For: Nova Scotia Duck Tolling Retrievers
Genetic Registries – Please read below about genetic registries established for Tollers. Additional fees may apply.
Genetic Registries - According to policy of the NSDTR Club of Canada, all Canadian owned and/or registered Tollers that test as Normal/Clear for prcd-PRA by the OptiGen Test for Tollers will be reported to the NSDTR Club of Canada Genetic Registry. Registration of Carrier and Affected results is the option of the owner, who may provide a copy of the test report to the Registry.
- Is there more than one type of PRA in Tollers?
Based on experience to date, there is only one form of PRA in Tollers, the form called progressive rod cone degeneration (prcd). Thus, the DNA mutation test is expected to identify all cases of Toller PRA. Several other breeds have this same type of PRA, however the typical characteristics of the disease can be somewhat different from breed to breed, for example, age of onset, severity, and rate of progression to blindness. Tollers show much variation in each of these characteristics.
- What is the usual age at diagnosis?
Tollers have been diagnosed with PRA over a very wide age range - as young as 3 years and as old as 8 years. The typical age of diagnosis is 4 to 6 years. Some dogs are quite old before the disease is seen. Other dogs might never show signs of PRA even though they are genetically affected. As more dogs are examined, it’s likely that even younger and older dogs will be discovered showing first signs of PRA.
- Are there any proven Toller cases of false positive in this test similar to other breeds?
So far there is no known case of a false positive in Tollers. There is no evidence so far that a Carrier dog might actually be Normal/Clear, nor that an Affected dog might actually be Carrier or even Normal/Clear. This situation is very different than for the initial prcd-PRA test in most other breeds where the rate of false positives was substantial.
In Tollers, both “expressivity” and “penetrance” of PRA play an important role in understanding PRA genetic status.
- What is variable expressivity?
Some diseases are very predictable in terms of age of onset and severity of symptoms. Such a disease is typically “expressed” in the same way in each affected individual. But Toller PRA doesn’t fit this description. It can have different ages of onset, different degrees of severity, and/or different rates of progression within the same line, the same pedigree, or even the same litter. One confusing result of reduced or variable expressivity is that a dog can test Affected, yet show no clinical signs of abnormal vision until much later, or show only mild and slowly progressing clinical signs of the disease. This dog must not be confused with a case of false positive.
- What is penetrance?
The extreme case of reduced expressivity is incomplete penetrance. An inherited disease has incomplete penetrance in cases where the individual is known to have the affected genotype, but never shows the disease. Even so, the clinical disease shows up again in its offspring. Clearly, the affected genes were present in the parent but the disease didn’t “penetrate” to a recognizable state. Again, this case must not be confused with a case of false positive. Toller pedigrees with incomplete penetrance have been documented.
- How frequent is PRA in Tollers?
CERF reports the combined frequency of PRA and PRA-suspicious status in Tollers as 7% of 693 dogs with CERF exam records between 1991-1999. This is a high disease frequency, and might suggest that a large proportion of dogs at risk for PRA are selected for CERF exams. If the CERF frequency is valid, it indicates a high rate of carriers in the Toller population, possibly as high as 40%. This will become clearer as larger numbers of Tollers are OptiGen tested.
- What Tollers were used in the research to develop their prcd-PRA test?
Thirteen families of Tollers, totaling almost 100 dogs, from the U.S., Canada and The Netherlands were studied to develop and validate this test. The test can be used on registered purebred Tollers worldwide.
In February 6, 2002 the Board of Directors of the Nova Scotia Duck Tolling Retriever Club (USA) approved the creation of a registry of US owned Tollers that have been OptiGen prcd-PRA tested & are Normal/Clear. They also approved the release of Normal/Clear results for US owned dogs to the Nova Scotia Duck Tolling Retriever Club of Canada.