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Veterinary Medicine Laboratories – Department of Clinical Sciences 

Ocular Immunology, Toxicology, and Drug Delivery – Equine Recurrent Uveitis

Equine recurrent uveitis (ERU) (also known as moon blindness, iridocyclitis, and periodic ophthalmia) is a major ophthalmic disease of the horse and is the most common cause of blindness in this species. Fortunately, recent advances in the treatment of horses with ERU have led to the successful management of this disease in some horses.

This page discusses some important facts about ERU, its causes, and potential treatment options for the affected horse.

What Is Equine Recurrent Uveitis?

Uveitis refers to inflammation of the uveal tract of the eye. The uveal tract consists of three components: the iris (colored portion of the eye responsible for opening and closing the pupil), the ciliary body (produces a watery fluid that maintains the shape of the eye) and the choroid (back portion of the eye responsible for supplying blood and nutrients). Equine recurrent uveitis is the term used for eye inflammation that recurs periodically. Equine recurrent uveitis occurs to varying degrees among horses and may affect one, or both eyes. After a horse is affected, ERU recurs at varying intervals, each time causing additional ocular damage that leads to permanent changes in the internal structures of the eye.

What Causes Recurrent Uveitis in Horses?

Any injury to a horse’s eye may result in the development of uveitis and subsequent ERU. The two most common causes of ocular injury are trauma and infectious diseases. Trauma to the eye may result from a blunt blow or by penetration of the globe (eyeball) by a sharp object. Other common causes include:

  • Bacterial organisms (Leptospira, Brucella, Streptococcus)
  • Viral agents (equine influenza, equine viral arteritis, parainfluenza type 3)
  • Parasites (Onchocerca, Strongylus, Toxoplasma)
  • Miscellaneous (endotoxemia, tooth root abscesses, neoplasia)

Regardless of the inciting cause, it is believed that the ERU is immune-mediated. This means that once the primary infection resolves, subsequent episodes of eye inflammation are the result of overaction of the horse’s own immune system in response to active immune cells such as T-lymphocytes.

What Treatment Is Available for Recurrent Uveitis in Horses?

The main goals of therapy for ERU are to preserve vision and reduce and control ocular inflammation in an attempt to limit permanent damage to the eye. In horses where a definite inciting cause has been identified, treatment is directed at eliminating the primary problem, and initial tests to isolate an inciting agent are performed. These tests may consist of a complete blood count, biochemistry profile, conjunctival biopsy, and serology for bacterial and viral agents. More often, however, one particular cause cannot be isolated. In these instances, therapy is directed at the allaying of symptoms and reducing ocular inflammation.

Because vision loss is a common long-term manifestation of ERU, initial therapy must be aggressive. In acute cases, treatment in the form of systemic and local therapy consisting of antibiotics, corticosteroids and anti-inflammatory drugs is used, many times simultaneously. Initial therapy is instituted for at least two weeks, and may be continued for an additional two weeks after the resolution of clinical signs. In severe cases, local subconjunctival injections of corticosteroids may be indicated as an adjunct to therapy. In most instances, a subpalpebral lavage catheter is placed to facilitate delivery of topical medications.

Unfortunately, certain horses are never able to completely eliminate their uveitis and for these horses alternative forms of therapy must be considered.

Although traditional therapy may be used to control recurrent flare-ups, no therapy has proven to be 100% effective in eliminating ERU. Some horses respond well to long-term topical and systemic therapy and are able to be managed in this fashion. Other horses, however, do not respond to traditional therapy and may experience recurrences of uveitis more frequently. Fortunately, studies investigating the use of an immunosuppressive drug known as cyclosporin are currently underway in an attempt to manage these horses. This treatment involves the ocular implantation of a cyclosporin delivery device, which requires general anesthesia.

How Will I Know If My Horse Has Recurrent Uveitis?

The equine eye is a highly specialized structure and therefore responds to inflammation in a variety of ways. One sign is a cloudy, red, painful eye. Horses with this condition are typically blepharospastic (holding the eyelids closed or squinting) and may be sensitive to light due to the painful nature of this disease. Other signs include ocular discharge (which may be evident as tearing), corneal edema (bluish tinge to the cornea) and conjunctival hyperemia or scleral injection (prominent vessels in the whites of the eye). Any of these signs may indicate a potential problem and should alert the owner and veterinarian to the possibility of ERU.

What Is the Long-Term Effect of Equine Recurrent Uveitis?

The recurrent nature of ERU allows multiple opportunities for damage to occur in the internal structures of the eye, and therefore the prognosis is guarded. Some manifestations of damage include cataract formation, degeneration of the globe with pthisis bulbi (shrinking of the eye) and blindness.

Your Veterinarian’s Role

Your horse’s veterinarian plays a key role in the successful management of ERU. Although vision loss is a common result of ERU, early recognition by your veterinarian, with possible referral to a veterinary ophthalmologist for persistent or nonresponsive cases, can have a positive effect on the eventual outcome.