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. The anterior uvea refers to the portion in the front of the eye: the iris and the ciliary body. Diseases that affect the anterior uvea often affect the choroid as well.
In horses, cysts of the iris may be present in the connective tissue. Many such cysts involve blue irises. Because vision is not usually affected, therapy is not typically necessary. However, cysts on the corpora nigra (tissue that extends from the iris into the pupil) may impair vision and can be mistaken for tumors. In such cases, surgical removal or deflation may be needed.
Inflammation of the iris and ciliary body (called anterior uveitis or iridocyclitis), can occur due to disease within the eye or in other parts of the body. Inflammation of the choroid in the back of the eye frequently occurs at the same time. Trauma and, rarely, tumors or worms within the eye are causes of inflammation of the uvea in only one eye. Common causes of inflammation of the uvea in both eyes of horses include immune-mediated diseases and infectious diseases such as leptospirosis (a bacterial disease), equine viral arteritis (a viral disease), and bacterial infections of the joints, navel, and gut of newborn foals.
When anterior uveitis is severe, it results in:
Complications, such as glaucoma (increased pressure in the eye), cataracts, cloudiness of the cornea, and blindness, can occur as a result, especially when the inflammation is longterm or recurs (equine recurrent uveitis, see below).
Your veterinarian will use a thorough medical history, complete examination of the eyes and the rest of the body, and blood tests to diagnose anterior uveitis and determine its cause. Treatment consists of topical, oral, or injectable medications to manage movement of the iris, any infections, and inflammation. Your veterinarian will select the most appropriate treatment program based on the cause the inflammation.
Bleeding inside the eyeball is called hyphema. It may appear as liquid or clotted blood.
Causes of hyphema include:
Sudden, severe bleeding usually has a good outcome if the cause is identified and treated promptly. Recurrent or longterm hyphema has an unknown or poor outlook because glaucoma or blindness is likely. Although no drugs are available to treat hyphema directly, certain medications may help relieve the signs.
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 some 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 is an autoimmune condition, meaning the horse's immune system is causing damage to its own tissues. The syndrome occurs after an initial episode of uveitis (see Anterior Uveitis, above), but not all horses with uveitis develop the recurrent form. It 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 (including leptospirosis), parasitic worms, viruses (including equine influenza), tooth root abscesses, hoof abscesses, and eye injuries. 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. The syndrome occurs more frequently in Appaloosas, warmbloods, and draft breeds, suggesting the condition might also be inherited.
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. The goals of treatment are to reduce inflammation, relieve discomfort, and prevent vision loss. 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, prompt therapy with both topical and whole-body anti-inflammatory medications is usually started to minimize the damage to the eye. Surgery to reduce inflammation longterm or other medications may also be necessary. 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, proper dental care, 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, prevent trauma to the eyes, and reduce environmental triggers. These steps should be taken in addition to providing the prescribed medications.