This superficial fungal ulcer gives the appearance that the cornea is not shiny, but dry. | Photo: Dennis E. Brooks, DVM, PhD, Dipl. ACVO

Editor’s Note: This article was revised by the author to reflect new and updated information in November 2017.

Fungi are microscopic plants that lack chlorophyll; they are commonly found in the hay, grasses, shavings, straw, and dust of a horse’s environment. They normally live in balance with bacteria on the surface of the horse cornea and conjunctiva. For example, nearly all healthy horses living in Florida (95%) have fungi living on the surface of their eyes; the percentage undoubtedly varies from one geographic area to another.

Fungal organisms are capable of causing severe corneal disease called keratomycosis in the horse following corneal trauma or inappropriate medical treatment of corneal ulcers. Horse corneas also might be more susceptible to fungal infection due to their large corneal surface areas (with more space available for infection and also injury), the prominence of the entire eye, and perhaps deficiencies of the horse eye’s immunology. Topical steroids decrease the horse’s immune protection such that microbes can more easily cause disease, as well as predisposing the cornea to fungal infection and decreasing the effectiveness of antifungal agents