Uveitis in horses 

By vet Hannah Marrow

Uveitis is inflammation of the uveal tract within the eye. The uveal tract is composed of the iris, ciliary body and choroid. 

The iris is the coloured part of the eye which alters the size of the pupil depending on the brightness of the light. The ciliary body is a small structure within the eye which produces fluid to keep the eye inflated. The choroid is a vascular structure located at the back of the eye behind the retina. It functions to supply blood and nutrients to the retina. As a result, uveitis results in a painful eye with a constricted pupil. The eye is often closed, particularly in bright light, as the horse tries to avoid light. Many owners notice that their horse’s eyelashes are ‘droopy’. 

Unfortunately, uveitis is the leading cause of blindness and ocular pain in horses. Signs to look out for are ocular pain, resulting in the horse keeping its eye, or eyes, closed, frequent blinking, increased lacrimation (tear production), reddening and/or swelling of the conjunctiva, clouding of the cornea and pupil constriction.

Breeds including Warmbloods and Appaloosas are more commonly affected by uveitis, suggesting that there is a genetic predisposition to this condition. 

Uveitis can also be seen in horses who are suffering from other eye diseases. For instance, ulceration of the cornea, trauma to the eye or irritation around the eyes. 

Prompt and appropriate treatment of eye disease is important to reduce the likelihood of uveitis developing.

The presence of flies on and around the eye can result in a condition called conjunctivitis, where the tissues surrounding the eyes become inflamed and reddened. This can lead to uveitis. Also, the presence of flies around the eyes and face can cause irritation, leading to eye trauma as the horse scratches their head.

If a horse suffers from two or more episodes of uveitis, they have a condition known as Equine Recurrent Uveitis (ERU). This is an auto-immune condition which is characterised by multiple recurrent bouts of inflammation requiring repeated episodes of treatment. Unfortunately, if a horse has an acute episode of uveitis, they are more likely to develop ERU in the future. Equine recurrent uveitis affects up to 10% of the horses worldwide and although many treatments are available to manage this condition, it may ultimately lead to a chronically painful and blind eye which will need to be removed.

Treatment of an acute case of uveitis requires anti-inflammatory medications applied to the eye surface, pain relief and atropine, a drug used to dilate the pupil making the eye more comfortable. 

UV face masks/meshes are recommended to limit the exposure of the eye to UV light and reduce the likelihood of recurrence, since horses suffering from uveitis are very sensitive to sunlight exposure. 

To reduce the likelihood of further episodes of uveitis occurring in the future, the control of flies is critical to reduce irritation to the eyes. Fly masks prevent the flies having access to the eyes and surrounding tissues. Although fly repellent sprays can be used on the legs and body, care should be taken to avoid spraying the eyes. 

Careful selection of pastures for grazing can be beneficial in reducing the exposure to flies; avoiding wet and boggy pastures and pastures with lots of tree cover where possible, as these act as good habitats for flies.

According to recent studies in the US and Canada, about a third of horses evaluated for ocular disease were already blind at time of examination, stressing the need for early diagnosis and treatment, even if the presenting signs are mild. If you have any concerns about your horse's eyes, it is recommended that you seek veterinary attention promptly.