by Emily McNally, VMD. Rappahannock Equine Clinic
The equine eye is not unlike the eyes of many other species. Because horses are prey animals, they have evolved to be constantly on the lookout for predators. Their eyes are very sensitive to movement. Compared to other animals, a horse’s eye is placed more laterally on their head with an elongated pupil, allowing them to have a wider field of vision than humans. They can see almost 360 degrees around their body but do have two blind spots: one 3 feet directly in front of their face and the other behind their head, extending down their back. The eye is made up of the retina, the globe, the eyelids, the third eyelid, the cornea, the iris, and the lens. Globe is another word for the eyeball itself, which houses all the important structures of the eye. The retina is on the back of the eyeball and is essentially an extension of the brain that is sensitive to light and transmits information via the optic nerve to the brain where perceptions are processed. The cornea is transparent without any blood vessels so it can refract light into the eye. The iris is the colored part of the eye and contains muscles that control the size of the pupil. Behind the iris is the lens that focuses images onto the retina.
Problems in the eye may appear as swelling, tearing, squinting, blinking, and itching. Even though some symptoms may appear minor, they may represent a serious issue with the eye and should be evaluated by a veterinarian as soon as possible. The eye is a delicate organ that does not respond well to inflammation and injury.
These ulcers occur when the thin outer layer of superficial cells of the cornea are disrupted usually by trauma, fly irritation, or a foreign body. Bacteria can then invade and break down the cornea further, leading to an ulcer. Corneal ulcers are extremely painful and the first sign is often a horse that is unwilling to open its eyelid all the way and exhibits excessive tearing. Treatment consists of frequent application of serum (or other protease inhibitors), antibiotics, and atropine (dilates the pupil) to the eye. Oral or IV nonsteroidal anti-inflammatory medication, such as Banamine, is also often administered. An ulcer is an emergency and should be examined by your veterinarian as soon as possible.
Uveitis is essentially inflammation inside of the eye and equine recurrent uveitis (also commonly known as moon blindness) can recur throughout a horse’s lifetime; it is the most common cause of blindness in horses. Signs of an acute attack include squinting, redness, cloudiness, and excessive tearing. How often and how severe an attack occurs varies from horse to horse, and it is the cumulative effect of many attacks that can lead to blindness. Treatment includes management of acute attacks and prevention of future attacks. There are newer techniques, including surgical implantation of slow-release steroid devices into the eye that can help horses with this.
Eyelid lacerations are fairly obvious because there is often visible blood and the edge of the eyelid can usually be seen hanging loose in front of the horse’s eye. It is important for these to be repaired by a veterinarian quickly, so the eyelid can continue its function of protecting the cornea and distributing the tear film. If you do see a piece of eyelid hanging from the horses eye, do not cut it off! No matter how gruesome it looks, these can often be repaired and do quite well.
The lens sits behind the colored portion of the eye. When there is a problem with the lens, it usually responds by becoming opaque, eventually leading to complete or partial blindness. This opacity is known as a cataract. Cataracts can be secondary to equine recurrent uveitis or can be a hereditary condition. Cataracts can be treated surgically either by removal of the lens completely or a procedure known as phacoemulsification.
The most common cancer of the equine eye is squamous cell carcinoma. These masses have a typical pink, wartlike appearance and can be found on the third eyelid, on the surface of the eye, or around the eye on the eyelids. This cancer is most commonly seen in horses with no pigment around their eyes, such as Appaloosas and Paints. Treatment includes removal, radiation therapy, freezing, and/or chemotherapy. Sarcoids and melanomas are other cancers that can invade the ocular area and cause problems with vision. The eye may be small, but it is made up of a lot of parts. Early detection and treatment is paramount to success of most equine eye problems. Working together with your veterinarian is the best way to have a positive outcome. Remember, if in doubt, have us out! Please let us know if you have any questions about your horses’ eyes by contacting us at 540.841.7171.