Ophthalmic disease is a debilitating condition that can affect your horse’s performance and quality of life. A basic understanding of the most common equine eye conditions can help you take steps to prevent them.
Good vision is critical for all horses, whatever their discipline. From a cutting horse working cows or an eventer navigating a cross-country course, to a trail horse crossing creeks or a school horse used for riding lessons – all these equines need optimal eyesight in order to do their jobs properly. Alterations in vision may affect their performance, and increase risk of injury to the humans riding, driving or handling them, since horses can react unpredictably as a result of changes in their eyesight.
Unfortunately, ophthalmic (eye) disease is quite common in horses. This is partially due to the prominent profile of their eyes, their flighty nature as a prey species, and the environments in which they are kept. Autoimmune disease of the equine eye is also common and can be blinding. When it comes to preventing and managing these eye conditions, the first step is learning what they are and how they can affect your horse.
Common eye conditions: causes and outcomes
Many ophthalmic problems in horses are due to trauma — scratching or wounding of the cornea, the normally clear windshield of the eye. Bacteria, fungi and yeast normally present in the environment can cause infection of the cornea. These infections can rapidly progress, leading to vision loss or even eye loss in severe cases. In older horses, or those with Cushing’s disease, “indolent” corneal ulcers can take months to heal. This delayed healing is not due to infection, but continued treatment is nonetheless required to prevent infection, and the horse will be uncomfortable. These indolent ulcers often require a minor surgical procedure to heal, and your veterinarian may recommend testing and treating for Cushing’s disease.
Horses also suffer from autoimmune diseases of the eye. Similar to autoimmune diseases in humans, such as lupus or rheumatoid arthritis, the immune system attacks its own cells as if they were foreign, like bacteria or viruses. Autoimmune eye diseases can be difficult to predict and treat, and may have a significant impact on vision.
Equine recurrent uveitis
Equine recurrent uveitis (ERU) is the most common cause of blindness in horses. “Uveitis” is inflammation within the eye, and occurs when the blood vessels inside the eye become damaged and leaky. ERU, also known as moon blindness, is an autoimmune disease in which the eye is the target of repeated attacks by the immune system. An initial episode of uveitis from any cause – trauma, corneal ulceration with secondary uveitis, infection with leptospirosis – is necessary for the development of ERU, which occurs usually months to years later. With each attack of uveitis, the eye is damaged. Cataract formation in the lens, retinal inflammation and retinal detachment eventually cause vision loss. The clinical signs can be as subtle as mild tearing and squinting of the affected eye.
Treatment during a flare-up involves topical and systemic anti-inflammatories. It is important to continue medications beyond the resolution of clinical signs. Some horses are candidates for the surgical placement of an implant that gradually secretes a medication (cyclosporine) to suppress the local immune system. This implant significantly reduces the frequency and severity of uveitis flare-ups.
Immune-mediated keratitis (IMMK) is a disease in which the cornea is targeted by an autoimmune attack. Whitish spots or haze and red blood vessels develop within the cornea, and can progress to cover 100% of the corneal surface. The presence of blood vessels and abnormal cells affects vision; because the normally clear cornea becomes cloudy and opaque, vision is impaired. Horses with IMMK are mildly painful to non-painful. Diagnosis is based on response to treatment or in cases that involve surgery, biopsy of corneal samples.
Treatment includes suppressing the local immune response with topical steroids and nonsteroidal medications. A topical immune-modulating medication (cyclosporine) can be used as a mainstay of therapy. This medication is also available as a surgically-placed slow-release implant, which can decrease the frequency and number of medications needed to control the disease. In some cases, topical medications are no longer required once the implant has been inserted.
Preventing equine eye disease
Preventing eye problems is ideal, but not always possible. Unfortunately, there is no definitive way to prevent ocular disease in horses – but there are ways to potentially minimize occurrence.
- Keep pastures trimmed below eye level to prevent corneal ulceration. If possible, remove burdock plants from pastures. The bristles can get in the eye and cause corneal ulceration.
- Do not allow horses to put their heads outside the trailer window while travelling. Debris, dust and insects may cause irritation to the eyes.
- Consider using a fly mask for turnout. Fly masks may be beneficial for preventing eye irritation from debris and dust when windy, or in horses that are irritated by flies. It is important to remove the mask daily to ensure there are no signs of irritation. It is also important to keep the fly mask clean – a mask caked in dried mud may cause more irritation than not using it at all.
- Use your veterinarian as a resource. Supporting your horse’s overall health with a good management program is critical, so the importance of yearly veterinary examinations, and a specific de-worming and vaccination program, should not be overlooked. Ensure a good plane of nutrition and an exercise program that supports healthy body condition.
- Minimize stress. Some stress is inevitable, especially in performance horses that are often travelling and competing, but you should always take steps to reduce stress where possible.
Even if your horse’s eyes look normal, consider a yearly screening examination performed by a veterinary ophthalmologist, similar to your own annual check-up with an optometrist. This examination detects any abnormalities that have the potential to progress and cause a problem. It’s also important for breeding animals, as it identifies ocular abnormalities, most notably cataracts, that may be inherited by offspring. If your horse displays any signs of ocular abnormalities, including squinting, tearing, discharge, redness or color changes to the surface of the eye, contact your veterinarian. Many eye problems can quickly worsen, so prompt diagnosis and treatment is critical to ensuring the most successful outcome for your horse.