The uvea (or the uveal tract) is the colored inside lining of the eye consisting of the iris, the ciliary body, and the choroid. The iris is the colored ring around the black pupil. The ciliary body consists of muscles that contract and relax to allow the lens to focus on objects; it is also the source of the aqueous humor, the clear fluid within the eye. The choroid is the inner lining of the eyeball. It extends from the ciliary muscles to the optic nerve at the back of the eye. The choroid also contains layers of blood vessels that nourish the inside parts of the eye, especially the retina.
Persistent membranes across the pupil, cysts of the iris, and inflammation of the iris and ciliary body (anterior uveitis or iridocyclitis) are all conditions that can affect the front of the uvea. In horses, cysts of the iris may be present in the connective tissue. Many such cysts involve blue irises. see Eye Disorders of Dogs: Disorders of the Anterior Uvea in Dogs
Common causes of inflammation of the uvea in both eyes of horses include immune-mediated diseases and infectious diseases such as toxoplasmosis (a disease caused by a parasite), systemic mycosis (a fungal infection), leptospirosis (a bacterial disease), equine viral arteritis (a viral disease), and bacterial infections of the joints, navel, and gut of newborn foals.
Equine Recurrent Uveitis (Periodic Ophthalmia, Moon Blindness)
Equine recurrent uveitis is one of the most common eye diseases in adult horses. Typically, there are periods of active inflammation of the uvea, followed by varying periods with no signs at all. During the so-called inactive periods, mild inflammation continues in most horses. If untreated, the inflammation eventually leads to harmful complications that make this syndrome the most common cause of blindness in horses throughout the world.
Equine recurrent uveitis has many potential causes, but the end result is damage to the uveal tract. Specific conditions or agents that may cause this disorder include certain bacteria, parasitic worms, equine influenza (a viral illness), tooth root abscesses, and hoof abscesses. Inflammation can be stimulated by dead or dying larvae of parasites that have unexpectedly migrated to the eye. Therefore, active episodes of uveitis are sometimes seen after normal deworming treatments.
Signs of active uveitis include frequent squinting, watering eyes, cloudiness of the cornea, and contraction of the pupil. An active episode may include inflamed cells infiltrating the retina or choroid, partial separation of the retina, retinal bleeding, and a hazy appearance to the vitreous (the clear “jelly” that fills the eye). One or both eyes may be affected. When both eyes are affected, it is common for one eye to be more severely inflamed.
Longterm equine recurrent uveitis leads to scarring of the cornea, formation of fibrous tissue in the iris, glaucoma, cataracts, and degeneration of the retina. The importance of careful eye examinations cannot be overstated. Horses with longterm uveitis can have few or no obvious signs of eye disease but may develop degeneration of the retina. To protect your horse's vision, eye examinations should be included in its routine care.
Your veterinarian will want to identify the underlying cause of the uveitis. Because an episode of uveitis (inflammation of the uvea) can be the first sign of generalized disease, a thorough physical examination is usually performed in addition to the eye examination. Blood tests are often included as part of the examination. Other tests may also be needed to identify the cause.
Therapy should begin as soon as possible. If a specific cause can be identified, your veterinarian will address it as part of the treatment plan. In addition, or in cases where no cause is identified, aggressive therapy with both topical and whole-body anti-inflammatory medications is usually started to minimize the damage to the eye. Your veterinarian will prescribe the therapy best suited for your horse. Be sure to follow the prescription instructions exactly to help your horse recover.
Good husbandry practices such as effective fly control, frequent bedding changes, routine worming and vaccinations, minimizing contact with cattle or wildlife, draining stagnant ponds or restricting access to swampy pastures, and maximizing nutrition have all been advocated as means to reduce the effects of equine recurrent uveitis. These steps should be taken in addition to providing the prescribed medications.
Last full review/revision July 2011 by Kirk N. Gelatt, VMD; David G. Baker, DVM, MS, PhD, DACLAM; Steven R. Hollingsworth, DVM, DACVO