Of the approximately 70,000 species of fungi, there are 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.

Fungal infections in the horse can represent a number of diagnostic challenges and therapeutic dilemmas, regardless of the class of fungi. 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. 

Fungal Agents Affecting the Horse

Many local fungal infections can represent an aesthetic nuisance and cause minor discomfort to the horse, while systemic fungal infections can be quite serious. The respiratory system, surface of the skin and deep tissue can be targets of mycotic infection, as can the hair. While subcutaneous (e.g., pythiosis, mycetoma) and deep mycoses (e.g., blastomycosis, coccidioidomycosis) are considered rare and limited to restricted geographical areas, dermatophytosis, cryptococcosis and aspergillosis are of wider concern due their worldwide diffusion and (in some cases) their zoonotic potential.4 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.

Certain skin conditions in the horse can be attributed to fungi, and in some cases, a given skin condition may be attributable to one or more different types of fungi. For example, dermatophytosis is a highly contagious fungal infection of the skin that can affect horses of all ages, and can be caused by two genera of fungi: Trichophyton (T. equinum var equinum and T. verrucosum), or Microsporum (M. gypseum and M. equinum). Dermatophytosis is usually limited to the hair and skin, but seldom invades the dermis in otherwise healthy horses. Transmission between horses generally requires contact with a source of contaminated material. The spores are very resistant, can survive for long periods in stable environments, and it has been reported that biting flies can transmit Microsporum spp.2

Although full-blown respiratory fungal infections in horses are rare,3 fungal infections of the upper respiratory tract are more common. These can be hard to diagnose and threaten to the horse’s overall health. Diagnosis of upper respiratory tract is usually based on clinical signs. 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 Equines

In recent years, there has been significant progress in the development of new antifungals in veterinary medicine and in the determination of pharmacokinetic profiles in horses.2 It is reported that systemic therapy typically results in an increased incidence of successful treatment of fungal diseases in horses. The newer antifungals differ in their mode of activity, toxicity, and propensity to interact with other drugs. Thus, antifungal therapy should 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.