Disclaimer: The following is only general information. If you believe that your horse has a problem please call your veterinarian as soon as possible.
The equine eye is the largest of any land mammal. Its visual abilities are directly related to the animal's behavior and the fact that the horse is a flight animal. (To understand how a horse’s eye works and what a horse actually sees, take a look at the article on ‘Eyesight’.) Horses that have eye damage, for whatever reason, can be a danger to themselves and those around them.
Poor eye health and disease can lead to reduced vision and, if left untreated, blindness. There are three basic structures within the eye, which can become diseased or injured. These are the eyelids (including the third eyelid), tear ducts (lacrimal structures) and the associated structures of the eye itself.
The most common ailments and diseases of the eye include conjunctivitis, chronic uveitis, abscesses, cataracts and common abrasion.
Conjunctivitis
Conjunctivitis (also called pink eye) refers to inflammation of the conjunctiva (the outermost layer of the eye and the inner surface of the eyelids). It is usually due to infection, which can be viral, bacterial or an allergic reaction.
Symptoms
Stringy opaque grey/yellow discharge from eye
Excessive tearing
Squinting, light sensitivity
Localised swelling
Inability to close the third eyelid
Crusting around the affected eye
Treatment
Always call your veterinarian to determine whether the infection is viral, bacterial or an allergic reaction. Your veterinarian may prescribe topical antibiotics, non-steroidal anti-inflammatory treatments and antihistamines depending on the cause of the infection, its severity or its persistence.
Prevention
Your horse’s eye health is critical and should be part of your grooming and health check schedule, whether daily or weekly, which should include wiping your horse’s eye with a mild saline solution. This will eliminate the main causes of local inflammation which lead to infection such as dust, pollen, hair etc. Close observation of your horse’s eye health will also allow you to identify whether there are any seasonal causes of infection which would indicate a particular allergen as the culprit.
Chronic Uveitis
Equine recurrent uveitis (ERU) is the most common cause of blindness in horses and is believed to affect approximately 10% of the equine population. The exact cause of ERU remains unclear, although researchers have shown that recurrent bouts of inflammation involving activated T-cells (cells that form part of the horse’s immune system) lead to destruction of the retina. There are many causes of uveitis. Leptospirosis, a bacterial disease that can also affect humans, is one cause. However, in many cases the cause is never found. Unfortunately, the end result is always the same: destruction of sensitive eye tissue by the horse's own immune system, possibly resulting in blindness.
Symptoms
Excessive tearing
Pain
Sensitivity
Squinting
The symptoms that accompany this disease are so mild that it can go unnoticed and untreated. If left untreated, it will lead to deteriorating vision and eventually blindness.
Treatment
Equine recurrent uveitis is one of the most difficult diseases to treat. Anti-inflammatory medications to decrease inflammation may be prescribed by your vet, however this rarely prevents the disease from recurring.
Even with treatment, many horses will eventually lose vision or, at the least, will experience repeated, painful episodes of the disease. For this reason it is important that a veterinarian is contacted at the first sign of any abnormality and/or discomfort.
Prevention
There are no preventative measures, however owners can monitor eye health as a part of their daily routine and note any abnormalities.
Abscesses
An abscess is a localized collection of dead tissue and white blood cells. A corneal abscess is a very serious and potentially vision-threatening condition that can follow minor corneal ulceration in the horse.
If an eye becomes punctured (such as a small corneal puncture from a branch or other foreign object), an ulcer will develop. The cells around the ulcer will try to cover the wound, but in doing so trap bacteria, fungi, and/or foreign bodies. The cells then form a barrier (like a scab on a wound), which protects the bacteria or fungi from topically administered anti-bacterial and anti-fungal medications. This makes the condition almost impossible to treat without aggressive therapies including surgical removal.
Symptoms
Yellow-white opacity of the eye
Various levels of corneal edema (swelling)
Pain and sensitivity
Secondary uveitis,
Blindness
Treatment
A veterinarian should be called as soon as any of these symptoms occur. Aggressive treatment, including surgery, may be required.
Prevention
As the most common cause of abscess is penetration wounds, all obvious hazards such as low branches, barbed wire and old machinery should be removed from the paddock. In the case of an abrasion injury involving the eye and surrounding structures, contact your vet immediately.
Cataracts
Cataracts block the visual image as they increase in size, but do not block the light. Congenital (present at birth) cataracts are frequent defects in foals; most affected foals have them in both eyes. In adult horses, cataracts might be caused by heredity, traumatic injury, nutritional deficiencies or toxicities, or be secondary to other inflammatory eye conditions such as uveitis.
Careful examination is needed to determine if eyes with cataracts also have recurrent uveitis. This is especially important when the eye is being considered for cataract extraction, since there is an increased risk of complications and a poorer outcome for vision when uveitis is the cause of the cataract.
Symptoms
Opacity (clouding of lens)
Varying degrees of blindness
Treatment
Cataract treatment is generally surgical. It is performed after assessment of overall eye health, if there is no evidence of infection or associated uveitis.
Cataract surgery requires general anesthesia. In this treatment, a specially-designed vibrating needle is inserted into the anterior chamber, the cataract is broken up with ultrasound, and the fragments of the cataract are aspirated (sucked out) from the eye through the dilated pupil. Recent advances in this technique have seen the success rate rise to over 80%.
You can find out more about what your horse sees in our article on ‘Eyesight’.
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