It’s not uncommon for horses to experience eye problems. Several conditions and diseases can affect vision and eye health in horses, including uveitis, cataracts, and conjunctivitis.

Horses experiencing eye issues may have symptoms such as swelling, tearing, drainage, discoloration, cloudiness, or sensitivity to light. Some conditions may not affect the eye, but instead, the eyelid or area around the eye. 

If your horse is affected by vision problems, this may result in poor performance, reluctance to move, nervous behavior, stumbling or clumsiness and an increased risk of injuries.

If your horse shows signs of any eye problems, have him evaluated by your veterinarian as soon as possible. With many eye conditions, early diagnosis and treatment will improve your horse’s prognosis and decrease recovery time and financial burden for the horse owner. [1]

The Horse’s Eye

The eye enables visual perception, so the horse’s brain can interpret its surroundings.

Horses have large eyes and horizontally elongated pupils to allow for maximum light capture.

Equine vision is adapted for peripheral motion detection and low light conditions.

This is attributed to their evolution as a prey species and their need to constantly monitor their environment while grazing. [3]

Anatomy

Damage to any one of the eye’s intricate structures can affect the rest of the eye and how it functions. [2]

The wall of the horse’s eye is composed of three layers: [3]

  1. An outer, fibrous tissue layer made up of the sclera (the whites of the eye) and the cornea (clear dome in the front of the eye);
  2. A vascular tissue layer called the uvea which is made up of the iris (colored part of eye), the ciliary body behind the iris, as well as the choroid which sits underneath the retina; 
  3. An inner layer of nervous tissue that makes up the retina

Parts of the Eye

Other important parts of the eye that can be affected by eye conditions include:

  • The pupil, the hole in the center of the iris through which light passes; 
  • The lens, a disc-like structure that sits behind the iris and pupil and is held by ligaments attached to the ciliary body;
  • A space in front of the lens filled with a clear, watery fluid called the aqueous humor. This fluid bathes the lens and cornea, bringing oxygen and nutrients to these structures and removing their wastes;
  • A large space behind the lens filled with a clear, gel-like fluid called the vitreous humor, which holds the retina in place;
  • The conjunctiva, a protective layer of tissue that covers the sclera and lines the inside of the eyelids as well as the third eyelid;
  • The lacrimal glands, which produce tears to keep the surface of the eye lubricated.
  • The orbits are the bony structures in the skull that house the eyeball, extra-ocular muscles, nerves, blood vessels, lacrimal glands, and fatty tissue. 

Understanding Equine Vision

Light enters the horse’s eye by passing through the cornea, aqueous humor, lens, and vitreous humor which are all clear. These structures also focus light on the retina by bending it.

Light then passes through the retina and gets converted into an electrical impulse. This signal travels through the optic nerve to the brain where it is interpreted into vision. [3]

Signs of Vision Problems

If a horse is experiencing vision problems, it may affect their performance as well as their overall well-being.

Signs of an eye condition affecting a horse’s sight may include:

  • Clumsy behavior
  • Self-trauma
  • Reluctance to move (especially from a lighted area to a dark one)
  • Spooky behavior
  • Head shaking

Additionally, horses with vision problems may be bullied by more dominant horses in their grouping. [3]

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Common Equine Eye Conditions

Many problems can affect the horse’s eyes and vision. These can range from minor to serious conditions. Usually, only one eye is affected, but in some cases, both eyes can be affected. 

The following are some of the most common eye conditions that veterinarians see in horses:

Corneal Ulcers

Corneal ulcers, which are breaks in the surface layer of the cornea, are one of the most common eye conditions seen in horses. Also known as ulcerated keratitis, this condition is caused by trauma to the eye, usually from plant material.

Symptoms include squinting, redness of the eye, eyelid swelling, discharge, or roughened or irregular areas on the corneal surface. [3][4]

Diagnosis

Veterinarians diagnose corneal ulcers by using fluorescein stain, which adheres to the eye and allows them to see the damage. [3][4]

There are three categories of corneal ulcers: simple, indolent, and complex.

  • Simple ulcers are acute, superficial and non-infected.
  • Indolent ulcers are also superficial and non-infected but are chronic in nature. [1]
  • Complex corneal ulcers are the most severe type. They include deep, melting, infected, or perforating corneal ulcers and may be acute or chronic. Complex ulcers often require referral to a veterinary ophthalmologist for surgery to avoid loss of the eye. [1]

Treatment

Treatment for corneal ulcers will depend on the severity and depth of the trauma. For small, superficial ulcers, a topical antibiotic is typically used as well as oral or topical anti-inflammatory drugs.

For more complicated ulcers, hospital referral may be needed so the eye can be medicated every few hours and monitored closely. [4]

Most corneal ulcers heal within 5-7 days. Longer healing time may indicate that the cornea is infected. [6]

Corneal ulcers can become infected by bacteria and fungi that live around the eye, and this may cause the ulcer to become larger or perforate through the cornea. Infected ulcers require more specific and frequent treatment. [3][4]

If the corneal ulcer does not respond to treatment, is significantly deep, or has a “melting” appearance, surgery is usually recommended to remove dead and infected tissue and place a conjunctival graft within the eye. [3][4]

Conjunctivitis

Another common eye condition is conjunctivitis, which involves inflammation of the inner lining of the upper and lower eyelids. This condition is especially prevalent during the summer months.

Signs of conjunctivitis include discharge that can be watery or thick, redness, and swelling of the eye. [4][5]

Several things, including allergies, insect hypersensitivity, bacteria, fungi, viruses, parasites, or trauma, can cause conjunctivitis. It can also be secondary to another eye problem.

It’s important to have your horse evaluated by a veterinarian to determine the exact cause of conjunctivitis and rule out other eye conditions. [4][5]

Depending on the cause, conjunctivitis is usually treated with antihistamines, topical antibiotics, and anti-inflammatories, as well as making changes to the horse’s environment. [4][5]

Uveitis

Uveitis, commonly known as Moon Blindness, is an immune-mediated, inflammatory condition that affects the middle layer of the eye.

Complications related to uveitis are the leading cause of blindness in horses worldwide. This condition can be classified as acute or chronic/recurrent. [4]

Uveitis can have multiple causes, including but not limited to bacterial or viral infection, equine influenza, and equine herpesvirus. It may also be caused by trauma to the eye or tumors. In some cases, the cause cannot be identified, however. [4]

Prevalence

Appaloosas, Warmbloods, Quarter Horses, Thoroughbreds, Hanoverians, American Paint Horses, and draft breeds appear at increased risk for uveitis.

Of these, Appaloosas are reported to be eight times more likely to develop equine recurrent uveitis (ERU) and four times more likely to go blind in one or both eyes as a result of the disease. [1][6]

Equine Recurrent Uveitis

In horses with ERU, the iris becomes stuck to the lens, cataracts may form, and corneal scarring, retinal detachment, retinal scarring, and retinal and optic nerve degeneration often occur. Eventually, this damage leads to permanent blindness in the horse. [3]

The immune system triggers new ERU episodes. However, in some cases, there is ongoing, low-grade inflammation that may never resolve. [3]

ERU has three sub-classifications:

  • Classic: Characterized by periods of active, painful inflammation in one or both eyes followed by a dormant phase. 
  • Insidious: characterized by persistent low-grade inflammation, usually in both eyes, with gradual and worsening destruction of ocular tissue. This form of ERU is not typically painful.
  • Posterior: Characterized by inflammation in the vitreous humor, retina, and choroid. Retinal degeneration is usually present. This form is most often seen in Warmbloods and draft breeds. It can be unilateral or bilateral. [7]

Symptoms

Common symptoms of uveitis in horses include:

  • Discharge from the eye;
  • Corneal swelling;
  • Blood or pus in the eye;
  • Squinting;
  • Redness;
  • Cloudiness of the cornea;
  • Yellowing of the normally blue iris or darkening of a brown iris, and
  • Constriction of the pupil (miosis). [3][4]

Treatment

Uveitis is diagnosed based on clinical signs and low intraocular pressure. In addition to a thorough eye exam, typical tests include a complete blood count, serum biochemistry profile, and serologic tests for specific infectious causes of the disease. [1]

Treatment for ERU may include topical medication to dilate the pupil, topical anti-inflammatories (NSAIDs), topical immunosuppressants, and systemic anti-inflammatories.

Recent surgical therapies include a supra-choroidal implant, which releases medicine into the eye and decreases the recurrence rate of ERU. [1][4]

Many horses with ERU develop glaucoma or a condition known as phthisis bulbi, which makes eye removal (enucleation) necessary.

Because uveitis is a painful condition, any permanently blind or inflamed eye should be removed surgically. Fortunately, many horses can successfully return to their discipline following this procedure. [3][4]

If the horse becomes blind in both eyes, however, euthanasia may be the best option, so the horse doesn’t become a danger to himself or others.

Cataracts

Cataracts are opacities within the lens of the eye. They may be small, causing little visual impairment or involve more than just the lens and cause complete blindness in the affected eye. Signs of a cataract include a white lens or white discoloration in the pupil opening. [3]

Cataracts can be inherited or occur after trauma or inflammation of the eye, such as with ERU. Foals may be born with congenital cataracts, either hereditary or related to maternal or environmental factors such as fever, malnutrition, or toxin exposure during pregnancy. [3]

Inflammation caused by cataracts is usually treated with anti-inflammatory medications.

For more severe cases, cataract surgery can be performed, and specialized prosthetic lens implants can be used to restore vision. This surgery is highly specialized and is only performed by veterinary ophthalmologists. [3]

Keratomycosis

Keratomycosis is a common fungal infection of the cornea that threatens a horse’s vision. It occurs mostly in warm or hot climates and is usually seen in late summer to early autumn.

Symptoms may include excessive blinking, sensitivity to light, pain, discharge from the eye, corneal haze, and/or white/yellow coloring to the eye. [2][6]

This condition comes in ulcerative and non-ulcerative forms and is grouped into three basic categories: [6]

  • Superficial
  • Stromal ulcerative
  • Stromal abcesses

Researchers believe that keratomycosis usually begins with corneal trauma that allows environmental fungal organisms to invade the cornea. However, seeding of fungi from a foreign body of plant may also be possible.

Treatment of corneal ulcers with long-term antibiotic therapy or corticosteroids may also play a part in developing keratomycosis. [6]

Treatment

Treatment for this condition is directed at the fungi, as well as inflammation of the iris and ciliary body (known as iridocyclitis). Treatment may be prolonged, and surgery may be needed for severe cases.

Despite increased awareness of keratomycosis and aggressive treatment, the prognosis for sight is still guarded. [6]

Squamous Cell Carcinoma

Squamous cell carcinoma (SCC) is the most common cancer of the equine eye region. It usually occurs on the third eyelid, conjunctiva, cornea, or eyelid but may spread to involve the orbit. [1][3]

SCC usually appears as a pink, raised, roughened mass or it may appear as erosive sores when it involves the eyelid. The area may be inflamed, ulcerated, or contain dead tissue. [4][5]

In rare cases, the tumor may spread. When metastasis does occur, cancer usually spreads to the lymph nodes, salivary glands, or thorax. [1][3]

Prevalence

Older horses with white skin around the eyelids, as well as those exposed to ultraviolet light are most at risk for developing SCC.

Draft horses, including Belgians, Clydesdales and Haflingers, also have a high prevalence of SCC, even without white skin around the eyelid. Other breeds most often affected by SCC include Appaloosas, American Paint Horses, Thoroughbreds, and Quarter Horses. [1][3][7]

Treatment

SCC is diagnosed through a biopsy. Veterinarians recognize several stages of the disease, including dysplastic, epithelium, carcinoma in situ, noninvasive, and invasive. [1]

Treatment for SCC will depend on the location and size of the tumor. Smaller tumors that involve the third eyelid, conjunctiva, and cornea are the easiest to treat successfully by surgical removal.

Eyelid and orbital SCC have a lower success rate and usually require removal of the eye and surrounding tissue. [3]

Unfortunately, recurrence of SCC after surgical removal is common. Therefore, repeat biopsies of suspicious lesions are recommended for 3-5 years after treatment. [1]

Periocular Sarcoids

Like SCC, sarcoids are another common eyelid lesion. They are thought to be caused by the bovine papillomavirus and often occur in younger horses.

A horse may have one or several sarcoids, and though these lesions are locally aggressive, they are benign (non-cancerous). [3][8]

Sarcoids can range from small wart-like lesions to large ulcerated growths. Aside from appearing around the eye, they can also occur at other locations of the body.

The initial lesion appears as a lump under the eyelid or a wart-like mass, but it can progress rapidly, invading the surrounding skin and underlying tissues. [3][8]

All horse breeds can develop sarcoids, but Quarter Horses, Arabians, and Appaloosas are at increased risk.

There are five types of sarcoids, including occult, verrucose, nodular, fibroblastic, and mixed, but eyelid sarcoids are usually nodular, fibroblastic, or mixed. [3]

Treatment

Sarcoids can be diagnosed by clinical signs alone or through biopsy. However, care should be taken as interfering with the lesion may cause it to spread or become more aggressive. 

The most common treatment for periocular sarcoids is surgical debulking and adjunctive therapy such as cryotherapy, immunotherapy, or chemotherapy. However, excessive debulking around the eye may be difficult or impossible.

The prognosis for periocular sarcoids is good to guarded, but they may have aggressive regrowth after treatment. [3]

Periocular Melanoma Tumors

Eighty percent of grey horses are affected by melanomas, which commonly occur around their eyes. Though rare, melanomas can also occur in non-grey horses and tend to be more malignant.

Melanomas usually develop slowly, but then growth may accelerate. There may be single or multiple eyelid tumors, but the initial tumor is typically a small, solid, spherical lump under the skin. [1][8]

Veterinarians diagnose periocular melanomas through biopsy. [1][8] Excision of the tumor is the most common treatment, and cryotherapy is used in some cases as well.

There is a canine melanoma vaccine that has been used successfully in some horses as well. However, the vaccine is expensive and doesn’t guarantee a cure, so it’s best used with caution. [1][8]

Diagnosing and Treating Eye Problems in Horses

Veterinary eye examinations are best performed under sedation. If the surface of the eye is involved, the exam usually includes fluorescent staining to identify the extent of the lesion. [2]

The goal of treating any eye condition is to identify and treat the cause, create an ideal healing environment, and prevent worsening of the lesion. Controlling secondary intraocular inflammation should be prioritized as well. [2]

Medications

There are three ways to deliver medications to the horse’s eye:

  • Topical
  • Local ocular (injected)
  • Systemic

The delivery method depends on which part of the eye needs treatment. The conjunctiva, cornea, anterior chamber, and iris are best treated with topical therapy.

The eyelids can be treated with topical therapy but often require systemic therapy. The back of the eye is treated with systemic therapy, as most medications cannot penetrate that area. [2]

Most ophthalmic drugs come as solutions, gels, suspensions, and ointments. Of these, solutions are the most commonly used as they are immediately available for absorption. Gels prolong the residence time, but may be more difficult to use.

Because eye conditions are usually painful and it may be difficult to keep the horse’s eye open to apply medications, regular topical treatment often becomes difficult, if not dangerous. [2]

A nasolacrimal lavage system, in which a catheter is passed through the bottom eyelid (under sedation), often provides the easiest method for applying topical medications. [2]

If the eye condition does not respond to medical treatment, surgical options should be discussed with your veterinarian. [2]

Managing a Horse with an Eye Condition

Horses with any ongoing eye condition should be fed from the ground. Stall bedding should consist of sawdust instead of straw to avoid further contaminating the eye.

Insect control is important, and fly masks work well to keep flies out of the eye so long as they are kept clean and don’t interfere with the lavage system. [9]

For horses stalled and experiencing light sensitivity, walking them during early morning, late afternoon, or evening hours is best to avoid bright sunlight. [9]

Visually Impaired Horses

For visually impaired horses, either permanently or temporarily, a safe environment is critical.

Remove hazards such as sharp objects, low hanging tree limbs, debris, and holes in the ground. Barbed wire and electric fences should be avoided. [4]

Visually impaired horses can be dangerous as they may be easily spooked, especially when first acclimating to their condition. Handlers should be cautious not to make sudden movements or noises and speak calmly when near the horse.

Keeping food and water in a consistent location and using consistent verbal commands are also important. [3][14]

A calm, sighted horse can work as a good companion to a visually impaired horse and can even act as a guide horse for them if needed. The good news is that many visually impaired horses do adapt to their disability in time. [3]

Nutritional Support

As with any health condition in the horse, diet is important. Good quality pasture and hay should be the foundation of the diet.

Omega-3 fatty acids can also be beneficial for ocular health. Although not extensively studied in horses, omega-3 fatty acids have been shown to benefit eye health in humans and other animals. In particular, docosahexanoic acid (DHA) supplementation in people helps delay age-related vision loss and retinal degeneraton. [10]

The anti-inflammatory effects of DHA could also support eye health in horses. DHA is only found in marine sources such as fish oil or algae. Plant oils such as canola or camelina oil will not contain DHA. Mad Barn’s w-3 oil provides 1,500 mg of microalgal DHA per 100 ml serving.

Another nutrient commonly associated with eye health is vitamin A. Night blindness has been reported in horses with severe vitamin A deficiency. Therefore, meeting their vitamin A requirement is recommended in order to optimally support eye health.

For a mature horse, the vitamin A requirement is estimated at 30 IU per kg of bodyweight (15,000 IU for a 500 kg / 1100 lb horse). [15]

Beta-carotene is a precursor to vitamin A that is abundant in fresh pasture but is partially lost when grass is cut to make hay. Horses on hay-based diets should be provided supplemental vitamin A to meet the requirement.

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References

  1. Bauer, B. Ocular Pathology. Vet Clin Equine. 2015. View Summary
  2. Hamor, R.E. and Whelan, N.C.  Equine Infectious Keratitis. Clinical Pharmacology and Equine Therapeutics. 1999. View Summary
  3. Miller, P.E. and Murphy, C.J. Equine Vision. Equine Ophthalmology, Third Edition. 2016.
  4. Robinson, B. Common Equine Eye Conditions. AAEP.
  5. Brooks, D.E. Equine conjunctival diseases: A commentary. Equine Vet Educ. 2010.
  6. Galera, P.D. and Brooks, D.E. Optimal management of equine keratomycosis. Vet Med (Auckl). 2012. View Summary
  7. Bellone, R.R.Genetics of Equine Ocular Disease. Vet Clin Equine. 2020. View Summary
  8. Dugan, S.J. Ocular Neoplasia. Vet Clin North Am Equine Pract. 1992.
  9. Michau, T.M. Equine Glaucoma. Vet Clin Equine. 2017. View Summary
  10. Georgiou, T. and Prokopiou, E. Role of Omega-3 Fatty Acids for Eye Health. Omega-3 fatty acids. 2016.
  11. Degroote, R.L. and Deeg, C.A. Immunological Insights in Equine Recurrent Uveitis. Front Immunol. 2021. View Summary
  12. Stein, L. et al. Squamous cell carcinoma with clear cell differentiation in an equine eyelid. J Vet Diagn Invest. 2019. View Summary
  13. Henrikson, M.D.L. et al. Equine corneal stromal abscesses: An evolution in the understanding of pathogenesis and treatment during the past 30 years. Equine Vet Ed. 2013.
  14. Sandmeyer, L. Understanding Equine Vision and Eye Disease . Horse Journals. 2022.
  15. Chapter 6: Vitamins. Nutrient Requirements of Horses. 2007.