infection. These cells havebeen shown to play an important role inproducing reactive oxygen species thatlead to selective destruction of oligo-dendrocytes as innocent bystander cells65,67. It has also been shown that CDVinfection leads to enhanced macrophageactivity and hence enhanced destructiveactivity30.

It is important to realise that viral clear-ance results in CNS recovery and onlypersistent infection of the CNS can lead toa progressive smouldering inflammatoryreaction. It is possible for virus to persistin white matter areas outside the inflam-matory demyelinating lesions or even inthe periphery of such lesions. Mecha-nisms allowing such persistence arepoorly understood but are obviouslyimportant to the evolution of disease andare the topic of ongoing investigation27.

Cytokines play a significant role in anydisease in which inflammation contrib-utes to the disease process. To date therehave been few studies evaluating theeffect of cytokines in CDV infections55,68.One of the hindrances to this type of workin dogs is the lack of species-specificreagents. One study evaluated mRNAexpression of interleukin 1(IL-1) , IL-6,IL-8, IL-12, tumour necrosis factor (TNF), interferon (IFN)and TGFinwhole blood using reverse transcriptasepolymerase chain reaction (rt-PCR)68. Nosignificant correlation between diseaseand cytokine mRNA expression wasdemonstrated. No cytokine transcriptscould be found in the blood of any of theacutely viraemic dogs with a high viralload. This may have been due to immunesuppression. Pro- and anti-inflammatorycytokines revealed no clear pattern ofexpression. It is highly probable that theform of disease, immune status of the in-dividual and time of sampling all impactstrongly on cytokine expression. Circu-lating levels of cytokine mRNA may alsobe poorly correlated with cytokine pro-tein production at tissue level. Anotherstudy examined mRNA expression ofvarious cytokines in the CSF of naturally-infected dogs55. IL-10 was found to be thedominantly-expressed cytokine, althoughboth pro- and anti-inflammatory cyto-

kines were co-expressed in most cases,making it very difficult to link expressionwith pathogenic effect. A further problemis the heterogeneous nature of CNS le-sions at any time. Both non-inflammatoryand chronic inflammatory lesions co-existin the same brain at any particular time.Furthermore, determining cytokinemRNA expression in CSF may not relateto what is actually taking place in thebrain tissue. One case with advancedchronic lesions expressed only TNF andIL-12. Ideally, it would be possible tostudy the cytokine kinetics in brain tissueover time at the same time as being able toevaluate inflammatory cell phenotype.

T-cell subsets in the brains of naturalCDV infections have been evaluated163,164.Acute demyelination (histologically non-inflammatory) is accompanied by a dif-fuse influx of CD8+ cells. It is suggestedthat these cells may be responsible forearly antibody-independent anti-viralcytotoxicity. The chronic inflammatoryperivascular cuffs consist mainly of CD4+lymphocytes, plasma cells and macro-phages. This may represent a phase ofantibody-dependent demyelination.These findings highlight the heteroge-neous and possibly, in part, immune-mediated plaque pathogenesis of CDVdemyelination.

CLINICAL SIGNSSigns vary depending on a number of

factors such as the age of the host, host im-mune competence, virus strain and organsystem affected45,74,96. It is estimated thatbetween 50 and 70 % of CDV infectionsare sub-clinical62. Mild forms of the dis-ease present with non-specific signs suchas listlessness, loss of appetite and fever.More specific presentations includeoculonasal discharge, coughing, dysp-noea, diarrhoea and vomiting. Thesesigns are not specific for CD and in theexperience of the authors Ehrlichia canisinfection may be an important differentialdiagnosis for practitioners in areas whereboth diseases are present. Both diseasescan present with similar secondary skinand chest infections and the typicalhaematological findings of leukopaenia,thrombocytopaenia and possibly a non-regenerative anaemia. Kennel cough maybe another differential diagnosis. It hasbeen the experience of the first author atthe Onderstepoort Veterinary AcademicHospital (OVAH) that some puppiesadmitted to the isolation ward for sus-pected parvovirus-induced diarrhoeawere subsequently found to suffer fromCDV infection. This presents a particu-larly severe problem, as the introductionof CDV to a group of severely immune-suppressed Parvovirus-infected puppies

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can be catastrophic. Problems with CDVinfection following discharge from hospi-tal of puppies that had parvovirus infec-tion is likely in these circumstances.

The form of the disease that presentsmost commonly depends especially onthe level of immune competence of thepopulation and age of the affected dogs.Clinical signs therefore vary widely154. Inunvaccinated dogs and unvaccinatedpuppies (of between 12 and 16 weeks ofage when maternal antibody titres havewaned) the severe generalised catarrhalform is most common. These dogs arefebrile, with mucopurulent conjunctivi-tis, upper respiratory tract signs, depres-sion, anorexia, vomiting and diarrhoea(which may be bloody). Severe dehydra-tion and weight loss may be present.Adequate treatment in this group mayreduce the mortality rate.

The most common form of the diseaseseen at the OVAH is the neurological formin older dogs. The disease occurs frequentlyenough to justify investigation for CDVinfection in all dogs showing non-surgicalspinal cord disease and in all dogs withbrain signs.

In an outbreak described in an urbandog population of the Copenhagen areain Denmark, more than half of the casespresented with the catarrhal form. Manyof these dogs also had CNS involvementand digital hyperkeratosis (so calledhard-pad)25. In an outbreak describedamongst unvaccinated sled dogs in north-ern Greenland (with a mortality of 1000 ofa population of around 3000), most diedwithin a week with classic catarrhal andnervous signs. Digital hyperkeratosis wasnot observed. Blindness was observed ina few survivors26. In a Finnish study of anoutbreak in approximately 5000 vacci-nated dogs with a mortality of 30 %, themajority showed classic catarrhal disease.A few cases of digital hyperkeratosis werenoted. Nervous signs were noted but theproportion was not reported51.

Typically, neurological signs followthe catarrhal signs within about 23weeks10,11,62. As demonstrated by previousstudies and observations cited above,many dogs will present for the first timewith neurological signs preceded by no oronly very mild signs. In one report, onlyone third of cases with neurologicaldisease had a previous history of systemicsigns, and another third had no previoushistory of disease at all148. Once CNSdisease manifests, it is usually progres-sive, without a relapsing course, althougha chronic relapsing course has been de-scribed73. Neurological disease oftenappears localised according to clinicalexamination, despite the diffuse viral andlesion localisation148. Signs are very varied

but commonly include seizures105, blind-ness, central vestibular disease52, cerebel-lar87,118,142,154, brain stem and spinal corddisease135. Myoclonus occurs in about onethird of cases and is most commonlyassociated with CDV, but should not beregarded as pathognomonic, as it is seenin other causes of menigoencephalo-myelopathies77,78,146. It is seen most com-monly in the muscles of mastication andthe limbs, but may be seen in any mus-cle group77,78. Naso-digital keratosis isreported to occur in about 8 % of caseswith neurological disease148. Bi- or unilat-eral optic neuritis is often the cause ofblindness. Swelling of the optic disc maybe seen in the acute disease. Ill-definedgrey to pink densities may be seen in thetapetal and non-tapetal regions. Chronicdisease may cause well-delineated hyper-refletic retinal areas24,57,88,107.The immunecompromising nature of the disease92,100,140

results in the frequent occurrence of sec-ondary bacterial infection of the skin andrespiratory tract which are features of theacute phase of the disease. The activationof other unusual and CNS infectionsowing to the immune suppression causedby CDV has been reported, and these in-fections include combinations of CDVand Toxoplasma gondii 5 0 , Neosporacaninum148, Filaroides hirthi32, cryptospori-diosis149, Pneumocystis carinii141 and canineinfectious hepatitis virus89.

CDV infection has been associated withinflammatory joint diseases and hypertro-phic osteodystrophy (HOD) in dogs20,116.In a study of experimental infection ofgnotobiotic dogs CDV infection of theproximal metaphyses of the humerus wasprominent. Antigen was abundant in themarrow, osteoclasts and osteoblasts,where it had a direct cytolytic effect, evencausing cell necrosis. There was a strongassociation between CDV antigen pres-ence and osseous changes. CDV antigenhas been reported in the bone of humanswith Pagets disease and an associationbetween dog ownership and this diseasein man has been postulated58. There is,however, very little evidence for this, andother studies have failed to support thishypothesis54,85,114,115,123,129. A similar linkbetween CDV infection and multiple scle-rosis in man has been suggested, but thereis no proof of this association, and CDVshould be regarded as an animal diseasewith no public health risk36,37,39,60,76,101,133. Inan earlier study linking CDV infectionand HOD, dogs injected with serum fromHOD patients developed CDV signs andthe infection was confirmed at necropsy69.CDV has also been demonstrated byrt-PCR, in situ hybridisation and southernblotting in the bone tissue of dogs withHOD112,113. It is possible that all dogs with

CDV infection develop sub-clinical bonedisease, especially considering that dogswith HOD develop constitutional signsof disease such as fever and malaise. In-fection during puppyhood may causeenamel hypoplasia in the permanentteeth seen in adulthood22,45.

A rare progressive condition that hascome to be called old dog encephalitis ispurported to occur in dogs that have sur-vived CDV infection. It is characterised bywidespread perivascular lymphoplasma-cytic infiltration in areas of demyelinationand neuronal degeneration. In somecases it has also been described as asclerosing panencephalitis with infiltra-tion and replacement of nervous tissue bya dense astrocytic network. Clinical signsare typically restricted to lesions typicalfor the cerebrum and exclude involve-ment of the brain stem, spinal cord andcerebellum104.

A link between juvenile pyoderma andCDV has been suggested. A pustular skindisease is not uncommon in CDV infec-tion and has been described as an allergicresponse to the virus6. In a recent report,3 of 4 dogs recently vaccinated with amodified live CDV vaccine developedjuvenile cellulitis and radiographic evi-dence of HOD. The skin disease wassuccessfully treated in all 4 dogs withprednisolone. Distemper-like skin lesionshave also been described followinginoculation with a modified live virusvaccine34. The concurrence of HOD andjuvenile cellulitis has been reported pre-viously35,106,117.

EXPERIENCE WITH THE DISEASE INSOUTH AFRICA

A retrospective study of cerebrospinalfluid samples submitted to the Depart-ment of Veterinary Tropical Diseases,Faculty of Veterinary Science, Universityof Pretoria, Onderstepoort, from dogswith clinical signs compatible with CDVinfection was conducted during 1998 and1999.

One hundred and thirty-three CSFsamples were submitted during this2-year period. Thirty-four (25 %) werefound to be positive for CDV-specific IgG.Of these 34 cases, 23 showed nervoussigns only (including tremors, vestibularsyndrome, hind-limb ataxia, progressiveataxia, seizures, myoclonus, depression,periodic neck pain, generalised body painand dementia). A combination of neuro-logical signs with gastrointestinal signswas seen in 4 cases. No dogs were ob-served to have a combination of neuro-logical and respiratory signs. The typicalnon-neurological signs seen were puru-lent oculonasal discharge (4 dogs) andrespiratory signs (2 dogs) that included

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pneumonia-induced coughing and in-creased respiratory rate. The gastrointes-tinal signs observed were anorexia,diarrhoea and vomiting (4 dogs). No casesof digital hyperkeratosis (so called hard-pad disease) were noted. Nineteen of the34 CSF CDV-specific IgG-positive dogswere less than a year old, and 12 of thesepresented with only neurological signs.The remaining 15 dogs were older than ayear, 2 being older than 7 years. Fourteenof these 15 dogs older than a year showedonly neurological signs. No seasonalincidence was noted over the 2 years. Theresults of this study highlighted severalimportant issues about the disease as it isseen at the OVAH. These include: CDV infection is sufficiently common in

the OVAH environment to justify rulingout this disease early in the course of theinvestigation of diverse neurologicalsigns. (See Diagnosis below).

Neurological signs are the most commonpresentation. Catarrhal signs combinedwith neurological signs are the nextmost common presentation. Catarrhalsigns alone are an infrequent presenta-tion. The neurological presentation isthe most common in all age groups andespecially common in dogs older thanone year.

Digital hyperkeratosis is an uncommonsign.At a satellite clinic of the Medical Uni-

versity of Southern Africa that served amore resource-deprived community thanthe OVAH, catarrhal signs alone and incombination with neurological signswere more common than neurologicalsigns alone 33,49,130.

In a retrospective evaluation of 154 dogbrains presented to the OVI for rabiesdiagnosis between July 1992 and July1993, a final diagnosis of CDV was madein 26 cases. Diagnosis was based onimmunoperoxidase staining for inclusionbodies in at least 1 of 6 standard sections.Other common diagnoses included cere-bral babesiosis, E. canis infection andgranulomatous meningoencephalitis (J Jvan der Lugt, pers. comm., 1999).

DIAGNOSISDiagnosing canine distemper in the live

animal with confidence remains difficult,yet very important because of the impli-cations of the disease. Identifying typicalclinical signs still remains the most impor-tant initial diagnostic step. Because thedisease often has a poor prognosis andhas a high infectious potential, antemortem confirmation of the diagnosis ispreferred. The many varied clinicalpresentations of the disease (especially inits neurological form) may also dictatethat clinical signs are by no means typical,

further emphasising the need for a defini-tive diagnostic test. Numerous differentmethods that have been used for diagno-sis testify to the difficulties experienced inthis regard.

The most common haematologicalfinding is lymphopaenia. In a local study86 % of 30 dogs with CDV infection werelymphopaenic33. Thrombocytopaeniamay also be present15. A left shift neutro-philia may be helpful in confirming thepresence of significant secondary bacte-rial infection. In the South African setting,because of the frequency of other concur-rent infectious diseases, haematologicalfindings may be varied if mixed infectionsare present. Other immune-suppressiveinfections (such as ehrlichiosis or parvovi-rus infections) can induce very similarhaematological abnormalities. This isespecially relevant because many of thedogs suspected of having distemperinfection live in developing communitieswhere infectious diseases are very com-mon and vaccination is seldom practised.Viral inclusions in the white or red cellsare rarely observed. Serum biochemistryis usually unremarkable. Radiology maydemonstrate an interstitial (viral) pneu-monia in early cases and an alveolarpattern with bacterial pneumonia later inthe course of the disease.

CSF and serological abnormalities andthe demonstration of viral inclusions inepithelial cell cytology (by fluorescentantibody techniques) are usually the mostpractical and useful means of confirminga diagnosis.

The diagnosis of inflammatory diseasesof the canine CNS is usually difficult, withat least one third of cases remainingundiagnosed in the live animal despitethorough investigation146. However, resultsof analysis of the CSF of CDV-infecteddogs are frequently useful in diagnosis.Raised protein content and lymphocyticpleocytosis are common1,41,64,81,139,146,156. Inthe study by Carter, 89 % of the 30 CDVpositive dogs examined had CSF proteinconcentrations in excess of 0.5 g/ (normalis

of CDV from a developing community.All dogs were euthanased and necropsiesperformed and CDV infection confirmedon histology of the brain. Analysis of theCSF for distemper yielded the followinginformation: The direct fluorescent antibody test for

viral antigens in cells from the CSF hada sensitivity of 27 % and a specificity of100 %.

The indirect fluorescent antibody testfor IgG in the CSF had a sensitivity of64 % and a specificity of 100 %. No IgMwas detected in any of the cases byusing this method.

89 % of dogs had a protein concentra-tion of 0.5 g or more.

Direct fluorescent antibody testing onconjunctival scrapings had a sensitivityof 37 % and a specificity of 100 %. Im-pressions made of the tissue of the foot-pad had a sensitivity of 23 % andspecificity of 100 %.Blood analysis showed:

An indirect fluorescent antibody (IFA)test for IgG in serum had a sensitivity of96 % but a specificity of 0 %.

IFA for IgM in the serum had a sensitiv-ity of 75 % and a specificity of 80 %.

86 % of cases had haematocrits below0.4 / and 49 % were thrombocytopaenic.

86 % were lymphopaenic.This study concluded that the following

tests, in order of preference, were helpfulin confirming the ante mortem diagnosis ofCDV:1. IFA for IgG in the CSF in cases of CDV

encephalopathy.2. IFA for IgM in the serum.3. Direct fluorescent antibody testing

for CDV antigen in conjunctivalscrapings.

4. Direct fluorescent antibody testing forCDV antigens in cells in the CSF ofcases with CDV encephalopathy.

These findings have proven useful andpractical in the resource-deprived com-munities of South Africa. A similar surveyof diagnostic tests in dogs from devel-oped areas has not been performed, andwould probably yield slightly differentresults because the acute catarrhal form ofthe disease is rare in this population ofdogs. Most CDV disease in developedcommunities is of the chronic neuro-logical type. In this setting, CSF IgG titresare most useful, but fluorescent antibodytesting of cellular material for analysis isless useful. Reverse transcriptase PCR ofCSF fluid may prove to be useful in thefuture.

CANINE DISTEMPER VIRUSINFECTION IN WILDLIFE

Although CD is an important disease indogs, there have been some alarming

distemper outbreaks in non-domesticspecies, which further highlights theimportance of control of the dog reservoir.Eight of the 11 families of the orderCarnivora are susceptible to CDV, with amortality rate that varies widely betweenspecies5,13,72. Of the species in which CDVinfection has been reported, the followingoccur in South Africa: lion, leopard, wilddog, hyaena, foxes and otters70,102,132. Ofthese species, exposure to CDV has beendemonstrated serologically in lions,leopards and wild dogs. However, nocatastrophic mortality due to CDV infec-tion has been reported in southernAfrica102,152. Distemper outbreaks havebeen reported in captive lions, tigers,leopards and jaguars in North America14.Most of these animals die of CNS diseasefollowing respiratory and gut involve-ment.

Outbreaks have been reported in marinemammals, including seals (so calledphocine distemper)125 and striped dol-phins. Thousands died in the Mediterra-nean between 1990 and 199243,46,84,125,150,161.It is most likely that these outbreaks origi-nated in terrestrial carnivores53.

A massive CD outbreak in the lionpopulation of Serengeti (Tanzania) killedabout 30 % of the lion population of thatarea in 1994. The same area experiencedCDV-related deaths in bat-eared foxes,African wild dogs, spotted hyaena, com-mon jackal and silver-backed jackal. Thesource of these infections is purported tobe the domestic dog population of about30 000 that surrounds the park132. Severalyears later 55 % of the lions of Maasai-Mara were found to be serologicallypositive for CDV, indicating exposure90.Preventing the spread of this disease towildlife would require vaccinating alldogs around the conserved areas orvaccinating the wildlife, which is notpractical in free-ranging wild animals.Developing a vaccine that is safe incaptive wildlife remains a challenge, asthere have been several reports of diseasecaused by the use of modified liveCDV vaccines40,47,79,110,119. It would beill-advised to use a live vaccine in captivewild animals. In this regard, inactivatedvaccines (which are no longer commer-cially available) are best used. Recombi-nant or subunit vaccines would bedesirable for captive wildlife83,126,127,138.

PREVENTIONVaccination remains the mainstay of

preventing CDV infection in dogs, andtherefore a large amount of research and alarge industry has developed around thispractice. Maternally-derived antibodyprovides temporary protection to neo-nates for a period that varies from days to

months. This phenomenon has been re-sponsible for the use of a modified livemeasles virus vaccine. The 2 viruses areantigenically closely related, and themeasles virus is not neutralised by thematernal antibody in 6-week-old pup-pies12. The second and all subsequentvaccines are modified live virus vaccines,as by this time maternal antibody titrehas waned to the point of being inconse-quential. This long-accepted practice ofheterotypic vaccine use has been ques-tioned, as ongoing usage of measlesvaccine means that progeny of bitchesvaccinated with measles vaccine maypossess maternal antibodies againstmeasles virus that would interfere withearly measles vaccine in puppies born tothem. It has been shown that a high-titreCDV vaccine is even more effective inprotecting 6-week-old puppies againstexperimental distemper challenge thanthe traditional human measles vaccine34.Itis generally accepted that vaccinationshould take place every 24 weeks fromthe age of 6 weeks until 12 weeks, andthen yearly thereafter with a modifiedlive virus vaccine7,31,44. Frequent vaccina-tion has been practised in an attempt toreduce losses early in life. It has beenshown that maximal vaccination (everyweek from the age of 2 weeks until 10weeks and then at 12, 16, 18 and 24 weeks)gave no better protection than a minimalregime (5, 9, 13, 17 and 22 weeks) in Grey-hound puppies. There was also no ad-verse effect on cellular immunity of themaximal protocol111. Inactivated vaccinesdo not provide protection against experi-mental challenge infection9. A canarypox recombinant vaccine has recentlybeen developed that provided goodprotection127 and is now commerciallyavailable.

The need for yearly booster vaccinationis often questioned, and in fact in devel-oped countries the recommendation isnow that booster vaccinations are onlyrequired every 3 years following theinitial puppy series and a booster at oneyear44. Boosters for geriatric animals arediscouraged44. Sufficient levels of neutral-ising antibodies remain in serum forlonger than 6 years after vaccination31. Ithas, however, been claimed that, on thebasis of serum antibody titres, annualvaccination should be maintained109. Onthe other hand, it has been suggested thathumoral immune mechanisms alone donot account for immunity and thatprotection lasts much longer than indi-cated by antibody titres63. In a countrysuch as South Africa, where most dogs areunvaccinated and reproductively active,and where a large wildlife resource ispotentially at risk, it would be unwise to

132 0038-2809 Tydskr.S.Afr.vet.Ver. (2001) 72(3): 127136

recommend anything other than yearlybooster vaccination.

Not all vaccines are equally effective, ashas been demonstrated in numerouscomparative studies on the efficacy ofvarious products. Outbreaks of diseasehave been described in well-vaccinatedpopulations, as indicated above. Despitethese reports, the chances of outbreaksare greatly reduced in well-vaccinatedpopulations128. It should, however, alwaysbe remembered that no vaccine is 100 %effective in 100 % of animals. When indi-viduals present with signs of CDV infec-tion, the disease cannot be ruled out onthe basis of an adequate vaccinationhistory.

There are numerous reports of adverseeffects of CDV vaccination. Introductionof a foreign protein will always be associ-ated with a certain amount of risk rangingfrom mild urticarial skin reactions tolife-threatening anaphylaxis. It has beenrecommended that caution should beexercised when using a modified livevaccine in dogs infected with Parvovirus,as this may predispose dogs to neurolog-ical disease99. Cases of post-vaccinal en-cephalitis were recorded in dogs invarious parts of Britain after the adminis-tration of a particular batch of vaccine38,108.It has also been shown that there is atemporal relationship between the use ofcanine vaccines (not specifically CDV)and immune-mediated haemolytic anae-mia48. Full-blown CDV infection withmortality has been reported following theuse of modified live virus vaccines invarious wildlife species, as describedabove.

MANAGEMENTThere is no effective specific antiviral

treatment. All treatment focuses on patientsupport and nursing care. Before treatingan infected dog, the infectious nature ofthe disease and prognosis should bediscussed with the owner. The possibilityof neurological signs developing shouldbe discussed with owners. This remainsan important reason for euthanasia butbefore this is done, it is worth consideringtreatment for a period of time in a selectgroup of patients, as some dogs mayrecover. Patients for which euthanasiashould be discussed include: 1) thosedogs with neurological disease that com-promises quality of life (such as intracta-ble seizure activity, severe myoclonus orincapacitating spinal cord disease), 2)severe and worsening catarrhal forms ofthe disease, or 3) dogs that come frommultiple-dog households where otherdogs may be at risk of infection.

Supportive treatment for dogs likely tobe shedding virus should be adminis-

tered on an outpatient basis as far as pos-sible unless the dogs can be barrier nursedin isolation. This is frequently a problemin the private practice situation in SouthAfrica where most isolation facilities arestill used for Parvovirus-infected puppies.CDV suspect dogs should not be treatedin proximity to these patients. Antibioticcover (broad spectrum and effective inthe respiratory tract), fluid and electrolytebalance (including anti-emetics if re-quired), nutritional support and seizurecontrol form the basis of a managementplan. Good nursing care such as keepingthe animal warm and dry and cleaningoculonasal discharges should be applied.Various homeopathic treatment claimshave been made. These are scientificallyunsubstantiated.

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