No matter how large or small, how brave or fearful, how bold or gentle, your dog is at heart a hunter. Dogs descend from wolves, consummate predators of animals as large as bison, and their eyes reflect this ancestry, as well as thousands of years of breeding by humans. In fact, some eye problems in dogs are the result of breeding them for specific traits, such as a flat face.
How Dogs See
Eyes work much like a camera. Light enters through the pupil (see diagram). The iris, a structure that can expand and contract, controls the amount of light allowed in. Light then passes through the clear cornea and lens, which focus the light on the retina, a light-sensitive layer. This layer contains color-sensitive cones and motion- and light-sensitive rods, all of which convert light into electrical signals. The cones and rods send these signals via the optic nerve to the brain, which constructs an image from them. Dogs have only two types of cones, compared with the three types in human eyes. As a result, dogs don’t distinguish as many colors as do people.
Eyes also contain structures not found in a camera, such as the gel-like vitreous humor that fills the eyeball and gives it shape.
Canine eyes have some structures that human eyes do not. The nictitating membrane is a thin whitish-pink tissue that acts as a third eyelid and protects the eye. The tapetum lucidum is a reflective lining behind the retina; it is what makes dogs’ eyes glow eerily when light hits them.
The visual streak is a horizontal band in the retina right above the optic nerve; this area has the highest concentration of rods and cones and vision is sharpest here. The visual streak varies greatly among breeds, and studies suggest that different breeds see the world differently.* In wolves and in dogs with long heads like wolves, the streak is wide, with the nerves evenly distributed. The shorter a breed’s head, the narrower (more circular) the streak tends to be. Pugs, for example, have a small spot of sharp vision—an “area centralis”—as humans do. Even within breeds, the visual streak can vary from type to type.
These features and others equip the dog to be a good hunter under various light conditions. The tapetum lucidum improves vision in poor light, as does the high proportion of rods to cones. A rod-dense retina also makes dogs excellent at detecting motion and shapes. Because most dogs’ eyes angle slightly to the side, they have a wider field of view than humans. When a wide field of vision combines with a wide visual streak, as in a German Shepherd, the dog can see the whole horizon at once (instead of having to scan the eyes back and forth as humans do).
As hunters of large prey with keen senses of smell and hearing, dogs don’t need to see well close up, and near vision is blurry in long-nosed dogs. (Short-nosed dogs, with their humanlike area centralis, do appear to see well close up. Though the area centralis may lessen their ability as hunters, it may make them better lapdogs, more able to “read” their owners’ faces.) Overall vision is also less sharp.
Common Eye Disorders
Some eye disorders occur more often than others. “As a general practitioner, I was often presented with problems such as conjunctivitis, dry eye and corneal ulcer,” says Christine Lim, DVM, Resident III in Comparative Ophthalmology at the William R. Pritchard Veterinary Medical Teaching Hospital at the University of California–Davis. “Now that I specialize in ophthalmology, I am more often presented with cataracts, glaucoma and retinal disorders.”
Following are a few of the more common canine eye problems. (Note: Some breeds are more prone to eye problems than others, and a mixed-breed with one of those types in the mix could also be affected.)
Conjunctivitis. A condition in which the lining of the eyelids and the front of the sclera (the white of the eye) become inflamed. It can be caused by infection, an object in the eye, an allergic reaction, dry eye, a scratch, or even smoke or dust, and can also be a symptom of other diseases. Treatment depends on the cause.
Dry Eye. A condition in which not enough tears are produced to keep the eyes properly lubricated. Dogs may inherit this condition; among the breeds at higher risk are the American Cocker Spaniel, English Bulldog, Pug, Lhasa Apso, Pekinese, Shih Tzu and West Highland White Terrier. Small, flat-faced dogs sometimes have eyes that bulge so much that their eyelids cannot close, which allows the surface of the eyes to dry out.
Dry eye may also result from an immune system reaction, an injury or a drug side effect. Dryness can be a serious problem because dry eyes are easily irritated and may develop conjunctivitis or corneal ulcers. Artificial tears, good eye hygiene, anti-inflammatory drugs and/or cyclosporine ointment (Optimmune) may help. If the cause is known, the veterinarian treats that as well.
Corneal Ulcer. A slow-healing sore on or in the cornea, accompanied by inflammation. Most ulcers are caused by injuries, and treatment often involves antibiotics. According to Samuel J. Vainisi, DVM, Diplomate, ACVO, of the Animal Eye Clinic in Denmark, Wisc., small dog breeds with very short noses and big eyeballs are more prone to eye injuries. “Because of that, we see a lot of ulcers on the eyes” of breeds such as the Boston Terrier, the Pekinese, and the Shih Tzu.
Cataract. A clouding of the lens that obscures vision. Cataracts are the most common cause of blindness in dogs. Most dogs with cataracts inherited the tendency to develop them. Inherited cataracts can occur in the Afghan Hound, American Cocker Spaniel, Boston Terrier, Chesapeake Bay Retriever, German Shepherd, Golden Retriever, Labrador Retriever, Miniature Schnauzer, Norwegian Buhund, Old English Sheepdog, Schnauzer, Siberian Husky, Staffordshire Bull Terrier, Standard Poodle, Welsh Springer Spaniel and West Highland White Terrier. Diabetes, injuries, poor diet and aging can also lead to cataracts.
Removing the lens allows light to again enter the eye. For best post-surgery vision, the natural lens is usually replaced by a plastic lens. “The surgery itself is not too stressful for the majority of patients,” says Dr. Lim. However, “the first few weeks postoperatively can be stressful because it is very intensive—the patient must wear an Elizabethan collar at all times, and several medications are required.”
Glaucoma. Elevated pressure of the intraocular fluid (fluid inside the eyeball) caused by fluid draining more slowly than it is produced. Glaucoma can damage the retina or optic nerve.
Most often, a dog gets glaucoma because she inherited an eye structure that leads to poor drainage. Breeds in which primary (inherited) glaucoma occurs include the Alaskan Malamute, American Cocker Spaniel, Basset Hound, Beagle, Boston Terrier, Bouvier des Flandres, Chow Chow, Dalmatian, English Cocker Spaniel, English Springer Spaniel, Great Dane, Labrador Retriever, Norwegian Elkhound, Poodle (all sizes), Samoyed, Shar-Pei, Shih Tzu, Siberian Husky and Welsh Springer Spaniel.
Primary glaucoma has no obvious cause and affects both eyes, although one eye may develop glaucoma earlier than the other. Secondary glaucoma is glaucoma that is caused by a dislocated lens, injury, tumor or other problem that decreases fluid drainage in the eye; it may affect just one eye.
Four types of glaucoma occur in dogs. In open-angle glaucoma, pressure builds and damage occurs slowly. The American Cocker Spaniel, Basset Hound, Beagle, Boston Terrier, Miniature Poodle, Miniature Schnauzer and Norwegian Elkhound are prone to this type. Narrow-angle (also called closed-angle) glaucoma is more common. It is an emergency in which glaucoma comes on quickly and painfully and causes serious damage within as little as a few hours. Dogs prone to this type are the Alaskan Malamute, American Cocker Spaniel, Basset Hound, Chow Chow, Dalmatian, English Cocker Spaniel, Fox Terrier, Great Dane, Poodle (all sizes), Samoyed, Siberian Husky and Welsh Springer Spaniel. The third type is goniodysgenesis, in which a ligament in the eye is defective and may cause partial blockage of drainage. The American Cocker Spaniel, Basset Hound, Bouvier des Flandres, Chihuahua, Chow Chow, English Cocker Spaniel, Norwegian Elkhound, Poodle (Toy and Miniature), Samoyed, Siberian Husky and Terrier (some breeds) are among the breeds prone to this type. In pigmentary glaucoma, an excess of pigment cells block drainage. Cairn Terriers are prone to this type.
Glaucoma treatments include surgery, pills, eye drops or (rarely) removal of the eyeball. “Glaucoma is still one of the more difficult things to handle,” says Dr. Vainisi. “Even though there are literally dozens of glaucoma procedures, there still is not that ideal one … even in humans.”
Retinal Disorders. “Progressive retinal atrophy” (PRA) is the name for a group of conditions in which rods and cones die off; there is no treatment. Dogs who get PRA do so because they’ve inherited a defective gene. Although PRA strikes more than 100 breeds of dogs, different genes are responsible. Therefore, breeds differ in the age at which the condition appears, how fast the condition progresses, and the ratio of males to females among affected dogs. PRA appears during puppyhood in the Cardigan Welsh Corgi, Cairn Terrier, Collie, Gordon Setter, Great Dane, Irish Setter, Miniature Schnauzer and Norwegian Elkhound. In contrast, some breeds usually don’t develop PRA until adulthood. These include the American Cocker Spaniel, English Cocker Spaniel, Labrador Retriever, Lhaso Apso, Miniature Poodle, Portuguese Water Dog, Tibetan Spaniel and Tibetan Terrier. PRA occurs mostly in males in the Siberian Husky and Samoyed. Genetic tests for PRA are available for several breeds.
Other retinal problems include detachment of the retina from the back of the eye, inflammation and abnormal development. Causes include infection and injury. Some retinal disorders have no treatment, while others can be helped by surgery or treatment of the cause.
Dr. Vainisi, a pioneer of veterinary retinal surgery, treated movie star Benji for a detached retina in 2004. Another small dog, a Shih Tzu with two detached retinas, was his first case in 1985. “This was the love of her [the owner’s] life, this little dog,” Dr. Vainisi says. He asked the owner to bring the dog to Grand Rounds at the medical school at the University of Illinois at Chicago, where he was a faculty member. None of the ophthalmologists were willing to tackle the case because the retinas were completely detached and the procedure would be very difficult. But afterward, one of the ophthalmology residents volunteered to help. He came to Dr. Vainisi’s clinic the next evening with equipment for human retina surgery borrowed from the university, and they operated. “Within a matter of a couple of days, the dog got his vision back. It was really like a miracle,” Dr. Vainisi says.
It’s no coincidence that both these cases involved small dogs. According to Dr. Vainisi, several small breeds of dogs, including Boston Terriers, Jack Russell Terriers and Shih Tzus, love to pick up toys and shake them hard. “Fluid goes violently back and forth in the back of the eye, and it just rips the retina right off,” he says. “One moment they’re seeing, and the next moment they can be totally blind.”
Time Is of the Essence
The best way to protect your dog’s vision is to catch eye disorders early, when they are most easily treated. A dog with eye or vision problems may paw at or scratch his eye, squint, bump into things, become afraid of the dark, or be frightened in situations that did not frighten him before. The eye may produce discharge, be red, look cloudy or be swollen. The nictitating membrane may partially cover the eye.
If your dog seems to have an eye problem, take her to the veterinarian right away. Your vet may have the knowledge and equipment to diagnose and treat the problem immediately; if not, she may refer your dog to a veterinary ophthalmologist, a specialist in animal eyes and their disorders.
“Veterinary ophthalmologists do a one-year general internship and then a three- to four-year residency with board-certified ophthalmologists, seeing nothing but ophthalmic cases,” says Ellison Bentley, DVM, Diplomate, ACVO, and clinical associate professor of comparative ophthalmology at the University of Wisconsin’s School of Veterinary Medicine. “After completing the residency, they must pass a board exam given by the American College of Veterinary Ophthalmologists to become board certified.”
As in other veterinary specialties, the veterinary ophthalmologists on the leading edge of their discipline are at universities. However, veterinary ophthalmologists who practice in the community keep up-to-date by going to conferences and attending continuing-education seminars.
Only about 300 veterinarians in the United States have board certification from the American College of Veterinary Ophthalmologists. As a result, if your dog needs a veterinary ophthalmologist, you may need to travel to see one. Some, but not all, veterinary ophthalmologists see dogs only by referral.
Despite the dog’s legacy as a hunter, the modern dog doesn’t have to hunt farther than her bowl to find her dinner. So good eyesight is not a necessity for a pet dog; her keen senses of hearing and smell can compensate when vision is impaired. Even so, it’s important that your dog’s eye problems be treated quickly so that she doesn’t suffer pain or develop worse problems. Work with your veterinarian to keep your dog’s vision in the best shape possible.
*P. McGreevey, T. Grassi, A. Harman, “A strong correlation exists between the distribution of retinal ganglion cells and nose length in the dog,” Brain, Behavior and Evolution 63(1):13–22.