Fungal infections in horses are considerably less common than infections caused by bacterial or viral agents. Fungal organisms are typically encountered in the animal’s normal environment, and the clinical manifestation of disease often reflects the route of exposure (skin, respiratory tract or reproductive tract). In some cases, immunodeficiency or immunosuppression may predispose an individual to fungal infection, while in other cases, there is no identifiable predisposing risk factor.

Overt signs of disease will typically be associated with an inflammatory response at the site of infection and may lead to generalized illness, abortion or death. A search of submissions to the University of Kentucky Veterinary Diagnostic Laboratory for the period from 2009-2019 turned up 241 cases that had fungal involvement. The most common sites of infection were the placenta, lungs, eye, guttural pouch, skin and nose. Less commonly represented were the gastrointestinal tract, nervous system and multiorgan infections.

Fifty-nine cases of fungal placentitis were identified. Gestational ages ranged from 180 to 351 days, and the majority of cases presented as abortions four to one month prior to the mare’s due date. The immediate/short term outcome for the foal was reported in 53 cases, of which 30 were born dead or euthanized. Live foals were frequently born early with gestational ages ranging from 310 days to full term. Five foals were born following premature placental separation, and three had a complicated delivery due to dystocia. A fungal organism was cultured in 18 cases, Candida sp. was isolated in two cases, Rhodotorula sp. in one case, and Aspergillus sp. in 15 cases. Age of the broodmare was generally not reported.

Thirty-nine cases of pneumonia were identified in horses ranging from 3 days to 25 years of age. Horses with fungal pneumonia often had a history of concurrent gastrointestinal disease (21/39 cases).

Gastrointestinal diseases included prior surgery for large colon volvulus, colitis and enteritis. Concurrent infectious agents identified in the gastrointestinal tract included Lawsonia intracellularis (one case), Neorickettsia risticii (one case), Clostridioides difficile (one case) and Salmonella sp. (five cases). The association between colitis, particularly due to salmonellosis, and pneumonia has been previously reported.

Horses with fungal pneumonia but without a history of gastrointestinal disease presented with a range of conditions including nerve paralysis, immune mediated hemolytic anemia and dystocia. Treatment protocols were not provided often enough to determine how many horses received corticosteroids as part of clinical management.

Cases of guttural pouch infection ranged from 2 weeks to 23 years of age. Of 33 cases, 23 did not have co-morbidities outside of the guttural pouch. Where present, additional diseases included encephalitis, laminitis, uveitis, pneumonia and colitis, neuropathy, equine multinodular pulmonary fibrosis, basioccipital bone fracture and small intestinal volvulus.

Twenty-six horses presented with fungal keratitis with an age range of 1-24 years. These were submitted as biopsy cases with minimal clinical histories and culture performed at external laboratories.

Twenty cases of gastrointestinal fungal infection were identified. Affected animals ranged from 3 days to 30 years of age and included four cases of fungal colitis, two of enteritis, nine of gastritis, one of enterocolitis, three of glossitis and one of esophagitis. All but two cases had a more significant primary disease that included acute respiratory distress syndrome, sepsis, Tyzzer’s disease or diaphragmatic rupture.

Cases of rhinitis (14 cases) and dermatitis (21 cases) were submitted through the biopsy service, and sufficient clinical histories weren’t routinely provided for analysis.

In summary, fungal infections, localized and systemic, continue to be an uncommon but important cause of disease in equids. It appears that primary infections of the dermis, nasal cavity and guttural pouch frequently do not present with co-morbidities, while those of the gastrointestinal and lower respiratory tract often occur with serious disease of other organ systems. Fungal placentitis continues to be a recognized cause of abortion and premature birth. In many cases culture is unrewarding and diagnosis is dependent on demonstration of the organism microscopically.

Rebecca Ruby, MSc, BVSc, Dipl. AVCP, assistant professor and veterinary pathologist at the UK Veterinary Diagnostic Laboratory. Source: April 2021 Equine Disease Quarterly.