VET CHECK: DISEASES OF THE CANINE EYE

diseases of the canine eye

Most of us tend to take our own eyesight for granted, until it starts failing.

The same goes for the eyes of our dogs. During almost three decades of mushing I did not encounter any eye problems in my dogs. That trouble-free period ended several years ago when three members of my team (one Siberian, two Alaskans) developed problems, all within a short time.

The Siberian shown in the picture contracted blastomycosis, a severe fungal disease that can affect all parts of the body, including the eyes.

One Alaskan was diagnosed with PRA (progressive retinal atrophy), a condition that causes gradual loss of vision. The second Alaskan developed pannus, a chronic inflammation of the cornea that can result in blindness if not treated. This article covers several common eye diseases in dogs, including known causes, symptoms, treatments and prognosis.

Eye diseases can be hereditary, or they may be due to bacteria, viruses, fungi, trauma, tumors, or allergies. Minor irritations may clear up with use of an eyewash or application of an artificial tear solution or ointment, or may resolve untreated. Some conditions must be treated immediately in order to prevent blindness. Many are painful.

Symptoms of a serious eye problem may include increased redness, squinting, drainage, or a change in vision.The structure of the dog’s eye is similar to that of the human eye. Many canine eye disorders are also similar. Due to space limitations this article covers only the most common conditionsBesides easily seen irritation/inflammation in the white parts and membranes of the eyes, cataracts are often easily noticeable. A cataract is a color change (or clouding) of the normally transparent lens. Since the lens is behind the pupil, an advanced cataract may appear as a white pupil. With development of cataracts, vision becomes blurred, gradually diminishes, and ends in blindness.

Cataracts can occur at any age. They can accompany other illnesses (i.e., diabetes mellitus). There are several ways in which to classify cataracts. Two important classifications include age of onset (i.e., congenital, juvenile, senile) and cause (i.e., trauma). Congenital cataracts are present at birth. Juvenile cataracts, also called developmental cataracts usually refer to cataracts that occur after birth but prior to six years of age.

Senile cataracts occur in older dogs. The two most common causes of canine cataracts include inheritance and diabetes mellitus. Unfortunately, Siberian huskies are on the list of breeds affected with inherited cataracts. Their cataract is juvenile in onset and typically starts around one year of age. In any breed, increasing popularity tends to bring out undesirable traits.In older dogs, it is important not to confuse nuclear sclerosis, which is a normal age-related change, with a true cataract. In nuclear sclerosis the interior portion of the lens hardens and becomes somewhat compacted, producing an indistinct grayish-blue haze.

Nuclear sclerosis requires no treatment and does not affect vision substantially.Some concerns have been raised that frequent exposure to microwave ovens, i.e., in house dogs appearing in the kitchen whenever food is being prepared, may cause cataracts, or that they may be caused by the flash of cameras. This has not been proven.Surgery is the only proven treatment for cataracts.

The procedure presently used in dogs is the same as that performed in people. It is a type of ultrasound (called phacoemulsification). An artificial lens implant can be placed in the eye to further improve vision. The success of cataract surgery in dogs – about 90% – is not as high as in people.Another frequently encountered condition is conjunctivitis, an inflammation or infection of the conjunctiva, the tissue layer lining the eyelids, third eyelid, and a portion of the eye itself. The conjunctiva can become irritated from airborne allergens, or plant materials that lodge in the eye. Bacterial conjunctivitis (like pink eye in people) is possible but uncommon. Reduced tear production (or dry eyes) is a common cause of conjunctivitis in older dogs and can be easily diagnosed with a simple test (the Schirmer tear test). Symptoms of conjunctivitis include redness of the eye, sometimes with a discharge.

The discharge is clear if the conjunctivitis is of an allergic nature. With infection or dry eye, the discharge may be yellow or green. The eyelids may stick together when closed, due to the accumulation of “pus.” Conjunctivitis is usually painful. Antibiotic drops or ointment are the usual treatment. Conjunctivitis due to dry eye requires very specific medication (cyclosporine) to stimulate tear production. Cortisone eye preparations (i.e., hydrocortisone or prednisolone) are available for the treatment of allergic conjunctivitis. However, these drugs should not be used if a corneal ulcer is suspected, as cortisone could worsen the ulcer. A special dye (fluorescein) should be applied to the eyes prior to using cortisone, to be sure there is no ulcer present. One of the most common causes of vision loss in dogs is glaucoma, a buildup of fluid in the eye resulting in increased pressure. In glaucoma the fluid in the front chamber of the eye (the aqueous humor), which is continuously produced and drained, can no longer drain efficiently.

This creates an imbalance between inflow and outflow of fluid, leading to a rise in pressure. Excessive pressure damages the ocular tissues (especially the optic nerve), rendering them nonfunctional with the final result of blindness. There are two types of glaucoma – primary and secondary. Primary glaucoma, the most common form in dogs, occurs in the absence of other eye disease.

Primary glaucoma usually has a genetic basis and occurs in several arctic breeds including the Siberian Husky, Alaskan Malamute and Samoyed. Primary glaucoma usually appears in one eye first, followed by the second eye months or years later. Secondary glaucoma occurs in association with another problem, i.e., injury to the eye, or a tumor. The onset of glaucoma can be rapid resulting in a red and painful eye. Other symptoms include a dilated pupil, a cloudy cornea, and vision loss of the affected eye.

The eye will enlarge due to the increased fluid pressure. If the glaucoma is detected within 12-24 hours, it may be possible to restore vision. Once a dog has developed glaucoma in one eye, the other eye should be treated to delay the onset of glaucoma in that eye. If you suspect glaucoma, see your veterinarian or veterinary ophthalmologist immediately. Early treatment is important in order to prevent blindness. Even if it is too late to prevent the loss of vision in one eye, you could save the other eye. Eye drops such as Timoptic and Xalatan are commonly used.

If all treatments fail, the eye may have to be removed (enucleated). This was required for my Siberian in the picture after contracting blastomycosis. After successful treatment, glaucoma suddenly developed in one eye. Vision could not be preserved despite aggressive treatment. Several months after the enucleation the second eye also developed glaucoma. The dog, retired from working in harness, is now completely blind but getting along well. Eye implants are available.

They are costly, of a cosmetic nature only, and do not save the eyesight. Another method to relieve the pressure in an already blind eye is called chemical ablation. With this procedure an antibiotic is injected into the eye under anesthesia. This destroys the fluid producing tissue, thus reducing the pressure and alleviating the pain. Chemical ablation is not always effective but has an estimated success of 95%. This procedure is not appropriate if there is a tumor or infection in the eye.Another condition that may lead to blindness is PRA, or progressive retinal atrophy (a type of retinal degeneration).

This hereditary condition occurs in many breeds, including arctic dogs. Lining the inside of the eye, the retina converts light into signals that travel to the brain via the optic nerve. The brain interprets these signals as vision. The retina contains light-sensitive cells (photoreceptors) called rods and cones, for night vision (black and white) and daytime (color) vision, respectively. In most dogs PRA starts between the ages of one and eight years, progressing slowly over time. The rods are affected first, meaning that night vision is the first to go. PRA is not painful and the eyes often look normal.

The dog may hesitate to walk into a dark area, pointing to night blindness. In later stages of the disease the pupils may appear dilated and there may be a reflection appearing like a light shining from the back of the eye. At this point the dog may walk more slowly in unfamiliar surroundings. In some dogs the lens may appear cloudy, as cataracts can form in association with PRA.Unfortunately, there is no treatment for PRA and no proven way to slow its progression, though oral vitamin supplementation is often suggested. For dogs at risk for PRA, some veterinary ophthalmologists recommend 400 units of vitamin E daily or some other vitamin supplement (i.e., Ocuvite). Vitamin A, zinc and lutein are considered beneficial to the eyes.

If you are already using supplements, check dosages so as not to overdose these supplements. Fortunately, most dogs adapt well to blindness. My dog with PRA, now nine years old, shows no change in behavior or performance in harness, in daylight or darkness. Affected dogs, as well as their siblings and parents, should be spayed or neutered to prevent accidental breeding. A screening eye examination can detect PRA once it is present, and new DNA tests can detect genetically affected dogs long before they develop the disease.

The DNA test requires that a blood sample be sent to a company called Optigen (www.optigen.com). Numerous additional eye problems can complicate life for dogs and those who love them. One is pannus, a chronic corneal inflammation. Pannus is most common in the German Shepherd, but can occur in any breed. Exposure to ultraviolet light and high altitude are important factors in its development. It usually manifests in dogs three years of age or older. One of the first signs is a pink fleshy discoloration in the lower outside corner of each eye. This is due to blood vessels and cells invading the cornea. If left untreated, the entire cornea can become affected. Eventually, the pink coloration will turn to brown (pigment).

Pannus cannot be cured but is usually controlled with treatment, i.e., prednisolone eye drops.Numerous other conditions can affect the canine eye, including retinal dysplasia, retinal detachment, uveitis, small eye, cross-eyes, even eyeworms. Space constraints prevent elaborating on these. Not all eye problems can be prevented, but we can all be observant. Healthy eyes are moist and clear. A veterinarian or veterinary ophthalmologist should check any redness, swelling, squinting or mucus. I remove the little “blobs” that are sometimes present in the corners of the eyes on cool mornings with a clean tissue.

When training on muddy trails, one should be extra vigilant for signs of trouble if the eyes get sputtered with mud. Dogs’ eyes should be protected when using any kind of chemicals on or around the dog. And remember, eye problems can be painful and may progress to blindness.

Karen Gadke, PhD (Health Science) is a clinical study specialist, medical writer, author and lecturer. She has been mushing/racing since 1979. She owns both Siberians and Alaskans.