equine bacterial and fungal diseases a diagnostic and therapeutic update

8/12/2019 Equine Bacterial and Fungal Diseases a Diagnostic and Therapeutic Update

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Equine Bacterial and Fungal Diseases:

A Diagnostic and Therapeutic UpdateStephen D. White, DVM, Diplomate ACVD

Bacterial and fungal skin diseases are important in the horse. Bacterial skin diseases

(pyoderma) are most often caused by Staphylococcusspecies,Corynebacterium pseudo-

tuberculosisor Dermatophilus congolensis. The most common clinical signs associated

with bacterial skin infections are crusts, papules, abscesses, and draining tracts; the latter

two lesions are more commonly associated with C. pseudotuberculosis. Ideally, antibiotic

treatment should be based on bacterial culture and sensitivity. Fungal infections are most

commonly caused by dermatophytes (ringworm) orSporothrix schenkii, although the role

ofMalasseziain equine skin disease is beginning to be investigated. The clinical signs of

fungal infections are variable and may include alopecia, crusts, papules, pruritus, nodules,

ulcers, and draining tracts. The latter three lesions are more commonly associated with S.

schenkiiinfection. Treatment is dependent on the organism cultured and may include both

topical and/or systemic treatment.

Clin Tech Equine Pract 4:302-310 2005 Elsevier Inc. All rights reserved.

KEYWORDShorse, skin, bacteria, fungi,Staphylococcus, Corynebacterium, Dermatophilus,

dermatophyte, ringworm, Sporothrix, Malassezia

Bacterial SkinDisease (Pyoderma)

Staphylococcus sp.

Bacterial folliculitis (superficial pyoderma) is usually

caused by a coagulase-positive Staphylococcus species.Both S. aureus and S. intermedius have been isolated.1,2 In onestudy,S. aureusaccounted for twice as many isolates as S.intermedius; interestingly, the same study isolated somestrains of S. hyicus as well.3 Many isolates are resistant topenicillin G. In another study, lysozymes from equine neu-trophils were only slightly bactericidal for S. aureus.4 Re-cently, methicillin-resistant, coagulase-negative staphylococ-cal species were cultured from healthy horses in Japan; theauthors concluded, These organisms must be considered apotential threat to horses and veterinarians who care forthem.5 In another study, occurrence of pyoderma waslinked to poor nutrition and husbandry.6 The author feels

that superficial pyoderma secondary to allergies is under-diagnosed in the horse.Clinical signs include crusts and/or alopecia, usually in a

circular pattern suggestive of dermatophytosis, which is per-haps the reason that equine pyoderma is under-diagnosed.

Circular skin lesions with an exfoliative border, as seen indogs with superficial pyoderma representing epidermal col-larettes, or encrusted papules, similar to the miliary derma-titis reaction pattern in cats, can also be seen (Figures 13).7

These infections tend to be variable in their intensity of pru-

ritus. Histology usually shows folliculitis and/or furunculo-sis, but bacterial colonies are not always seen.1A truncal form

Department of Medicine and Epidemiology, School of Veterinary Medicine,

University of California, Davis, CA.

Address reprint requests and correspondence to Stephen D. White, Room

2108 Tupper Hall, UC Davis, Davis, CA 95616. E-mail:

[email protected] Figure 1 Staphylococcal folliculitis: crusts in a circular pattern.

302 1534-7516/05/$-see front matter 2005 Elsevier Inc. All rights reserved.

doi:10.1053/j.ctep.2005.10.004


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of bacterial folliculitis (contagious acne, contagious pustu-lar dermatitis, Canadian horsepox) is often associatedwithpoor grooming and trauma from tack and saddle, warm wet

weather, and heavy work. It is painful and interferes withworking and riding. It is usually caused by a coagulase pos-itiveStaphylococcusspecies but may also be attributable toCorynebacterium pseudotuberculosis (Figure 4).8 Folliculitis of-ten develops in the saddle and lumbar region, particularly inthe summer (Figure 5). The affected area initially may beswollen and very sensitive; this is followed by formation offollicular papules and pustules, which may become confluentor rupture, forming plaques and crusts.

Bacterial pastern folliculitis, often caused by a coagulase-positive Staphylococcus species, must be considered as one ofthe multiple differential diagnoses of the disease presentationcommonly referred to as grease heel or scratches. Thelesions are usually limited to the posterior aspect of the pas-tern and fetlock regions, and one or more limbs may beinvolved. The initial lesions consist of papules and pustules.If left untreated, the lesions coalesce and may produce largeareas of alopecia, erythema, ulceration, and suppuration,which may be quite painful (Figure 6).

Diagnosis ofsuperficialpyoderma is based on clinical pre-sentation, ruling out dermatophyte infection, and responseto antibiotics. Although skin biopsy for histopathology is

often not performed in a clinical setting, it may be helpful

when the clinical signs are not classic or the horse has notresponded to empirical use of antibiotics. In a recent retro-spective report, cocci were found on thesurface of specimensfrom 23% of horses with skin disease but only 7% fromhorses with healthy skin. Bacterial folliculitis had a higherprevalence of surface bacteria than any other disease.9

Whether this could translate into excessive numbers of coccion cytology remains unknown.

Deep pyoderma, furunculosis as well as folliculitis, causedbyStaphylococcusspecies is uncommon in the horse. It maypresent as a nodular disease termed botryomycosis, whichmimics a deep fungal infection (Figure 7). These lesions mayrequire surgical excision as well as long-term antibiotics. Di-agnosis ofdeeppyoderma is based on clinical presentationand biopsy.

Antibiotic treatment of staphylococcal skin infectionsshould be based on bacterial culture and susceptibility; how-ever, empiric use is common. The most commonly used an-tibiotic in equine bacterial dermatitis is trimethoprim-sulfa(30 mg/kg q12 hour PO for 2-6 weeks; longer for deep infec-tions). Interestingly, dosing intervals forintravenousadmin-istration of trimethoprim-sulfamethoxazole in horses may

Figure 2 Staphylococcal folliculitis: epidermal collarettes.

Figure 3 Staphylococcal folliculitis: widespread, co-

alescing areas of alopecia and scaling.

Figure 4 Corynebacterium pseudotuberculosis folliculitis: circular areas

of crust and alopecia.

Equine bacterial and fungal diseases 303


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not be appropriate for use in donkeys or mules. Donkeys

eliminate the drugs rapidly, compared with horses.10In casesof Staphylococcus sp resistance to TMS, enrofloxacin may begiven (7.5 mg/kg PO once daily; poultry or large animalinjectable formulation). Use of enrofloxacin in young horses(less than 2 years old) should be avoided, due to concerns ofarticular cartilage damage.11

Recent work in four horses identified a novel plasmid-

borne gene (designated qacJ) encoding resistance to quater-

nary ammonium compounds in three staphylococcal speciesassociated with chronic infections.12This suggests horizontal

transfer within and between different equine staphylococcalspecies. If this is applicable to antibiotic resistance as well,clinicians may find the use of empiric antibiotics compro-

mised in the future.For localized lesions, mupirocin ointment 2% (Bacto-

derm Pfizer) or silver sulfadiazine cream (Silvadene,Marion Merrell Dow) may be effective. For an antibacterial

shampoo, the authorfinds ethyl lactate (Etiderm, VIRBAC)helpful, although preparations containing chlorhexidine (2-

4%) may also be used.

Corynebacterium PseudotuberculosisSolitaryor multiple abscesses, or nodules with many drainingtracks that progress to diffuse cellulitis, areoftencausedby C.

pseudotuberculosis. When the disease affects the pectoral re-gion or inguinal regions, it is sometimes termed pigeon fe-

ver and dry land distemper, respectively, in the USA. Thedraining nodules or abscesses are especially common in the

pectoral region, and occasionally affect the face, neck, axilla,groin, and limbs; they begin deep and enlarge, often with

much edema, and rupture in 1-4 weeks, discharging viscid,creamy pus, a major source of contamination (Figure 8).

These may sometimes be the sequella of wounds.13Abscessesmost often rupture externally.

This type of deep C. pseudotuberculosis infection may becontracted from areas where caseous lymphadenitis is com-mon in sheep, although proximity to sheep is not a require-

ment. Various strains of this organism are responsible fordifferent outbreaks14; these seem to occur more commonly in

the western USA. The disease may be seen seasonally wheninsect populations are elevated. In a recent report using asensitive molecular assay for detecting the phospholipase D

exotoxin gene of C. pseudotuberculosis, potential insect vec-tors were identified, including Hematobia irritans (horn fly),

Stomoxys calcitrans (stable fly), and Musca domestica (housefly). C. pseudotuberculosis was identified in up to 20% ofhouse flies in the vicinity of diseased horses.15

Diagnosis is based on clinical signs, with the infection

Figure 5 Bacterial folliculitis: circular areas of alope-

cia in the saddle area (used with the permission ofDr. P. Bourdeau).

Figure 6 Pastern folliculitis.

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readily identified by bacterial culture of aspirate samplesfrom abscesses. The synergistic hemolysis inhibition test isuseful for diagnosis of internal abscesses; however, it is un-reliable for the diagnosis of external abscesses.16 Treatmentdepends on the area of the body affected. For example, if theabscess is in the axilla and thus painful on movement and/orpreventing locomotion, establishment of drainage is very im-portant and antibiotics are indicated. Procaine penicillin

(20,000-50,000 IU/kg/d) with rifampin (3-5 mg/kg 12qhourPO) or trimethoprim sulfa (30 mg/kg q12 hour PO) may beused. Treating with TMSandrifampinconcurrentlymayleadto a greater incidence of colitis, and should be avoided. If thedecision is made to use antibiotics, but drainage cannot bereadily established (for example, an axillary abscess wheretheowneris unwillingto allow the veterinarian tousea trocarand drain), the antibioticsmustbe used for aminimumof 1month. If theabscess is solitary andnot causing pain or fever,antibiotics are usually not necessary; rather, bringing the ab-scess to a head with hot packs or heat-inducing agents (ich-

thymol) is important. Once any abscess has drained, gentle

cleaning with tamed-iodines or chlorhexidine is indicated.

Dermatophilus CongolensisThis actinomycete bacteria ia a well-known cause of skin

disease in horses. Three conditions must be present forDer-

matophilusto manifest itself: a source of infection, moisture,

and skin abrasions. Chronically affected animals are the pri-mary sourceof infection; however they only becomea serious

source of infection when their lesions are moistened, which

results in the release of zoospores (infective stage). Mechan-

ical transmission of the disease occurs by both biting and

nonbiting flies, ticks, and possibly fomites.

Dermatophilosis is usually seen during the fall and winter

months, corresponding to rainfall. The dorsal surface of the

animal is most commonly affected (Figure 9). Occasionally,

the lesions involve the lower extremities when animals are

kept in wet pastures (dew poisoning), or if horses are left in

Figure 7 Botryomycosis (deep pyoderma) (usedwith the permission of Dr. V. Fadok).

Figure 8 Corynebacterium pseudotuberculosis: drain-ing abscesses on the ventrum of a horse.



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the stall while the stall is cleaned with high-pressure waterhoses. In theearly stagesof thedisease, thick crustsunder thehair coat can be palpated more than they can be seen. Re-moving the crusts and attached hair exposes a pink, moistskin surface. The under surface of the crusts are usually con-cave with the roots of the hairs protruding, often in the shapeof a paintbrush (Figure 10).

Diagnosis is made by impression smears whereby a por-tion of one of the crusts is minced and mixed with a few dropsof sterile water on a glass slide, gram stained (a modifiedWrights stain such as Hema 3 Solution; Fisher Scientific,Houston, TX may also be used), and examined microscopi-cally for the characteristic railroad track pattern of cocci

(Figure 11). Alternatively, bacterial culture or histopathologymay be utilized for diagnosis. The latter reveals a thick crustcomposed of alternating layers of parakeratotic stratum cor-neum, dried serum, and degenerating neutrophils with a su-perficial folliculitis.1 In gram stained histologic sections, thebranching, filamentous organisms can be observed in thecrusts and in the follicles. Treatment is removal of the horsefrom the wet environment, removal of crusts (carefully, as

this may be painful), washing with iodophors or lime sulfur,and antibiotics (penicillin: 22,000 mg/kg procaine penicillinG intramuscularly twicedailyor trimethoprim-sulfaorally: asabove for staphylococcal pyoderma) for 7 days.17As thecrusts are an important contagion, these should be disposedof rather than simply brushed on to the ground.

Miscellaneous InfectionsOther bacteria causing skin disease in equines have beenrecently reported. These have been deep or systemic infec-tions, with Streptomyces sp causing fistulous withers in don-keys and systemic infection with Salmonella sp caused infarc-tion and cutaneous necrosis.18,19

Fungal Skin Disease

Dermatophytes (Ringworm)The most common equine dermatophyte species isolatedfrom horses are Trichophyton equinum, M. equinum, T. men-tagrophytes, and T. verrucosum.1,3,20 Tack (bridles, halters, sad-

Figure 9 Dermatophilosis: severe scaling and alope-

cia (used with the permission of Dr. V. Fadok).

Figure 10 Dermatophilosis: Paintbrush lesion ofhair, crust, and pus (used with the permission of

Dr. M.M. Sloet van Oldruitenborgh-Oosterbaan).

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dle blankets) often act as fomites. The lesions usually appearfirst in the axillary/girth area and may spread over the trunk,rump, neck, head, and limbs. Lesions may be superficial ordeep. Superficial infections are much more common and aremanifested by the development of thick crusts, or more gen-

erally a diffuse moth-eaten appearance with desquamationand alopecia, sometimes in a ring pattern (Figures 12and13). A small crust may form over the follicle and the hair islost, but extensive alopecia and crust formation do not occur;this process may cause some irritation and pruritus. Occa-sionally, the initial lesions may be very urticarial in nature,progressing to multiple focal sharply demarcated areas ofalopecia and scaling (Figures 14and 15). Rarely, dermato-

phytes may be a cause of coronary band disease (Figure 16).Infections of Trichophyton verrucosum in humans due totransmission from horses have recently been reported.21

Diagnosis is by fungal culture; biopsy is less reliable.1 In-terestingly, Trichophyton species occasionally may cause ac-antholysis, mimicking pemphigus foliaceus on histopath-ology.22For fungal culture, use forceps to acquire hairs thatappear stubbled and broken, especially at the advancing pe-riphery of an active, nonmedicated lesion. In addition, for-

ceps or skin scrapings may be used to gather surface keratinfrom similar areas. The hair and surface keratin of horseshave large numbers of saprophytic fungi andbacteria. Hence,some clinicians recommendgentlycleansing the area to besampled with water or alcohol, and allowing it to air drybefore sampling.

The veterinarian may choose to send the samples to amicrobiology laboratory or perform in-house culture. If the

Figure 11 Dermatophilosis: branching chains of cocci (railroad

tracks) modified Wrights stain,100(usedwith thepermission ofDr. V. Fadok).

Figure 12 Dermatophytosis: circular alopecia and

scaling due to Trichophyton mentagrophytesinfection.

Figure 13 Dermatophytosis: circular alopecia in a foal due toTricho-

phyton equinum(used with the permission of Dr. J. Traub-Dargatz).



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latter, Dermatophyte Test Medium (DTM) is often used.DTM is essentially modified Sabourauds dextrose agar towhich the pH indicator phenol red has been added. How-ever, T. verrucosum does not grow on DTM.23

Topical treatment alone is often curative. Whereas 50%captan (2 tablespoons of the powder in 1 gallon of water) hasbeen recommended in the past, and is certainly safe for tack,its potential for carcinogenicity and effectiveness has beenquestioned. Lime Sulfur (LymDyp; DVM, Miami, FL) 1 cupto 1 gallon of water, or bleach 1:10 with water, are both

effective, but messy, odiferous, and staining. Miconazoleshampoos are becoming more widely used, and may be aseffective. In Europe and Canada, an enilconazole rinse(Imaverol; Merial) is highly effective.

Systemic treatment is occasionally needed. The efficacy

and proper dose of griseofulvin in horses has not been thor-oughly researched. However, a dosage of 100 mg/kg daily for7 to 10 days has been advocated, and has been used withgood success on a small number of horses by the author.Griseofulvin is a teratogen, and should not be used in preg-nant mares. Alternatively, 20% NaI may be given IV (250mL/500 kg horse every 7 days, 1 to 2 times). This also iscontraindicated in pregnant mares as it may cause abortion.

Vaccination to T. equinum mayreduce theincidence of newinfections and protect a high percentage (80%) of vacci-nates from infection. These data are based on results with aninactivated vaccine containing both conidia and mycelial el-ements.24Such vaccines are not available in the US.

MalasseziaThe exact species ofMalasseziayeast growing on horses skinis beginning to be investigated.25In one study, the Malasseziasp.isolated were identifiedas M. furfur, M. slooffiae,M. obtusa,

Figure 16 Dermatophytosis: scaling of the coronary band due to

Microsporum gypseuminfection (used with the permission of Dr. V.Fadok).

Figure 17 Intermammarydebris in a pruritic mare with Malassezia sp

infection.

Figure 14 Dermatophytosis: urticarial lesions due to Trichophytonmentagrophytesinfection.

Figure 15 Dermatophytosis: urticarial lesion due to Trichophyton

mentagrophytes infection, transitioning into circular area of alopecia.

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M. globosa, and M. restricta.26The author has examined sev-

eral mares with intense pruritus due to a Malassezia infection

between their mammary glands. The mares rubbed their tail,

perineum, and ventral abdomen. Physical examination

showed a dry, greasy-to-the-touch crust and exudate (Figure

17). Cytology of the exudate showed numerous yeast organ-

isms, which were identified on culture as Malassezia species

(Figure 18). Treatment with a topical 2% miconazole/chlo-

rhexidine shampoo was curative. The author is aware of

other, similar cases. However, healthy nonpruritic mares may

also have large numbers of yeasts in the intramammary area.

In these cases, M slooffiae and a species tentatively named

Malassezia equi have been identified.27

SporotrichosisA more serious disease is caused by another yeast,Sporothrixschenkii. This condition presents as a nodular to ulcerative,lymphatic-cording disease, often initially noted on the distal

legs (Figures 19and 20). S schenkii gains access at woundsites; the organism has been reported from most geographicareas. Diagnosis is by demonstrating the fungus on histopa-thology, impression smears, and/or culture. This disease is azoonosis, so care shouldbe taken in handling suspectedsam-ples. The organism may be difficult to find on histopathol-ogy, even with special stains. Successful therapy with a num-ber of different systemic iodine preparations (NaI, KI) hasbeen reported. The organic iodides have proven to be supe-rior in efficacy to the inorganic iodides in the treatment of

Figure 18 CytologyofMalassezia sp from intermammarydebris from

a healthy mare.

Figure 19 Sporotrichosis causing multiple ulcers and distal limb

edema.

Figure 20 Sporotrichosis causing severe ulceration (used with thepermission of Dr. M-R. Paradis).

Figure 21 Pythiosis: severe ulcerative, nodular lesion on theventrum

of a horse (used with the permission of Dr. V. Fadok).



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equine sporotrichosis, with ethylene diamine dihydroiodide(EDDI: EDDI Equine; The Butler Company, Dublin, OH)being the drug of choice. This product is in the form of a feedadditive and can be mixed with a small amount of grain andadministered at a dosage of 1 to 2 mg/kg of the active ingre-dient given once to twice daily for the first week, then reduce

the dosage to 0.5 to1.0 mg/kg once daily for the remainder ofthe treatment. In general, lesions will begin to regress duringthe first month of treatment; treatment should be continuedforat least 1 monthbeyond the complete resolution of allcutaneous nodules and the healing of any ulcerated lesions.Discontinuing therapy prematurely will invariably result inan unnecessary relapse of the disease. During treatment, thehorse should be closely observed for any evidence of iodidetoxicity (iodism): excess scaling andalopecia, a serous ocularor nasal discharge, excess salivation, anorexia, depression,coughing, nervousness, or cardiovascular abnormalities.Should any of these signs develop, the treatment should bediscontinuedfor 1 week, andthetreatment resumed at three-quartersof thedosage at which theiodism wasnoted. In most

instances, the treatment is subsequently well-tolerated.28

PythiosisNot a true yeast, but rather a protista, Pythiumsp (Pythiuminsidiosum) is considered to be the causative agent of swampcancer (aka Florida horse leech, bursattee, kunker). Thisorganism is found in tropical and subtropical areas aroundthe world. The pythiosis lesions occur most commonly onthe limbs, abdomen, neck, and lips, and consist of densegranulation tissue containing masses of yellowgray necrotictissue, which are sometimes calcified with the masses oftenbeing present as cores in the fistulae that can be removedintact. Such masses are known as leeches or kunkers. The

granuloma ulcerates and extends peripherally and may reacha very large size in a short time (Figure 21); the overlying andadjacent skin is destroyed both by the inflammatory reactionand self-mutilation by the horse. Pythiosis and habronemia-sis (summersore) may produce similar lesions. Histopatho-logic examination of affected tissue reveals pyogranuloma-tous inflammation directly surrounding the organism.Isolation of organisms from the lesions is necessary for theirfurther identification and study, but histologic demonstra-tion of theprotozoa within tissues that areobviously reactingto its presence is critical in establishing the causal relation-ship in an individual lesion. Recent advances in ELISA ormolecular techniques offer better potential for organism de-tection and identification.29Wide surgical excision com-

bined with immunotherapy has the best chance of success,although recent work using a vaccine alone is promising.30

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equine bacterial and fungal diseases a diagnostic and therapeutic update

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