Horses’ skin and coats are regularly exposed to numerous fungal organisms via the air and soil. Like bacterial and viral infections, horses are more susceptible to fungal infections when they are immunocompromised by stress or other infections. When they become established, some fungal organisms can produce significant infection under these conditions. Fungal infections such as ringworm can be more or less a nuisance, while others range from irritating to life-threatening. 

Fungi are eukaryotic organisms with a cell wall comprised of chitins, glucans, and mannans. The plasma membrane within the cell wall contains ergosterol, a sterol that regulates the permeability of the cell membrane and activity of membrane-bound enzymes. It is also frequently targeted by antifungal agents.1,2

There are approximately 70,000 species of fungi, with around 50 that can cause disease in horses.1 Among these, there are four classes:

• Superficial— Affect the skin

• Cutaneous— Affect the skin and/or hair

• Subcutaneous— Spread from the surface of the skin to deep tissue

• Deep mycosis—Affect the upper and lower respiratory systems

Pathogenic fungi can also be divided into three groups: 

• Multinucleate septate filamentous fungi

• Nonseptate filamentous fungi

• Yeasts

Depending on environmental conditions, dimorphic fungi can morph between forms. In soil and decaying matter, the mycelial form is usually present and is composed of a collection of hyphae.2 The mycelia produce infective spores that can infiltrate vertebrate tissue.3 Fungal infections in the horse can represent a diagnostic and therapeutic challenges regardless of the class of fungi. 

Fungal Agents Affecting Horses

Subcutaneous and deep mycoses tend to be chronic, progressive diseases.1 Their clinical signs include extensive, painful lesions which can resemble other types of infection.

Although full-blown respiratory fungal infections in horses are rare,3 fungal infections of the upper respiratory tract are common but are often hard to diagnose. Treatment options are contingent upon on the etiologic agent, the site and extent of the infection, accessibility to surgical intervention, known antifungal susceptibilities and evidence-based study results.2

Fluconazole for Horses

There has been significant progress in the development of new antifungals in veterinary medicine in recent years, and in the determination of pharmacokinetic profiles in horses.2 Systemic therapy typically results in an increased incidence of successful treatment of fungal diseases in equines. These novel antifungals differ in their mode of activity, toxicity, and propensity to interact with other drugs. Thus, it is recommended that antifungal therapy be tailored to the etiologic agent whenever possible.

Oral fluconazole at a loading dose of 14 mg/kg, followed by 5 mg/kg q24h, yields concentrations in plasma, cerebrospinal fluid, synovial fluid, aqueous humor, and urine above the minimum inhibitory concentration reported for several equine fungal pathogens. However, fluconazole reportedly has minimal activity against filamentous fungi (Aspergillus and Fusarium spp). Low-dose oral fluconazole (1 mg/kg PO q24h) for at least 10 to 15 days has been anecdotally successful in treating fungal keratitis.5 

Where to buy Fluconazole

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

Fluconazole carries numerous 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.

2vetfolio.com.

3Merck Veterinary Manual.

5Latimer F.G., et. al. Pharmacokinetics of fluconazole following intravenous and oral administration and body fluid concentrations of fluconazole following repeated oral dosing in horses. Am J Vet Res 2001; 62:1606-1611.