Eye Diseases and Your Dog

Although your dog relies heavily on his nose and ears to learn about the world, he does use his eyes to help him get around and to know when he is supposed to bark at the squirrel running along the fence.  So, what happens when your dog’s eyes get sick?  How will you know when he needs to go to the vet?  And what can you do to keep your dog’s eyes healthy?

Normal Canine Eye Structure

Each eye sits in a bony structure of the skull known as an orbit and is made up of three layers.  The outermost layer is fibrous, the middle layer is vascular (made up of blood vessels), while the inner layer is made up of nerves.  The fibrous layer is made up of the sclera or the white of the eye, and the cornea, a transparent cover on the front of the eye.

The middle layer is also called the uveal tract.  It contains the choroid, the ciliary body, and the iris or colored portion of the eye.  In the center of the iris is the pupil which dilates and shrinks to control the amount of light which enters the eyeball.  The lens is right behind the pupil, where it serves to focus light onto the retina.  The inner layer, located at the back of the eyeball is the retina, which sends electrical images through optic nerves to the visual cortex of the brain.

Inside the eyeball, there are three chambers, two containing aqueous humor, a watery substance, and one containing the more gelatinous vitreous humor which helps the eye to keep its shape.  The front or anterior chamber lies between the iris and the cornea, while the middle chamber sits between the iris and the lens.  The posterior chamber is located behind the lens, but in front of the retina.  It is the chamber where the vitreous resides.

Just as in humans, canine eyelids are flaps of skin that extend over the eye to protect it.  The inside portion of the eyelid is covered with the conjunctival membrane, and the eyelashes are attached to the edges of the lid.  As an extra protective measure, dogs have a third eyelid known as the nictitans, which comes up from the inside corner of the eye.  The nictitans contains an auxiliary tear gland which helps the lacrimal gland produce tears and drain them away from the eye.

How does the eye work?

When light is reflected off of an object such as a chew toy, the light goes into the dog’s eyeball through the cornea to the pupil, where they are focused by the lens and sent through the vitreous to the retina.  On the surface of the retina are receptor cells known as rods and cones.  Rods work best in dim light, while cones see bright lights and colors.

Contrary to popular belief, dogs are not color blind.  However, they have very few cones, which means they don’t have a sharply developed sense of color.  In addition, they can only see the primary colors of blue and yellow, not the red, blue, and yellow that humans see.  So, for example, a dog can see green (a combination of blue and yellow), but not orange (a combination of red and yellow).  Dogs do have a much higher concentration of rod cells than humans do, making them much better at seeing in dim light.  That’s why a dog never tries to move the entire coffee table with his pinkie toe like you do in the middle of the night.

What eye diseases are common in dogs?

The most common eye diseases in dogs result from the minor traumas that occur every day when your eyes are so close to the ground.  The conjunctiva may become inflamed, causing conjunctivitis or “pink eye”, the cornea may be scratched which in turn may cause a corneal ulcer or corneal inflammation known as keratitis.  Although you may need veterinary treatment to resolve these problems, they are not generally considered serious health issues.

However, other diseases are quite serious because they may cause your dog to lose his sight.  The most common cause of blindness in dogs is cataracts.  With age, the lens becomes clouded over, preventing light from passing through to the retina.  Injuries and diabetes may cause cataracts, as can genetics.  The only treatment for cataracts is to remove the clouded lens and replace it with an artificial one.

Glaucoma

Under normal conditions, the aqueous humor inside the eyeball circulates freely.  As new intraocular fluid is made, the older fluid drains out of the eye.  However, sometimes the “drain” becomes blocked from inflammation, tumors, or a misplaced lens, causing the pressure inside the eyeball to increase.  The drain may also be blocked by a genetic malformation of the eye structures.  The increased pressure destroys the retina and optic nerve, resulting in blindness.

If your dog begins to squint and rub his eyes, it is vital to get him checked by a veterinarian as soon as possible.  Irreversible damage can be done to the dog’s eyes in a matter of hours.  Medication can be given to reduce the production of aqueous humor, but if that is not effective, surgery to open the drain may be required.

Prolapsed Third Eyelid Gland

When the tear gland of the third eyelid becomes detached from its normal position, it may bulge out, forming a bright red ball in the inner corner of the eye.  The bulging gland is so red it resembles a cherry, giving rise to the more common name of this condition:   Cherry Eye.  It is not particularly painful, but the exposed gland may become irritated or infected.  To prevent the condition from becoming more serious, your vet may perform surgery to put the gland back into place and tack it down.

Cherry eye usually occurs in puppies, and is most common in Cocker Spaniels, Lhasa Apsos, Shih-Tzus, Poodles, Beagles, and Bulldogs.

Diseases involving a dog’s eyelashes and eyelids

The main function of eyelashes is protective.  They are designed to keep foreign bodies out of the eye, protecting the cornea from scratches.  However, there are times when the lashes themselves become foreign bodies.

Distichiasis occurs when eyelashes grow from the ducts of glands inside the eyelids.  These lashes rub the eye’s surface, causing irritation, redness, squinting, and sometimes discharge from the eyes.  The condition is most common in retrievers, spaniels, Poodles, Shih Tzus, and Weimaraners.  Mild cases are treated by placing ointment on the eyes to prevent irritation of the cornea, but more severe cases may require surgery to remove the lashes and destroy the hair follicles so they don’t grow back.

Entropion is very similar in symptoms to distichiasis because the eyelid rolls inward, pushing the eyelashes against the cornea.  It is a genetic condition, common in Shar Peis, Chow Chows, Bulldogs, retrievers, and Rottweilers.  The treatment is surgery for your puppy at 4 to 6 months of age to remove a portion of the skin and muscle around the eyelids, making them align more normally.

Blepharitis is the medical name for an inflammation of the eyelids, most often caused by infection.  The edges of the eyelid become sore and red, while a thick discharge seeps from the eyes.  The disease may be the result of entropion, allergies, flea bites, infections, or disorders of the endocrine or immune systems.  Treatment involves getting rid of the underlying problem or antibiotics if the problem is a bacterial infection.

Common genetic eye diseases

Progressive retinal atrophy (PRA) refers to a condition where the rods and cones of the retina degenerate.  Although there are different types of PRA, nearly all forms of the disease lead to complete blindness.  Irish Setters and Norwegian Elkhounds who have the genetic make-up required for PRA generally show signs of the disease very early as the rods and cones never even begin to develop correctly.

The more common type of PRA for most breeds has a later onset, with dogs starting to show symptoms between four and seven years of age.  Breeds susceptible to progressive rod-cone degeneration (PRCD) are Poodles, Cocker Spaniels, Portuguese Water Dogs, and Labs.  The rods and cones develop normally, but then begin to degenerate as the dog ages.

Rods typically begin to degenerate before cones, meaning that the dog will ose his night vision first.  He may begin to bump into things when he wanders the house at night.  As the disease progresses, he will begin to lose daytime vision as well, eventually going completely blind.

Your vet will do an electroretinogram (ERG) to detect electrical signals given off by the retinal cells when light hits them.  The retina of a dog with PRA will give off weaker electrical signals than a dog with normal eyes.

The gene for PRA is recessive, meaning that two copies of the gene must be present in order for the dog to be affected.  This makes it very difficult to remove the disease from the breeding pool.

How do I find a puppy without inherited eye diseases?

A board-certified veterinary ophthalmologist can perform an examination of prospective breeding dogs to detect many genetic eye diseases before the dogs are bred.  The special examination follows a protocol set forth by the Canine Eye Registration Foundation, and a dog who passes the examination is said to be CERF certified.  Once the dog has been certified free of these diseases, he may be safely bred.

However, because many dogs reach breeding age before some of the genetic eye diseases begin to manifest symptoms, passing a CERF examination is not an absolute guarantee that puppies from CERF-certified parents will be free of all eye diseases.  Nevertheless, it is wise to ask your breeder to show you the CERF certificates from both parents before placing a deposit on a puppy.