Ringworm is a superficial, highly contagious skin infection caused by a closely related group of fungi known as dermatophytes. Dermatophytes have the ability to utilize keratin as a nutrient source (i.e. they have a unique enzymatic capacity [keratinase]). There are several different species and strains of dermatophytes, therefore the type and severity of the infection differs from horse to horse. Some dermatophyte species are zoophilic, meaning they are able to cause infections not only in horses, but also humans or other animals. Certain animal species are more prone to infection by a particular dermatophyte species. In horses, recent studies have found that each species of dermatophyte tends to invade a particular area of the horse and cause their own specific reaction.
|Species||Natural Habitat||Where found on horses||Clinical signs|
|Microsporum canis||Cats||Generalized, saddle area||Multiple areas of hair loss|
|M. equinum||Humans, Horses||Saddle, flank and girth areas (pressure areas)||Discrete, raised areas of crusting, progressing to areas of patchy hair loss|
|M. fulvum||Humans||Head||Area of hair loss|
|M. gallinae||Chickens||Flank and girth||Circular area of hair loss|
|M. gypseum||Soil/Dead animals||Limbs and saddle area||Lesions|
|Trichophyton equinum||Horses||Saddle area||Single area of pathy hair loss|
|T. soudanense||Humans||Girth area||Areas of hyperkeratosis and patchy hair loss|
|T. vanbreuseghemii|| ||Generalized||Areas of hair loss ("moth eaten" appearance)|
|T. verrucosum||Cattle||Limbs and rump||Areas of inflammation|
Although ringworm can occur in horses worldwide, it is more prevalent in hot, humid climates than in cold, dry regions.
Dermatophytes produce enzymes called keratinases that break down the protective barriers of the outer layer of the horse's skin and hair, allowing itself entry to establish infection.
Ringworm is characterized by the appearance of circular areas of hair loss, scaling, and crusting. Although in can occur anywhere on the horse's body, it most often occurs on the face, legs, girth, shoulder, and chest.
Ringworm is easily transmitted, and occurs through direct or indirect contact with skin of other infected animals, insects, people, soil or fomites (tack, blankets, grooming equipment, etc.). The fungus has been known to survive on saddle-girths for 12 months.
The incubation period for ringworm varies from several days to a couple weeks.