PRA Article

(from the CWCCA Web Site)
About the Cardigan>PRA Article

Progressive Retinal Atrophy
in the Cardigan Welsh Corgi

Dr Simon Petersen-Jones DVetMed PhD DipECVO MRCVS
Department of Small Animal Clinical Sciences
College of Veterinary Medicine
Michigan State University
D-208 Veterinary Medical Center
East Lansing, MI 48824-1314
Tel: (517) 353-3278
Fax: (517) 355-5164 Email: peter315@cvm.msu.edu

Progressive retinal atrophy – PRA- was a term that used to strike terror into the hearts of many dog breeders. How distressing it must have been to know that a beautiful puppy that you had bred was going to go blind, to say nothing of the anguish felt by the new owners of the pup. Until recently the only options available to breeders with PRA in their lines was to either do a test mating to check that a dog that might have PRA in its background was not a carrier of the disease, or to completely avoid lines with any hint of PRA in the background. Neither approach was ideal, test mating is time consuming and expensive and potentially results in PRA-affected animals. Avoiding affected lines could mean that some outstanding animals could not be used and that the available “gene pool” was narrowed, this itself could lead to a different set of problems.

Cardis from across the world suffered from PRA. The problem was first reported in the veterinary press in the early 1970s and we know now that cases occurred in Europe, North America and Australasia. Thanks to the co-operation of breeders from across the world we were able to collect enough blood samples from affected dogs and carriers to allow us to identify the gene mutation that causes PRA in Cardis. We know that PRA in the breed results in a progressive loss of the cells in the retina that register light (photoreceptors). There are two sorts of photoreceptor, called rods and cones. They convert light into electrical messages that are sent to the brain via the optic nerve. The rod photoreceptors are designed to function in dim light and do not register colors. In contrast the cone photoreceptors work in brighter light and are responsible for color vision (and yes, dogs do see in color, although not quite in the same way as humans). PRA in the Cardis affects a gene that works in the rods. The gene defect leads to an abnormal accumulation of material in the rod photoreceptor causing it to die. This affects the rods very early in life, indeed we suspect that they may not even form correctly. The result of this is that affected puppies can be shown to have defective nighttime vision from as early as 6 weeks of age (testing vision at this young age is not easy!). The PRA gene is not normally functional in the cone photoreceptors, so it is surprising that these cells also die. Their death is probably a result of the mass death of the surrounding rod cells. As the cones die the affected dogs also lose their daytime color vision. It takes a while for this to occur and the affected dog is 2 to 3 years of age before he/she has severe problems seeing in bright light. Currently there is no treatment for PRA.

Once we had identified the gene defect that caused PRA in Cardis it was a relatively easy matter to come up with a diagnostic test. This test is capable of distinguishing between affected dogs, carrier dogs and genetically unaffected (normal) dogs. All that is needed is a small DNA sample from a blood sample or a cheek swab, so the test can even be performed on young puppies. We have recently altered our PRA test so that it can be run at a lower cost and can be performed on cheek swabs. We can therefore reduce the price of the test to $35/test. We can send out cheek swab collection packs at a charge of $4/kit (contact details at end of article). The most important time, however, to test dogs is when planning to breed. To ensure that no PRA-affected puppies are born one has to ensure that both parents are tested and that two carriers are not mated. A litter from a carrier x carrier mating will on average have 25% of puppies affected and 50% carriers. Now that a test is available, carriers can be used for breeding, so long as they are not mated to another carrier. This enables those breeders with PRA in their lines to eradicate the problem from their lines over a few generations without losing the good features that their line exhibits. The concept is that by testing prior to breeding they can separate the “good” genes that their dogs have from the “bad” gene (i.e. the PRA gene). We have currently tested nearly 600 dogs and are finding a carrier rate of about 8.5%.

Here at Michigan State we are continuing to study PRA and search for the gene defects that cause the disease in many other breeds of dog. The work is laborious and expensive, but we are confident that we will be able to identify the cause of PRA in other breeds and develop further tests for this distressing disease. If you would like to support the gene searching in other breeds we would welcome any donations to this work and we are currently fundraising to develop a major center for eye research at Michigan State which will look into many other hereditary eye diseases in dogs as well as PRA. If you would like to support the PRA research please send checks made out to Michigan State University and marked PRA research to Dr Petersen-Jones. If you would like to discuss contributing towards the major animal eye research center please contact Dr Petersen-Jones in the first instance.

People wishing to submit samples from Cardigan Welsh corgis for PRA testing can obtain the submission forms via the web page of the CWCCA or by contacting Dr Petersen-Jones.

Test kit for collecting cheek swabs – send check for $4.00 (payable to Michigan State University) to:

Dr Petersen-Jones (Corgi test)
Dept. Small Anim. Clin. Sci., Michigan State University, D-208 VMC,
East Lansing MI 48824-1314

Cheek swab PRA test for corgis - $35.00/test.

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