IF A PUPPY'S PARENTS ARE CLEARED OF EYE DISORDERS DOES THAT GUARANTEE THAT THE DOG WILL NOT DEVELOP EYE DISORDERS?

No. The screening process for eye disorders can only determine "phenotype" (genetic expression) but cannot determine "genotype" (actual genetic make-up). Because disorders like RD and gPRA are results of recessive genes, it is possible that a dog may carry the recessive gene without expressing it. In such a case, the recessive gene for the disorder is present but "masked" by the dominant gene for normal eye structure and function.

Such a dog would be called "a carrier" of the disorder. To be affected by the disorder, the dog must receive a recessive gene from both parents. For these reasons, eye disorders attributed to recessive genes are often difficult to completely eradicate from the gene pool because "a carrier" may be bred many times before it is realized that the dog does indeed carry the disorder as shown by afflicted offspring.

In order for the disorder to affect offspring, the mate must also be a carrier. Over the years, a carrier will produce many more carriers. The more carriers in the gene pool, the greater the risk of producing offspring which will be afflicted by the disorder. In a litter born of screened parents, there is a 0-50% risk (0-5 out of 10 puppies) of the litter being carriers of a recessive disorder such as RD; there is a 0-25% risk (0-2 out of 10 puppies) of the litter will actually develop an eye disorder such as RD. For these reasons it is recommended that dogs suspected of being carriers through expression of the disorder in offspring no longer be allowed to breed.

Dominant genes are more easily eradicated from the gene pool because individuals carrying the gene will always express it. However, one complication lies in the fact that some dominant disorders are not observed until later in life, and an individual may be bred prior to discovery of the disorder.

Once a hereditary eye disorder has been diagnosed, eliminating the affected individual from the breeding program and alerting owners of offspring to the potential risk may help in preventing future generations of developing hereditary eye disorders.

What is glaucoma?

Glaucoma causes the animal significant pain and vision loss usually before it is detected by the owner. This disease is still being investigated by the Siberian Husky Club of America to determine it's significance in the breed. They have not yet (as of February 2006) recognized it as a hereditary disease in the Siberian Husky.

Glaucoma is a leading cause of blindness in dogs. It is the result of increased fluid pressure within the eye (elevated intraocular pressure or IOP). If the pressure can not be reduced, there will be permanent damage to the retina and optic nerve resulting in visual impairment. Complete blindness can occur within 24 hours if the IOP is extremely elevated or can occur slowly over weeks or months if the the elevation is mild. Glaucoma is usually very painful.
Glaucoma may be primary (inherited) or secondary to a number of eye disorders including luxation of the lens, tumours of the eye, and uveitis (inflammation of the eye).

Primary/inherited glaucoma causes an elevation of pressure within the eye because of abnormal drainage of fluid through the iridocorneal angle. When the angle at which the iris and cornea join is wide, the glaucoma is classified as open angle. If the base of the iris is pushed forward, the glaucoma is described as narrow angle.

What is progressive retinal atrophy?

Progressive Retinal Atrophy (PRA)

PRA is the most common disorder affecting the retina of the dog and is a result of the reduction of retinal blood vessels and atrophy of the receptor cells of the retina. There are two types of PRA: generalized-PRA and centralized-PRA. In generalized-PRA, there is overall retinal function loss. Both eyes are affected, though one may be at a more advanced stage than the other. The condition is progressive, as indicated, though the rate of progression varies from breed to breed and individual to individual; however, the end result in all cases is blindness. Dogs afflicted with g-PRA often can only recognize objects immediately in front of them as there is early loss of peripheral vision. Centralized-PRA also affects both eyes and is progressive, however, dogs may retain peripheral vision for several years, but there is an early loss of central vision.

The cells of the retina receive light stimuli from the external environment and
transmit the information to the brain where it is interpreted to become vision. In progressive retinal atrophy (PRA), deterioration of the retinal cells causes blindness.

The retina lines the back of the eye. The inner layer is the neural retina (called simply the retina) which has 9 layers, the outermost of which consists of the photoreceptor cells - the rods and cones. The outer layer of the retina is the retinal pigmented epithelium (RPE). In dogs the retina is not mature until 6 or 7 weeks of age.

The term progressive retinal atrophy covers several types of inherited degeneration (deterioration) of the retina. Sub-classifications of PRA are based on the age at which dogs show signs of the disease and the type of retinal cell which is affected.

Generalized PRA:These diseases affect primarily the photoreceptor cells. Both eyes are similarly affected and dogs eventually become totally blind.

Age of onset: g-PRA: 2-3 years; c-PRA: 4-10 years

Symptoms:

g-PRA: night blindness, "tunnel vision".
c-PRA: gradually failing vision, tendency to collide with stationary objects in dog's path, poor distance vision, slow pupillary reflexes, dilation of pupils even under daylight conditions.

Treatment: Currently there is no successful treatment for slowing down or reversing the degeneration processes associated with either form of PRA. Treatment with vitamin A has been reported to improve vision in dogs in early stages

What are cataracts?

A cataract is any opacity or loss of transparency of the lens of the eye. The opacity may be confined to a small area of the lens or capsule, or it may affect the whole structure. A complete cataract affecting both eyes will result in blindness, whereas small non-progressive cataracts will not interfere with vision. Primary cataracts occur in some breeds; in other breeds the cataract may develop secondarily to another inherited disorder such as progressive retinal atrophy.

Most cataracts are inherited. Non-hereditary cataracts also occur, as a result of other diseases, trauma, toxicity, or metabolic disturbances

How are cataracts inherited?

The genetics have not yet been defined for most affected breeds. In others, the mode of inheritance is autosomal recessive or with incomplete dominance.

What are juvenile cataracts?

According to CERF, the incidence of juvenile cataracts in the breed checked by ACVO veterinarians is around 15-18%. The actual incidence is probably higher as many long time breeders discover the anomaly in young dogs early and never certify them. With a typical juvenile cataract, the dogs vision is not usually substantially affected, and they lead a full, happy, albeit it neutered, life. However, a more aggressive cataract also exists, which progresses quickly and may cause blindness by 2 to 3 years of age.

What is corneal dystrophy?

Corneal dystrophy affects the cornea or the outer transparent portion of the eyeball. In most cases, Siberian Huskies with this disorder have an abnormal collection of lipids in the clear cornea of the eye which results in a hazy or crystalline opacity. Ophthalmologists describe the location of the opacity as anterior, mid, or deep stromal. The Siberian Husky is prone to deep stromal dystrophy which involves triglyceride deposits. Annular dystrophy also occurs and appears as a doughnut shaped opacity in the peripheral cornea. Corneal dystrophy is usually seen in young adult dogs and may affect females more than males. Vision is seldom affected and no effective therapy for the condition exists at this time. Recent genetic tests are suggesting that a recessive gene with variant expression transmits this disorder. Some corneal dystrophy is often not present or detectable until age 4 to 6 years, at which time the dog could easily have produced a few litters and perpetuated the problem. This will prove to be a difficult genetic disease to eliminate from the breed.

More on PRA/CPRA

Progressive retinal atrophy (PRA) and central progressive retinal atrophy (CPRA) affects the retina, the light-sensitive inner lining of the posterior part of the eyeball. The retina contains two types of specialized cells called rods and cones. The rods are necessary for sight in dim light or night light, and the cones are utilized in in bright light vision. The Siberian Husky has a unique type of PRA that is only found in Siberians and man. This type of PRA is called XLPRA (X Linked PRA) since it is transmitted through the "XX" chromosome of the female. It will cause a loss of night vision followed by a loss of day vision, eventually blindness. The recessive gene for XLPRA is situated on the "X" chromosome of the female. Females who inherit a defective gene on the "X" chromosome from one parent and a normal gene on the other "X" chromosome from the other parent, will not be seriously affected. They will be carriers with very subtle retinal defects and no loss of vision. The male puppy from a carrier dam will receive either a defective gene or a normal gene, depending on what chromosome was copied in the DNA replication. If he has the defective gene, the dog will be affected with PRA since males carry an "XY" chromosome. The disease in males can be devastating with loss of vision as early as 5 months of age.