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Progressive Retinal Atrophy - PRA

Progressive Retinal Atrophy (PRA) is an inherited condition known to occur in many breeds of dogs. The initial onset of disease occurs at different ages in different breeds of dogs. Dogs with PRA gradually lose eyesight due to degeneration of the retina. There is no treatment for PRA. The retina is a layer of cells in the back of the eye which absorbs light, and sends the appropriate electrical signals to the brain. Two types of cells are involved in light reception, rods and cones. Rods absorb light in low light situations, and are therefore essential to night vision. Cones allow better visual discrimination and color vision (in species with color vision), but require greater levels of light to function.

PRA is characterized by a loss of rod cells initially. The effect of this is that the dog may have, or appear to have, normal vision in daylight, but is cautious and disoriented at night. This disease is sometime referred to as "night blindness." Eventually the cones are also affected, and the dog gradually loses day vision as well. Most PRA affected dogs eventually become blind entirely.

Because of the dog's ability to move around well in familiar surroundings, vision loss is often quite advanced before the owners notice the dog has a problem. Often the vision loss becomes obvious only when the dog is taken to a strange environment.

The eyes have no obvious signs of abnormality, and the condition is not painful. As the disease progresses, the owner may notice the dog's pupils remaining dilated, as the eye attempts to take in more light. The eye may seem to "glow" in the dark. This is due to the retina reflecting light back out through the pupil. As it loses its layers of rods and cones, the underlying tissue allows light to be reflected, instead of absorbed. This is called hyperreflectivity. The eyes may resemble those of a cat, which normally has a very reflective retina.

Diagnosis is made by doing an ophthalmic examination of the eye. The veterinarian looks for the increased reflectivity of the retina, and a decrease in the numbers and size of blood vessels supplying the retina. An electroretinogram (ERG) is a special test which measures the electrical impulses created by the retina. An ERG can be used to diagnose PRA before the characteristic changes are seen on ophthalmic exam. While useful, ERG is usually performed under general anesthesia, and is only available at large veterinary teaching hospitals.

The cause of the retinal degeneration is not yet understood. However, the genetic nature of the condition is well described. Seven different genetic types of PRA are known, but all eventually lead to the same result. All but one is known to be a simple recessive mode of inheritance. This means any affected animal has two parents which are either affected or carriers, and that at least half of an affected dogs littermates are probably carriers. Rod-cone dysplasia-1 (rcd-1) is a type of PRA found in Irish Setters. Rod-cone dysplasia-2 (rcd-2) occurs in collies, rod dysplasia (rd) and early retinal degeneration (erd) occur in Norwegian Elkhounds. Siberian Huskies have a X-linked form of the disease, which means the gene occurs only on the X chromosome.

The most common type of PRA is progressive rod-cone dysplasia (prcd) which is known to affect Labrador retrievers and poodles and possibly as many as sixty other breeds. Italian Greyhounds definitely have PRA in their gene pool, and prcd is believed to be the form of PRA affecting Italian Greyhounds.

While PRA can be easily diagnosed, it poses a dilemma for breeders for two reasons. First, dogs may not develop signs until late in life, making it difficult to avoid using affected animals as breeding stock. No research has been done specifically on IG's, but it appears the age of onset in IG's is 3-4 years. Second, there has not been a good way to diagnose carriers, making it very hard to remove them from the gene pool. Test breeding a suspected carrier to an affected animal will confirm carrier status if affected offspring are produced, but this is not a very practical solution.

In March of 1994, Dr. Gustavo Aguirre and his associates at the Baker Institute for Animal Health, took a giant step forward in the eradication of PRA. They successfully identified the gene causing rcd-1 in Irish Setters, and developed a blood test which can identify dogs with normal DNA, dogs which are carriers, and dogs which are genetically affected with PRA, even though they do not yet show signs. For the first time, a method of identifying both carriers and affected individuals is now available. This testing is being administered through the Canine Eye Registry Foundation (CERF) at Purdue University. Dogs with normal DNA receive a Canine DNA Registry number, and that information is forwarded to the AKC.

At this time, there are no blood tests for the other types of PRA, although the Baker Institute is actively pursuing this line of research. The Baker Institute is one of the few research entities doing research on animal health problems. Many research institutions make progress in animal diseases as they search for solutions to human problems, but the Baker Institute is dedicated solely to animal health.

Little is specifically known about the age of onset and incidence of PRA in Italian Greyhounds. More data is needed in order to form a clear picture of this disease. You can help in this process by having a CERF exam done annually on your dogs.

Having your dog's eyes examined by your regular veterinarian during his annual physical exam is a useful tool in helping maintain your dog's health. The eyes often reveal signs of underlying disease. However, to gather data about, and diagnose, inherited eye diseases in dogs, a yearly exam should be performed by a board certified veterinary ophthalmologist. Ophthalmologists have specialized equipment not found in generalized veterinary practices, and years of extensive training recognizing and diagnosing inherited eye disease. In addition, only veterinary ophthalmologists are permitted to enter data into the CERF database.

Veterinary ophthalmologists often give discount rates for eye exams if they are done for CERF. The advantage of a CERF exam is that a copy of the information about every dog's eyes is sent by the veterinarian to CERF. There it is entered into a large database which contains information about eye disease in dogs. As more IG's have their eyes examined, more data will be available about the diseases present in the breed.

The CERF database is what is known as a "closed" database, which means that only statistics are released, not information about individual dogs. The exception to this lies in the process of registration. At the time of the exam, a copy of the data is also given to the owner. If the owner chooses to send in his/her copy along with the identifying information about the dog, any dog with no major inherited eye abnormalities will receive a registration certificate, good for one year.

The names of dogs whose eyes are registered are released by CERF, and this information is passed on to the AKC where it appears on registration certificates, pedigrees and may now be added to a dog show catalog.

What should you do? Have as many dogs' eyes as possible examined by a veterinary ophthalmologist, using the CERF form. If you do not know of an ophthalmologist near you, your veterinarian or a local veterinary association should be able to refer you to the closest one. Read your premium lists carefully. Eye clinics are sometimes held at all-breed shows, and offer very reduced rates.

Having CERF exams performed will add to our knowledge of inherited eye diseases in dogs. Have the exam repeated annually, or as often as you can. As the onset of PRA is late, a normal exam at one year may only create a false sense of security.

Encourage people who buy dogs from you to have their dog's eyes CERF'd as well. Even if they are not used as breeding stock, they add to our knowledge of the gene pool, and the incidence of inherited eye diseases. CERF publishes an informative pamphlet, Eliminating Heritable Eye Disease in Purebred Dogs. It is available at no charge from CERF at: CERF, 1248 Lynn Hall, Purdue University, W. Lafayette, IN 47907-1248. (765) 494-8179 Fax (765) 494-9981.

Here's what we do know about inherited eye disease in IG's. From January 1991 through December 1994, a total of 343 IG's were entered in the CERF database. Of these, 65 (19%) had one or more eye abnormalities. Not all of these abnormalities may have been considered major, nonetheless, 1 in 5 IG's examined did not have completely normal eyes. There are eye problems in the breed. The more we learn about them and their incidence, the better off we are.

Food for thought. Consider this scenario. You are interested in finding a stud dog to breed to your precious angel. You shop around and select two five year olds you think are compatible. You ask about eye certification. Dog A had a normal CERF exam at a dog show at 18 months. Dog B has a current CERF number, from an exam conducted when he was 4 1/2 years old. Additionally, his parents have current CERF numbers at 8 and 9 years of age respectively. Two of his three littermates have current CERF numbers as well. The third was sold as a pet and was lost to follow up. Which dog would you be more comfortable breeding to? Which is statistically less likely to be a PRA carrier? By the way, what do you know about your bitch and her relatives?


 
 

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