When an eye infection is not going well or there is ongoing mechanical irritation, an ulcer forms. An ulcer is where the outer surface of the cornea is disintegrating, usually because of enzymes released by the organism and inflammatory cells. If allowed to go unchecked, the ulcer can perforate the cornea resulting in loss of the eye. Ulcers usually indicate infection, but chronic trauma, severe anterior uveitis, and eosinophilic keratitis can cause ulcers also.

A particularly difficult to treat ulcer are those infected with funguses. These sometimes result from improper use of antibiotics or steroids being used in infected eyes. Any progressive ulcer not responding to antibiotic treatment needs to be considered for antifungal therapy quickly.

A common cause of painful, cloudy eyes is a condition known as recurrent uveitis. This condition is caused by the body's immune system attacking the eye and is recognizable as recurrent episodes of tearing, squinting, and cloudiness of the cornea.

Recently, viral infections of the horse's eye have been better documented and Equine Herpes-2 (EHV-2) has been isolated from horses with this problem. Generally this is a cause of pain and slight cloudiness. The cloudiness is often small spots or forms small lines on the cornea. This can be a long term, low grade condition or resolve only to return later. The cases of this I have seen had a mild cloudiness that on close inspection, was made up of small spots of cloudiness on the cornea. This should be considered in mild to moderate cases of corneal edema, non-responsive to treatment for bacteria or autoimmune disorders.

Both idoxuridine and trifluorothymidine have been used successfully to shorten episodes but reoccurence is common. Cyclosporine should be avoided.

Some newborns are born with the lids folding in on themselves resulting in irritation of the cornea. Tearing, squinting, and cloudiness if severe are first seen and on careful inspection the eyelid, usually the lower, is turned in on the eye with the lashess contacting the cornea.

J Am Vet Med Assoc 1996 Oct 1;209(7):1283-1286

Eosinophilic keratoconjunctivitis in seven horses.

Yamagata M, Wilkie DA, Gilger BC

Eosinophilic keratoconjunctivitis was diagnosed in 7 horses at The Ohio State University between 1976 and 1994. All horses had moderate-to-severe blepharospasm, chemosis, and conjunctival hyperemia; epiphora; and extensive yellow-to-white caseous mucoid discharge. Corneal ulcers associated with this disease were perilimbal and extended centrally. All ulcers were covered with a white necrotic plaque firmly attached to the underlying cornea. Other ophthalmic abnormalities were not detected. Corneal scrapings examined cytologically contained numerous eosinophils interspersed between epithelial cells, few mast cells, and neutrophils. Microbial organism were not seen. Bacterial and fungal cultures were negative for ocular pathogens. The initial diagnosis of eosinophilic keratoconjunctivitis was made on the basis of clinical and cytologic findings. In 5 horses, the condition completely resolved after topical treatment with corticosteroid (0.05% dexamethasone) and triple antibiotic ointments. However, the duration of treatment was prolonged, with a mean treatment time of 64 days (range, 45 to 106 days). All corneal ulcers remained superficial, and despite the prolonged duration of treatment, none of the horses developed secondary bacterial or fungal keratitis. One horse underwent superficial keratectomy and had the shortest resolution time (14 days).