Equine Recurrent Uveitis (Moon Blindness) in Donkeys

Equine recurrent uveitis, or ERU, is repeated inflammation of the uveal tract inside the eye. The uvea includes structures that help nourish the eye and control the pupil. In donkeys, as in horses, these flare-ups can be very painful. They may come and go, or smolder at a low level between obvious episodes.

The term moon blindness is older language for the same condition. Despite the name, the disease is not linked to moon phases. What matters is that inflammation keeps returning or never fully settles. Over time, repeated inflammation can scar delicate eye tissues and reduce vision.

ERU is one of the most important causes of blindness in equids. Some donkeys show dramatic pain during a flare. Others may be more stoic, so the eye can look only mildly abnormal even when significant inflammation is present. That is one reason any painful or cloudy eye in a donkey deserves prompt veterinary attention.

This condition is often manageable, but it usually requires long-term planning with your vet. The goal is not only to calm the current flare, but also to protect comfort and vision over time.

ERU is usually considered an immune-mediated disease. That means the donkey’s immune system continues to drive inflammation inside the eye, even after the original trigger has faded. In equids, leptospirosis is one of the best-known risk factors. In North America, leptospiral infection is strongly associated with many ERU cases in horses, and the same concern applies when evaluating donkeys.

Other possible triggers include previous eye trauma, corneal ulceration, systemic infection, or inflammation elsewhere in the body. In some cases, your vet may suspect a prior infectious trigger but still treat the current problem mainly as immune-driven eye inflammation. Not every donkey will have a single confirmed cause.

Environmental exposure matters too. Standing water, wildlife, and livestock urine can increase leptospirosis exposure risk on some farms. Fly irritation and dusty conditions do not directly cause ERU, but they can worsen eye discomfort or make it harder to notice early changes.

Because donkeys can mask pain, the exact starting point is not always obvious. Your vet may focus on identifying treatable contributors, ruling out look-alike conditions, and building a plan to reduce future flares.

Diagnosis starts with a careful history and a full physical exam, followed by a detailed eye exam of both eyes. Your vet will look for signs such as corneal edema, aqueous flare, pupil changes, synechiae, cataract formation, vitreous changes, and evidence of old scarring from previous episodes. A history of repeated painful eye flare-ups is a major clue.

Two common tests are especially important. Fluorescein stain helps rule out a corneal ulcer, and tonometry measures eye pressure. ERU often causes low eye pressure during active inflammation, while glaucoma can raise pressure and needs a different plan. These tests help your vet choose safer medications.

Depending on the case, your vet may recommend bloodwork, serum chemistry, and selected infectious disease testing such as leptospiral serology. These tests can support the overall picture, but they do not always give a complete answer about what is happening inside the eye right now. In more complex cases, referral to a veterinary ophthalmologist may be the most useful next step.

Advanced evaluation may be needed if vision is threatened, the eye is not responding as expected, or surgery is being considered. Referral centers can help assess whether long-term medical management, a cyclosporine implant, vitrectomy, or removal of a blind painful eye is the most practical option.

Not every case of ERU can be prevented, because the disease often has an immune-mediated component. Still, there are practical steps that may reduce risk or help catch flare-ups earlier. Good general herd health matters. Work with your vet on routine preventive care, prompt treatment of eye injuries, and fast evaluation of any squinting, tearing, or cloudy eye.

Because leptospirosis is an important risk factor in equids, reducing exposure is sensible where possible. That can include limiting access to stagnant water, improving drainage in muddy or swampy areas, discouraging wildlife contamination of feed and water, and managing contact with livestock environments that may increase urine contamination.

Daily observation is one of the most useful tools for donkey pet parents. Donkeys are often stoic, so subtle changes can be easy to miss. Check for tearing, light sensitivity, a smaller pupil, or a slightly hazy eye. A well-fitted fly mask may help protect the eye from sunlight, dust, and insects in donkeys with previous flare-ups.

If your donkey has already had ERU, prevention usually means flare reduction and early intervention, not a guaranteed cure. A written monitoring plan with your vet can help you act quickly when signs return and may improve long-term comfort and vision.