There are tens of thousands of species of fungi, but only around 50 have the ability to cause disease in animals and humans. Of these, there are four classes of fungi:

• Superficial – Affecting the skin

• Cutaneous – May attack skin and hair

• Subcutaneous – Can spread from the skin to deep tissue

• Deep mycosis – Attacks the respiratory system

Additionally, fungi are often classified as primary or opportunistic. Primary fungi can affect any horse. Opportunistic fungi affect horses that are immunocompromised by other illnesses. The chief source of most fungal infections is soil, where they can be acquired via inhalation, ingestion, or through the skin.1

In general, there are around 10 fungal infections that are commonly found in horses. The most frequently reported in healthy horses are Conidiobolus coronatus, Cryptococcus neoformans, Blastomyces dermatitidis, Histoplasma capsulatum and Coccidioides immitis. In immune compromised horses, the most common are Pneumocystis carinii, Aspergillus spp., Candida spp, Fusarium spp and Emmonsia crescens.1

The symptoms of fungal infections vary depending on the type of fungi. The most common indicators of fungal infections in horses are:

• Rapid breathing

• Cough

• Nasal discharge (may be mucous or blood)

• Lack of appetite

• Respiratory distress

• Hemoptysis

• Chronic

• Weight loss

• Lesions on skin

• Ulcerated skin2,3

The most commonly referenced 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 Ringworm is highly contagious and can spread to humans. 

Candidiasis 

Candidiasis 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 with infections being rare in adult horses. 

Guttural Pouch Mycosis (Aspergillosis) 

The guttural pouches are sacs formed by the auditory tube connecting the horse’s middle ear with the back of the throat. Guttural pouch mycosis is a fungal infection caused by several Aspergillus species. Nosebleeds, nasal discharge and difficulty breathing or swallowing are common signs of guttural pouch mycosis.1,3  

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. 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 Horses

The treatment of choice for a variety of 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. Griseofulvin 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 100 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.