Eye Diseases of the Horse (AAEP 2003)

Dennis Brooks, DVM, PhD, Dipl. ACVO, is a professor of ophthalmology at the University of Florida. He opened his talk at the AAEP's Horseman's Day with a short video of an upper level dressage horse and rider. The horse is competing in fourth level completely blind. This unusual case was caused by the horse stepping on a nail and resulted in a systemic infection that got in his eyes and resulted in glaucoma.

"I always dedicate my lectures to the horses that I couldn't fix," said Brooks. "When I first got in practice, I failed often because we thought horse eyes didn't heal. The horse eye heals tremendously. We just have to learn."

He stressed that horse owners should begin to watch their horses' eyes because the sooner you see something wrong, the sooner your vet can get there and the sooner the horse can be healed.
Brooks said he doesn't like removing horses' eyes, and often if you wait and treat them appropriately, they will heal.  "I think 90-95% of them can get better," he stressed. "The key is the horse owner recognizing there is a problem. You also have to have a vet who isn't afraid of eyes."

Vision of the Horse
What does the horse see? A horse has a total visual field of about 350 degrees when his head is facing forward. He has a front binocular (sees with both eyes) field of 65-80 degrees with small blind spots near his face and tail. When his head is up he is using binocular vision; when his head is down he is using peripheral (to the side) vision. Horses don't see both at the same time, explained Brooks.

Horses have very good detailed vision, especially in front. The peripheral vision is mostly for detection of movement.

People have calculated on the human eye chart what a horse sees. A horse viewing this chart at a distance of 20 feet can see the letters in line six. Humans with normal vision can see down to line eight. How does that compare to other animals? The average horse has 20/33 vision (can see at 20 feed what a human sees at 33 feet); cats 20/100; and dogs have 20/75.

The horse is believe to see blue and red, but not yellow and green. Some Appaloosas don't see well at night.

The cornea is the windshield of the eye. It is incredibly tough and thin, yet the disease processes that attack it are some of the most powerful that attack any animal, said Brooks. That means therapy has to be aggressive. Ointment once or twice a day isn't enough. The iris (colored part of the eye) is full of blood vessels. The pupil is the hole in the center, and it moves in response to changes in light level. Get to know your horse's pupil size because it tells you a lot about the health of the horse eye.

The most common disease of the lens is the cataract (clouding of the lens). Behind the lens is the retina, wherein lies the nerve supply.

Tears drain nasally (down through a tube in the skull into the nasal cavity). If the horse is tearing down his face or getting a build-up of pus or gunk in the inner corner of the eye, then he probably has a tear drainage problem. Once in a while the tear draining apparatus does not develop properly in foals and it takes a while to notice. The horse's eyes will stink and you suspect a drainage problem because of smell, said Brooks.

Horses have three eyelids, including the nictitating membrane (the thin, film-like one that moves from the inside corner of the horse's eye). The lower eyelid does not move in the horse. Eyelids move to distribute tears.

Disease and injury of the upper eyelid is more important than disease of the lower lid because of that movement and tear distribution.

Brooks said horse owners should learn the position of the eyelashes on upper lid. They almost always stick straight out. While Brooks said ponies can look through their lashes, having lashes turned down usually is an indicator of pain in the eye. "Know what is normal for your horse," he advised. "Horses tolerate eye pain tremendously. When they start to show pain, they are having pain that would drive you and me crazy."

When horses start to get painful, their eyelashes start to droop down. Some horses do this naturally, so get to know your horse when he's normal. When eye starts to get better, the eyelashes will straighten up.

Entropion is when the lower eyelid rolls inward. There are hairs there so if not corrected, they can rub the eye. The cornea is one of the most sensitive areas of the body in adult horses, but not in the foal. They don't show signs of pain and you might not notice a problem until it has progressed. (Ectropion is when the lid rolls out.)

Make sure your veterinarian looks at the foal's eye when he is young, cautioned Brooks. Remember, looking for corneal disease in foals is tricky because they don't show signs of pain.

"Lid lacerations need to be repaired surgically!" said Brooks. "Sometimes I see these and someone has cut them off. It doesn't matter how old they are, even at three, four, or five days after injury I can still fix it. You need to fix it because the horse can still blink and he is not at as much risk of developing other problems. You can't build a new eyelid."

When dealing with squamous cell carcinoma (cancer), don't let them get out of hand before being treated, advised Brooks. He said these tumors are often seen in horses without any pigment around their eyes (white). He said while usually this is found in older horses, that's not always the case. Ultraviolet light causes this in horses, dogs, cats, people, etc.

Test, Test, Test
Fluorescein dye test is the basic eye test. Every horse eye exhibiting signs of pain should be stained, stressed Brooks. This orange stain detects a corneal epithelial defect or ulcer. There are other things to do, but you as a horse owner should demand this, he said. Detecting a corneal ulcer when it is small gives it the best chance to heal quickly.

There are bacteria and fungi normally living on the surface of the eye. When the eye gets a scratch, these bacteria and fungi can attack.

Rose Bengal stains exposed cells when the tear film is not normal. We know there are some horses that don't produce enough tears, said Brooks. We learned the hard way that some horses don't need to have an ulcer to get an infection on the surface of their eye, they just have to have a lack of tears.

Diseases of the Eye
"There are really only two ophthalmic disease: Corneal ulcers and everything else," said Brooks. Therapies are different, and some therapies for the "everything else" can make ulcers worse, cautioned Brooks.

Melting ulcers occur when there is an eye injury and the horse's eye begins to digest its own cornea. It doesn't take long to digest a hole all the way through and cause a rupture.

There are no blood vessels in normal cornea. In the melting area the blood vessels grow in and heal that area. So, we want to encourage the blood vessels to move into the ulcer site--the sooner they move in, the quicker they heal, noted Brooks.

"What is in blood vessels that causes ulcers to heal?," asked Brooks. "If you talked to your grandparents, they'll tell you that if a cow or sheep had an injury to eye they would make a nick in the eyelid and let blood leak onto the eye. Blood works well as eye drops. It speeds up healing.

"Sometimes a melting cornea gets out of control and you have to do corneal transplants," said Brooks. "I've done more than anyone else. I'll go to the wall to try and save these eyes."

Some melting ulcers cause a hole in the cornea, and the iris comes through and "plugs" the eye.

Brooks said he has been seeing more corneal stromal abscesses--or small spots of infection--in horse corneas. These horses are painful and fluorescein stain doesn't work. For some reason, there is a small puncture in the cornea with introduction of microbes or a foreign body. Epithelial cells migrate over the hole or defect and form an abscess. This is a major problem that is often treated incorrectly.

"Since the horse didn't have ulcer, it is often misdiagnosed," said Brooks. "Many cases are fungal, so they are medically non-responsive. These horses need corneal transplants."

Brooks said that when a horse dies, the owner is asked for the horse's cornea to help another horse, and most owners are very responsive. "We have a cornea bank," said Brooks.

Moon Blindness
One of the most common causes of equine blindness in U.S. horses is recurrent or persistent uveitis, also known as moon blindness. This is an autoimmune system disease, and it is worse in the Appaloosa (with some statistics showing a 20% occurrence in both eyes in the Appaloosa population). This is a very painful disease that is also very old. It was first described in 300 A.D.

Brooks said this problem doesn't show a positive reaction to stain, and that all parts of the eye are damaged. He also said not all moon blindness cases look the same or have the same signs.

Medical therapy works in short term, he says. The problem is that this is a disease that doesn't go away. He said in Appaloosa and Paint horses the problem can be quite prolonged.

There are several treatments for moon blindness. In cases where the horse can still see, Brooks can make an incision in the white part of eye and put in a plastic implant that continually releases cyclosporine (an immunosuppressant drug). This procedure was developed at North Carolina State University.

Brooks said he will do acupuncture for moon blindness, and some horses seem to respond.

Tonopen is a great glaucoma test for the pressure inside the eye. Normal pressure inside the horse eye is higher than any other land mammal, said Brooks. Glaucoma is quite common and very treatable.

Brooks' closing remarks were to remember that sight is valuable. Protect it in your horses!

Question and Answer:
Q. Are fly masks useful in white-faced horses?
A. Yes. They help keep the sun off the white skin and prevent cancer.

Q. Can allergies result in recurrent tearing?
A. Yes, horses often get allergies. Fly masks can help allergic horses. Keep the fly masks clean.

Q. What type fly mask is best? Some collapse and rub the eye.
A. They make hard ones that don't collapse. Sanitary napkins attached to the inside of the mask can hold it away from the eye and absorb moisture.

Q. What's the connection between leptospirosis and moon blindness
A. The leptospirosis organism has proteins that are normally found in the eye of horses. Testing for lepto is basically testing the immune system of the horse. If he's positive, it's a bad prognosis. If he can see, he should be on medication the rest of his life. If he's a negative lepto Appaloosa, there's a better chance he'll not go blind. Treat him.

Q. How detrimental is it to clip eyelashes in show horses?
A. I don't think that's a problem, just be careful. Eyelashes grow really fast.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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