This condition is characterised by an inflammation or the iris surrounding the pupil. It can present as a one-off problem, or because of residual after-effects within the eye an affected horse can be prone to bouts of recurrent inflammation and eye discomfort. In days gone by the condition use to be termed "Moon Blindness"; quoting from Modern Practical Farriery, published in 1882; "It was a superstition that the periodical attacks were influenced by the monthly return of the moon."

Even then it was realised that this was not the case, and many suggestions have been made since then and we still do not have a clear answer. Uveitis is an immune mediated disease; the inflammatory response has been associated with a multitude of possible causes including responses to viral [such as Influenza], bacterial [such as Leptospirosis] and parasitic infections [such as Onchocerciasis], and an injury to the eye can certainly start off the initial inflammation within the iris. Evidence suggestive of an association with Leptospirosis includes affected horses being more likely to have detectable Leptospira antigens on a blood test, when compared to unaffected horses. Experimental infection of ponies with Leptospira was found to induce recurrent equine anterior uveitis in infected animals.

Symptoms include inflamed conjunctiva, a tightly constricted pupil and cloudiness of the cornea and contents of the front part of the eye. This initial bout of inflammation may set up a series of events that makes the eye more likely to succumb to a further episode.

If this situation goes unrecognised or untreated it can lead to the more long term damaging problems which involve the iris becoming attached to the lens of the eye, which it immediately overlies. This unfortunately can lead to cataract formation and partial or complete blindness in the affected eye. Occasionally the eye itself can deteriorate and become shrunken and sightless; in these circumstances the eye can be removed under general anaesthesia to remove the source of the chronic irritation. However it is contraindicated to consider removal of the cataratous lens in a previously inflamed eye as this can lead to a profound inflammatory reaction and a worse problem than you started with.

The condition is intermittently the centre of research at various specialist centres. The condition is not congenital or hereditary. Treatment involves anti-inflammatory drugs such as corticosteroids, applied both to the eye and by injection, and drugs to dilate the iris so as to reduce the chance of it becoming attached to the lens. It has been noted that the condition has become less common over the past few years and it has been postulated that this may coincide with the use of a wormer containing an ivermectin , as this is known to be effective against the Onchocerca parasite. Therefore it may be wise to ensure that wormers containing ivermectins are part of a worming programme.