Equine recurrent uveitis (ERU) – also known as moon blindness, recurrent iridocyclitis, or periodic ophthalmia – is an acute, nongranulomatous inflammation of the uveal tract of the eye, occurring commonly in horses of all breeds, worldwide. The causative factor is not known, but several pathogeneses have been suggested. It is the most common cause of blindness in horses. In some breeds, a genetic factor may be involved.