Ringworm – Not a Worm
The medical term for ringworm, dermatophyte, means a fungus that infects the skin. Once an infection (dermatophytosis) is established, the offending fungus announces its presence, usually in the form of an itchy, circular pattern of hair loss (the “ring”), sometimes accompanied by redness or a rash. In horses, it commonly occurs on the girth and saddle areas and can spread to other parts of the body. The skin reactions are easy to see, but the multiple species of dermatophytes that can infect horses are less than forthcoming about their everyday presence, hiding out of sight in the soil or stowing away on fomites such as shared grooming equipment and tack. The fungus can be present on horses for a few weeks before clinical signs appear and can spread from one horse to another by direct contact. These sneaky tactics mean ringworm can also infect people or other animals, spreading through otherwise seemingly innocuous routes.
Diagnosing ringworm involves taking a hair sample for a fungal culture in the laboratory. It is important to confirm the diagnosis, as other conditions can mimic ringworm in appearance, and the most effective treatments can be very different.
Although ringworm is a common equine skin condition, it is not the only possible cause of hair loss and rashes. Ringworm look-alikes include bacterial (staph) skin infection, occult sarcoid (a type of skin cancer), and alopecia areata, a rare autoimmune skin disease where the body’s immune system mistakenly attacks the hair bulb. Rain scald (dermatophilosis) can also resemble ringworm.
Ignore It and It Will Go Away?
In some cases, ringworm resolves on its own without treatment, but it can take a month or more. Therefore, it is important to treat horses as soon as possible to limit their discomfort and avoid spreading the fungus to other animals and people.
Treatment might consist of antifungal shampoos, dips, or topical therapies. Oral treatments such as griseofulvin and terbinafine are also available, but potential unwanted side effects, such as liver damage, evidenced by elevated liver enzymes, and birth defects, should be taken into account. It is important to follow veterinary and product instructions to get the maximum effect. Treatment also extends beyond the horse. Tack, blankets, grooming equipment, and other potentially contaminated surfaces should be disinfected to limit the spread of disease. If a known soil-borne fungal species is identified through laboratory culture, the ground and/or stall floor should be treated with bleach to try to eliminate the organism.
Whereas bacteria usually grow quickly in culture, fungi grow in the laboratory over a period of weeks. It is not uncommon for clinical signs to resolve before the results of the culture are known. However, the results might still be beneficial for cases that are difficult to treat or to provide information on appropriate treatments for other affected animals on the premises and limit the potential spread to humans.
This article was originally published in the Horse Report from the Center for Equine Health at the UC Davis School of Veterinary Medicine in Spring 2022.