Progressive Retinal Atrophy DNA Testing Frequently Asked Questions
What is Progressive Retinal Atrophy (PRA)?
PRA is a progressive deterioration of the cells of the retina responsible for vision. Because of the progressive nature of the disease, this condition ultimately leads to complete blindness. In the English Springer Spaniel (ESS), the cells responsible for low light vision (the rod cells aka, “rods”) deteriorate first, leading to diminished low light vision. Eventually the cells responsible for bright light vision (cone cells, aka “cones”) deteriorate and bright light vision becomes diminished. Over time as these photoreceptors die off the dog eventually goes completely blind.
What is the difference between Early Onset and Late Onset PRA?
Originally is was thought that ESS affected with PRA would be completely blind by 3-5 years of age, when dogs are still relatively young. When our DNA cord1 test was discovered, we found that majority of the dogs that were DNA affected with this mutation were not blind by the expected age. Instead they were not losing vision until much later in life, at around 8-10 years of age, if at all. We were not aware that DNA affected dogs could retain vision that long, so essentially we learned of a second form of PRA which we now term “Late Onset PRA”. The original form of PRA is now termed “Early Onset PRA”.
If my dog tests affected for the Cord1 Mutation, but remains visual, does that mean the test is wrong?
Because some dogs that have tested as “DNA cord1 affected” retain vision until old age, many breeders have concluded that the DNA cord1 test is not valid in the ESS. This is not a true statement. We need to look at this from a different perspective.
The early form of the PRA has been greatly diminished in the ESS due to diligent screening for the disease through annual CERF (ACVO) eye examinations on our breeding stock over the years. Now we have a form of PRA that becomes a problem much later in the dog’s life, is more insidious in its onset and is not always visibly apparent on the ACVO eye exam. We now believe that the reason we don’t see vision loss or visible changes in the eye may be due to the fact that the dogs just don’t live long enough to show signs of affliction. Perhaps if they lived to the age of 25 yrs, we would see complete blindness, but since their life expectancy is not that long, many never completely lose their vision.
How Can I Use The DNA cord1 Test Results In My Breeding Program?
You should use the test results in the same manner you would use the results of any other health screen we perform on our breeding stock. It is best to look at the pros and cons that any dog has to contribute to a breeding and if the pros out weighs the cons, then use the dog. Keep in mind that PRA is NOT a life threatening disease, so if your dog is affected, but not afflicted, don’t necessarily scrap the dog from your breeding program. Remember, if you breed a DNA affected animal to a clear animal, the worst you will
produce are carriers. On the one hand, this increases the pool of carriers, but this would allow you to retain some of the exceptional qualities of your DNA affected individual, and ultimately move forward. If you then were able to take one of those carriers and breed that to a clear individual, you will further decrease the number of carriers, by hopefully producing some clear (DNA Normal) individuals, as well as more carriers. It is also possible to breed a DNA affected individual to a DNA carrier, the catch is that you could produce some affected puppies as well as carriers in this type of breeding. The ideal approach in this scenario would be to keep one of the carriers and move forward with that individual as described above. The most important thing to keep in mind is that the DNA test allows us to avoid eliminating some of the great dogs that may test DNA cord1 affected, and still continue to strive to eliminate the problem from our breeding program. If we start with an affected individual, it may take a generation or two longer, but we can still eliminate the problem. So don’t “throw the baby out with the bath water”.