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Equine Recurrent Uveitis or Moon Blindness

Uveitis means inflammation of the uvea. That doesn’t tell you much, does it? The uveal tract of the eye is comprised of the vascular (blood vessel) and pigmented structures of the eye. Equine Recurrent Uveitis (ERU) is the leading cause of blindness in horses worldwide. The prevalence of this devastating disease is reported to be between 2 and 25%!

Statistics for the United States state that between 10 and 25% of horses will suffer from uveitis. That higher number is associated with the staggering incidence in Appaloosa horses. Other breeds affected at a higher rate include paints, warmbloods, and draft horses.

Equine Recurrent Uveitis

Leopard Appaloosas are more at risk than Appys with blankets or more solid-based patterns. Appaloosas are much more likely to have disease in both eyes. The really dismal statistics show that Appys are 8 times more likely to get ERU than other breeds and will very likely become blind in one or both eyes.

The significantly higher risk factor in Appaloosas points towards a genetic component, but there is no direct proof of this yet. Although there are only theories, the current thinking is that ERU is an autoimmune disease caused by a combination of genetics and environment. The relationship is not well understood, but some theories suggest that the bacteria Leptospira spp has some role in creating disease.

A Kansas State Veterinary ophthalmologist sums up the challenges of understanding this disease:

Our understanding of the ERU disease process is constantly evolving and ERU is an area of intense research within veterinary ophthalmology. 

Jessica Meekins, DVM, MS, Diplomate ACVO, Assistant Professor of Ophthalmology, Kansas State University Veterinary Health Center
Symptoms of Equine Recurrent Uveitis
Equine Recurrent Uveitis

A horse with an unhappy eye looks just like a human with the same problem. Excessive tearing, squinting, sensitivity to light, and redness of the eye may be the first visible signs. Alternatively, you might just notice slight tearing and a swollen eyelid. Other possible indicators of a problem are a cloudiness to the eye, or the pupil of the eye might be very small or constricted. The pupil size might not be the same in both eyes.

Signs of inflammation can occur in one or both eyes. One eye may look worse than the other. This is not a contagious disease, so it won’t “spread” from one eye to another, or to another horse. I have stated this before, but it bears repeating:

Any signs of pain, trauma, or trouble with the eye is an emergency. Eye issues can get worse very quickly. This is not a time to delay calling your veterinarian!

Three types of Equine Recurrent Uveitis

The symptoms described above are common to the classic form of ERU. This is the most painful and most common form of ERU. Periods of active inflammation, visible to the horse owner, are followed by periods of relative quiet with no clinical signs. There may be invisible low grade inflammation occuring during these “quiet” periods. Unfortunately, the repeated inflammatory cycle may ultimately result in partial or complete loss of vision. This form of ERU most often occurs in warmbloods and Icelandic horses.

The insidious form of Equine Recurrent Uveitis is the one most commonly seen in Appaloosas, and has been diagnosed in draft breeds as well. This form causes persistent low-grade inflammation in one or both eyes. It does not appear to be painful; however, the result is the same… destruction of the structures of the eye, leading to vision loss.

The posterior form of ERU is most often diagnosed in warmbloods and draft breeds. It is characterized by destruction of the structures at the back of the eye. Often, retinal degeneration results from this form of ERU.

Diagnosis of Equine Recurrent Uveitis

As I stated previously, there is much about ERU that is not understood. For a horse to get a diagnosis of ERU, at least two bouts of uveitis must have occurred. Ocular tumors and trauma can trigger uveitis, among other things.

A veterinarian must diagnose ERU using a complete ocular exam. This exam includes assessing the pressure within the eye (tonometry), the integrity of the cornea (fluorescein stain), and a thorough exam of the other structures of the eye. In addition to evaluating the eye, your veterinarian will likely do some blood work and lab tests to identify systemic disease and check for parasitic infections such as Leptospira spp.

Treatment of Equine Recurrent Uveitis

I spent four years in Veterinary school, and I can tell you without hesitation that treating my horse’s eye multiple times a day when he had corneal ulcers was a nightmare. I put drops in my German Shepherd’s eyes twice a day to treat his pannus… easy peasy! Getting my thousand pound beast to drop his head and allow me to mess with his painful eyes… not so easy.

Treatment goals are to reduce or eliminate inflammation and pain, preserve vision, and hopefully minimize reoccurrences. Anti-inflammatory drugs such as NSAIDs (non-steroidal anti-inflammatory drugs) and steroids are used both topically (in the eye) and sometimes systemically. Ophthalmic atropine relaxes the eye and reduces pain.

Horses treated with NSAIDs long-term need regular monitoring of blood work for kidney damage, as well as careful attention to the potential for GI ulceration. It was because of NSAIDs that Mica developed a stomach ulcer.

There is a minor surgical procedure that can make treatment much, much easier. A subpalpebral lavage catheter can be implanted, allowing you to medicate your horse without having to directly insert the drug into the eye(s).

Another option is a cyclosporine implant. This technique implants a small disc in the sclera of the eye that releases an immunosuppressive drug over a long period of time. Check out the linked article to see if your horse is a good candidate for that approach.

Prognosis of Equine Recurrent Uveitis

This UC Davis article has this to say about the prognosis for horses with ERU:

Early diagnosis and intervention are associated with the best prognosis for ERU-affected horses. Long-term prognosis is guarded. Current treatments can slow the progression of inflammation in the eye, but are not curative. More than 60% of affected horses are unable to return to previous levels of work and approximately 56% of ERU-affected horses eventually become blind. ERU-affected horses with glaucoma or cataracts are more likely to become blind and are also more likely to require removal of the affected eye (enucleation).

Amy Young

Sadly, removal of the eye is sometimes necessary. It is one thing to remove one eye, but if the horse has ERU in both eyes, there are few good options. I was prompted to write this post because a friend recently lost a horse to this persistent and devastating disease.

Here are some additional articles if you’d like further information.

Equine Recurrent Uveitis, Or “Moon Blindness”

Equine recurrent uveitis (ERU)

Equine Uveitis

Overview of Equine Recurrent Uveitis (This is Merck Veterinary Manual and is more of a scholarly article… more medical terminology!)

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