PRA – How Much Do We Really Know?

© Bernadette Merchant

Progressive Vision Loss in the Australian Cattle Dog and the Australian Stumpy Tail Cattle Dog – How Much Do We Really Know ?

My name is Bernadette Merchant and I have been involved in the ASTCD for more than 20 years, while maintaining a continuing interest in the health of both breeds.

I have always done annual eye certificate examinations through an ophthalmologist and in 1999 I started asking questions and researching prcd-PRA, the recognised genetic variant responsible for most cases of spontaneous vision loss in the ACD. PRA is a progressive disease which causes total blindness, often in younger dogs. Because I needed clear answers to assist with my own breeding choices in Stumpies (with a much smaller working gene pool), I began sending ASTCD blood samples to Dr Greg Acland at Cornell University in USA in the same year.

My annual eye examinations continued as I began to record the results from all of my related breeding animals. The DNA revolution was about to make a major break through in genetic disease testing for many breeds (including the prcd-PRA test for the Australian Cattle Dog) and with the assistance of Noreen Clark, I was able to gain the co-operation of Dr Greg Acland and others at Optigen in the USA, keen to establish whether the gene pattern responsible was in fact the same for both breeds – as one might reasonably expect.

In 2002 I presented 19 ASTCDs to Dr Bruce Robertson for my annual eye clinic and he diagnosed two dogs as showing classic signs of PRA bilaterally. I was devasted. One would think that with at least 16 years of “clear” eye certificates that you were on the right track.

Blood was drawn from around 20 ATSCDs and sent to Dr Greg Acland at Cornell, seeking to establish whether in fact they showed the same chromosomal aberration that causes PRA in the ACD, already classified as one of the prcd-PRA breeds. The evidence proved conclusive and consequently a prcd-PRA test was released ten months later for the Australian Stumpy Tail Cattle Dog.

We had a DNA test and the means to begin testing for affected and carrier animals, which should have allowed us to gradually eliminate the defect – problem solved, or so we thought.

Despite having a DNA test and 16 years of clear eye certificates, I decided I would continue to do ophthalmologist eye exams every second year, knowing that there is more than one form of eye disease in many dog breeds and the whole purpose of having a national eye certification scheme is to be able monitor new or unexpected developments in any breed.

In 2008 I was notified of a young ASTCD male that could not be affected from the known form of prcd-PRA (because his genetic status was known), who was going blind and was not yet three. A subsequent DNA test confirmed that he was `clear’ for the known form (prcd PRA). A second dog from the same litter has also been diagnosed with this `non-standard or atypical form’ of PRA. Both dogs have donated blood to further research and no dog from this litter has been bred from. In due course, Optigen researchers expect to be able to isolate or identify the DNA code for this unexpected variant – the exception that proves the rule, so to speak.

The DNA revolution is a wonderful tool and of great benefit to breeders but it should not encourage complacency. There are many other vision-threatening conditions that are not always recognized as a potential threat, which is why annual eye certificates are essential (at least for all active stud dogs and breeding bitches) to guard against all those conditions for which there is currently no test available.

There is evidence that the `other’ form of PRA is also shared by the Australian Cattle Dog, just like the known form of prcd-PRA.

I have decided to share this story, just like I shared the story of prcd-PRA more than a decade ago, so that other breeders will be on their guard to protect the breed and the owners of the breed from these devastating conditions. There is no doubt in my mind that our awareness of the possible risk, plus the methodical steps we have undertaken to identify and select against known affected and carrier animals, has greatly strengthened our position as we work to consolidate healthy lines in the ASTCD .

I urge all breeders to be vigilant in the diagnosis of this condition and generous with their support to genetic research. There is no one `silver bullet’ solution – DNA testing for prcd PRA alone is of little real benefit UNLESS breeders also submit their active breeding animals for regular eye certification under the ACES. Eye certificates (at least bi-annually) and the discovery of genetic abnormalities BOTH contribute to the systematic development / refinement of reliable DNA tests for these conditions.

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