Fungal Ulcers in the Equine Eye

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.

Keratomycosis refers to "kerato," meaning corneal disease, and "mycosis" referring to fungal infection. The term keratomycosis refers to any corneal disease caused by fungus; these diseases include ulcers, abscesses, and iris prolapses. It is more common--and the clinical signs are the most severe--in horses living in warm, humid geographic regions such as Florida, Georgia, Louisiana, and Alabama (warm, moist conditions are more favorable for fungal growth). It is less common in cold climates, and infrequently diagnosed in other animals such as dogs, cats, and cows. Some scientific reports have indicated a tendency for keratomycosis to occur in the summer and fall months. This could be due to temperature and humidity factors favoring fungal growth, or related to the amount of time horses spend stalled, which increases the risk of keratomycosis.

Seasonality of infection in the southern United States shows that fungal ulcers occur throughout the year, with the majority of cases occurring in October through January.

Fungal organisms include mushrooms, filamentous fungi or molds, and yeasts. The filamentous fungi are the most common fungi that infect the horse cornea. The filaments are also called hyphae, which are divided by cross-walls or septa. Each hypha has a surrounding cell wall made up of chitin. Chitin, a physically strong structural compound, also forms the body of insects. Several species of septate filamentous fungi are common to the equine eye (Fusarium, Aspergillus, and Penicillium).

Corneal ulcers are disruptions of the corneal epithelium. The pathogenesis of fungal ulcers (ulcerative keratomycosis) commonly begins with slight to severe corneal trauma resulting in the removal of corneal epithelial cells, exposure of the stroma, and stromal invasion by the fungal organisms living on the surface of the horse eye. The fungi can also directly seed the stroma from a plant that contacts the cornea (such as a branch brushing the eye while trail riding). Further corneal destruction results from the release of powerful enzymes from the fungi, white blood cells, and corneal cells. Therapy for the destructive effects of these enzymes has been limited in the past, but is critical for the rescue of the fungal-infected horse eye. Fungal hyphae are frequently found deep in the equine cornea rather than on the surface.

Clinical Signs

Clinical signs associated with ulcerative keratomycosis include corneal swelling, a droopy upper eyelid, a small pupil, squinting, and increased tearing. Ulcers infected with fungi range from minor corneal epithelial abrasions (above left) to superficial plaques, to extremely deep, severe ulcers (above center). Diagnosis of keratomycosis is based on finding fungal hyphae or yeast on at least one of the following: 1) cytologic (cellular) examination of a corneal scraping (above right), 2) culture of the corneal lesion, or 3) surgical biopsy.

Clinical suspicion of fungal involvement and response to anti-fungal therapy can also aid the diagnosis. Cytology (86% positive) and culture (85% positive) are effective means of diagnosing keratomycosis. However, because of the propensity for deep invasion by the fungal organisms (to where they can't be collected with a surface scraping), your veterinarian might have a negative cytology and/or culture result from superficial corneal specimens.

Treatment Options

Treatment must be directed against the fungi as well as against the corneal and intraocular inflammatory responses that occur following fungal replication and death of the hyphae. Anti-fungal drug treatment is often required for an extended period to achieve complete fungal destruction and resolution of the clinical signs, as the anti-fungal drugs used to fight equine keratomycosis tend to slow growth of the fungi rather than kill them quickly.

I currently initiate anti-fungal therapy for equine keratomycosis with topical miconazole or natamycin three to four times a day. Itraconazole, silver sulfadiazine, amphotericin B, and fluconazole also can be used in temperate, cool geographic areas, but are not effective in the southern United States. Antibiotic treatment is also indicated with keratomycosis as concurrent bacterial infection is relatively common.

Uveitis (inflammation of the uvea and other fibrous outer layers of the eye) or iridocyclitis (inflammation of the iris and the ciliary body) is always found with keratomycosis in horses, and must also be aggressively treated. Iridocyclitis is present any time a horse has a corneal ulcer or abscess, and it can escalate in intensity following fungal death after anti-fungal therapy is started. Flunixin meglumine (Banamine) is the most frequently used non-steroidal anti-inflammatory agent in my patients for systemic treatment of iridocyclitis. One percent atropine sulfate is used carefully for its ability to dilate the pupil and diminish eye pain in horses with corneal ulceration. Atropine can contribute to colic in horses, and should therefore be used carefully.

Serum from the blood of the horse can be extremely beneficial in treatment of fungal ulcers in horses, as it contains anti-proteinase enzymes to speed corneal healing. Acetylcysteine and other drugs can reduce the enzyme activity found in the tears of horses with keratomycosis. Unfortunately, this anti-enzyme activity is not utilized as often as necessary in horses because acetylcysteine is irritating to the eye, is expensive, has a foul odor, and needs refrigeration; however, it will be used when all other medications have failed.

Topical corticosteroids are contraindicated in keratomycosis in horses as they can worsen the disease. Topical corticosteroid use in corneal ulcers in horses has been implicated in enhancing fungal replication, predisposing the cornea to further fungal infection, and decreasing the effectiveness of anti-fungal drugs. A concern with topical antibiotic therapy is that it can suppress susceptible bacterial strains, while allowing multiplication of resistant organisms. This makes conditions more favorable for fungal replication. Keep in mind that all drugs should be used carefully in a horse with an eye ulcer.

Combined medical and surgical therapy is indicated if ulcers are extremely deep, if they are not responding to medical treatment, or if they worsen in spite of medical treatment. Approximately half of the horses with keratomycosis will require combined medical and surgical therapy to save the eye and vision. Surgeries for keratomycosis include conjunctival grafts and full-thickness corneal transplantation. Conjunctival grafts involve transplantation of the conjunctiva to cover a corneal ulcer. Corneal transplantation involves suturing a piece of cornea from another horse into the diseased cornea of the affected horse, and to make the cornea transparent following corneal scarring.

Will the Horse See?

The veterinary literature is replete with cases of poor visual outcomes of ulcerative keratomycosis in horses, as fungi cause serious, sight-threatening diseases in horses. Vision following ulcerative keratomycosis in horses might be retained in as few as 50% of the eyes, with nearly half of these eyes reported to either become blind and/or require enucleation (removal of the eye).

This high failure rate is, in my opinion, due to a lack of aggressive medical and surgical treatment early in the disease when fungal ulcers can be most effectively treated. Aggressive medical and surgical therapy for ulcerative keratomycosis in horses should result in a positive visual outcome and survival of the eye in more than 90% of cases. Despite this chance of success, therapy is quite prolonged and scarring of the cornea might be prominent. 

Editor's note: This is the sixth article in a series of eye articles by Dr. Brooks. See the first article, "Eye Anatomy and Physiology," article Quick Find #2797 at, for more information on eye anatomy.

About the Author

Dennis E. Brooks, DVM, PhD, Dipl. ACVO

Dennis E. Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He has lectured extensively, nationally and internationally, in comparative ophthalmology and glaucoma, and has more than 140 refereed publications. He is a recognized authority on canine glaucoma, and infectious keratitis, corneal transplantation, and glaucoma of horses.

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