“Moon blindness” is a chronic, painful eye disease, and it’s the most common cause of blindness in horses. It was so named during the 1600s because people thought recurring attacks were related to phases of the moon. This eye disease might be one of the first veterinary diseases ever documented. In the pyramids at Giza (Egypt), there are depictions of ocular problems in cavalry horses of that time, 4,500 years ago, showing uveitis or a disease very similar. This eye problem has also been called iridocyclitis and periodic ophthalmia, but the current term is equine recurrent uveitis (ERU).

During the past several centuries, horse owners thought the main causes were heredity, damp stables, bad feed, and marshy pastures. It was noted that land drainage reduced the incidence and that there was more moon blindness among horses on farms irrigated with town sewage. These factors pointed toward bacteria, yet the most popular theory a few decades ago was that moon blindness was due to lack of riboflavin (a B vitamin) in the horse’s diet.


According to Brian C. Gilger, DVM, MS, Dipl. ACVO, a professor of ophthalmology at North Carolina State University who recently published Equine Ophthalmology, ERU can be caused by several factors. One of the most common causes is infection with Leptospira, spiral-shaped bacteria, or spirochetes, that can be found in areas with stagnant water.

“It’s not the infection that causes the uveitis, but the immune response to the initial infection,” says Gilger. “Lepto is one of the initiators of this immune response.”

Other causes include trauma to the eye, and bacterial infections such as brucellosis, Streptococcus, Rhodococcus equi, and Borrelia bergdorferi (the spirochete that causes Lyme disease).

Viral infections such as equine influenza, equine viral arteritis (EVA), and equine herpe