The genetic disorder, prcd-PRA, causes cells in the retina at the back of the eye to degenerate and die, even though the cells seem to develop normally early in life. The “rod” cells operate in low light levels and are the first to lose normal function. Night blindness, results. Then the “cone” cells gradually lose their normal function in full light situations.
Most affected dogs will eventually be blind. Typically, the clinical disease is recognised first in early adolescence or early adulthood. Since age at onset of disease varies among breeds, you should read specific information for your dog. Diagnosis of retinal disease can be difficult.
Conditions that seem to be prcd-PRA might instead be another disease and might not be inherited. OptiGen’s genetic test assists in making the diagnosis. It’s important to remember that not all retinal disease is PRA and not all PRA is the prcd form of PRA. Annual eye exams by a veterinary ophthalmologist will build a history of eye health that will help to diagnose disease.
Unfortunately, at this time there is no treatment or cure for PRA. If your dog is affected, you may find it helpful to read about other owners’ experiences living with blind dogs. (Suggested links:www.eyevet.org and www.blinddogs.com)
Prcd-PRA is inherited as a recessive trait. This means a disease gene must be inherited from each parent to cause disease in an offspring. Parents were either “carrier” or affected. A carrier has one disease gene and one normal gene and is termed “heterozygous” for the disease.
A normal dog has no disease gene and is termed “homozygous normal” – both copies of the gene are the same. And a dog with two disease genes is termed “homozygous affected” – both copies of the gene are abnormal.
It’s been proven that all breeds being tested for prcd-PRA have the same disease caused by the same mutated gene. This is so, even though the disease might develop at different ages or with differing severity from one breed to another.
Although prcd-PRA is inherited, it can be avoided in future generations by testing dogs before breeding. Identification of dogs that do not carry disease genes is the key. These “clear” dogs can be bred to any mate – even to a prcd-affected dog which may be a desirable breeding prospect for other reasons.
The chance of producing affected pups from such breedings depends on the certainty of test results. Again, you’ll find the specific information on the certainty of test results for your dog by linking to breed-specific information.
The Genetic Test
The OptiGen prcd test is done on a small sample of blood from the dog. The test analyses the specific DNA mutation causing prcd-PRA. The OptiGen test detects the mutant, abnormal gene copy and the normal gene copy.
The result of the test is a genotype and allows separation of dogs into three groups:
- Normal/Clear (homozygous normal)
- Carrier (heterozygous)
- Affected (homozygous mutant).
Normal / Clear
- Genotype: N / N [Homozygous normal]
- The dog is non-carrier of the mutant gene.
- The dog will never develop prcd – PRA (Progressive Retinal Atrophy ) and therefore it can be bred to any other dog.
- Genotype: N / PRA [ Heterozygous ]
The dog carries one copy of the mutant gene and one copy of the normal gene.
- The dog will never develop prcd – PRA (Progressive Retinal Atrophy ) but since it carries the mutant gene, it can pass it on to its offspring with the probability of 50%.
Carriers should only be bred to clear dogs. Avoid breeding carrier to carrier because 25% of their offspring is expected to be affected (see table above)
- Genotype: PRA / PRA [Homozygous mutant]
- The dog carries two copies of the mutant gene and therefore it will pass the mutant gene to its entire offspring.
- The dog will develop prcd – PRA (Progressive Retinal Atrophy ) and will pass the mutant gene to its entire offspring.