Ringworm is very common in horses and may be confused with many other skin diseases due to its variable appearance. Veterinarians refer to ringworm as dermatophytosis, meaning an overgrowth of specific microbes that live on the skin. Not caused by a worm at all, ringworm is a fungal infection of the skin and hair follicles caused by the Trychophyton and Microsporum genera of fungi, closely related to dermatophytes known for causing raised ring-shaped skin lesions on people and other animals. In horses, the disease generally appears as localized patches of hair-loss that grow larger as the infection spreads.
In many cases, horses seem undeterred by ringworm infections. The lesions may become irritated and inflamed however, and may start to itch and flake causing the animal to rub and scratch. The two species of fungus most commonly associated with ringworm in horses are T. equinum and M. equinum, but other genus / species of ringworm may spread to horses from other animals, including dogs, cats, pigs, and humans, especially if the horse is immuno-compromised. Equine ringworm is a zoonotic disease, meaning it has the potential to spread to and infect people.
Ringworm fungus grows on keratin-rich hair shafts near or just beneath the surface of the skin. The fungus causes the hair to become devitalized and break off, leaving a fine stubble in a dime to quarter-sized circular pattern. It may also invade the outer layer of the skin, causing flakes and scales to form. Hairs at the lesion’s borders pluck easily because of damage to the follicles.
The incubation period for ringworm – the time between exposure and the onset of symptoms – is between a few days and one month.
The fungus is spread most effectively by direct contact with other horses. Contaminated equipment, tack, blankets, and even handlers may spread the disease as well. Adult horses may become carriers of the fungus, never exhibiting obvious symptoms but spreading disease none-the-less. Younger animals are more susceptible to ringworm infection than their adult stable-mates.
Because shared blankets and tack can be a source of contamination, patches of hair-loss may first appear in the shoulders, flanks, and girth. Horses sweat under tack, lending warm, moist environments ideal for fungal growth. A species of Trychophyton carried by rats and mice can occasionally contaminate feeds and feeding supplies, causing lesions that originate on the face. Ringworm may then spread to other parts of the body as isolated circular lesions or generalized regions of hair-loss.
Because the disease is so highly contagious, horses suspected of having ringworm should be immediately segregated from the herd. All equipment, tack, and grooming supplies must be thoroughly disinfected with dilute bleach or an appropriately labeled anti-fungal cleaning product. Bedding should be properly disposed of to avoid contamination, and blankets should be scrubbed with bleach solution and completely dried before storage or reuse. The veterinarian may prescribe a shampoo containing miconazole, which is used daily for one week, then weekly until the lesions heal. Alternatively, chlorhexidine or betadine sprays may be employed to control the infection.
Ringworm infection should be confirmed by a veterinarian prior to treatment by a fungal culture on DTM, or dermatophyte test medium. Hair samples from the newest lesions are plucked, and while treatment is ensued, the fungus is grown for a week or more on DTM agar plates until it can be isolated and identified by species. Ringworm testing may help to preclude spreading of the disease to the rest of the herd (and handlers), as the horse may be contagious for several weeks while being isolated and treated.