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Equine recurrent uveitis (ERU), also known as moon blindness, is a condition that affects the horse’s iris within the eye. If the iris becomes damaged, resulting in inflammation this is called uveitis. This can occur as a primary disease, as a result of trauma to the eye, or as part of another disease. When the condition happens repeatedly it is called equine recurrent uveitis.

The iris is the part of the eye that gives it its colour. In most grey, bay, brown, chestnut or black horses, the iris is a dark nutmeg brown colour. In cream or coloured horses (and occasionally in other colours) it can be a variable blue colour, and there may be different colour distributions in the iris of the same eye.

The iris provides a barrier in front of the lens that controls the light that enters the back of the eye. It dilates in reduced light conditions to collect more light and constricts in bright light to limit the amount of light.

Equine recurrent uveitis has been recognised in horses for thousands of years. It was one of the first ophthalmic diseases recorded in the horse. It was historically known as “moon blindness” because it recurred at regular or irregular intervals.

ERU is fairly rare in the UK, with around half of those horses that do succumb to the condition suffering a recurring problem. ERU is more commonly seen in the USA and in central Europe, particularly in Germany and the Czech Republic. Much of the research into treatments of the disease has been carried out in these countries.

Signs of Equine Recurrent Uveitis

The signs of ERU are those of an inflamed and painful eye: blinking, watering, cloudiness, and partial or complete loss of vision in the affected eye. Generally, only one eye is affected at any one time, but both eyes may be involved on occasions. Signs to look out for include:

  • Swollen eyelids
  • Sensitivity to light
  • A cloudy cornea
  • Colour changes in the iris
  • A constricted pupil
  • Material within the front chamber of the eye such as blood, fibrin or pus
  • Reddening around the eye
  • A yellow-green discolouration deep within the eye
  • Pain, which shows as eye closure and increased tearing

In order to minimise damage to the eye and preserve sight as far as possible, rapid, aggressive treatment of the condition is essential.

Causes of Equine Recurrent Uveitis

The cause of the disease is as yet unclear, as Dr Andy Matthews, an equine vet and one of the authors of the standard veterinary textbook on horses’ eyes, explains.

“Usually there is no history of external trauma or injury to the eye, so we are talking about an endogenous disease,” says Dr Matthews. “In other words, a disease that is occurring from within the animal’s own body – an autoimmune type of disease, put crudely, a disease caused by the animal’s immune system ‘attacking’ itself.

“Any inflammation of the structures within the eye can lead to severe and permanent damage to its function of providing sight. Because of this, its immune responses are very specific – almost ‘damped’ down to prevent too much inflammation occurring.

“In horses with Equine Recurrent Uveitis it appears that the eye has lost this immune privilege, which results in it giving a more generalised inflammatory response. This leads to the signs and subsequent permanent damage to the eye that we commonly see.”

How to prevent Equine Recurrent Uveitis

“It seems likely that there may be different trigger factors for the condition in different parts of the world, which would also account for the stark differences in frequency with which it is found in different places,” says Dr Matthews.

“In both Germany and the USA specific types of bacteria known as Leptospira species appear to be implicated. There’s no evidence of Leptospira species occurring widely in Britain, which could account for the reduced incidence of the disease here.”

Treatment of Equine Recurrent Uveitis

“In Britain, we rely heavily on medical therapy,” says Dr Matthews. “Treating the condition early is essential and owners should seek veterinary advice straight away when confronted with a horse that has a watering and blinking eye.

“The use of anti-inflammatory drugs such as corticosteroids are the main line of attack, and these can be administered using drops or as an injection of a slow-release compound under the lining of the eyelids. In addition, intramuscular injection of corticosteroids delivers the medicine via the horse’s bloodstream.

“In Germany, surgery in the form of vitrectomy (removing the vitreous fluid from behind the lens), has a good record of preventing recurrence of the disease, but how successful it is at preserving vision is uncertain.

“More recently, the Americans have been experimenting with treating the condition using an immunosuppressive agent known as Cyclosporin A. Researchers in the USA and Italy have tried using slow-release Cyclosporin A implants which have been placed in the eye through a tiny stab incision in the cornea. These implants last about 8-12 months and have had success at preventing recurrence.”

Breeds susceptible to Equine Recurrent Uveitis

“It appears that Appaloosa and Appaloosa-derived breeds have an increased susceptibility to the disease, and clinical cases in these horses can be very difficult to manage. In addition, there is certainly a familial history of Equine Recurrent Uveitis in some breeding lines of other breeds.

“If the condition doesn’t improve, the horse can be left with a permanently painful and unsightly eye. In these cases removing the eye completely is the way forward. This removes the source of discomfort and many one-eyed horses can return to almost full work, including jumping.”

What to look out for

  • ERU usually occurs in horses who are more than four years old
  • ERU often, but not always, just affects one eye
  • Look for a partially closed, inflamed eye with watery discharge
  • If the eye can be seen, it may look cloudy with a constricted pupil
  • The horse may be sensitive to bright light and feel more comfortable in a dark stable

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Horse & Hound (29 July 2004)