(Part 2 of a 2-Part Report) - by Wendy Shepard Chisholm, V. M.D.
Progressive Retinal Atrophy
Generalized progressive retinal atrophy is an inherited trait affecting many breeds (Chesapeake Bay Retriever, Labrador Retriever, Golden Retriever, Irish Setter, Toy and Miniature Poodle, Norwegian Elkhound, Samoyed, American Cocker Spaniel, Collie, Miniature Schnauzer, Welsh Corgi and Saluki).3 In the Chesapeake, PRA is recognized as a significant retinal disease. PRA is a degenerative disease of the retina visual cells which leads to blindness. This abnormality may be detected by an electroretinogram before it is clinically apparent. In all breeds studied to date, PRA is recessively inherited.
In the Chesapeake, PRA can develop and be diagnosed in dogs between eight and twelve months of age, with blindness occurring by two and three years. Other affected dogs show ophthalmoscopic lesions at a later age (four to seven). It is possible that two different types of PRA (early onset and late onset) are present in the breed.
PRA develops due to a missing chemical in the retinal cells which interferes with the recycling of chemicals necessary for the retinal cells to function. This leads to eventual retinal nerve cell degeneration. The rod cells are usually involved first. Rods are important for night vision and because of their early involvement with PRA, night blindness is an early indicator of the disease. Affected dogs may become lost in the dark, but function normally under daylight conditions. Eventually the cone cells become involved, leading to a slow loss of day vision. There is no known treatment. The disease is not painful, and most affected, (blind), pets have no problem functioning in a familiar environment.
The disease is inherited in a recessive fashion; therefore many dogs with normal vision, with no signs of PRA, may be carriers. Tom Cox wrote an article for the ACC Bulletin in Jan/Feb. 1992 issue on "Patterns of PRA Inheritance" which I would recommend everyone read. I have also tried to illustrate in Chart II the genetics of the inheritance of PRA. PRA is believed to be a single-gene recessive trait. Therefore, for an animal to develop PRA, both parents have to carry the recessive PRA gene. It is impossible to determine by physical exam if an animal is a carrier, or normal, non-carrier, as they both have the came phenotype (same external characteristic). "Affected dogs should not be used for breeding, nor should the dogs which produced the affected dog (carrier or affected dogs themselves) because the condition is thought to be inherited by a recessive mechanism." 1
Only through breeding trials with PRA affected dogs can carrier dogs be identified. This would be impractical to conduct and for this reason, PRA will prove very difficult to eliminate from purebred dogs.
Several forms of PRA are also present in the Labrador Retriever, each differing in the age of clinical onset. In addition, although uncommon in Labradors in the U.S., Central Progressive Retinal Atrophy (CPRA) is a common cause of retinal degeneration in Labradors in England. As more English labs are imported to the U.S., it is being seen more frequently.1 In CPRA central vision is usually affected primarily and affected dogs have difficulty marking stationary objects. The peripheral retina is affected later, thus dogs are able to see moving objects until late in the course of the disease. Vision will deteriorate but does not necessarily lead to complete blindness.1 This condition can be diagnosed by a veterinary ophthalmologist in dogs between the age of one and one half years to three years, but is usually not noticed by the trainer until four to eight years of age. Some researchers believe the disease is inherited in a dominant fashion (with incomplete penetrance), while others believe it is polygenetic (involving many genes).
This paper would be incomplete without discussing retinal dysplasia. While it is not a common problem in Chesapeakes, several cases have been brought to my attention in the past year. Retinal dysplasia is the most important retinal disease affecting Labrador Retrievers used for hunting and field trial work. Retinal dysplasia is a widespread inherited condition in the Labrador. During CERF clinics in Minnesota, there is a 10 to 20% incidence in the Labradors examined.1 Most dogs have the mild form of the disease. The condition is congenital and may or may not be associated with retinal detachment depending on the extent. (It is relatively uncommon in lines of Labradors used for conformation work.) Many people in my training group (who all own and run Labradors) are continually plagued with this problem. (I also know of at least four Chesapeake field trialers who currently run Labradors as well, so it is important to be aware of this eye disease.)
Retinal dysplasia involves abnormal development of several structures of the visual system. Dogs may be very mildly affected and demonstrate folds in the retina. These are areas where extra retina develops and instead of forming a thin membrane over the back surface of the eye, the extra retina develops into folds. This fold results in a blind spot. Often times the retina is also undernourished and an area of retinal degeneration will occur. Dogs with mild changes (i.e. a few retinal folds), usually have no visual compromise. Subtle changes on the part of the dog, on the positioning of the head while marking a bird, help affected Labradors make use of normal areas of the retina. Larger blind spots may cause dogs to miss a mark or miss stationary objects, while these dogs are able to perceive moving objects with less difficulty.
Labradors with a more severe form of retinal dysplasia may result in blindness due to large areas of retinal folds or degeneration. Retinal detachment can also develop resulting in blindness. The more severe form of retinal dysplasia can occur with retinal separation, cataracts, and eye enlargement in dogs which inherit the gene from both the bitch and stud dogs. These dogs also may suffer from skeletal dysplasia or dwarfism, as the same gene for retinal dysplasia (which works in a dominant fashion for the eyes) cause skeletal dysplasia (in a recessive fashion). 1
Retinal dysplasia in Labradors is the result of a dominant gene.1 Dogs with only a single dose of the genetic information usually develops the mild form of the condition with retinal folds. These folds can be seen early in life (the best time for an ophthalmologist examination is between eight and sixteen weeks of age.) Because retinal dysplasia is a dominant trait for the eyes, a concentrated effort should be made by dog enthusiasts, by careful selection of dogs for breeding, who do not come from lines with the condition and by using dogs who were examined at an early age and found to be clear. Examination of a two year old dog prior to breeding does not necessarily prevent the introduction of this condition into your line of dogs because very small folds noted at an early age can "straighten out" with growth, making the condition clinically impossible to diagnose in the older dog, although the dog has genetic information to produce other dogs with mild to severe forms of retinal dysplasia.1 Most of the dogs that are mildly affected suffer no visual compromises and can make excellent hunting dogs or pets.
Retinal dysplasia in the Chesapeake does not resemble retinal dysplasia in the Labrador Retriever. In the Chesapeake the only component of the disease is the presence of single to multiple folds in the retina. These folds may result in a blind spot. Very uncommonly will large areas of degenerative progress to full retinal detachment (complete blindness), (see Chart 1). The optional time for evaluation of retinal dysplasia is twelve to sixteen weeks. The other component of retinal dysplasia noted in Labradors (cataracts, enlarged eyes and skeletal involvement) are not part of retinal dysplasia which develop in the Chesapeake.1 Dogs with only a few folds have no detectable vision problem while dogs with geographical dysplasia would likely suffer a blind spot.1 A CERF number is not given to a dog with retinal dysplasia, as retinal dysplasia is known to be inherited in many breeds.5
In conclusion, I have tried to summarize the most important eye diseases present in retrievers. I believe the most serious eye problems affecting the Chesapeake Bay Retriever is Progressive Retinal Atrophy. On a personal note, my first experience with PRA came in 1971, when I spent my junior year abroad at Stirling University in Scotland. I bought a beautiful English black Lab pup, named "Skye", and brought her home with me. In 1976, when I was working at the Veterinary Diagnostic Laboratory at the University of New Hampshire, "Skye" developed night blindness. My boss, a veterinary pathologist, examined her and diagnosed PRA. By the time I enrolled in veterinary school in 1 979 at the University of Pennsylvania, "Skye" was completely blind, and was developing cataracts as well. (Skye was a great dog, and adjusted well to blindness.) In 1980 these "mature cataracts" caused glaucoma in both eyes. (Glaucoma is an elevation of intraocular pressure (IOP), a painful condition, which when sustained, causes intraocular damage and blindness). While glaucoma is hereditary in certain breeds, such as the beagle, in this case the elevated IOP occurred due to the clogging of the drainage angle of the eye with the mature cataract particles. For about two years, with the help of veterinary ophthalmologists, we were able to treat the glaucoma medically, with timoptic eye drops, which decrease aqueous fluid production. Eventually, one of Skye's eyes became painful and surgery to remove the affected eye was performed. Skye went on to live to fourteen, with one blind eye. I have no regrets for owning a blind dog for seven years. However, because Progressive Retinal Atrophy is such a serious, hereditary eye disease that leads to blindness, with major complications such as occurred in "Skye", it is in the best interest of our breed to eliminate this debilitating eye disease. Breeders can identify and incorporate into their breeding program as many non-carriers as possible. A study of pedigree can result in this identification of "clear non-carrier" animals, and help the breeder in attempting to eliminate PRA.
CHART I - taken from CERF Publication
Abnormal development of the retina at birth and recognized to have 3 forms:
1) Retinal dysplasia - folds - foci of retinal folding that may be single or multiple
2) Retinal dysplasia - geographic - any irregularly shaped area of abnormal development representing changes not accountable by simple folding
3) Retinal dysplasia - detachment - either of the above described forms of retinal dysplasia associated with separation (detachment) of the retina.
The two latter forms are associated with vision impairment or blindness.
CHART IIThe Genetics of Progressive Retinal Atrophy
Examples of inheritance when normal animals are bred with carriers, and to animals affected with PRA.
R - represents normal, non carrier (dominant gene)
r - represents gene carrying PRA (recessive gene), this gene is expressed as PRA when paired with another recessive gene (r)
RR - represents a normal paired gene, that does not have PRA, or is not carrying PRA
Rr - represents a paired gene that is a carrier of PRA. This animal is clinically normal (shows no signs of PRA) but is carrying the PRA gene. The dominant R gene is masking the expression of the recessive r PRA gene.
rr - represents a paired gene that represents an animal with clinical signs of PRA.
1. Normal male (RR) bred with PRA female (rr)
2. Carrier male (Rr) bred to a carrier female (Rr)
3. Carrier male (Rr) bred to a normal female (RR)
4. Carrier male (Rr) bred to PRA female (rr)
1. Olivero, Dennis: "Eye Problems in Retrievers" Retriever Field Trial News June,1993.
2. Dziezyc, Joan and Brooks, Dennis: "Canine Cataracts" Compendium for Continuing Education Vol. 5, No. 2, February 1993.
3. Peiffer, Robert: "Inherited Ocular Diseases of the Dog and Cat" Compendium for Continuing Education , 152 Vol. 4, No. 2, February 1982.
4. Miller, William and Albert, R.A.: "Canine Entropion" Compendium for Continuing Education Vol. 10, No. 4, April 1988.
5. American College of Veterinary Opthalmology 1992, "Ocular Disorders Proven or Suspected to be Hereditary in Dogs" (a CERF publication).
6. Cox, Tom: "Patterns of PRA Inheritance: A Primer" American Chesapeake Club Bulletin, Jan/Feb 1992.
7. Rubin, L.F.: "Inherited Eye Diseases in Purebred Dogs" Williams & Wilkins, Baltimore 1989.
8. Helper, Lloyd: "Canine Ophthalmology" Lea and Febiger, Philadelphia 1989.