Most horse owners have heard about strongyles, bots, ascarids, and various other parasites that live in the equine gastrointestinal tract. Collectively known as worms, these parasites can be kept in check by periodic use of commercially available deworming products.

Less familiar are other equine parasites such as eye worms. These parasites belong to the genus Thelazia, and numerous related species are found in various types of birds and mammals, including humans, throughout Asia, Europe, and North and South America. Research of 106 dead horses in Kentucky found that 28 animals, or 26% of the total, were infected with these parasites.

Eye worms are spread to horses and cattle by face flies. After the larvae are deposited in a horse’s eye, the worms achieve their mature length of 18 mm (3/4 inch) in about 10 weeks. Infection is benign in many horses, though some equines develop conjunctivitis, irritation of the tear ducts, and a chronic clear or yellowish discharge. In rare cases, horses might show sensitivity to light, squinting, or ulceration and perforation of the cornea.

Eye irritation and drainage in horses can be caused by dust, pollen, or other foreign substances as well as the presence of parasites. A veterinarian may be able to isolate the cause and prescribe an effective treatment, though it may be difficult to prevent the condition from recurring. Wiping fly repellant onto the horse’s face (while avoiding the tissues in and around the eyes) may be helpful, and fly masks are also useful in keeping flies away from a horse’s eyes.

For more horse nutrition information from Kentucky Equine Research visit their website.