While scientists estimate that there may be millions of fungal species in the world, there are over 70,000 species of fungi that have been identified, with around 50 of these potentially giving rise to infections (mycoses) in animals and humans. Of the four classes of disease-causing fungi, the superficial and cutaneous types can infect the skin of a horse.1 The superficial fungi affect the skin alone, while cutaneous fungi attack the skin as well, but can also affect the hair. Fungal skin infections in the horse are generally categorized as dermatophytosis and dermatomycosis. Dermatophytosis involves infection of the keratinized tissues including the hair and are usually caused by fungi such as Microsporum, Trichophyton or Epidermophyton. Dermatomycoses are fungal infections of the hair, hoof or skin, and are caused by non-dermatophyte fungi.2 

Ringworm is one of the most common fungal skin infections affecting horses. It is caused by a dermatophyte fungus (usually Microsporum or Trichophyton) and presents with skin lesions which usually start as small raised spots from which the hair is lost.3 Over time, the lesions can spread from these spots and become thick and scabrous.1 Occasionally, the lesions can be sore and itchy. Often, only two or three lesions may be seen, but if left untreated, the condition can become extensive. Ringworm is highly contagious and can be spread to humans. In some cases, entire groups of horses can become infected in an outbreak, often through the process of systematic grooming in a facility.

Ringworm Occurrence and Diagnosis

Ringworm is typically transmitted from horse to horse by direct contact, tack, grooming equipment or contact with stables or trailers where the fungi are present. The fungi that cause ringworm are highly resistant to environmental factors and can remain on viable on surfaces for long periods of time. They can remain on the skin for up to 21 days before clinical signs develop; thus, this condition can be spread well before there are signs of infection. Very often, a new horse will introduce ringworm to a facility.3 It is thought that younger horses are more susceptible than older ones, although geriatric or debilitated animals are also considered to be more at risk as well. More often than not, horses that have been infected develop a strong immunity and are usually not subsequently infected, even if exposed.1,3

Ringworm lesions are usually (but not always) characteristic, and can resemble other skin conditions like rain scald or folliculitis. The veterinarian can confirm a diagnosis by collecting a skin scraping and subsequent analysis of skin cells, debris and hair for ringworm spores. Since ringworm spores take several weeks to culture however, the veterinarian will usually recommend treatment prior to confirmation.

Most ringworm cases spontaneously resolve within 6 to 12 weeks3 however, given the high risk of spreading ringworm within a facility, treatment is the more prudent option. Additionally, horses with active cases of ringworm are prohibited from competing or racing and are unable to travel abroad. Thus, it makes more sense to treat cases as soon as they arise.

Where to buy Griseofulvin 

Griseofulvin is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

Griseofulvin carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1wagwalking.com.

2Padalino, B. et. al. Rare Generalized Form of Fungal Dermatitis in a Horse: Case Report. Animals: an open access journal from MDPI vol. 10,5 871. 17 May. 2020.

3vcahospitals.com.