Horses’ eyes are amazing. They are big, beautiful and perfectly positioned on the sides of the head to enable vision practically all around, without the horse having to move. Horses do have a blind spot behind them (which is, of course, important to remember so you don’t startle them by coming up directly to their tail end) and there is a very small area they can’t see, directly in front of them.
Most of what horses and ponies see is the view through just one eye, which is called monocular vision. Only about one fifth of their field of vision is binocular vision – the view through both eyes together. We humans have a much small field of vision overall, though with much more binocular vision and less monocular vision. This may explain why sometimes horse spook or shy at something they can see, which we are totally oblivious to.
Horse handlers tend to pay a lot of attention to the eyes, noting if they look soft and kind, or bright and wild, and being especially careful if the white of the eye is showing, as this can denote anxiety or stress. Be aware though, that in some horses the white of the eye shows all the time, due to less pigment.
Problems can occur in horses’ eyes due to injury or disease, and these will need veterinary attention to prevent long-term damage or even blindness. Veterinarians can examine the structures of the eye using different equipment, and with the aid of some medications. There are very good treatments for a lot of the conditions that can affect the eye, and some of the common conditions of the eye will be discussed below.
However, in many cases it is difficult to work out whether (or how much) a horse’s vision has been affected. There isn’t an easy eye test for horses, and they are very good at compensating for vision loss, which would have been very important in the wild.
The eye itself is complex but can, in basic terms, be divided into a few main structures.
Eyelids: Eyes have upper and lower eyelids, with long eyelashes on the upper. The eyelids contain very strong muscles which can hold an eye tightly shut if it is sore (and/or if you or your vet is trying to examine it). Fortunately, there are straightforward nerve blocks that allow the eyelid to be more easily held open, and local anaesthetic can be placed where needed to relieve pain. In addition, horses have what is known as a third eyelid (or nictitating membrane). You can sometimes see this pink-coloured structure coming across the eye from the inner corner. These three eyelids together help protect the eye from external damage.
Tear ducts: The eyelids produce tears to clean, protect and keep the eye moist. A small channel called the nasolacrimal duct drains the tears into the nostril. This duct can become blocked (it is very narrow), causing a runny eye.
Cornea: The surface of the eye is called the cornea. This contains several layers of cells and is another barrier to things getting deeper into the eye. It can get damaged, either from something like a knock to the head or a scratch from a sharp piece of hay or a stick. Damage to the cornea can be just a scratch on one or two layers, or a much deeper puncture where this is a risk of the eyeball bursting. Holes in the layers of the cornea are called ulcers, and we will discuss them more later in this article. Usually, the cornea heals very quickly but any infection (bacterial, viral or fungal) can slow down and even prevent healing.
Lens and iris. The iris is the coloured part of the eye and it is a ring of two layers of muscle that act to adjust the size of the pupil depending on how much light there is. You may have seen that your horse’s pupils become bigger in darker conditions which allows more light through to the back of the eye and in very bright conditions the pupil becomes much smaller. The clear lens is just behind the pupil and this has the job of focussing the light on the back wall of the eye, the area called the retina.
Corpora nigra. An unusual feature of the equine eye is the corpora nigra; the strange, dark, knobbly structures that you can see at the top of the iris. They are thought to give the back of the eye more protection from the sun.
Retina. At the very back of the eye is the retina where the images are detected by lots of specialised cells and the information from these travels down the optic nerve to inside the brain to be processed.
Normal variations
Horse’s eyes are most commonly brown in colour but can be blue or more uncommonly green, amber or hazel. You can get horses with one brown and one blue eye, and can even get individual eyes that are part brown and part blue.
Corpora nigra can be quite obvious in some horses and ponies; in fact, sometimes people can get worried when they first notice them. Corpora nigra can become cystic which will be discussed below.
Common veterinary eye problems
Horses, as we know, can be accident-prone and unfortunately the eyes are in a prominent place to get injured, so we do see eye injuries quite commonly.
Eyelid injuries
These can occur from getting caught on something like a small hook, a sticking-out nail or twig, or from pulling back. They can look very dramatic with a lot of blood, or can just involve a very small piece of the edge of the eyelid. Either way, it is extremely important that your vet attends and stitches the eyelid back to normal.
The most important part is the very edge of the lid, the part that moves over the eye, as this needs to be smooth and to cover the eye with no gaps. Fortunately, the eyelids usually heal really well once surgically repaired.
Corneal injuries/ulcers
Again, injuries to the surface of the eye are common. They can be from all sorts of causes; a knock to the head, fly irritation, hay or even grass seeds can get into the eye. The break in the cells of the cornea is painful (and even more so in bright light). Signs you will see are a runny eye, eyelids partially or fully closed, and there may be swelling. This needs urgent veterinary attention to prevent infection and inflammation causing the damage to become deeper. Part of the examination will usually be applying a special fluorescent dye to the surface of the eye; this makes even tiny breaks in the surface of the cornea more visible. It’s important, because the approach to treatment is different depending on whether or not there is a break in the cornea.
Other conditions
Uveitis. Uveitis is inflammation of thefront areas of the eye, including the iris. This can, and often does, happen whenever there is an injury to the eye. It can also occur for other reasons and with other triggers; some horses have a chronic condition called Equine Recurrent Uveitis which flares up every so often and can lead to permanent changes and blindness. Any uveitis needs to be treated urgently, as the longer it is left the more chance of permanent damage to the eye. The signs include a weeping eye, eyelids partially closed, swelling and redness of the eyelids, and a cloudy appearance to the eye itself.
Cystic corpora nigra. The corpora nigra can turn into cysts, which are usually slow growing (though occasionally they can grow quickly), and it can be concerning that vision is being blocked or affected, especially in bright light when the pupil is small. It is variable as to whether the horse is bothered by them. Some seem to react, and others don’t appear to. If you do see an unusually large dark structure in your horse’s eye, get it examined by your vet. It could be a cyst, in which case it will need to be assessed as to whether it is affected your horse’s vision, or there is a much less likely possibility it could be a type of tumour in the eye, a melanoma.
Tumours. The most common tumour we see involving the eye is a type of skin cancer called squamous cell carcinoma. Sarcoids can also appear near the eye. Squamous cell carcinomas are more common when there has been sun on light-coloured tissues (it appears that pigment can give some protection), and so it is more common in Appaloosa and paint horses who lack pigment in the skin around their eyes. The tumours can appear on the eye itself or on the lids, including the third eyelid. It is important to get these seen by a vet while they are still small to allow easier removal, and try to prevent further spread.
Allergies. Some allergies can show up as runny eyes. Your vet will need to determine whether there is any infection involved and will also check that the tear ducts aren’t blocked.
Abscesses. Sometimes, damage to the cornea can result in a small abscess forming in the layers of cells. This is due to a small sharp object introducing infection, then the cells have healed over the top, trapping the infection in. This can be treated, but sometimes requires surgery.
Cataracts. A cataract is when the clear lens becomes cloudy. There are lots of reasons for horses getting cataracts; they can be hereditary, some foals are born with them, and in some it can follow from a traumatic injury or be as a result of inflammation. Cataracts affect vision in varying levels but can cause blindness as they progress.
Treating an eye injury
Most eye conditions are veterinary emergencies. Apart from being very painful for the poor horse, the longer an eye condition is left untreated the more serious the damage can become. If it becomes very serious, blindness can be the final outcome. A horse with a sore eye often holds it closed, and it can be very difficult for an owner to see what is going on.
If the eye is that sore, it needs urgent veterinary treatment, and your vet can give pain relief and local analgesia to allow examination.
Treatments for an eye can be difficult to give, most horses don’t tolerate eye drops or ointment being put into their eyes several times a day very well (and fair enough!).
For those who need intensive treatment, or are very difficult to treat, we can place what is called a subpalpebral lavage tube. This is a small tube inserted through the eyelid with the other end well away from the eye, usually along the mane. Then treatments can be given down without having to go near the eye directly. This is a good system, as some hospitalised horses with serious eye problems need treatment every one to two hours and even the best-behaved horse would get sick of their eyes being messed with that often.
Eyes are so important. If you are at all concerned, call your vet even just to discuss whether your horse needs to be seen urgently. It is much better to do this than to wait and have to potentially treat a much bigger problem later.
