A horse's skin and coat can be contaminated via air and soil by various fungal organisms. Though also found on normal horses,
some of these fungal organisms can produce significant fungal infection under the proper conditions. From the commonly recognized
ringworm to the more insidious pythiosis, horses can be infected by these potentially nasty fungal infections, which are not
only irritating, but also in the case of pythiosis, can be life-threatening.
The dermatophyte Trichophyton equinum is the most common cause of dermatophytosis or ringworm in horses, but it may also be associated with Microsporum species (M. gypseum). Ringworm is especially seen in hot, humid climates during moist, warm weather, and it is common to confined animals during
fall and winter. It may be prevalent in horses in larger groups at breeding, training and racing facilities. Dermatophytes
are transmitted between animals from contact with infected hair or from fungi in an environment contaminated by infected horses.
Equipment such as brushes, combs, blankets and tack, all can pass an infection readily from one animal to another. Within
a barn, rodents and companion animals also can spread dermatophytosis to horses.
Dermatophytosis is the only fungal disease seen with any frequency, according to Peter Ihrke, VMD, Dipl. ACVD, chief of dermatology
services at the University of California-Davis. Most of their cases are Trichophyton equinum, M. canis from barn cats, and occasionally T. veruccosum from cows.
"The further south and the higher the humidity, the more dermatophytes one sees," he says.
Once exposed to the dermatophyte fungus, an infection can establish due to disruption of the skin surface of even minor abrasions,
facilitating penetration and invasion into the hair follicles.
Although often seen in adult animals, young horses are especially susceptible to symptomatic ringworm infection, especially
those kept indoors under poor management conditions or those horses on a poor dietary regimen.
Pythiosis lesion shows the characteristic kunkers.
Animals with immune suppression are more prone to establishment of fungal infection, such as those treated with glucocorticoids
for other conditions. Horses contaminated by other skin conditions, such as prevalence of lice or those in an environment
with a high incidence of biting flies. Those housed in crowded, moist or filthy conditions are highly susceptible, too. The
presence of skin abrasions, however slight, or horses with areas subject to skin friction such as girth or bridle irritation,
are also at risk for ringworm.
The symptoms of T. equinum usually start with patches of raised hairs in a circular or ring pattern. The hairs give way easily when plucked, and hair
loss follows, leaving a silvery scaling of the underlying skin. The lesions are pruritic in the early stages of the disease
but can remain sensitive to the touch for longer periods. The initial ringworm lesions can spread outward to cover large areas
of the body, if no appropriate measures are taken. The disease usually resolves spontaneously within six to 12 weeks, but
the fungal spores that have contaminated the stable environment and equipment can persist for years.
Signs of M. gypseum may appear suddenly as raised patches in the haircoat or as increased skin scaling in one or two spots or over the entire
body surface. It then may progress to crusty, itchless and painless, patches of hair and skin one inch in diameter, spreading
all over the body. It may also be localized to the lower legs, especially the pasterns. Lesions of T. equinum are usually present on the face, neck, dorsolateral thorax and girth. The legs are less commonly affected.