Periodic ophthalmia, otherwise known as recurrent uveitis, uveitis, or moon blindness, can be a devastating disease of the equine eye. It also, unfortunately, is a disease that we really don’t know much about. The hypothetical causes have been sporadically researched over the years, but we aren’t really much closer to understanding this inflammatory ocular disease.

Classic appearance of long-standing uveitis. Note the scarring of the iris and the white reflective cataract. Photo by Dr. Michael A. Ball

The term moon blindness comes from the ancient belief that the disease was associated with the changes of the lunar cycles. The "recurrent" or "periodic" part of the disease is the result of the propensity of this disease to recur in a rather unpredictable manner after the animal has once experienced the disease. The medically descriptive name for this disease is uveitis (pronounced you-vee-itis), and I will refer to it by that name for the rest of this article