Uveitis is a generic term used to describe inflammation of the blood vessels and vascular tissues within the eye.

In the horse, the disease can separately involve the front of the eye (when it is termed iridocyclitis) or the back of the eye (chorioretinitis), but in many cases the entire eye is affected to some extent.

The inflammation can arise from trauma or blood-borne infection, but in the horse the disease is most commonly endogenous — meaning there is no identifiable external cause. In these cases, evidence is mounting that the disease has an autoimmune origin.

This arises when the normally low-key immune responses within the eye — maintained that way to avoid triggering inflammation likely to damage the very sensitive and vision-critical structures — fail to recognise these ocular tissues as “self” and mount an aggressive response against them.

What causes this failure to recognise “self” is unknown, but it seems probable that in some cases, infection can initiate the disease by altering the immune signature of the eye tissues. The vigilance of the horse’s immune system does the rest.

Is bacteria to blame?

Scarring of the eye

Scarring of the eye

The infection currently receiving most attention as a likely cause of equine uveitis is leptospirosis.

In central Europe, where uveitis is reported to affect some 10% of horses, the Leptospira bacteria are implicated in up to 80% of cases. In the UK, the disease affects less than 0.2% of horses. The Leptospira types usually associated with the disease are absent from our wildlife population (the most important reservoir for infection), contributing to reduced incidence.

However, uveitis was once very common here in the UK and its prevalence among cavalry horses around the time of Waterloo was about 5%. Interestingly, in the mid-19th century a law was enacted in the UK preventing equines affected with the disease from entering horse shows, effectively removing these horses from the breeding pool. This may be largely responsible for the welcome decline in the disease over the past 150 years.

Recent studies have shown that there is a breed-related susceptibility to the disease, with warmbloods and appaloosa types being particularly affected. In the latter, the disease can take an insidious course, frequently grumbling along undetected.

However, it is highly likely that there are more and as yet unrecognised causes of uveitis in the horse, with each causal pathway leading to the same outwardly similar clinical presentation — inflammation within the eye.

A recurring problem

Tube system designed to apply medication onto the eye remotely

Tube system designed to apply medication onto the eye remotely

When it is recurrent, the disease is called equine recurrent uveitis. This is the common form in mainland Europe.

In the UK the disease is less commonly recurrent, which could reflect a genetic tendency within our horse population towards restoring the immune mechanisms in the eye to their normal low-key status after a single episode of the disease.

The structures within the eye are highly sensitive to injury or disease and have limited capacity to recover their original transparency or light-capturing functions.

Iridocyclitis, where the disease affects the front of the eye, is the most common presentation and is typically very painful. The horse’s eyelids may be tightly closed with evidence of marked tearing, while the cornea may appear opaque or reddened.

This is a real emergency in every sense. The earlier the diagnosis, the much greater the likelihood of successfully treating the disease — and the far lower the chance of long-term damage in the eye.

Unfortunately, the insidious disease seen in appaloosa types and the form of uveitis that affects only the back of the eye are not particularly painful, meaning these conditions often go undetected until the eye is irreversibly damaged.

Once the initial disease has settled down, the eye is left with signs of “healed” damage. These can range from being only detectable on biomicroscopic examination through to blind, internally scarred eyes with extensive cataracts present.

After the initial episode of acute disease, there is no reliable means of predicting its recurrence. If the disease is shown to be associated with Leptospira, which requires laboratory testing of samples of fluid taken from the eye, experience from continental Europe suggests recurrence is more likely. Similarly, where the disease initially affects only one eye, there is no means of predicting whether the opposite eye will be affected.

Experience in the UK indicates that if either event has not occurred within one year of the disease first appearing, then the chances of these events happening lessen significantly. However, lifelong vigilance for signs of recurrence is required.

Treatment challenges

Effectively treating the acute disease requires anti-inflammatory medication delivered directly on to the eye, in addition to anti-inflammatories and pain suppressants given by injection or in the horse’s feed.

Administering medication on to the eye is usually resented because of the pain, and failure effectively to medicate these cases is a common cause of what seems like treatment failure. For this reason, vets often fit a delivery tube through the eyelid to allow administration of medicines without going near the eye, and may hospitalise the horse for treatment by dedicated nursing staff.

Most eyes will get better with aggressive treatment, but some seem to respond temporarily or not at all. Frustratingly, that is the nature of this complex disease. In “healed” eyes, there is no means of removing the damage caused by the acute disease.

Recently, methods of surgically implanting medication delivery devices directly into eyes have evolved. These are used in recurrent disease to reduce the frequency and severity of episodes, in some cases eliminating them.

Where Leptospira is proven to be involved, surgically removing the vitreous gel from the back of the eye may help prevent recurrence — although most eyes will subsequently develop cataracts.

A blind eye, particularly where uveitis is recurrent, is best removed for the welfare of the horse.

Ref: Horse & Hound; 6 August 2015