Moon blindness is a common and painful disease of one or both eyes in horses and the most common cause of blindness in horses worldwide. Moon blindness, so named because in the 17th century it was believed that the chronology of the disease followed the phases of the moon, is also known by the medical term recurrent equine uveitis or ERU. Owners may notice their horses blink or tear open their eyes, and the cornea or clear layer covering the eye may turn whitish. Horses with mole blindness can go blind if the disease is left untreated. Some breeds, such as Appaloosas, are at higher risk. Appaloosas are eight times more likely to develop ERU than other breeds due to an inherited genetic risk factor. 1 Learn how to watch for the symptoms of moon blindness and how your veterinarian can diagnose and treat the disease here. ERU can be treated to relieve symptoms and slow the progression of the disease, but there is no cure.
What is moon blindness?
Moon blindness (URE) is a group of immune-mediated diseases that cause repeated bouts of inflammation in the inner part of the eye.2 This inflammation causes severe pain and often blindness. While many processes cause a single bout of uveitis or inflammation inside the eye, two or more episodes lead to a diagnosis of moon blindness. This means that once your horse has an attack, it is important to watch carefully for another one in the months or years that follow.
The anatomy of the eye is very complicated, but some of the most important features are the cornea, which is the transparent outer layer that allows light to pass through, and the anterior part that lies just below the cornea. The deepest layer of this anterior part is the iris, which expands and contracts to depend on the amount of light in the environment. Behind the iris is the lens, which allows the animal to focus on an object, followed by the posterior part, the vitreous, and finally the retina, which receives the image and transmits it to the brain.
The area affected by moon blindness is called the choroid. It consists of the iris, the muscles that control the iris (ciliary body), and most of the choroid, the blood vessel layer that surrounds the vitreous body and supplies blood to the outer retina.
Inflammation of the eye causes pain and a loss of the barrier between the blood vessels and the inside of the eye. This leads to clouding of the inside of the eye and loss of clarity of the cornea. Initially, the intraocular pressure drops from normal. Recurrent attacks lead to blindness through various mechanisms. The lens may shift and/or develop cataracts, intraocular pressure may increase (glaucoma), and corneal and retinal scarring may occur. Moon blindness usually affects one eye, but about 20% of horses with ERU have both eyes affected.
Symptoms of moon blindness vary depending on whether the disease occurs at a relatively early stage (acute signs) or whether the eye has already had several attacks of URE and the disease is in a late stage (chronic signs). Most acute signs can be picked up by an alert owner. In contrast, most chronic signs other than a small eye or blindness require an ophthalmologic (ophthalmic) examination by a veterinarian.
In some horses, low-grade (subclinical) inflammation occurs, which can cause the eye to show chronic signs without acute signs. A veterinarian should check your horse’s eyes for chronic signs during the annual exam.
Moon blindness affects 2-25 percent of horses worldwide2. The causes of moon blindness are multifactorial and poorly understood3. It is thought that an inflammatory insult, such as infection, parasite, or trauma, may trigger the initial inflammatory episode or uveitis. The horse’s immune system then triggers an abnormal and damaging inflammatory cycle that can increase (relapses) and decrease (periods of normality). When there is no genetic predisposition in predisposed breeds, Leptospira bacteria are often the triggering factor.
Any horse can develop URE, but certain breeds are at higher risk. These include Appaloosas, Warmbloods, American Quarter Horses, and draft horses.
The most important first step in diagnosing a horse with the acute symptoms described above is to distinguish between moon blindness and a corneal ulcer. Moon blindness is an autoimmune disease that causes abnormal and exaggerated inflammation, while a corneal ulcer occurs when the horse scratches the eye with a foreign body, causing a cut in the delicate cornea. In the case of an ulcer, squinting eyes, tearing, corneal edema, and water retention are symptoms that can lead to a proper diagnosis and facilitate treatment.
If you notice acute signs of lunate blindness, you should call your veterinarian. Your veterinarian will examine the eye closed with a special lamp. He will then apply a dye (fluorescein) to the eye that will be combined with a corneal ulcer. If there is no corneal staining, a horse with the above acute signs probably has uveitis and not an ulcer. To diagnose glaucoma associated with moon blindness, intraocular pressure is measured with a gentle, highly specialized instrument called a tonometer. The veterinarian may also take blood samples to test for leptospirosis.
The main goals in treating moon blindness are to preserve vision, reduce pain during an attack, and reduce the frequency of a recurrence. After the initial diagnosis of uveitis, your veterinarian will prescribe an ointment to be applied to your horse’s eye that contains corticosteroids. Corticosteroids are medications that dampen the abnormal immune response in lunar blindness. Therefore, it is important to distinguish between ERUs and corneal ulcers, as topical corticosteroids can delay the healing of corneal ulcers and lead to superinfection with fungi or bacteria.5
Another ointment your veterinarian can apply to your horse’s eye is called atropine, which relieves painful spasms that occur with inflammation of the iris and ciliary body. Your veterinarian will also likely give a non-steroidal anti-inflammatory drug called flunixin meglumine (Banamine) orally to control pain and reduce inflammation. If your horse is positive for Leptospira, antibiotics may be given. Treatments usually continue for 2-4 weeks and require regular follow-up visits to your veterinarian during this time.
If your horse has recurrent outbreaks, your veterinarian may refer you to another veterinarian (ophthalmologist) who specializes in eye problems. He or she will perform a complete eye examination with special instruments and make recommendations. These recommendations may include antibiotic injections directly into the eye6 or surgery to implant a weak immunosuppressant (cyclosporine)7.
If your horse’s pain cannot be controlled or if the eye is blind, it may be more comfortable to remove the eye (enucleation).
The long-term prognosis for vision improves if diagnosed early in the disease, but slightly less than 50% of horses with moon blindness eventually lose vision.8 This can last for years, so a horse with ERU can often have a normal quality of life for some time if treated well. However, once chronic symptoms develop, the pain associated with ERU and the ocular changes may cause the horse to become more anxious and irritable, or unable to return to its previous level of performance, even if it can still see something. Horses that can receive cyclosporine implants usually have better long-term outcomes.
Some horses have to have one eye removed for uncontrollable lunar blindness. A horse with one eye can have a good sporting career if it uses the other acute senses and the wide field of vision of the remaining eye. Horses that require removal of both eyes or are blind in both eyes can rarely ride but can have a good quality of life with some environmental adaptations.
Uveitis is difficult to prevent, mainly because it has many causes. However, the caregiver must recognize the acute signs to prevent uveitis from recurring or causing lunar blindness.
If you suspect your pet is sick, call your veterinarian immediately. Always consult your veterinarian about health issues, as he or she has examined your pet, knows his or her health history, and can make the best recommendations for your pet.