PRA

Australian Cattle Dogs – more information:

1. Is there more than one type of PRA in ACDs?
The predominant form of PRA in ACDs is prcd-PRA. There are a couple rare cases that appear to have “typical” PRA but do not test affected with the OptiGen test. They do not appear to be inheriting their disease as a prcd allele. They might have inherited a rare type of PRA that is not prcd. Or, the most likely explanation is the dogs have an acquired (non-inherited) form of retinal degeneration. Acquired retinal disease appears to be frequent in working dogs and could lead to mis-diagnosis of prcd-PRA on a clinical basis.

2. What is the usual age at diagnosis?
ACDs have been diagnosed with prcd-PRA over a very wide age range – as young as 3 years and through 8 years or older. Although the typical age of diagnosis is 4 to 6 years, a dog cannot be considered free of prcd-PRA until at least 8 years of age with a clear eye exam. As more dogs are examined, it’s likely that even younger and older dogs will be discovered showing first signs of prcd-PRA.

3. How frequent is PRA in ACDs?
There isn’t a good estimate of frequency of PRA in ACDs yet, but the carrier frequency is expected to be quite high, possibly approaching 50%. More about this will be learned as testing goes on. Since there could be a high carrier rate, owners should choose to test key dogs in their lines first.

4. Are there any proven cases of false negative in this test?
Two dogs out of 250 research dogs did not show as “affected” Pattern C using the prcd-PRA test for ACDs, even though they appeared to have PRA upon exam by Dr. Acland. Upon further pedigree research, it is believed that these dogs do not have prcd-PRA. If this is accurate, they are not false negative for prcd. Rather, they are “positive” for another disorder and do not have prcd-PRA. They may have another, rare form of PRA or an acquired retinal degenerative disorder. OptiGen will follow-up on any ACDs that do not test as Pattern C but subsequently are diagnosed with PRA.

5. Are there any proven cases of false positive alleles in this test similar to other breeds?
So far there is no known case of a false positive allele in ACDs. There is no experience so far that a Pattern B dog might actually be Pattern A, nor that a Pattern C dog might actually be Pattern B or even A. This situation is very different than for the initial prcd-PRA test in other breeds where the rate of false positives was substantial. In ACDs, the risk of a false positive result is estimated to be less than 0.5%, based on the theoretical possibility of recombination between the prcd gene and the prcd markers. With more extensive testing of new pedigrees, it is possible that the issue of false positives in Pattern B or C might need to be reconsidered.

6. What ACDs were used in the research to develop their prcd-PRA test?
Several lines of ACDs from the U.S., Australia and Europe were studied to develop and validate this test. To the best of our experience so far, the test can be used on purebred, registered ACDs worldwide.

Australian Cattle Dog Links: Australian Cattle Dog Club of America

Genetic Registries – ACDs: According to the policy of the ACDCA, the Orthopedic Foundation for Animals (OFA) will serve as database administrator for the Australian Cattle Dog Health Registry, including the database for prcd-PRA results. OptiGen will send OFA results on all U.S. owned ACDs. OFA will post the results on OFA’s public database. Pattern A results for samples received at OptiGen up to and including July 15, 2002 will be released automatically; OFA will contact the owners of all Pattern B and C dogs to request written permission to release their results. For samples received July 16, 2002 and thereafter, OFA will release automatically all results in accordance with the revised ACDCA registry policy. The $15 fee for this registry must be ADDED to the payment to OptiGen and is passed entirely on to OFA.

· Expected results of breeding strategies using OptiGen mutation tests for inherited recessive diseases
Parent 1 Genotype	Parent 2 Genotype
Parent 1 Genotype	Normal	Carrier	Affected
Normal	All = Normal	1/2 = Normal 1/2 = Carriers	All = Carriers
Carrier	1/2 = Normal 1/2 = Carriers	1/4 = Normal 1/2 = Carriers 1/4 = Affected	1/2 = Carriers 1/2 = Affected
Affected	All = Carriers	1/2 = Carriers 1/2 = Affected	All = Affected

Our breeders are flabbergasted about the amount of B's and C's. This just cannot be true--and the results certainly aren't "occasional!" If these dogs are really B or C, we would have had many more blind dogs or dogs with PRA diagnosis, and we don't. Why?

Indeed, this situation is being reported more often than "occasionally." This most likely means that the false allele is more common than we initially predicted based on results from the pedigrees in our research group. If we could rewrite our literature, knowing what we know now, we'd suggest a more frequent occurrence of the false allele--even though we still can't put a number on that frequency. Part of this situation is due to prcd-PRA being a recessive disease and part is due to the false allele. Consider: Part I, Recessive inheritance of prcd-PRA: If--for example only--the real, true frequency of prcd-PRA in a particular breed were something like 5%. In such a case, the expected frequency of carriers in the population-at-large would be 35%. That is just a fact of inheritance based on calculations of gene frequencies of recessive conditions. Even that number might seem high to breeders. To our knowledge, the real frequency of prcd-PRA isn't known. We have used 5% only as an example to show that even with a low frequency of prcd-PRA, the carrier frequency would be considerably higher. Part II, that ignominious false allele: The false allele is still hidden among the Pattern Bs and Pattern Cs. We don't know what percentage of dogs have false alleles, but it surely inflates the actual numbers of Bs and Cs. Keep in mind, though--the real reason we offered the test in its current form is to identify Pattern As, or genetic normals: those dogs that definitely have no prcd gene; cannot pass on a prcd gene; will never become affected. This provides the foundation for safe breeding of all dogs. Would we be doing breeders a favor by withholding this test? Likewise, the test is valuable in reliably identifying Pattern B dogs that will never become affected with PRA. - 03/31/2000