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Equine recurrent (periodic) uveitis (moon blindness, recurrent iridocyclitis) 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.

Epidemiology
A genetic component exists in some breeds of horses, notably the Appaloosa, which has a higher than usual incidence of blindness. 1-2% of horses in the United States are thought to be so severely affected by ERU to cause vision loss.

Aetiology
Several aetiologies are suggested, and any combination of these may be present in any given case.
 * Vitamin deficiency (A, B or C)
 * Viral infection
 * Bacterial infection
 * ''Leptospira
 * ''Streptococcus equi
 * ''Brucella abortus
 * Borrelia burgdorferi
 * Rhodococcus equi
 * Parasitic infection
 * Strongyle
 * Onchocerca cervicalis
 * Protozoal infection
 * Toxoplasma gondii
 * Autoimmune disease

Leptospira is considered the most important pathogen associated with ERU, and uveitis usually occurs 12-24 months after exposure. The disease has been suggested to be primarily autoimmune in nature, being a delayed hypersensitivity reaction to any of the above agents.

Clinical signs
In the acute stage of the disease, a catarrhal conjunctivitis is present, with signs of ocular pain, usually blepharospasm, increased lacrimation, and photophobia. Miosis is also usually present. After a few days, this will progress to a keratitis and iridocyclitis. Other ocular problems may also occur, including conjunctival and corneal oedema, and aqueous flare.

After an acute flare-up, no clinical signs of disease may be seen for a prolonged period, which can vary from a few hours to a few years. With frequent acute incidents, though, additional clinical signs may be seen, including anterior and posterior synechiae, poor pupillary responses, cataracts, and a cloudy appearance to the vitreous humour.

Diagnosis
The cornerstone of diagnosis is an accurate history, and a good clinical examination of the eye, to eliminate traumatic uveitis. Ultrasonography is a useful tool, as it can detect a thickened iris, but only in the hands of an expert.

Medical therapy
During an acute flare-up, therapy is targeted at reducing the inflammation within the eye and providing pain relief. Since the majority of the pain induced by ERU is secondary to spasm of the ciliary body and iris, topical mydriatics are used to dilate the pupil and relieve the spasms. Atropine is the most commonly used mydriatic. Atropine increases the horse's risk of colic, so the drug is tapered as the horse's clinical signs (miosis) improves.

Anti-inflammatory therapy is usually given both systemically and topically. Topical steroids, such as prednisolone acetate or dexamethasone, are potent and considered the preferred drug for use in an acute flare of uveitis, but predispose the horse to severe corneal ulceration (a "melting" corneal ulcer) if they develop any damage to the epithelium of the cornea. They are only used when the horse has no evidence of corneal damage. Topical NSAIDs, such as flurbiprofen, bromfenac, and diclofenac, are less potent relative to topical steroids but are safer to use if a corneal ulcer is present. Horses are also placed on systemic NSAIDs, usually flunixin meglumine, which has been shown to be more effective than phenylbutazone or aspirin. Anti-inflammatories are usually given for 1-2 weeks until the horse is no longer experiencing clinical signs, then slowly tapered for an additional 2 weeks, as sudden discontinuation predisposes the horse to redeveloping inflammation.

Cyclosporine sustained-release device
Cyclosporine is an immunosuppressant that has been shown to reduce the reoccurrence of ERU episodes and improve visual outcomes when administered to the eye. It may be loaded into a reservoir that slowly releases the medication. This device is surgically implanted, either under general anesthesia or while standing and sedated. The device is considered very safe, although it has a limited amount of drug and may require re-implantation within the following 24 months.

Vitrectomy
Vitrectomy is a surgical procedure that removes the vitreous from the posterior chamber of the eye. It has been shown to be highly successful in horses that develop ERU secondary to Leptospirosis but is less-successful for other causes of ERU. It is primarily used as a treatment in Europe, and has had limited success in the United States, possibly due to the a difference in leptospira-infected animals. Vitrectomy is thought to help by removing the causative agent (reducing or completely eliminating further episodes) as well as by removing inflammatory cells in the vitreous that may impede vision. Vitrectomy has potentially serious complications, including retinal hemorrhage, retinal detachment, and cataract formation.

Prognosis
Vitrectomy can offer a cure to a percentage of horses affected by Leptospirosis-induced ERU. However, most horses require lifelong management of the disease. Prompt treatment during flares is especially important. In one study, 56% of horses experienced vision loss, with a higher percentage of Appaloosas developing vision loss relative to other breeds, and a higher percentage of Leptospira-positive horses developing vision loss relative to animals that were negative for the organism. Cataract formation was a common cause of vision loss in horses. Other serious sequelae associated with the disease included lens luxation, vitreitis, corneal scarring, and glaucoma. A small percentage of horses (4%) required enucleation.