Equine Recurrent Uveitis in Mules: Moon Blindness and Immune-Mediated Eye Disease

Equine recurrent uveitis, or ERU, is inflammation inside the eye that comes back over time. It is commonly called moon blindness. In mules, the disease behaves much like it does in horses: flare-ups may be obvious and painful, or they may be quieter and still cause damage in the background. The inflamed tissues are part of the uveal tract, which includes structures that help control light entry and nourish the eye.

ERU is one of the most important causes of blindness in equids. During a flare, your mule may squint, tear, avoid bright light, or develop a cloudy-looking eye. Repeated episodes can leave behind permanent changes such as scarring, cataracts, glaucoma, retinal damage, or a shrunken blind eye.

Many cases are considered immune-mediated, meaning the body keeps reacting against eye tissues even after the original trigger is gone. In North America, prior exposure to Leptospira bacteria is a well-recognized risk factor in some equids. Because mules are less studied than horses, your vet will usually apply horse-based evidence while tailoring care to your mule’s exam findings, comfort, handling needs, and long-term use.

ERU is usually thought of as a multifactorial, immune-mediated disease. That means there may be more than one trigger, and the immune system continues to drive inflammation after the initial event. In equids, recognized contributors include prior Leptospira exposure, trauma, other infections, and inflammation that starts elsewhere in or around the eye.

Leptospira is especially important to discuss with your vet. In North America, Leptospira interrogans serovar Pomona has been strongly associated with ERU in horses, and similar concern applies to mules because they are equids with comparable eye anatomy and disease patterns. Not every mule with ERU will test positive, and not every positive test means Leptospira is the only cause. Still, it can influence how your vet thinks about prognosis and referral options.

Other possible triggers include blunt or penetrating eye injury, corneal ulcers, parasite-associated inflammation, and less commonly systemic infectious disease. Genetics are well documented in some horse breeds, but mule-specific risk data are limited. In practice, your vet often focuses less on finding one single cause and more on identifying active inflammation, ruling out look-alike emergencies, and building a plan to reduce future flare-ups.

Diagnosis starts with a careful history and a full eye exam. Your vet will ask whether the problem has happened before, whether one or both eyes are affected, and whether there has been trauma, recent illness, or changes in behavior or vision. Because mules can be stoic, subtle signs from the pet parent or handler are often very helpful.

The eye exam may include a bright-light exam, magnification, fluorescein stain to check for corneal ulcers, pupil assessment, and evaluation of the front and back of the eye when possible. Your vet is looking for signs such as aqueous flare, a constricted pupil, corneal edema, fibrin, synechiae, cataract formation, retinal changes, or evidence of chronic scarring. In some cases, sedation is needed for a safe and complete exam.

ERU is often diagnosed when uveitis has happened more than once or when the eye shows chronic changes that suggest repeated inflammation. Additional testing may include intraocular pressure measurement, ocular ultrasound if the inside of the eye cannot be seen well, and selected blood or ocular fluid testing when Leptospira or another underlying cause is suspected. A positive blood test can show exposure, but it does not always prove that Leptospira is driving the current flare, so results need to be interpreted alongside the exam.

Not every case can be prevented, because ERU often has an immune-mediated component. Still, there are practical steps that may lower risk or help catch flare-ups early. The most helpful habit is daily eye observation. Compare both eyes in the same light and watch for tearing, squinting, cloudiness, swelling, or a change in pupil size.

Protecting the eyes from irritation also matters. A well-fitted fly mask can reduce exposure to flies, wind, dust, and bright sunlight. Good pasture and barn management may also help reduce trauma risk. If your mule develops any eye injury, corneal ulcer, or unexplained tearing, early treatment may reduce the chance of secondary inflammation becoming more serious.

Because Leptospira exposure is linked to ERU in many equids, talk with your vet about local risk. Risk reduction may include limiting access to standing water when possible, discouraging wildlife and rodent contamination of feed and water, and discussing whether leptospirosis vaccination makes sense in your area and for your mule’s lifestyle. Prevention is not always perfect, but fast response to even mild eye changes can make a major difference in long-term comfort and vision.