Fungi are parasitic, spore-producing organisms that obtain their sustenance by absorbing food from their hosts. Many species of fungus exist in the environment, but only some of these can cause infections. The chief source of most fungal infections is soil, where they can be acquired via inhalation, ingestion, or through the skin.1

Certain fungal infections can cause disease in healthy horses, while others require a host that is immunocompromised by stressors such as poor nutrition, bacterial or viral infections in order to cause infection. Fungal infections may be localized or they may affect the entire body (systemic or generalized).

Some common fungal infections of horses include:

Ringworm

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.2,3 The lesions can spread from these spots and become thickened and scabrous.1 Often, only one to three lesions may be seen, but if left untreated, the condition can become extensive. Ringworm is highly contagious and can spread to humans. 

Guttural Pouch Mycosis (Aspergillosis) 

Guttural Pouch Mycosis is a fungal infection caused by several Aspergillus species. It is a respiratory infection that may become generalized. Aspergillosis is found worldwide and in almost all domestic animals as well as in many wild animals, but the susceptibility to fungal infections varies among species. The guttural pouches are sacs formed by the auditory tube connecting the middle ear with the back of the throat. Nosebleeds, nasal discharge and difficulty breathing or swallowing are common signs of guttural pouch mycosis. 

Candidiasis 

This is a localized fungal disease which affects the horse’s mucous membranes and skin. Caused by species of the yeast-like fungus, Candida albicans, it is distributed worldwide in a variety of animals and is most commonly. Superficial infections in the mucous membranes of the intestinal tract have been reported in foals.1,2 Infections are rare in adult horses. 

Coccidioidomycosis (Valley Fever) 

Coccidioidomycosis is a dustborne, noncontagious infection caused by the fungus Coccidioides immitis. This fungal infection occurs primarily in the drier regions of the southwestern United States, Mexico and Central and South America. Inhalation of fungal spores is the established route of infection. Coccidioidomycosis can vary from infections with few or no signs to progressive and even fatal forms. In horses, the most common signs include loss of weight, coughing, fever, musculoskeletal pain, and abscesses of the skin.

Cryptococcosis 

This systemic fungal disease is caused by Cryptococcus neoformans, which exists in the environment in a yeast form. It is found worldwide in soil and bird manure. Transmission is by inhalation of spores or contamination of wounds. While less common in horses, it can cause obstructive growths in the horse’s nasal cavities. 

Griseofulvin for Fungal Infections in the Horse

The treatment of choice for ringworm and other skin infections in horses is griseofulvin, a fungistatic antibiotic widely used in veterinary medicine, primarily for ringworm and other dermatophytic infections.4 In veterinary species, griseofulvin has been FDA-approved for use in dogs and cats to treat dermatophytic fungal infections of the skin, hair and claws, and to treat ringworm in horses.

Griseofulvin acts on susceptible fungi by disrupting the structure of the cell’s mitotic spindle, arresting the metaphase of cell division.4 Griseofulvin has activity against species of Trichophyton, Microsporum, and Epidermophyton. Griseofulvin has no antibacterial activity and is not clinically useful against other pathogenic fungi, including Malassezia ssp. Griseofulvin is contraindicated in patients hypersensitive to it or with hepatocellular failure. It should not be used in pregnant animals. It should not be used in pregnant animals or horses intended for food.

Where to buy Griseofulvin 

Griseofulvin is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. GRISEOFULVIN 200 MG/ML ORAL SUSPENSION (500ML) by NexGen Pharmaceuticals is indicated for the treatment of fungal infections in the horse. 

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.

4Plumb’s Veterinary Drugs.