Understanding moon blindness

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Andrew Coe MRCVS

11 August, 2004

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 beautiful 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.

Unfortunately it can become damaged, resulting in an inflamed iris – called uveitis. This can occur as a primary disease, as a result of trauma to the eye, or as part of another disease.

Equine Recurrent Uveitis

By far the most feared condition that affects the iris is Equine Recurrent Uveitis (ERU). This condition has been recognised for thousands of years and 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 rare in the UK, with a prevalence of 1.0–2.5% cited. Of those horses that do succumb to the condition perhaps 50% may suffer 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.

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. 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 the condition

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 ERU 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."

Preventative measures

"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."

Treatments available

"In Britain, we rely heavily on medical therapy," says Dr Matthews. "Treating the condition early is essential and owners should seek veterinary advice straightaway 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 been carried out for around 10 years. It 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 with increased susceptibility

"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 ERU 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

This veterinary feature was published in full in Horse & Hound (29 July)

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