Eyeworms (Thelazia species) are common parasites of horses in many countries, including several areas of North America. Horses are infected primarily by Thelazia lacrymalis.
The face fly, which feeds on secretions from the eye, transmits eyeworms in North America. Eyeworm larvae are swallowed by the fly and become infective in 2 to 4 weeks. The infective larvae are then deposited in the horse’s eye by the fly during feeding. The larvae mature into worms in 10 to 11 weeks. Female worms then deposit larvae into eye secretions, restarting the life cycle. Infections may occur year-round, but disease outbreaks usually are associated with the warm season activities of the flies.
Eyeworms can be found in the tear gland and its ducts, less commonly in the third eyelid and the nasal tear ducts, and also on the cornea, in the conjunctival sac, and under the eyelids. Infections with no obvious signs in horses appear to be typical of eyeworm disease in North America. However, irritation and inflammation of the eye is likely due to the rough outer layer of the worms. Inflammation of the tear ducts and sac has also been reported in horses. Mild to severe inflammation of the conjunctiva and inflammation of affected eyelids can occur. Inflammation, swelling, and cloudiness of the cornea, slow-healing sores (ulcers), holes, and permanent fibrous tissue, may develop in severe cases.
Currently, there is no reliable technique for detecting adult eyeworms in horses. Inspection of the eyes may reveal the worms; however, Thelazia lacrymalis in horses tends to be more invasive and less apt to be seen. Topical anesthetics may be administered to allow the veterinarian to detect and remove worms from the eye. Microscopic examination of tears for eggs or larvae may be attempted. Certain other parasites (such as Onchocerca microfilariae, or the larvae of stomach worms) that affect the eyes of horses must also be excluded as causes of the inflammation.
Your veterinarian may be able to remove eyeworms with forceps after using a local anesthetic. Flushing the eyes with an iodine solution or applying an iodine ointment may also be effective. An antibiotic-steroid ointment to treat the inflammation and any bacterial infection is often recommended. Certain whole-body drugs that destroy or flush out parasitic worms have been shown to work against eyeworms.
To reduce the chance of eyeworm infections, good fly control measures—directed especially against the face fly—are critical.