Fungal Skin Disease Dermatophytosis – Large Animals

Dr.Kedar Karki

General Considerations:

Dermatophytosis is a fungal infection of the superficial keratinized layer of the epidermis, hair, and hooves.

Transmission is by direct contact between animals or by contact with infected hair and crust on fomites or in the environment.

Young horses under 4 years of age are more susceptible due to lack of immunity.

The disease is more common in cattle less than 1 year of age.

It is becoming increasingly more common in lambs (called Club Lamb Fungus).

It is more common in the winter months – especially in confined animals.

It is a zoonosis.

Important Facts

Dermatophytosis occurs more often in horses under 4 years of age and in cattle less than one year old.

It is a zoonosis.

Etiology and Pathogenesis:

Species important in the horse include: Trichophyton equinum; Trichophyton mentagrophytes; Trichophyton verrucosum, Microsporum gypseum; and Microsporum canis.

Most common species in cattle and goats: Trichophyton verrucosum; Trichophyton mentagrophytes.

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Clinical Signs:

Equine:

The clinical features of typical dermatophytosis in the horse include:

Focal lesions.

Scaling and crusting.

Alopecia.

Pruritus (which may or may not be present).

Tendency for peripheral spread.

The lesions are almost always multiple and occur most commonly in the saddle and girth region.

Initially, the lesions may appear as urticarial-like swellings.

Soon afterwards, the hair comes out easily in the affected areas, and the alopecia is accompanied by scaling and crust formation.

“Atypical” forms of equine dermatophytosis occur.

Infection with Microsporum gypseum tends to be associated with much thicker crusts, more inflammation, suppuration and ulceration.

In some horses, there will be extensive scaling and crusting without significant alopecia.

Occasionally, the lesions are limited to the posterior aspect of the pastern region and are erroneously diagnosed as “scratches” or “grease heel”.

Important Facts

Typical lesions of dermatophytosis in horses is characterized by multifocal areas of alopecia, scaling and crusting.

In horses, lesions are often seen in the saddle and girth regions.

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Initially, lesions may look like urticarial-like swellings.

Bovine:

Trichophyton verrucosum – usually around head and neck and usually associated with a thick “keratotic” crust.

Trichophyton mentagrophytes – multiple ½ to 1 cm lesions – not crusty.

Diagnosis:

Frequently given by clinical appearance alone.

Wood’s lamp.

Not of much value.

Microsporum canis is the only dermatophyte affecting horses that will produce fluorescence (remember, only 50% of the strains fluoresce).

Direct Examination.Fungal Cultures.

The most definitive method of diagnosis.

Helps determine source of infection.

Perform as in small animals.

Vitamin enrichment media is needed to culture Trichophyton equinum and Trichophyton verrucosum. Trichophyton equinum requires nicotinic acid and Trichophyton verrucosum requires thiamine and inositol. Important Facts

Wood’s lamp will give a negative result in most cases. Remember! Only 50% of Microsporum canis strains fluoresce under the Wood’s lamp.

Fungal culture is the most reliable diagnostic tool.

Pluck broken hairs localized to the periphery of the lesion for fungal culture and also for direct examination.

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Trichophyton equinum requires nicotinic acid and Trichophyton verrucosum requires thiamine and inositol.

Treatment:

Most cases will heal spontaneously in 1 to 3 months unless the animal is debilitated by illness, parasites, malnutrition, etc.

The goal of the treatment is to decrease the course and severity of the disease, prevent spread of the infection, and decrease environmental contamination.

Good nutrition and adequate exposure to sunlight is advised.

Treatment includes fungicidal therapy for the animal and decontamination of the environment and Fomites (tack, brushes, blankets, etc). Chlorox can be used in the environment.

All in contact animals should be treated.

Topical fungicides most often recommended due to self-limiting nature of the disease.

Lime sulfur – 3 to 5%.

Enilconazole, a beta-substituted imidazole, shows excellent efficacy against dermatophytes. It is currently used in Europe as a topical and environmental fungicide for large and small animals. It is currently available in the US as an environmental fungicide for poultry industry (Clinafarm).

Povidone iodine.

Chlorhexidine – 0.5 to 2 %.

Captan 3% (available as a 50% powder – Orthocide) – 1 oz (2 tablespoons) per gallon of water (not approved in food animals).

Owners should wear rubber gloves when applying; Captan may cause a contact dermatitis in some people.

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Topical fungicides should be applied daily with a sponge or brush to the entire body for one week then twice weekly until clinical resolution.

All crusts should be removed to facilitate treatment.

Treatment should be continued for at least 6-8 weeks. A good rule-of –thumb is to treat for 2 weeks past clinical resolution.

Affected animals should be handled last (10 mg/kg daily for at least 30 days has been suggested).

Systemic therapy with griseofulvin – dose unknown in the horse.

Actual efficacy unknown due to high rate of self-cure.

Very costly.

Teratogenic – do not use in pregnant animals.

60 mg/kg daily for 5 weeks was effective in calves.

Important Facts

Most cases will heal spontaneously in 1 to 3 months unless the animal is debilitated by illness, parasites, malnutrition, etc.

The goal of the treatment is to decrease the course and severity of the disease, prevent spread of the infection, and decrease environmental contamination.

Treatment includes fungicidal therapy for the animal and decontamination of the environment (tack, brushes, blankets, etc).

Treat all contact animals.

Affected animals should be handled last.

Vaccination – used in Europe.

Trichophyton verrucosum modified live vaccine.

Life-long immunity.

Calves vaccinated at 2 weeks of age.5

No apparent interference of maternal antibodies.

Not protective against other dermatophytes.

Experimental Trichophyton equinum vaccine in horses was 91% effective. Horses that developed ringworm only developed single lesions.

Consider vaccination in chronic recurrent epidemics.

Prevention:

Do not share tack or brushes.

Routine disinfection of equipment.

Avoid overcrowding.

Good health care practices.

References

Scott, DW. Large Animal Dermatology. In: Fungal Diseases. W.B. Saunders, Philadelphia, PA, 1988, p 172 – 182.

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