Common equine eye conditions

Author: Hannah Dyball
Published: Thursday 25th September 2014

Whether we consider ourselves ‘horsey’ or not, most of us will have heard of conditions such as laminitis and colic. We might not know the ins and outs of the problems, but we sure as anything know they are serious. How many of us then, will have heard of corneal ulcers and recurrent uveitis? They might not be so talked about, but as far as health conditions go, they are pretty jeopardising.

The equine eye is the largest of any land mammal, and one of the most delicate. The positioning of the eyes (on the side of the head) facilitates a 360 degree view, which allows horses to see everything around them. For this to work, the eye sockets must be particularly shallow, meaning the eyes protrude somewhat more than other species. It goes without saying then, that the horse is highly susceptible to eye traumas and bacterial infections, particularly when turned out to pasture.

Healthy eyes should always look clear and bright, without excessive tearing or discharge. They should blink when a hand is waved in front of them and stay open in sunlight rather than closed or half-closed. If the eye is damaged it becomes very sensitive to light and thus the horse will keep it shut or squint (photophobia). Other signs to look out for include visible swelling around or involving the eye, redness, cloudiness or sunkenness, and evidence of injury.

Thankfully, most eye problems are eye-catching (if you’ll pardon the pun) enough not to go unnoticed. Still, if you are well acquainted with the eyes of your steed through checking them regularly, you are in a much better position to notice a change. The change could be anything from a slight clouding of the cornea to an unmissable, bloodied laceration of the eyelid. The sooner the problem is identified, the sooner treatment can begin.

While most horse owners will possess a basic enough knowledge of equine health problems to be able to patch up a simple cut or graze, or judge when a situation is minor or severe enough to warrant a veterinarian, few will be able to assess the severity of an eye problem and know when to seek help. You will find there is very little information online about treating equine eye conditions, and this is simply because, unless you are a qualified vet, you shouldn't be going anywhere near the affected eye with the intention to treat it.

If you notice anything wrong with or different about your horse's eyes, contact your vet immediately. Only a trained professional will know the appropriate course of action to take that doesn't jeopardise your horse's sight. After examining the eye (which may be done under sedation if the horse is visibly in pain), the vet will then assess vision, using a combination of tests that gauge light sensitivity (dazzle response), pupil response to light (pupillary light response), and whether the horse avoids a hand moving towards the eye (menace response).

If a vision problem is suspected, the vet may construct an obstacle course to test how easily the horse negotiates it. An ophthalmoscope may be used to examine the retina and the back of the eye. Drops will temporarily dilate the pupil to make the examination easier. At this time, it is best to keep your horse stabled and out of sunlight. Finally, the vet is likely to apply a dye or 'stain' called fluorescein to the eye to check for problems with the cornea and tear duct. The fluorescein dye will not penetrate the cornea unless there is a break in its integrity, something which is usually caused by a corneal ulcer. Where an ulcer is present, the eye will stain green.

Here are some of the most common eye disorders affecting horses:

Corneal ulceration is a relatively common eye condition that has the ability to progress very quickly, threatening the horse's sight. For this reason, early detection is imperative if a mild ulcer isn't to become a serious, invasive ulcer. The cornea is one of the most delicate tissues in the body and is easily damaged, being only 8-10 cell layers thick. No matter how minor the ulcer appears during routine examination, it should always be treated aggressively.

Most cases of corneal ulceration are the result of eye trauma, which is not uncommon in horses oweing to their prominent eye position. Lacerations and abrasions can easily be acquired in the field, especially if two horses aren't getting along. In many cases, a foreign object i.e. a plant, dirt, or other debris, will have become lodged in the eye, causing irration and eventual ulceration. Less commonly, infections (bacterial, viral and fungal), insect stings, and eyelid abnormalities are the cause of corneal ulcers.

Symptoms that suggest a corneal ulcer include squinting and/or sensitivity to light, excessive or increased tearing, discharge from the eye, manifestations of discomfort and pain, and corneal cloudiness. If you notice any of these signs, contact your vet immediately. Time is of the essence when it comes to eye disorders, particularly corneal ulcers as they progress quickly and, in extreme cases, can lead to loss of sight.

Conjunctivitis or 'red eye' is very common, either as a primary problem or as the result of existing eye disease. Horses with corneal ulcers, for example, are highly likely to have a degree of conjunctivitis. Other disorders with conjunctivitis as a clinical sign include glaucoma and uveitis. It is important for your vet to assess the cause of the problem in order to determine whether something more serious is at play. If discharge is presented from one eye only, this is more likely to be the case. Primary conjunctivitis is far less serious and usually affects both eyes.

During the summer months when flies are everywhere, minor cases of conjunctivitis are a common occurrence. Redness, swelling and discharge from the eye are the usual symptoms of the condition. To discount other problems, various tests may be performed. If the conjunctivitis is primary, treatment will include topical medications, usually eye drops. A bacterial cause will necessitate a course of antibiotics. To ease your horse's discomfort, gently clean away the discharge twice daily, using cotton wool and a saline solution/sterile water.

Uveitis, otherwise known as 'moon blindness,' is a serious condition affecting horses. Despite being relatively common, little is understood of the disease and treatment remains difficult. It is often referred to as 'recurrent uveitis' as it is characterised by periodic episodes of eye pain.

As with conjunctivitis, uveitis is classified as either primary or secondary, because it does tend to accompany other eye diseases. The condition involves the inflammation of the uveal tract (comprising the iris, ciliary body and choroid), although the exact reason for this is unclear. Generally speaking, uveitis is broken into 3 causal categories: ocular (includes trauma and corneal ulcers), systemic (includes viral, bacterial or parasitic disease, resulting in systemic illness), and immune-mediated (ERU), which is a syndrome involving chronic recurrence.

The condition can affect one or both eyes and is usually signalled by squinting or closed eyes (blepharospastic), ocular discharge and/or tearing, and eye cloudiness or redness. If your horse has any of these symptoms, consult with your veterinarian right away.

Because uveitis is a recurrent problem, it poses a greater risk to your horse's vision than other eye conditions. In fact, uveitis is the leading cause of blindness in horses. For this reason, it is essential every episode of inflammation is assessed by your veterinarian at the earliest opportunity.