The eye is a complex structure uniquely built for capturing light and transmitting visual stimuli to the brain. The globe itself has three tunics or layers: there is an outer fibrous tissue layer which is the sclera or the whites of the eye, and the cornea, the clear dome in the front. The vascular tissue layer is made up of the iris (the coloured part of the eye), the ciliary body, and choroid. These three structures are continuous with each other. The inner most layer is the nervous tissue layer which makes up the retina. The lens is a disc-like structure suspended by the ciliary body behind the iris. The pupil is the hole in the centre of the iris. In front of the lens there is a space filled with clear water-like fluid called aqueous humor. Behind the lens there is a large space filled with a clear gel-like fluid called vitreous humor. Conjunctiva is a layer of tissue that covers the sclera and lines the inside of the eyelids. The eyelids cover the eye in front. The eye is held in the bony orbit by muscles that attach to the sclera. These muscles are responsible for moving the eye. 

Visual stimuli (light) enter the eye by passing though the clear structures of the cornea, aqueous humor, lens, and vitreous chamber. It is passed through the retina where it is converted to an electrical impulse. This electrical impulse leaves the eyes in the optic nerves which extend from the retina, through openings in the skull into the brain. The optic nerves cross over at an area called the optic chiasm so that some information from each eye goes to both the right and left sides of the visual cortex of the brain. In the brain, the electrical impulse is interpreted into vision.  

 

Horse vision is particularly adapted for peripheral motion detection and low light conditions. This is likely due to the species evolution as a prey species and the need for constant monitoring of the environment while grazing. The horse has a very large eyes and a horizontally elongated pupil which allows for maximum light capture. The elongated pupil serves to protect the retina in very bright light but also allows a wider view of the horizon. The lateral position of the eyes allows for a large visual field of about 325º, this reduces the binocular visual field to about 65º. By comparison, the human binocular visual field is about 120º. Blind spots exist directly behind the head, between the eyes and directly under the body to about 4 feet in front of the legs. Visual acuity in the horse is difficult to determine but is estimated to be about 20/30 or 0.6 times that of humans. This is actually better than many common domestic species. Vision in the dark is enhanced by a reflective layer called the tapetum and a pupil which dilates widely to allow maximum light entry. The horse retina has a 9:1 ratio of rod photoreceptors (responsible for vision in the dark), to cone photoreceptors (responsible for color vision and visual acuity). Color vision is poorly developed in the horse. Horses are dichromats (seeing 2 basic colors) while people are trichromats (seeing 3 basic colors). The horse sees green/yellow and blue/gray but does not see red. It is also thought that the colors seen by the horse may be “washed out” compared to the vibrancy that humans would see.

Vision problems are common in horses and have many causes. Vision disturbances may be more easily detected if both eyes are involved than if only one eye is affected. Ocular diseases often have an impact on the performance of a horse. Signs of a vision problem include clumsy behavior and self trauma; horses may be resistant to move from one place to another, and this may be especially prominent when moving from a lighted area to a dark one; they may shy away or spook easily and occasionally have head shaking behaviors. There may also be a change in herd behavior; horses are herd animals that use visual cues to establish dominance order within a group. Animals with vision problems may be bullied by more dominant individuals.

Diagnosis of a vision problem begins with a complete eye examination. In some cases referral to a veterinary eye specialist (ophthalmologist), is necessary. Ophthalmologists will have expertise in the diagnosis and treatment of eye diseases. If the cause of the visual disturbance is not evident on examination alone, an electrical test of retinal function (electroretinogram or ERG) may be required. Refraction to look for short-or far-sightedness is also done if necessary. 

Causes of visual problems are many and can be separated into those that impair the ability of light to enter the eye (opacity of the cornea, the fluid in the anterior chamber or vitreous chamber, or cataracts in the lens), inability to convert the light impulse into an electrical signal (diseases of the retina), inability to transmit that signal to the brain (optic nerve disease), and inability of the brain to process the signal (diseases of the visual cortex). It is important to seek veterinary care if a vision problem is suspected. This may require referral to a veterinary ophthalmologist.

Management of a visually impaired horse requires special care. The horse must be provided with a safe environment. This requires removal of any hazards such as sharp objects, low-hanging tree limbs, debris, and holes in the ground. Barbed wire and electric fences should be avoided. When placed in new enclosures, demonstration of boundaries is important. Visually impaired horses can be dangerous as they may be fearful and easily spooked, especially early on. Signs should be posted so that people are aware of their condition. Handlers should be cautious to avoid sudden movements or noises and speak to them calmly when near them. Consistency in care and handling the animal is important. For example, keeping food and water in a consistent location and using consistent verbal commands in training. Choosing appropriate turn-out companions is also important. A calm, sighted horse can be good company and even act as a guide-horse for the other. Horses will adapt to visual impairment but this takes time and patience.   

2. Signs of Ocular Disease in the Horse 

Ocular disease is common in the horse and may manifest as vision problems or with ocular redness, tearing, squinting, or cloudiness of the eye. These may be signs of serious eye disease and should never be ignored. 

Corneal ulcers: Corneal ulcers are one of the most common acquired ocular diseases in the horse. A corneal ulcer is a break in the surface layer of the cornea. Ulcers usually develop secondary to trauma, often from plant material; for example, tree branches or straw scratching the eye. Signs of a corneal ulcer include redness of the eye, tearing, squinting, opacities in the cornea, and roughened areas on the corneal surface. Simple ulcers that are treated early on with antibiotic drops often heal uneventfully. However, ulcers may become infected by the bacteria and fungi that normally live around the eye and these can cause ulcers to enlarge, deepen, and even perforate. This may even result in loss of the eye. Infected ulcers require specific and frequent antibiotic therapy as well as, anti-inflammatory treatment and occasionally, surgery to repair. 

Uveitis: Uveitis is inflammation of the uveal tissue inside the eye. The uvea is made up of the iris tissue (the colored part of the eye) as well as the ciliary body (a part that is behind the iris) and the choroid (which sits underneath the retina). The clinical signs of uveitis include redness of the eye, squinting, and tearing, as well as cloudiness of the cornea or ocular fluid, a small or constricted pupil, and iris color changes (e.g. yellowing of a normally blue iris or darkening of a brown iris). Uveitis is treated with topical and systemic anti-inflammatory medications and drops to dilate the pupil.

There are many causes of uveitis. Equine recurrent uveitis (ERU) is uveitis that reappears over and over with new episodes triggered by the immune system. Each time there is inflammation inside the eye there is some permanent damage done. This includes the formation of adhesions of the iris to the lens, cataract formation, corneal scarring, retinal scarring, retinal and optic nerve degeneration. Eventually, these permanent changes lead to blindness and this often eventually involves both eyes. ERU is the most common cause of blindness in the horse and it is a painful condition. There is no cure for ERU but an attempt is made to minimize damage to the inside of the eye with prompt and aggressive treatment to control the inflammation.  Because uveitis is a painful condition, a blind inflamed eye should be removed surgically or a specialized surgery to remove the ocular contents and replace them with a silicone ball (intrascleral prosthesis), can be completed.

Glaucoma: This is a condition in which the fluid in the eye cannot drain properly and builds up causing increased pressure inside the eye. This leads to permanent damage to the retina and optic nerve and eventual blindness. Glaucoma may occur on its own, but is most commonly a secondary effect of chronic uveitis in the horse. Glaucoma is treated with pressure lowering medications, as well as anti-inflammatory medications. Laser surgery to destroy the cells that make the fluid inside the eye may also be effective in controlling the pressure, but may only work temporarily. Eventually, most eyes with glaucoma become blind. Because glaucoma is a painful condition, a blind glaucomatous eye should be removed surgically or a specialized surgery to remove the ocular contents and replace them with a silicone ball (intrascleral prosthesis), can be completed.

Cataracts: Cataracts are opacities within the lens. They may be small and cause very little visual disturbance, or they may involve more of the lens and cause blindness. When they are large they will also cause inflammation inside the eye (a form of uveitis called lens-induced uveitis). Inflammation associated with cataracts should be treated with anti-inflammatory therapy. Cataract surgery can be performed on horses and there are specialized prosthetic lens implants that can be placed at the time of surgery. This surgery is complex and only performed by veterinary ophthalmologists. There are also many potential complications of the surgery, but it can be very rewarding when successful.  

Squamous cell carcinoma: This is the most common cancer of the equine eye.  It occurs commonly on the third eyelid, conjunctiva, cornea, eyelids and may spread to involve the orbit. Older horses, those with white skin around the eyelids, and those exposed to ultraviolet light are most at risk. The tumor may appear as a pink, raised, roughened mass. It may also appear as erosive sores when involving the eyelids. It is common for this tumor to spread into local tissues, and it may spread to distant sites as well. Treatment depends on the location and size of the tumor. Small tumors involving the third eyelid, conjunctiva and cornea are the most easy to treat successfully with local surgical removal. The eye can usually be spared if these are treated early on. The eyelid and orbital squamous cell carcinomas have a lower success rate, and often require removal of the eye and surrounding tissue.  

Summary

In addition to vision loss, it is very common for serious ocular disease to start with some tearing, squinting, and redness of the eye. Cloudiness of the ocular surface or inside the eye is also a sign of a problem. Lumps or bumps around or on the eye may also indicate serious ocular disease. Many eye conditions can be successfully managed if treatment is started early. Delaying treatment may lead to permanent vision loss or loss of the eye. It is important to seek veterinary attention if you suspect anything is wrong with the eye. Veterinary ophthalmologists are specialists in diagnosing and treating ocular disease. There are several ophthalmologists in Canada and many in the USA. You can locate an ophthalmologist by visiting the website of the American College of Veterinary Ophthalmologists www.acvo.org. 

References

Grahn BH, et al. Veterinary Ophthalmology Essentials. Philadelphia: Butterworth-Heinemann. 2004
Miller P. Murphy C. Equine vision: Normal and abnormal.
In: Gilger B. Ed. Equine Ophthalmology.St. Louis: Elsevier. 2005: 371-408.
www.BlindHorses.org
Dwyer A. Practical management of blind horses. In: Gilger B. Ed. Equine Ophthalmology. St. Louis: Elsevier. 2005: 449-456.
Grahn BH, et al. Veterinary Ophthalmology Essentials. Philadelphia: Butterworth-Heinemann. 2004