Eyeworm Disease (Thelaziasis) in Donkeys

Eyeworm disease, or thelaziasis, is a parasitic infection of the eye and nearby tear structures. In equids, the usual parasite is Thelazia lacrymalis. Although most published guidance focuses on horses, the same parasite biology and eye anatomy make donkeys susceptible as well. The worms may live in the conjunctival sac, under the eyelids, around the third eyelid, or in the tear ducts and gland.

These worms irritate the eye mechanically as they move and feed in tear-rich tissues. Some donkeys have mild signs at first, while others develop obvious conjunctivitis with tearing, discharge, and discomfort. In more severe cases, the cornea can become cloudy or ulcerated, which turns a nuisance problem into a painful eye emergency.

For pet parents, the tricky part is that eyeworm disease can look like many other eye problems. A donkey with redness and discharge may have flies, dust irritation, trauma, a corneal ulcer, or another parasite-related condition. That is why any painful or persistently irritated eye should be examined by your vet rather than treated as a routine "pink eye" at home.

Eyeworm disease is caused by infection with Thelazia worms, most often Thelazia lacrymalis in equids. The parasite depends on flies that feed on tears and eye secretions. In North America, face flies are the main vector described for equine infections. The fly picks up larvae while feeding on an infected animal's eye secretions, the larvae develop inside the fly over about 2 to 4 weeks, and then infective larvae are deposited into another animal's eye during the fly's next meal.

Once in the eye, the larvae mature over roughly 10 to 11 weeks. Adult female worms then release larvae into tears, continuing the cycle. Because flies are central to transmission, cases tend to cluster in warm months and in environments with high fly pressure, manure buildup, nearby cattle, or limited fly control.

Donkeys kept on pasture, in mixed-species settings, or in areas with persistent face flies may be at higher risk. A donkey does not need poor care to get eyeworms. This is an exposure-related parasite problem, and prevention usually focuses on reducing fly contact rather than blaming management.

Your vet will start with a full eye exam and compare both eyes. Because painful equine eyes can hide ulcers or deeper disease, the exam often includes topical anesthetic, eyelid eversion, inspection of the conjunctival sac and third eyelid, and fluorescein stain to check for corneal damage. In some cases, sedation is helpful so the eye can be examined safely and thoroughly.

Diagnosis may be straightforward if your vet can directly see and remove the worms. That said, Thelazia lacrymalis can be harder to spot in equids because it may hide in the tear ducts or gland. Your vet may also examine tear fluid microscopically for larvae or eggs, especially if worms are not obvious on first inspection.

Just as important, your vet will rule out other causes of a painful or inflamed donkey eye. These can include corneal ulceration from trauma, foreign material, recurrent uveitis, habronemiasis, onchocercal eye disease, bacterial conjunctivitis, and eyelid abnormalities. That broader workup matters because treatment choices change if the cornea is ulcerated or if another parasite or inflammatory disease is involved.

Prevention centers on fly control, especially control of flies that feed on eye secretions. Practical steps include prompt manure removal, reducing wet organic material where flies breed, using approved fly repellents as directed by your vet, and considering well-fitted fly masks designed for equids. During peak fly season, daily observation of the eyes is worth the effort because early cases are easier to manage.

Pasture and housing management also matter. If donkeys share space with horses or cattle, overall fly pressure may be higher. Feeding areas, water sources, and loafing areas should be kept as clean and dry as possible. Some farms benefit from combining sanitation, physical barriers, and targeted insect control rather than relying on one method alone.

Routine parasite control should be discussed with your vet as part of the whole-herd plan, but deworming alone does not replace fly management. If one donkey develops persistent tearing or discharge, early examination can help catch eyeworms before corneal injury develops. That approach is often the most practical and cost-conscious prevention strategy.