Progressive retinal atrophy (PRA) is the name given to a group of hereditary retinal diseases in dogs. Although there are several classifications of the disease according to the age of onset of the disease and the types of retinal pathology which occur, almost all forms of PRA eventually lead to complete blindness. In some breeds of dogs, such as the Irish Setter and Norwegian Elkhound the disease begins very early in life (as early as 6 weeks of age.) In this type of PRA, there is a problem with the initial development of the rods and cones (the light receptor cells in the retina). The puppies with this disease often show behavioral signs associated with decreased vision as early as 12 weeks of age and may become completely blind by 1 to 2 years of age.
Miniature and Toy Poodles, American and English Cocker Spaniels, Portuguese Water Dogs, and Labrador Retrievers have a form of PRA called progressive rod-cone degeneration (prcd). This is the most widespread hereditary retinal disease leading to blindness in dogs. It usually has a later onset, and affected dogs may have decreased vision or blindness at 4 to 7 years of age. In this type of PRA the rod and cone cells of the retina develop normally but gradually degenerate.
The first sign of most types of PRA is night blindness. This is because the rods (the cells which allow vision in reduced light) degenerate before the cones (the cells which allow vision in the bright light). Often dogs will bump into objects in a dimly lighted room; a room in which a person can see well enough to avoid the object. Gradually dogs with PRA will lose their ability to see in lighted rooms and will go completely blind. They will frequently have dilated pupils. Sometimes owners will notice increased shininess or hyperreflectivity to the back of the eye.
Veterinary ophthalmologists who examine dogs with PRA will see a decrease in the size and number of the retinal blood vessels and a change in the reflectivity of the tapetum (the shinny membrane behind the retina). An important test done by ophthalmologists to diagnose PRA is an electroretinogram (ERG). This test detects the small electrical signals given off by the cells of the retina when they respond to light. The ERG is done by placing a contact lens on the eye and 2 small electrodes on the head. A bright light is then flashed into the eye. If the retina is normal, a distinctive signal is given off by the retina which is amplified and measured on a computer. If the retina is abnormal, the signal will be reduced in amplitude.
In all breeds except Siberian Huskies, PRA is inherited by an autosomal recessive gene. In Huskies it is thought to be a sex linked recessive trait. Since it takes 2 recessive genes (one from the mother and one from the father) in the same puppy to produce a dog with PRA, this inherited disease is difficult to remove from a breed. The PRA gene may be “hidden” in the genome and the disease may not occur in many generations of puppies, only to show up when 2 dogs carrying the PRA gene are bred together. In many breeds, the only way to determine if a dog or bitch carries the PRA gene is to test breed him or her. This involves breeding a known affected dog to a dog that is suspected to be a carrier and having the puppies examined for PRA. This is expensive and time consuming (especially in breeds with late onset PRA).
Fortunately, Drs. Gustavo Aguirre and Gregory Acland at the James A. Baker Institute at Cornell University have localized the gene for prcd in some breeds of dogs to chromosome 9. This has led to the development of a blood test for PRA. The test will be available in the fall of 1998 for Portuguese Water Dogs. Other breeds for which the genetic test will be available in the future include the Labrador Retriever, American Cocker Spaniel, and English Cocker Spaniel. The ability to identify carriers of the gene for PRA by a blood test will be a tremendous aid to dog breeders.