Hereditary diseases of the eye in dogs are common and much more common than in the cat or horse. Some eye diseases are inherited and, as in man, the dog suffers from several inherited eye conditions. Hereditary diseases can be divided into congenital conditions, ie those present at birth, and non-congenital conditions, ie those occurring later in life. It must be understood that not all congenital conditions are inherited and not all hereditary conditions are congenital. Typical of congenital conditions is considerable variation in the degree of severity from animal to animal - some animals are born blind, whereas others only show very minor defects with no apparent effect on sight and no change throughout life. Typical examples of such congenital conditions are collie eye anomaly and multifocal retinal dysplasia. Non congenital conditions may not occur until the animal is adult and sometimes several years of age. Typical of non-congenital conditions is progression so that the conditions starts as a mild problem but often progresses ultimately to total blindness. Examples of such conditions are the progressive retinal atrophies and cataract, although it must be remembered that not all cataracts are inherited.

It should be noted that some hereditary eye diseases are painful, examples being glaucoma and entropion whereas others are not eg progressive retinal atrophy and cataract, although the resulting blindness from these conditions may well make the dog nervous or even afraid. Hereditary eye diseases usually effect both eyes but just occasionally, perhaps surprisingly, only one eye is involved or the two eyes may be affected to differing degrees. Hereditary eye diseases can involve all parts of the eye. Sometimes the whole eye is implicated, eg microphthalmos, but more usually one part of the eye or periorbital region (the eye surrounding the eye), eg eyelid, cornea, iris, lens, retina.

Perhaps the best known hereditary eye disease is PRA (generalised progressive retinal atrophy) in which the retina, the light sensitive membrane at the back of the eye, degenerates from puppyhood during life. The condition is always bilateral and the first sign noticed by the owner is often one of night blindness or poor vision in subdued light, but thus progresses over months or years to total blindness and there is no treatment that will either halt or reverse the degeneration. Breeds affected include the Miniature and Toy Poodle, Lhasa Apso, Tibetan Terrier and Spaniel, English and American Cocker Spaniel, Labrador and Golden Retriever and Irish Setter amongst others.

Perhaps the commonest hereditary eye disease is entropion, or inturning of the eyelid. This affects different breeds in different ways and one, two, three or all four lids may be involved. The inturned eyelid rubs on the cornea, the front window of the eye, and causes irritation, inflammation and pain and ultimately ulceration. Entropion is a condition which can be cured by surgery and breeds affected include the Chow, Bulldog, Shar Pei, Bloodhound, St Bernard and sometimes Retrievers and Setters. Entropion is often related to the size, shape and position of the eye and the amount of skin on the head.

Hereditary cataract affects different breeds in different ways and the form of the cataract is often breed specific. It is usually, but not invariably, bilateral and may, or may not, progress to total blindness. The condition is pain free and cataract operations are frequently performed in the dog. There are many causes of cataract, not all being inherited, but examples of hereditary cataract are to be found in the Boston Terrier, Golden Retriever, Miniature Schnauzer, Norwegian Buhund, Siberian Husky and American Cocker Spaniel.

Another hereditary eye condition affecting the lens is lens luxation, in which, usually in dogs of three to five years of age, the lens in one eye, invariably followed by the second, falls away from its attachment within the eye and sets up a pressure, or secondary glaucoma, which is both painful and blinding. Surgery to remove the dislocated lens can be successful and prevent the formation of glaucoma.

Although some hereditary eye diseases can be treated it is much better to control these diseases and ultimately eradicate them by breeding from sound dogs. With this in mind eye certification schemes for the control of hereditary eye diseases are to be recommended. Breeding animals are examined by a specialist veterinary ophthalmologist and certified free from hereditary eye diseases which affect their breed. This has lead to the reduction of hereditary eye diseases in some breeds, examples being cataracts in the Golden Retriever and Afgan Hound and collie eye anomaly in the Shetland Sheepdog. However, most hereditary eye diseases are due to recessive genes and the carrier state cannot be diagnosed by ophthalmoscopic and slit lamp examination However, the advent of molecular biology and gene testing will, in the future, make eradication of these diseases, with the co-operation of dog breeders and the Kennel Club, a definite possibility.

Eye Anatomy
To review the anatomy of the eye, the eye functions to collect images and pass this information to the retina. The retina are the cells that translate the image to the brain, which creates visual images in the nervous system. The eyelids protect the surface of the eye, especially the cornea, which is the clear outer window of the eye. The ophthalmologist looks through the cornea to evaluate the lens and retina of the eye. The cornea protects the eye and also helps to focus light into the pupil or opening in the iris. The lens is located behind the iris and is important for the fine focusing of light images into the back part of the eye where the retina is located. The canine retina contains mostly rod cells (18:1) and only a few cone cells are present. For this reason researchers believe that most dogs only have limited color vision. However, the ability of the dog to see at night is likely to be superior to that of a person.¹

Inherited eye diseases may be congenital (present at birth), or develop later in life. Generally congenital defects involve structural alterations. Congenital lesions are not necessarily inherited, nor is acquired inherited disease easily distinguishable from non-inherited conditions.³ This distinction is of major concern for the conscientious breeder who has purchased or produced an animal with non-inflammatory ocular disease. Breed, age, history, examination of parents and litter mates, and characteristics of the lesion all help in making this determination. If doubt remains, test breedings may be recommended. In cases in which the distinction is not clear, and the disease is potentially serious in terms of clinical manifestations, this condition should be regarded as hereditary until control test breeding proves otherwise." It is important that one goes beyond treatment of the individual patient to include elimination of animals affected with significant ocular disease and carrier animals from the genetic Pool. Unfortunately, acquired genetic disease may not appear until middle or old age, by which time genetic dispersion has already occurred.

Basic Genetics
To review basic genetics, chromosomes in cells carry genes that program the development of the body. All genes come in pairs. The only exception are the sperm and the egg cells which contain genes not in pairs. In this way, one set of a pair of genes is contributed by the bitch and one set from the stud. Genes come in two basic varieties, the dominant gene and the recessive gene. A recessive gene is one which is "masked" or not expressed when it is paired with a dominant gene. Its genetic code is "covered up" by the other gene. The condition which is coded for by a recessive gene will only develop when both the bitch and the stud contribute the recessive gene. A dominant trait is one which can develop when only one (or both) of the pair of genes is for the trait. Dominant genes "cover up" and mask the expression of a recessive gene.

Mode of inheritance may be recessive or dominant and autosomal (carried on one gene), or polygenetic (carried on multiple genes),and expression of the disease has variable penetrance.

The majority of the documented hereditary conditions are autosomal recessive, resulting in a population of genetically normal animals, normal appearing heterozygous carrier animals, and affected dogs and cats. Carriers are usually recognizable only by test breeding the animal in question with an affected mate and observing the offspring.³

Conjunctivitis
Conjunctivitis is probably the most common non-hereditary eye problem in retrievers. Conjunctivitis is the inflammation of the membrane which lines the inside of the eyelids and also covers the white or sclera of the eye called the conjunctiva. Poor tear production, bacterial or viral infections, irritation from dust and dirt, and allergies can all cause conjunctivitis in dogs. The most common cause in retrievers is exposure to dust and dirt, during performance.

This problem is easily treated with ophthalmic ointments.

Corneal Trauma
Corneal trauma is also a common problem in retrievers. The first aid kit for the working dog should contain an eye wash solution, (a saline solution obtained from your vet), to flush debris from the eye, and a triple antibiotic ointment or solution (without cortisone), to place in the eye every few hours until veterinary care becomes available. There are no eye diseases which can be made worse by placement of saline flush followed by antibiotic drops in the eye. There are many diseases however, which will continue to progress despite this treatment and can lead to complete loss of vision rapidly. For this reason, it is important to seek prompt veterinary attention following the development of signs of eye pain or irritation.

Entropion
Entropion is the medical term which indicates that the eyelids roll inward toward the surface of the eye. This "in-rolling" of one or more eyelids may cause ocular irritation. Entropion can result in corneal ulcers or scarring of the surface of the cornea due to chronic irritation from the hairs of the skin touching the cornea during blinking.

Entropion is most commonly classified as either spastic or congenital.⁴ Spastic entropion is caused by ocular irritation that initiates continued spastic contraction of the orbicular muscle resulting in inversion of the lid margin.⁴ A cycle of irritation and spasm results. If the condition is treated early by correcting the initial problem, surgical correction may be unnecessary. Examples of causes of spastic entropion are distichia, (see below), and trauma to the face. Chronic inflammation such as untreated conjunctivitis can also cause entropion.

The congenital form can develop in pups to young adults. Usually the lower lid is affected. One or both eyes may be involved. Some pups with entropion will eventually grow out of the disease as the head enlarges during the first several months of life. For this reason, primary surgical correction which involves removal of extra skin from the affected lid, is usually not performed until dogs are six months of age or older.¹ If the condition is mild prior to six months, ophthalmic ointment can be used. This softens the hairs and thickens the tear film on the surface of the cornea to reduce the abrasion of the cornea by hairs. Temporary tacking sutures can also be placed in the eyelids of puppies to roll the lids out away from the surface of the eye until it can be determined if surgical correction is indicated.

Congenital entropion occurs in dogs that are conformationally predisposed.⁴ Inherited entropion may appear anytime prior to maturity, whereas mild entropion may improve spontaneously with growth of the pup. The inheritance of entropion is not clear. It is likely that it is influenced by several genes (polygenetic) defining skin and other structures which make up the eyelids, the amount and weight of the skin and other structures which make up the eyelid, the amount and weight of the skin covering the head and face, the orbital contents, and the conformation of the skull.⁵

Because the inheritance is not defined, and entropion does not result in a vision threatening condition, dogs with entropion are not disqualified from obtaining a CERF number. However, entropion must be recognized early in the course of the disease to prevent corneal damage. It is easily correctable by simple surgical procedures if indicated.

Distichlasis
Dictichiasis is the medical term for extra eyelashes which grow from the edge of the eyelid margin. Eyelashes normally grow from the external surface of the eyelid and grow up and away from the surface of the eye. In most cases the anomalous cilia (eyelashes), float in the tear film with minimal irritation, and treatment is unnecessary. In some breeds, the hairs begin to grow from the edge of the eyelid, and if the hairs are stiff or grow towards the cornea, they will irritate the cornea causing excess tearing, chronic conjunctivitis, and at times spastic entropion and corneal ulceration If corneal irritation does result, veterinary intervention is indicated.

Distichia is an inherited disease seen in a many dogs.³ The condition is probably inherited as a dominant trail. It is very common in the Golden Retriever. About 75% of Golden Retrievers examined can be found to have dictichia.¹ In most cases the hairs are thin and soft and curl away from the surface of the eye and should be left alone. In cases where clinical signs are seen, surgical treatment to permanently remove the hair should be considered. Hairs that are simply pulled out usually grow back thicker and stiffer in three to four weeks. Cryosurgery or a freezing procedure is the treatment of choice to destroy the hair follicle and prevent regrowth.

Dictichia is noted occasionally in Chesapeakes. Usually the upper eyelids of both eyes are affected. The inheritance is unclear. As with other eyelid abnormalities in retrievers, it alone will not usually result in CERF disqualification from registration. When diagnosed it should be recorded and breeding discretion is advised.⁵

Cataracts
Cataract is the medical term referring to opacity (cloudiness) - in the lens. it is due to a breakdown of the normal structure. Cataracts can range in size from a barely detectable spot to involving the entire lens. Even small opacities in the lens can result in blurred vision or glare problems.

The lens is an avascular structure whose primary purpose is to help focus light onto the retina. Because of constant growth and compression of the lens epithelial cells, the density and volume of the lens increase with age. The increased density of the lens in older dogs cause the eyes to appear "bluish gray" and hazy. This phenomena is called "nuclear sclerosis" and should not be mistaken for a form of cataract, as the dog will continue to have clear vision. Nuclear sclerosis also occurs in older people, who you may notice have a bluish gray color to their eye.

Cataracts can be classified according to age of onset: congenital, developmental (juvenile), and senile A congenital cataract is present at birth. It may be inherited or teratogenic (a teratogen is any substance that causes abnormalities in the developing fetus). Developmental cataracts appear from birth to about six years of age. Most of these are hereditary. Senile cataracts are seen in old age, and their cause is unknown. Many factors may be involved.

The majority of cataracts seen in dogs under six years of age are inherited cataracts.² Other causes of cataract formation include nutritional, metabolic, toxic, traumatic, and infectious causes, endocrine dysfunction, and intraocular disease. Cataracts have been reported in experimental dogs given diazoxide (an antihypertensive drug) and DMSO (dimethyl sulfoxide). Diabetes mellitus will cause cataracts in dogs. A traumatic incident such as thorns, or twigs penetrating the lens capsule can cause temporary or permanent cataracts.²

The age of onset for the inherited developmental or juvenile cataracts affecting Chesapeake Bay Retrievers is from six months to eight years. Usually the cataracts are first diagnosed in dogs six to eighteen months of age. These cataracts resemble the so-called classic "retriever cataract". This is the triangular-shaped cataract, positioned in the middle of the back part of the lens. It is usually slowly progressive and often remains small enough not to affect vision. The inheritance has been suggested to be by an autosomal dominant gene with incomplete penetrance as reported in other breeds.⁵

In the Labrador Retrievers several inherited cataracts have been described. The juvenile or developmental "retriever cataracts" can be diagnosed at six to eighteen months of age. Again, this cataract is slowly progressive and often remains small enough not to affect vision. This form of cataract is the result of a dominant gene. Most dogs have only one "dose" of the dominant gene. However, if both parents contribute the gene such that the offspring contains two "doses" of the gene, the cataract may progress to involve most of the lens. This results in vision loss and only cataract surgery with lens replacement with an artificial lens will return functional vision to the involved eyes.

Another from of cataract in the Labrador Retriever is a small to medium sized cataract which is very slowly progressive. It first appears in the front part of the lens. This cataract rarely results in loss of vision. It usually develops in dogs five years or older. The pattern of inheritance is currently unknown.¹

Golden Retrievers are also affected with hereditary cataracts. As in Labradors and Chesapeakes, they can develop the "juvenile retriever cataract" which does not result in a vision problem. Or they may be affected with complete corneal progressive cataracts that cause a severe visual deficit. It is suspected that cataracts in Goldens are transmitted as a dominant trait. Animals with triangular cataracts are heterozygous (one dose of the gene) and those with mature cataracts are homozygous (two doses of the gene).² In the Golden the "retriever cataract" is more likely to involve the entire lens and progress to vision loss, thus it appears the gene is more widespread in this breed.

Cataracts due to old age develop in dogs as they do in people. After six or seven years of age the lens of the dog begins to harden. This can result in relative farsightedness and dogs may seem to be out Of focus for close vision. People with hardening of the lens require glasses to read. Eventually the lens becomes so hard that the proteins in the lens begin to deform, take on water and a senile cataract develops. Senile cataracts are not inherited but are the result of aging of the eye. If they become sufficiently advanced, cataract surgery can be done as long as the retina and other structures of the eye are functional.¹

Animals diagnosed with cataracts should not be used for breeding and will not be given a CERF number. "The prudent approach is to assume cataracts to be hereditary except in cases known to be associated with trauma, other causes of ocular inflammation, specific metabolic diseases, persistent pupillary membranes, persistent hyaloid or nutritional deficiencies."⁵