The discovery of a cataract — typically during routine examination of the horse or at a pre-purchase exam (PPE, or vetting) — can be confusing and alarming for an owner.

Very broadly, a cataract is any opacity involving the lens of the eye. Cataracts vary widely in type and size, from small, single opacities present at birth to extensive and dense opacities obstructing the visual axis of the eye.

The condition can affect any type or age of horse and is common. A recent survey of racehorses in Australia found cataracts in 17% of the horses examined.

What causes cataracts?

The ocular lens is a highly complex structure made up of tightly and very precisely arranged transparent and metabolically active cells.

These cells are continually produced during the horse’s life and are required to remain alive and functional throughout, albeit the process slows with age. Biologically speaking, this is a huge ask of any tissue — it is remarkable that abnormalities of the lens are not more common.

Anything affecting the precise geometric laying down of lens cells during life, from the foetus to the adult, or interfering with the metabolic processes on which the transparency of these cells depends, will result in opacity and cataract.

In the horse, most cataracts result from the former and in some ways are similar to the types of cataract found in human babies and young adults. These cataracts tend not to enlarge over time.

Cataracts are less commonly caused by overt eye disease. This could be trauma or, very rarely, intraocular tumours. Most commonly, the cause is uveitis or intraocular inflammatory disease, a group of as yet incompletely understood diseases, including equine recurrent uveitis or periodic ophthalmia (so-called moon blindness). These cataracts can progress if the disease process cannot be effectively controlled.

Rarely, cataracts develop in adult horses for no apparent reason. Age-related or senile cataracts can be found in most horses over the age of 18, but appear to have little effect on their behaviour.

Weighing the evidence

Vision in horses is very difficult to assess objectively.

Horses appear able to interact with their immediate environment in a way which uses senses in addition to visual input, but which we forward-facing and vision-dependant primates assume to be primarily vision-based. This facility in horses is sometimes referred to as functional vision.

Most cataracts in horses do not appear to interfere with functional vision. Most affected horses behave normally, some competing at a high level. However, any cataract must, by default, interfere with the passage of light on to the light-sensitive photoreceptors of the retina. In turn, this must result in some degree of physiological disruption of normal eye function.

In fact, most horses seem to adapt to this readily, particularly where the cataract is present from early life — the exception being where a complete cataract, that is when the whole lens becomes opaque, occurs suddenly and usually as a consequence of uveitis.

The result of this is that attributing behavioural problems to cataracts in any one horse is a process fraught with subjective difficulties.

There is no medical treatment for cataracts. In selected instances, the opacity — in particular an extensive or total cataract — can be removed surgically and an artificial lens implanted. This surgery is not routine and is technically complex, expensive and involves aggressive aftercare, as well as carrying the risk of blinding.

Successful results have been achieved, particularly in foals and yearlings. Still, the long-term results are less promising. The most recently published results from two groups in the US record that overall only 25% and 48% of operated eyes remained “visual” after two years.

Ref: H&H 7 May, 2015