Equine recurrent uveitis (ERU) is an important and common condition of the equine eye, with a reported prevalence of 2%–25% worldwide. The classic form of ERU is characterised by episodes of active intraocular inflammation (inflammation within the eye) followed by quiescent periods of a variable length. However, some horses experience subclinical ocular inflammation where the inflammation within the eye persists without obvious signs of discomfort. Continued chronic inflammation causes secondary ocular changes such as cataracts, lens luxation, glaucoma and retinal degeneration. As a result, ERU is the most common cause of blindness in horses in the world.
ERU is an autoimmune disease where the body's inflammatory processes damage the eye. There are two forms of the disease. Acute or traumatic uveitis and chronic recurrent uveitis. Not every horse with a single bout of uveitis will develop ERU but horses that have experienced acute uveitis are at a greater risk. The exact disease mechanism is still unknown but many bacterial, viral, parasitic and non-infectious agents have been implicated including leptospira spp. There is no gender or age predisposition for ERU. However, appaloosas, warmbloods, and draft breeds are at a greater risk, which suggests there may be an inherited genetic component. ERU can be unilateral or bilateral and can affect the eyes asymmetrically.
The disease process is based around a change in the eye's blood barrier that allows white blood cells to enter the inner fluid within the eye. The white blood cells (leukocytes) release proinflammatory chemicals (cytokines, prostaglandins and leukotrienes). These inflammatory mediators disrupt the delicate balance within the eye and cause the clinical signs. The most common signs of acute ERU are puffy, watering eyes, squinting and red blood vessels at the sides of the eye, horses will be very sensitive to light and in some cases there will be a cloudiness with a blue or green tint. Acute signs also include, head shaking and potentially ulcers. Corneal scarring, glaucoma, cataracts and retinal degeneration are all signs consistent with chronic ERU changes are responsible for the clinical signs seen within the eye such as muscle spasm, constriction of the pupil, oedema within the eye, clouding of the fluid within the eye and thus reduction of vision.
The primary goals of treatment are to reduce inflammation, relieve discomfort, and prevent vision loss. If possible, the specific underlying cause should be identified and managed as best as possible. Aggressive treatment with systemic and topical anti-inflammatory medications is initiated immediately to reduce the inflammatory processes and to minimise damage from intraocular inflammation.
Topical steroid eye drops are a first line treatment that may need to be put in 6-8 times a day if the episode is severe. As the eye improves, the frequency of administration of topical steroidal can be gradually decreased. However, therapy should continue for at least 1-2 months to remove any underlining sub clinical inflammation. Topical atropine causes dilation of the pupil and decreases the risk of chronic changes in the eye, helps restore the eye's blood barrier and provides pain relief. Atropine is applied topically 2–3 times daily until the pupil is widely dilated. Atropine decreases gut motility in horses and so it is vital to monitor horse’s faecal output and monitor for signs of colic. Management using oral painkillers and anti-inflammatories can help in chronic and acute episodes.
As your horse will be sensitive to light, it is important to keep your horse in a dark area with no direct sunlight to avoid further aggregating the condition. It is also important to continue with practice whilst the horse is being treated with atropine as the horse will not be able to constrict the pupil and the eye will be damaged by too much light being allowed into the back of the eye.
There are surgical treatments for chronic unresponsive cases that involve either a cyclosporine implanting the eye that provides sustained-release (about 3 years) of the drug cyclosporine A, an immunosuppressive drug. Horses with implants have markedly fewer uveitic episodes than they did before surgery, and this device results in effective long-term control of ERU. The other procedure involves removing the fluid within the eye and replacing it with sterile water, the idea is to remove the inflamed fluid and allow the eye to almost "restart".
In summary ERU is a common and potentially blinding condition that requires quick effective veterinary treatment. Every sore eye should be seen by a vet before any drops or medications are given. Finally, ERU is not a condition that can be cured only managed.