Filed under: Corneal ulcer, Emergency, Eye Problems, Laceration, Uveitis
Hunter Ortis, DVM
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The most frequent eye related problems in horses are corneal ulcers (abrasions to the surface of the eye), eyelid lacerations and uveitis. Clinical signs of an eye emergency include excessive tearing or discharge, spasmodic squinting, holding the eye partially or completely closed, or constriction of the pupil. Additionally, changes in appearance of the eye and its surrounding structures such as swelling of the eyelids, increased redness of the usually pink tissue around the eye, and cloudiness of the cornea are signs of an eye problem.
The most common injury of the eye is a corneal ulcer. Ulcers occur when the surface of the eye comes in contact with a foreign object such a hay stem, stick or surface of a stall wall, leaving a defect in the integrity of the surface of the eye. A horse with a corneal ulcer will usually have symptoms of a painful eye and/or a hazy bluish appearance of the surface of the eye. Corneal ulcers are diagnosed with a fluorescing stain applied directly to the cornea during examination. Treatment usually includes topical antibiotics because the cornea is easily infected. Other medications may be indicated on a case by case basis.
Eyelid lacerations are commonly caused when the eyelid simply gets snagged on a piece of stall hardware, an overlooked raised nail or piece of wire. These injuries are much easier to diagnose than most other eye emergencies but are equally important to seek rapid veterinary attention. Most lacerations involving the margin of the eyelid will require repair with suture. The integrity of the eyelid margins is vital for maintaining a proper tear film on the surface of the eye. An irregular eyelid margin can cause recurring and chronic eye irritation.
Uveitis, or inflammation in the interior portion of the eye, occurs when inflammatory mediators are released causing leakage of damaging proteins into the internal structures of the eye. There are many different causes of uveitis including blunt or penetrating trauma, corneal ulcers, systemic diseases, and unknown causes to name a few. Most horses with uveitis will show signs of a painful eye that appears cloudy. Diagnosis is usually made with findings from a complete eye examination including the use of an ophthalmoscope. Rapid and aggressive treatment, often with multiple topical and systemic medications, is essential in minimizing the possibility of permanent damage to the internal structures of the eye.
Injuries of the equine eye should be considered an emergency, and owners should consult their veterinarian immediately when a potential problem has been identified. Delaying treatment or instituting improper treatment on some ocular conditions can lead to permanent damage to the eye and loss of vision.
Nathan Voris, DVM
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Did you know that the horse has the largest eye of any land animal? This fact, along with the eye’s prominent location-on the side of the face, at the widest part of the head-makes it prone to injury and infection.
A frequent question asked by horse owners when an eye problem is first noticed is “do I need to have my vet take a look at my horse’s eye?” The answer to that question is almost always “Yes” due to the aggressive nature of many ocular diseases. Your veterinarian should examine the eye if your horse is unable to comfortably hold the eye open completely, there is any discoloration of the eye, swelling is noticed around the eye, or if there is increased tearing from the eye. These signs are non-specific indications of ocular pain that need to be carefully investigated through ophthalmic examination and special staining to determine the origin of the discomfort. Glaucoma, uveitis, conjunctivitis and corneal ulceration can all cause these signs, and all are treated differently. In fact, medications used to treat uveitis or conjunctivitis should be avoided in the presence of a corneal ulcer as they will increase the risk of infection and delay healing.
If you notice that your horse is showing any indication of discomfort around the eye, contact your veterinarian as soon as possible. Many ocular problems can quickly cause irreversible damage or even loss of the eye in a very short amount of time if not correctly addressed.
Rob Foss, DVM
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Understanding how horses see helps us understand, predict, and modify an animal’s behavior in certain instances. We have a tendency to assume that a horse’s view of the world around us is similar to our own, but this is not the case. Unfortunately science does not give us an exact picture of what a horse sees; only a horse could do that (which of course they can’t). Evaluation of the anatomy and physiology of the horse’s eye does give us some very good hints as to what they see.
The vision of the horse has evolved to help avoid predators while grazing and allow for fast flight on difficult terrain. The eyes of the horse are set very far to the side and relatively far back on the skull, allowing him to see behind while grazing. When the head is held normally there is a narrow blind area behind the rump, although with head movement, the horse is also able to view the blind area. The horse can see straight ahead with binocular vision and to each side and rear with monocular vision. There are even two different neurological visual pathways, binocular and monocular.
Lateral vision is enhanced by the fact that the horse’s pupil is horizontal, so that its constriction in bright light does not decrease the lateral field of vision. Peripheral vision is more important for the horse than man and the peripheral retina appears relatively more useful, as it is effectively stimulated by movement. This ability to see well almost 360 degrees around the horse when considered in conjunction with the peripheral retina being stimulated by movement and the horse’s defense mechanism of flight explains the propensity of horses to spook easily from movement behind them.
Horses do see some color. They appear to differentiate yellow, green, and blue but have difficulty with red, similar to red-green color blindness in humans. How color vision is important for horse survival is unclear, but it is apparent the equine eye has evolved to handle both very bright days and dark nights. The equine pupil constricts quickly but does not dilate as rapidly as humans. This is why horses do not adapt quickly when walking into a dark barn or after a light is turned off. Large dark granules on the edge of the pupil called corpora nigra are thought to aid the constricting pupil to shield excessive light on bright days. A special reflective layer of the retina behind the sensory nerves, called the tapetum lucidum, allows dim light to be reflected back onto the nerves after it has passed through once, improving night vision.
The globe of the horse’s eye is not spherical in shape. This allows the lens to focus different distances more easily when viewed from certain angles. This means that a horse may focus on certain things of interest by tipping its head to certain angles. Movement of the head can also affect binocular and monocular vision and the neural pathways involved. One example of how vision plays into behavior is exhibited by catching a nervous horse. Approaching from the rear usually evokes a startle response, the horse will trot off then turn to face his stalker. He will view with binocular vision and usually move the head up and down to focus. Usually if you slowly approach from the side in monocular vision, when the horse finally resolves itself to being caught it will turn its head to binocular vision and relax. The way the horse views the world around it shapes behavior. Understanding the horse’s view should leave us better able to manage its behavior.