Conjunctivitis is inflammation of the pink tissues that line and surround the eye (conjunctiva), and associated structures. Anything irritating or inflaming these tissues can result in swelling and reddening, with the eye watering and/or squinting.

Conjunctivitis usually results from the accumulation of foreign material in the eye or fly irritation. Bacterial infection can follow these underlying problems. It can also be caused by other problems within the eye, like inflammation of the nasolacrimal duct or third eyelid.

Conjunctivitis can also be a feature of many diseases, such as equine recurrent uveitis (ERU) or corneal ulcers.

Unlike other species, contagious bacterial conjunctivitis (pinkeye) is not common in otherwise healthy horses, but it can occur.

More commonly, conjunctivitis is caused by parasitic (Onchocerca, Thelazia and Habronema) or viral infections (EHV, influenza, EVA and others).

Diagnosis is by ophthalmic exam, ruling out the presence of foreign bodies, and response to treatment. In some cases, swabbing for cytology is indicated. Rarely, bacterial or fungal culture and more advanced testing may be necessary.

Treatment involves removal of foreign material (if present), use of antibiotic ointments and treatment of the underlying cause. Sometimes systemic anti-inflammatories and, less commonly, antibiotics are used.