E224
Eye Diseases and Problems

anatomy and terms | problems associated with the nasolacrimal system | proptosis of the eyball | glaucoma | blepharitis | trauma to the eyelids | conjunctivitis | trauma to the cornea | keratitis | ulcerative keratitis | bacterial keratitis | fungal keratitis | cataract | lenticular sclerosis | luxation and subluxation of the lens | equine recurrent uveitis (moon blindness) | equine ocular thelaziasis (eye worms) | equine ocular onchocerciasis | equine ocular habronemiasis | equine sarcoid | emergency tratment for eye injuries | eye medications

Introduction: It is important to examine horses’ eyes on a routine basis. Repeated examination allows one to become familiar with the normal appearance of the eye, so any abnormalities will be noticed immediately. Signs of an eye problem vary tremendously and may include cloudiness, tearing, squinting, discharge, redness, blinking, swelling, an increase in blood vessels, or changes in the size or shape of the pupil. Horses may try and rub the eye on objects in an attempt to relieve irritation and itching. Any change in the eye or surrounding tissue may signal a problem and should be a cause for concern.

Many different problems can result in the same set of disease signs, so diagnosis cannot be made by clinical signs alone. Physical examination and special tests are needed to properly identify the cause of the problem. Tests may include a fluorescein dye test, tests to measure pressure within the eyeball, and ocular examination with different types of lenses. The eye may be dilated to allow for proper visualization of the back of the eye.

Eye problems should be brought to the attention of a veterinarian immediately. Prompt diagnosis and treatment can prevent further eye problems that can lead to loss of sight. In addition, changes in the eyes may be a sign of whole body disease. By immediately identifying and reporting any changes, diseases can be diagnosed early and treatment can begin.

Although all eye problems should be reported to a veterinarian, it is important for the horse owner to identify and recognize common eye ailments. The eye is composed of several parts, all of which can be injured or become diseased. Eye problems and diseases can affect one portion of the eye or simultaneously be found in several areas.

Basic Anatomy and Terms: The eye is protected by upper and lower lids, as well as a third eyelid, called the nictitating membrane. Glands which produce tears are located under the lids. The front portion of the eye itself is covered with a thin, clear covering called the cornea. The remainder of the eye is covered with dense white tissue, the sclera. The margin of the cornea and the sclera is called the limbus. The episclera is the outside surface of the sclera. The conjunctiva is the tissue which reflects from the inside of the eyelids onto the globe. Glands which produce tears are also located in the conjunctiva.

The iris is the colored portion of the eye; the black open space in the iris is the pupil. Behind the pupil is the lens. The lens is attached to the ciliary body. The back of the eye is covered with a layer of tissue called the retina. The inside of the globe is filled with a clear fluid called aqueous humor. This fluid is produced by the ciliary body and nourishes the eye while helping to maintain its shape. This fluid is continually produced and drained from the eye. Drainage occurs at the iridocorneal angle, also called the drainage or filtration angle.

1. Anterior chamber - The space in the eye bounded in front by the cornea and behind by the iris. It is filled with aqueous humor.
2. Anterior uvea - The front portion of the uvea, made up of the iris and ciliary body.
3. Anterior uveitis - Inflammation of the iris and ciliary body.
4. Anti-inflammatory - A medication that reduces inflammation.
5. Aqueous humor - The clear, watery fluid which fills the eye.
6. Blepharitis - Inflammation of the eyelids.
7. Blepharospasm - Spasm of the muscles that control the eyelids.
8. Cataract - An opacity of the lens or its capsule, or both.
9. Choroid - The part of the vascular layer of the eye (uvea) located at the back of the eye. It helps to nourish the retina. It is the posterior uvea.
10. Ciliary body - The part of the vascular layer of the eye (uvea) located between the iris and the choroid.
11. Conjunctiva - The mucous membrane lining the back of the eyelids and the front of the eye, except for the cornea.
12. Conjunctivitis - Inflammation of the conjunctiva.
13. Cornea - The transparent portion of the outer layer of the eye, which allows light to enter the eye. It attaches to the sclera.
14. Corneal ulceration - An open lesion or sore on the surface of the cornea.
15. Enucleation - Removal of the eye.
16. Epiphora - The abnormal flow of tears over the face.
17. Fibrous - Composed of cells that form tough, connective tissue.
18. Fluorescein dye - A special dye that is applied to the cornea that will not penetrate intact cornea or conjunctiva, but will adhere to and outline areas of ulceration.
19. Glaucoma - Increased ocular pressure resulting from altered flow of aqueous humor.
20. Globe - The eyeball.
21. Intraocular - Within the eyeball.
22. Iris - The colored, circular portion of the eye located behind the cornea. It is part of the uvea.
23. Keratoplasty - Corneal grafting used as a repair technique.
24. Keratectomy - Removal of part of the superficial layers of the cornea.
25. Keratitis - Inflammation of the cornea.
26. Lacrimal apparatus - The structures responsible for tear production.
27. Lens - A transparent structure suspended in the front portion of the eye. It refracts light that passes through the cornea on its way to the retina.
28. Lens luxation - Displacement of the lens.
29. Miosis - Contraction of the pupil so that the pupil becomes small.
30. Miotic - A medication that causes the pupil to become small.
31. Mucoid - This is a general term for types of secretions that are made up of mucous materials.
32. Mucopurulent - A secretion or exudate that is made up of pus and mucous.
33. Mydriatic - A medication that causes the pupil to become large.
34. Mydriasis - A dilated pupil.
35. Nasolacrimal - The system of the eye responsible for proper drainage of tears out of the eye and into the inside of the nose.
36. Nictitans (nictitating membrane) - The third eyelid.
37. Ophthalmoscope - A device for studying the interior of the eyeball.
38. Optic nerve - The nerve that originates from the retina and carries light signals to the brain.
39. Posterior uvea - The portion of the uvea made up of the choroid.
40. Posterior uveitis - Inflammation of the choroid.
41. Pupil - The circular opening in the center of the iris through which light passes into the eye.
42. Primary - The first, or foremost. Used in this case to describe problems that occur first and may be followed by other, secondary problems.
43. Proptosis - Prolapse of the eye.
44. Retina - The innermost coat of the eye, formed of cells sensitive to light.
45. Retinal detachment - Separation of the retina from the underlying tissue, the choroid.
46. Sclera - This is a layer of dense connective tissue that supports the eye. It is the white portion of the eye that connects to the cornea. The cornea and sclera make up the fibrous tunic of the eye.
47. Secondary - A problem or disease which occurs after the first or primary disease or problem.
48. Tonometry - The measurement of intraocular pressure.
49. Topical - Medication that is applied locally on an external portion of the eye.
50. Uvea - The portion of the eye which contains many blood vessels. It is comprised of the iris, ciliary body, and choroid. It is called the vascular tunic.
51. Uveitis - Inflammation of the uvea.

 

Figure #1: Vertical cross-section of the eye . Anterior chamber Cornea Suspensory lig. Ciliary body Sclera Choroid Vitreous chamber Optic disc Retina Lacrimal gland Eyelid Pupil Iris Lens

Copyrighted graphic used by permission from Anatomy of Domestic Animals, Sudz Publishing (email: sudzpub@mac.com)

Introduction: The lacrimal and nasolacrimal duct system function together to ensure that the eye is continuously bathed in protective tears and that the tears are removed from the eye. The lacrimal system is responsible for the production of tears. Glands in the eye produce tears that act to lubricate the eye and cornea, protect the eye from environmental debris, supply nutrients to the eye, and wash away irritants. Both the lacrimal glands and the gland of the third eyelid contribute to tear production. After the tears protect and bath the eye, they are drained via the nasolacrimal system. This duct system is responsible for drainage of tears from the inner corner of the eye into the inside of the nose. Health of the cornea and conjunctiva depends on the continuous production of tears and their uninterrupted removal from the eye. If there is a problem with any part of the system, drainage of tears from the eye may be impeded. If the tears do not drain into the duct system, they will spill over onto the face, creating a condition called epiphora.

1. Absence of the Puncta: There can be many different causes for obstructions in the nasolacrimal duct system. Blockage will occur if the openings to the ducts, called the puncta, are absent or closed. Foals may be born with closed or absent puncta. Tests may include the use of fluorescein dye to observe the movement of tears, a nasolacrimal flush, and passage of a nasolacrimal catheter. If a potentially functional duct system exists, treatment involves opening the puncta with surgery. Once the duct is created or opened, a catheter can be placed for one to two weeks to ensure it remains open. The eye is also treated simultaneously with topical medication to reduce inflammation and prevent infection.
    
2. Obstruction of the Puncta or Ducts: Other causes of obstructions may include inflammation that closes down any part of the duct system. This obstruction can be caused by conjunctivitis or by physical irritants such as sand, plant stems, and other foreign objects. Diagnosis is based on ocular examination and appropriate testing. Treatment would include antibiotic and anti-inflammatory medical therapy, removal of any foreign objects, and flushing of the duct system. Additional causes of obstruction may include space-occupying tumors. Treatment would depend upon the type and location of the tumor and may include surgical removal.
   
    

Problems with the lids can result in pain, swelling, redness, excessive tearing, and drainage from the eye. Problems with the lids can lead to additional problems with closely associated structures such as the cornea, conjunctiva, and nasolacrimal drainage system of the eye.

1. Blepharitis: Blepharitis is inflammation of the eyelids. It can be caused by any condition that results in inflammation, including allergies, trauma, parasites, immune-mediated disease, trauma, neoplasia, and infection. Infection can be caused by bacteria, viruses, or fungi. Examples of allergic conditions that may lead to blepharitis include dietary sensitivities or insect stings and bites. Parasitic diseases that potentially cause blepharitis include thelaziasis and onchocerciasis. Blepharitis may occur alone, but is often accompanied by inflammation and infection of surrounding tissues leading to conjunctivitis and keratitis.
   
   All animals can suffer from blepharitis. It may involve one or more lids, and be localized to one spot or involve large areas of the involved lids. Signs may include eyelid spasm, squinting, pain, ocular discharge, discoloration of the lids, hair loss surrounding the lids, redness, crusting, abscesses, pustules, and swelling. There may be concurrent irritation and scarring of the cornea, and redness and swelling of the conjunctiva.
   
   Diagnosis is based on physical examination and specialized tests. A Schirmer tear test, cultures and sensitivities, cytology, skin scrapings, and biopsies may be performed. Underlying conditions, such as fungal infections, parasitic skin infections, and bacterial skin infections, must be diagnosed and eliminated. Once a diagnosis is reached, treatment is designed to eliminate the underlying cause and remove the inflammation from the eyelid. Treatment may involve medications such as antibiotics, antifungals, and anti-inflammatories, along with topical medications that are instilled into the eye and on the lids. Topical therapies may include appropriate antibiotics, corticosteroids, and combination antibiotic/anti-inflammatory medications.
    
2. Trauma to the Eyelids: Eyelid injuries are somewhat common in equine medicine. They are usually caused by sudden head movements that catch and tear the eyelid and surrounding structures. These injures should be repaired as soon as they are noticed. This usually means that surgical repair with stitches is often required. Care should be taken to ensure that eyelids are sewn back in place so that the eyelashes do not contact the eye and that the eyelids provide adequate protection to the eye from wind, dust, debris, the sun, insects, etc. Antibiotic ointments are often given for 5-7 days after the injury to help prevent infection and give lubrication to the eye. Fly repellants should also be used to reduce the chances of infection and additional irritation in the eye.

1. Conjunctivitis: Inflammation of the conjunctiva is termed conjunctivitis. Any animal can suffer from conjunctivitis. Conjunctivitis may be primary or secondary to other problems. Primary conjunctivitis can be caused by immune-mediated diseases or allergies, as well as foreign objects in the conjunctiva or environmental irritants (dust, chemicals). Primary bacterial or viral conjunctivitis can occur, but is uncommon. The majority of the cases of conjunctivitis are secondary to other eye problems such as lid malformations (distichiasis or trichiasis) or nasolacrimal diseases. Others occur following glaucoma, cancer, uveitis, corneal inflammation or immune-mediated diseases.
   
   Signs of conjunctivitis may include spasm of the eyelids, swelling and redness of the conjunctiva, and discharge from the eyes. The discharge may be clear, mucoid, or mucopurulent. Horses with conjunctivitis may be sensitive to and avoid light.
   
   Conjunctivitis is diagnosed on physical examination. Additional specific tests are performed to identify other eye problems and to rule out other eye diseases that can lead to conjunctivitis. These include a Schirmer tear test, fluorescein dye, intraocular pressure test, bacterial culture and sensitivity, and if indicated, conjunctival cytology. Steps are taken to identify any underlying or accompanying disease situations that contribute to the conjunctival inflammation.
   
   Treatment involves both medicating the conjunctiva and treating any underlying or secondary problems. For example, horses with a foreign body imbedded in the conjunctiva will require sedation and removal of the object.
   
   Depending on the cause of the conjunctivitis, treatment may include topical medications to control inflammation and infection, eye washes to remove discharge, lubricants to add moisture to the eye, and medications to control infection and inflammation. Topical therapy may include antibiotic agents, corticosteroids, or combination medications. All discharge should be flushed from the eye before treatment is attempted to allow the medications to contact the surface of the eye and the conjunctiva.
   
   Prognosis depends on the underlying cause and severity of the condition. Simple bacterial conjunctivitis is typically very responsive to treatment with the appropriate antibacterial medications. Secondary conjunctivitis may not respond until the underlying cause is identified and treated. Some secondary conjunctivitis problems may be controlled, but not totally eliminated.
    
2. Trauma to the Conjunctiva: Injuries that only involve the conjunctiva usually do not require surgical closure unless extensive damage has occurred. Small foreign objects can be flushed out of the eye with saline. The eye can also be protected with topical antibiotic/anti-inflammatory ointments.

1. Trauma to the Cornea (abrasions and lacerations): Any animal can suffer from a traumatic corneal abrasion, scratch, or laceration. Mild injuries can result in a superficial abrasion. Deeper wounds are identified as penetrating or perforating. A penetrating laceration involves the cornea, but does not pass through it. A perforation goes entirely through the cornea.
   
   If wounds are caused by a penetrating or perforating object, the object may still be in the cornea. Wounds can be caused by any object that can contact the eye. Common injuries occur as a result of horses being hit by pellets from guns, scratched by wire, sticks, or coarse hay, and running through tree branches and weeds. Blunt trauma from being hit or kicked by another horse or running into an immovable object can also lead to lacerations of the cornea.
   
   Signs of a corneal wound include pain and inflammation of the eye. The eyelids may be kept closed and the horse may resist examination. The wound may be visible on the cornea, along with swelling, cloudiness, and hemorrhage. Facial trauma may also be evident. Diagnosis is made by history and physical examination. An immediate diagnosis can be made if the object that caused the wound is still in the eye. Additional testing is done to determine the severity and extent of the wound, assess vision, and identify trauma to other portions of the eye and face. Bacterial culture and sensitivity may be necessary. A specific test called a fluorescein dye test is performed using special stain to ascertain if the cornea is intact or if it has been perforated.
   
   Treatment depends on the severity, depth, and age of the wound. Care should be taken to apply only minimal pressure to the face and eye to prevent rupture of the eyeball. Any foreign body should be flushed out, removed with a dampened cotton-tipped applicator, removed with forceps, or surgically removed from the eye. Superficial wounds can be treated with topical medications to prevent infections and calm the eye. Topical eye medications include antibiotics and atropine.
   
   Some lacerations may require additional therapy with a soft contact lens. Extensive corneal lacerations and corneal penetrations require surgical repair. Surgical repair will also be needed for those injuries that contain a foreign body that is not easily removed. All corneal wounds should be rechecked at specific intervals to monitor healing. Superficial wounds can be checked at 3 day intervals; deep wounds should be checked daily for the first several days.
   
   Prognosis depends on the extent of the wound and the time elapsed between injury and veterinary treatment. The deeper the wound, the poorer the prognosis. Most superficial corneal abrasions and wounds heal quickly and satisfactorily. Deeper wounds or those created by blunt trauma may result in permanent loss of vision, inflammation, and pain. The full extent of the damage caused by the wound may not be apparent for days to weeks following the injury, so re-examination is necessary.
    
2. Keratitis: Keratitis refers to inflammation of the cornea. Signs may include swelling (edema), cloudiness, and pigmentation. Keratitis is typically divided into two major categories. This classification is determined by the presence or absence of ulceration of the cornea.
   
   1. Ulcerative Keratitis: This refers to inflammation of the cornea accompanied by ulceration of the surface of the cornea (a corneal ulcer). This is a very common condition in many eye problems. Ulcerative keratitis can be caused by any condition that disrupts the layers of the cornea, including trauma, entropion, ectropion, trichiasis, damage to the nerves of the face, burns caused by chemicals and cleaners, and diseases of the immune system. Infection by bacterial or fungal organisms can occur secondary to other irritations and injuries.
      
      Ulcerative keratitis is a very painful condition. Signs of disease include tearing (lacrimation), spasms of the lids, squinting, spilling of tears onto the face (epiphora), avoidance of sunlight (photophobia), redness, and discharge from the eye. One or more defects in the cornea may be visible and are referred to as corneal ulcers.
      
      Some ulcerative keratitis cases have a mushy, gelatinous appearance as the cornea breaks down due to excessive enzyme production. Chronic ulcers may show vascularization, scarring, pigmentation, and swelling. Healing ulcers may have a hazy, white appearance. Fluorescein dye testing helps to determine the depth, severity, and number of ulcers.
       

      Figure #3: The white arrows identify a circular corneal ulcer (ulcerative keratitis).

      Diagnosis is based on ocular examination and fluorescein dye testing. If needed, specific examination of the interior of the eye and cytology can also aid in the diagnosis. Other tests, such as a Schirmer tear test, are performed to rule out additional or contributory eye diseases.
      
      Treatment involves elimination of the cause, along with specific treatment for the ulceration and inflammation. Underlying eye problems, such as entropion or ectropion, should be treated appropriately. Treatment of the ulcer may include topical antibiotics to prevent infection, topical atropine to control pain, specific medications to control fungal or viral infections, and if indicated, specific medications to prevent collagen breakdown. Horses may be placed on systemic non-steroidal anti-inflammatory (NSAIDs) medications such as phenylbutazone and flunixin meglumine.
      
      Some ulcers are treated with protective contact lenses. Others may require surgery to trim (debride) the ulcer edges. Additional surgical procedures include punctuate keratotomy, conjunctival flaps, and flaps created from the nictitans. Eyes should be rechecked at approximately 3-day intervals; those with deep ulcers should be rechecked daily until satisfactory healing is observed.
      
      The prognosis for ulcerative keratitis depends on the underlying cause, the severity of the ulceration, the type of treatment employed, and response to therapy. Simple, superficial ulcers often heal nicely in approximately 1-2 weeks. Deeper ulcers treated with surgical techniques may require 4-6 weeks to heal; those treated without surgery may take longer, or never heal satisfactorily. Untreated or incorrectly treated corneal ulcers can progress, resulting in rupture of the cornea and loss of vision. This often results in removal of the globe.
       

   2. Bacterial Keratitis: This problem occurs when bacteria invade a corneal ulcer. The bacteria that are commonly involved are Pseudomonas, Enterobacter spp., Streptococcus spp., and Staphylococcus spp. Some of these organisms, particularly Pseudomonas, can cause extensive damage to the cornea to the point that the outer layers of the ulcer seem to "melt" away. In as little as 24-48 hours, a corneal lesion can progress to become corneal perforation. All of the same signs that are associated with routine corneal ulcers can be present.
      
      Aggressive therapy is required to properly treat one of these infections. A culture and sensitivity of the cornea and ulcer should be performed. Initially, the ulcer can be treated each hour with a concentrated solution of gentamicin ophthalmic solution. Atropine is also given to help reduce the pain. Treatments using cefazolin or tobramycin can also be used. Ideally, all antibiotic decisions should be based on culture and sensitivity results. Solutions that contain acetylcysteine, disodium EDTA, or serum can also be given to help reduce the destructive or "melting" nature of the ulcer. Flunixin meglumine is also given to help reduce the inflammation in the eye.
       
   3. Fungal Keratitis: This problem occurs when fungal agents invade a corneal ulcer. The fungi that are commonly involved are Aspergillus, Penicillin spp., Phycomyces spp., Mucor spp., and Fusarium spp. In many cases of eyes infected with fungal keratitis, there is a history of having an ulcer that does not seem to heal despite proper antibiotic or anti-inflammatory treatments.
      
      Diagnosing fungal keratitis can be accomplished by identifying fungal or yeast components from a scraping or biopsy. A culture of the ulcer can also identify fungal involvement. Certain cases can be difficult to identify. Treatment usually involves antifungal eye treatments applied every 2 hours for several weeks. Products that contain natamycin or miconazole are commonly used. Antibiotics, atropine, and flunixin meglumine are also commonly given. Most eyes with fungal keratitis have a guarded to poor prognosis.
       

      Figure #4: This is an example of fungal keratitis causing an ulcer and conjunctivitis.

1. Cataract: Cataracts refer to either opacities within a lens or an entire lens that is opaque. The term cataract means "to break down," referring to the loss of normal architecture of the fibers in the lens or its capsule, resulting in an increase of fluid in the lens and loss of transparency. In horses, most cataracts are caused by something else causing inflammation in the eye such as uveitis. Occasionally, foals are born with cataracts. Horses over 20 years of age commonly develop lenticular (nuclear) sclerosis.
   
    

   Figure #5: This eye has a cataract and some mild conjunctivitis.

   Diagnosis is made by examination of the eyes. Cataracts can be classified by location in the lens and degree of development. Examination of the retina should accompany any examination for cataracts.
   
   Treatment is reserved for cataracts that cause blindness. There is no effective medical therapy. Treatment involves surgical removal of the lens. The lens can be replaced with a prosthetic lens if desired. Although several techniques can be used, phacoemulsification is commonly used today. This technique involves the use of a small ultrasonic probe that is placed into the eye. It shatters the cataract and then removes the broken-down debris by suction.
    

2. Lenticular Sclerosis: Lenticular sclerosis, also called nuclear sclerosis, is a normal aging change in older horses. It is commonly seen in horses 20 years of age or older. As the horse ages, the lens thickens and becomes less flexible. This thickening creates haziness in the eye that can be confused with cataract formation. Light will pass through a sclerotic lens; the cloudiness does not obscure vision. Nuclear sclerosis can be differentiated from a true cataract on ocular examination. No treatment is necessary.
    
3. Luxation and Subluxation of the Lens: This occurs if the lens separates from the zonules that hold it in position. A partial separation results in a subluxation of the lens, leaving the lens either in or near its normal position. A total luxation results from a complete rupture of the zonules and total separation of the lens. The lens will then displace either into the anterior or posterior chamber. This condition can occur either by itself or secondary to intraocular problems including uveitis, glaucoma, neoplasia, or trauma.
   
   Signs of lens subluxation or luxation vary depending on the degree of lens movement and its location. A subluxation may not result in signs that are noticed by the owner. Luxation may result in a painful eye with photophobia, edema of the cornea, cloudiness of the cornea, and redness. Examination of the eye reveals changes in the depth of the anterior chamber of the eye, iridodonesis (the iris moves rapidly or trembles when the eye is moved), and an aphakic crescent. An aphakic crescent is a crescent-shaped area of the pupil that is lacking the lens. This occurs because the luxated lens is no longer sitting directly behind the pupil, but has shifted its position. Other eye conditions, such as glaucoma or uveitis, may be present.
   
   Diagnosis is based on ocular examination. The anterior chamber will be abnormally deep or shallow and the aphakic crescent will be present. Iridodonesis will be observed. Additional tests for ocular disease may be performed, including pressure tests for glaucoma and dilation of the eye to check for other abnormalities. Because lens luxation can occur in both eyes, the "normal" eye should also be examined.
   
   Treatment depends on the location and displacement of the lens. Subluxations may not require any treatment, except for continual monitoring. Subluxated lenses can progress to total luxations or contribute to glaucoma. Lenses that have displaced into the posterior chamber of the eye may be managed with topical medications, such as 0.125% demecarium bromide, that keep the pupil contracted.
   
   The majority of luxated lens are treated with surgical removal. If surgery is delayed, topical anti-inflammatory medications, such as 0.1% dexamethasone, are indicated. Medications to reduce intraocular pressure and prevent glaucoma, such as dichlorphenamide (a carbonic anhydrase inhibitor), can also be given. Surgery should be performed as soon as possible to prevent loss of vision. If accompanying glaucoma has resulted in blindness or a neoplasia is present, treatment may involve removal of the eye.
   
   Prognosis depends on the degree of displacement and location of the lens, accompanying ocular disease, and time between onset of the problem and treatment. The best prognosis is reserved for those cases that are treated early after onset with surgery and do not have accompanying glaucoma. The poorest prognosis accompanies those cases with glaucoma and a delay between onset and treatment.
   

Because the uvea is highly vascular, it is very reactive to changes in the body and is easily inflamed. Inflammation of the uvea is called uveitis. Specifically, inflammation of the iris and ciliary body is termed anterior uveitis. Posterior uveitis refers to inflammation of the choroid. Inflammation may be limited to only the anterior or posterior uvea, or involve both portions. Inflammation of the uvea allows particles to cross the blood-aqueous barrier and enter the aqueous humor. This causes an inflammatory response in the aqueous which can lead to a reduction or total loss of vision.

1. Equine Ocular Thelaziasis (eye worms): Thelaziasis is caused by Thelazia parasites. In horses, this problem often goes unnoticed, with many horses showing no signs. The infective Thelazia larvae are deposited near the eyes by flies (face flies) that feed on the moist discharge from the eyes. Once the larvae are deposited by the flies, they migrate into the tissues of the eye such as the conjunctival sac or the lacrimal ducts and glands. Once in these areas, the larvae mature to the adult stages and start the cycle all over again. Conjunctivitis, keratitis, and sensitivity to light (photophobia) are common clinical signs. Diagnosis of this problem can be done by directly observing the adult worms in the conjunctival sac or the lacrimal ducts and glands. Treatment for this infection involves manually removing the adult worms with forceps and saline flushes after an eye anesthetic is administered. Daily treatments with fenbendazole (10 mg/kg for 5 days) has been used to kill the worms. Proper fly control methods should also be implemented. Studies indicate that ivermectin is probably not effective.
    
2. Equine Ocular Onchocerciasis: This eye disease is caused by the parasite Onchocerca cervicalis. Infections most often occur in adult animals. The Onchocerca parasites cause eye problems when the microfilaria larvae of the parasite migrate through the body tissues and eye. The migration and then death of the microfilaria in the eye causes inflammation and an immune response. This causes conjunctivitis and keratoconjunctivitis. Uveitis and other ocular changes can also be noticed. To diagnose this problem, samples of the conjunctiva or cornea must be collected and examined for evidence of microfilaria organisms. The initial treatment for this condition is to get the inflammation in the eye under control. This usually involves corticosteroids that are given topically or systemically and anti-inflammatory agents such as phenylbutazone or flunixine meglumine. After the inflammation is under control, then treatment to kill the microfilaria is started. The typical treatment includes oral ivermectin at 0.2 mg/kg.
    
3. Equine Ocular Habronemiasis: This eye disease is caused by the migrating larvae from Habronema or Draschia stomach worms. The larvae from these stomach worms are deposited near the eyes by flies that feed on the moist discharge from the eyes. Once the larvae are deposited by the flies, they migrate into the tissues of the eye causing inflammation. The larvae cause raised, non-healing lesions on the corner of the eye nearest the nasal passages. These lesions have small, yellow nodules (sometimes called "sulfur granules") that seem to bleed easily. Changes and swelling can also occur to the cornea and surrounding structures. These infections occur more commonly in the summer months when fly numbers are the highest. Diagnosis of this problem is commonly done by simply examining the lesions directly, and/or examining samples taken from the affected tissues with a microscope. Oral ivermectin at 0.2 mg/kg of body weight is the treatment.
   
    

   Figure #7: This eye has been infected with ocular habronemiasis. Notice the lesions in the corner of the eye identified by the white arrow.

    

4. Equine Viral Atteritis (EVA): EVA infections can sometimes cause discharge from the eye, swelling of the eyelids and surrounding structures, corneal changes, and avoidance of light (photophobia).
    
5. Bacterial infections of Salmonella, Rhodococcus equi, and Streptococcus equi can also cause inflammation to various parts of the eye.
    
6. Viral infections of adenovirus and equine herpesvirus infections can also cause infections in the eye.

* Many of the horse specific eye diseases were referenced from Moore CP: Diseases of the Eye. In Smith BP: Large Animal Internal Medicine, 1996 Mosby-Year Book, Inc., pages 1353-1401.

Cancers

Introduction: Cancers may develop in any structure in the eye. They can be found in animals of any age, but are generally more common in older animals. A cancer may be primary and arise from tissue in the eye, or be secondary to a cancer somewhere else in the body. Cancers located anywhere else in the body may migrate (metastasize) to the eye. Tumors may be localized nodules or locally invasive. In addition to occupying space, tumors may cause infection and inflammation of involved tissues. Common eye tumors include adenocarcinomas, papillomas, equine sarcoids, squamous cell carcinomas, melanomas, and lymphosarcomas.

A mass in or near the eye will cause signs that reflect the involved area of the eye. For example, animals that have a mass on the lid or conjunctiva will demonstrate signs of blepharitis and conjunctivitis. A mass located on the retina may cause blindness and a dilated pupil. Signs associated with eye disease are not specific to one diagnosis, so ocular examination and diagnostic tests are necessary for proper diagnosis. Diagnostic tests may include special examination, biopsy, and histopathology. Microscopic examination will allow differentiation between inflammation, benign tumors, and malignant tumors. Treatment depends on the type and location of the growth and may include surgical removal, chemotherapy, and removal of the eye. Prognosis is dependent of the location and type of growth. Early identification and removal of malignant tumors will increase the probability of a successful outcome and reduce the risk of tumor spread.

1. Equine Sarcoid: Sarcoids are nonmalignant tumors that commonly occur on the eyelids. They can appear as smooth firm masses or be ulcerated and irregular. They usually do not spread to other areas of the body and do not go away on their own. There are many different ways of treating these tumors, all with various levels of success. Details about the various treatment options for sarcoids can be found on page E770 of the equine manual.

Figure #8: This is a case of equine sarcoids surrounding the eye.

 

Figure #9: This is a picture of squamous cell carcinoma. Most of the tumor is associated with the cornea.

 

Figure #10: The dark area on the left side of this eye is a melanoma tumor that is spreading across the eye.

 

Figure #11: Another example of squamous cell carcinoma.

 

Figure #12: A third example of squamous cell carcinoma.

* All of the previous pictures were used with permission from Colorado State University Ophthalmology Service.

Emergency Treatment for Eye Injuries: When a horse suffers any type of eye injury, it is important to evaluate the injury accurately and seek veterinarian attention quickly. With very few exceptions, all eye injuries should be examined by a veterinarian. Following are a few suggestions that can be implemented before veterinary attention is given.

Step #1: Gently examine the eye and surrounding structures. Many horses will be very resistant to having an injured eye examined. This may mean sedation by a veterinarian before a complete exam can be done. For these horses, simply look at the eye without handling it. If the horse will allow, a more extensive examination of the injury can be done. Identify if the eye itself has been injured or if structures around the eye have been damaged. There are different treatment steps that should be taken depending on the type of injury.

Step #2: Clean and protect the eye from additional injury. If only the surface of the eye has been injured, it may be difficult to see any problems on the eye itself. However, the horse may squint, there may be discharge (tearing), and the horse will not want to have the eye touched. Even though the eye may appear normal, it should still be examined by a veterinarian. For these types of injuries, the horse should be allowed to keep the eye closed. A fly mask can be gently placed over the head to help protect the eye and prevent additional irritation from flying insects.

If the eyelid or surrounding structures have been injured or lacerated in some way, the wound should be cleaned. A very dilute solution of betadine (1 part povidone iodine solution to 50 parts water) can be used to gently irrigate the injured area. For lid lacerations and other full thickness cuts or abrasions, it may help to try and cover the wound. A bandage made of betadine soaked gauze can be placed over the injured area or the entire eye, if necessary. The gauze should be soaked with diluted betadine and then taped into position. It is often helpful to not soak the edges of the gauze. This gives a dry place to help anchor the tape. The gauze and the injured area should not be allowed to dry. Every few minutes, additional diluted betadine can be dripped onto the gauze. Not allowing the edges to dry will greatly help the veterinarian when it comes time to suture things back together. It is also helpful not to cut or remove any lose tags of skin. These may be needed when closing the wound.

For mild cuts or abrasions that do not completely go through the skin, the wound should be cleaned and antiseptic ointment can be placed on the wound. The area can then be covered with a bandage. This time, however, the gauze should not be soaked in betadine. Care should be taken to ensure that none of the ointment enters the eye.

For eyes that seem to swell and are badly bruised, cold ice packs can be used to reduce the swelling. To help prevent additional tissue injury, the surface of the ice pack should not be placed directly onto the wound. The ice pack should first be wrapped in sterile gauze or a clean cloth and then applied to the area. The ice pack should not be left in contact with the skin for longer than 15-20 minutes at a time.

Step #3: All injuries that involve some or all of the eye should be examined by a veterinarian. Most injuries will heal well if they are given prompt and proper treatment.

Eye Medications: Eye medications can be delivered by several methods. Topical medications are applied directly to the eye surface. The topical medications may be available as eye drops and ointments. This method of administration is appropriate for both hospital and home treatment of eye diseases in horses. In addition, veterinarians may administer medications via injection into the eye. Common sites for these injections are subconjunctival (beneath the conjunctiva), retrobulbar (behind the eye), or intraocular (into the eyeball).

In addition, diseases of the eye may be treated with medications that are given directly to the horse, either by mouth or by injection. Finally, eye diseases may not be limited to the eyes; they may be a sign of disease that is affecting the entire body. In this case, the veterinarian will prescribe medication to treat the primary illness, as well as to control the problems in the eyes.

The following tables list commonly used eye medications. Depending on the combination of related eye problems present at one time, a specific medication may need to be combined with other medications or be inappropriate for its original, intended use. All eye medications should be used under the guidance of a veterinarian. Page B224 of this manual has information on how to properly clean the eye and administer products.

Summary: The eye is a complex structure that processes images for transfer to the brain. It is composed of several interrelated structures. A problem that affects any portion of the eye can result in loss of vision. A problem that affects one portion of the eye may also affect adjacent structures. Because different disease processes can cause the same signs in the eye, examination by a veterinarian is necessary for proper diagnosis and treatment. Prompt examination and treatment can prevent severe, progressive disease and loss of vision. Animals should be examined by the veterinarian at the first sign of any eye discomfort or abnormality.