Equine Ophthalmology

Question: My 8-year-old Paint horse gelding went from being very confident to jumpy and spooky over several months. He was in professional training and I genuinely believe he received kind care. After ruling out physical trauma, I had his eyes examined by an equine opthmologist in 2008. All structures were found to be OK. He is bald faced with pink skin, but both eyes are dark brown. Are there any new tests for equine vision that might be able to help him?

Answer: Assessing vision in horses (or in any animal, really) is one of the hardest things for veterinarians and veterinary ophthalmologists to do. If horses spoke english, things would be so much simpler! Not even an eye chart made of carrots will get a horse to tell you what they can see. Since we cannot directly ask horses what they see, we have to find other ways to indirectly test their vision. One way to assess vision is maze testing, placing a series of obstacles in the horse's path that they must step over, walk through or avoid. This can be difficult in some horses due to temperament or age. Objective assessment can also be difficult. The maze must be altered after every pass through, since horses memorize things very quickly! This testing can be performed at your barn. Other ways of assessing vision are measurements of eye structure and function. Retinoscopy or skiaoscopy is the process of testing for refractive errors. This is the same test your eye doctor performs to test if you are near sighted or far sighted. Most horses are neither near nor far sighted, but a veterinary ophthalmologist can perform retinoscopy to ensure that your horse does not have one of these refractive errors. Unfortunately, since glasses or contact lenses are not available for horses, correcting refractive errors is more difficult in horses than it is in people. Retinoscopy is performed with no sedation or mild sedation in a dim stall/room. Electroretinography assess the electrical function of the retina to ensure that it is responding normally to a light stimulus. The electrical response of the retina to light is the first step in the visual pathway to the brain, so ensuring that this critical stage is functioning normally is one way to assess vision. This testing is performed under heavy sedation by a veterinary ophthalmologist in a dark room. A more advanced test, called a Visual Evoked Potential, is similar to an electroretinogram but assess the response of the brain, not the retina, to light. It is rarely performed in horses, even in a research setting. I'm glad that you had your horse examined by an ophthalmologist, and I hope that if you cannot find an eye abnormality that you can resolve his behavioral issues some other way!

Question: One of my horses, (20+ gelding) shows signs of cataracts. Does every horse get them? Is there help for it, and how will cataracts hinder his vision?

Answer: Yes, most horses (and dogs and humans) get some degree of cataract as they age (we call them senile cataracts). They do not appear to affect horses' vision the way it affects our human vision, however! Horses also get an age-related change of their lens called nuclear sclerosis. Nuclear sclerosis causes a hardening of the lens and decreased lens flexibility, and is the most common reason for humans to need bifocals as they age. Nuclear sclerosis can be confused with cataract, but does not appear to cause visual deficits in horses. If you are noticing any changes in your horse's vision, I definitely recommend a complete eye exam by a veterinarian or a veterinary ophthalmologist.

Question: My horse is a 7-year-old Arabian with one brown and one blue eye. The blue eye tears regularly and currently is being treated for conjunctivitus. I've read that cancer is a danger with a blue eye. He wears a fly mask during the day. What more can I do to protect the eye? Also, what fly mask offers the greatest protection?

Answer: Horses with blue eyes tend to have very pink eyelids without pigmentation, and horses with pink (not black) eyelids are definitely more likely to get squamous cell carcinoma (a type of skin cancer). I would definitely recommend turning all horses with pink eyelids or blue eyes out in a UV-protective fly mask, such as the Guardian Mask or the Cashel Crusader fly mask. You can consider turning your horse out at night or in a shady paddock to limit UV light exposure. Regular eye exams by your veterinarian to look for early signs of squamous cell carcinoma is also very important. The disease is much easier to treat when it is caught early.

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Reprinted with permission from AAEP. To view the entire article please visit www.aaep.org/ask_the_vet.php