Blindness (prcd-PRA) in the Australian Cattle Dog
PRA Disease
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 recognized 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.
Inheritance
Prcd-PRA is inherited as a recessive trait. This means a disease gene must be inherited from each parent in order 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.
The Genetic Test
The OptiGen prcd test is done on a small sample of blood from the dog. The test analyzes 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) and Affected (homozygous mutant).
| Possible results using the OptiGen prcd test | |||
| Genotype | Risk Group | Significance For Breeding | Risk of prcd Disease |
| Homozygous Normal | Normal/Clear | Can be bred to any dog, extremely low risk of producing affecteds | Extremely low |
| Heterozygous | Carrier | Should be bred only to Normal/ Clear to remove risk of producing affecteds | Extremely low |
| Homozygous Mutant | Affected | Should be bred only to Normal/ Clear to remove risk of producing affecteds | Very high |
Breeding Strategies for prcd-PRA Test:
This table highlights all the desirable breedings that include at least one Normal/Clear parent. All other breedings are at risk of producing Affected pups with an extremely high probability of developing prcd during their lifetime. However, all dogs can be bred safely. It isn't necessary - or even desirable - to remove dogs from the breeding population. But when choosing pups to retain as potential breeding stock, it is important to select for Normal/Clear dogs and select against Affected dogs.
| Expected results for breeding strategies using the OptiGen prcd test | |||
| Parent 1 Status | Parent 2 Status | ||
| Normal/Clear | Carrier | Affected | |
| Normal/Clear | All = Normal/Clear | 1/2 = Normal/Clear 1/2 = Carrier | All = Carrier |
| Carrier | 1/2 = Normal/Clear 1/2 = Carrier | 1/4 = Normal/Clear 1/2 = Carrier 1/4 = Affected | 1/2 = Carrier 1/2 = Affected |
| Affected | All = Carrier | 1/2 = Carrier 1/2 = Affected | All = Affected |
The benefits of genetic disease testing are clear. With informed breeding practices, breeders immediately can avoid producing affected pups, yet use any dog in their program regardless of genetic disease status. And since genetic testing can be done at any age, each dog’s genetic status can be known before clinical disease signs are recognized. Over several generations of selection away from the disease gene, breeders can even eliminate a disease gene completely from their line.
BUT, there are basic limits for any and all DNA genetic tests. Whether a test is mutation-based or marker-based, it identifies only the specific mutation being tested or the association between a specific marker set and the disease. For example, a mutation test detects one specific mutation in one specific gene. If there are several different mutations or several different genes that can cause the same condition, one must discover and then test for each mutation and each gene. Likewise, a marker test uses one marker or set of markers to define a specific condition. If the condition is associated with several different marker combinations, one must discover and then test for each marker combination. It can be difficult or even impossible to know how many mutations or how many marker sets exist in all the members of a specific breed. As more and more dogs are tested, previously unknown variations may come to light.
In the case of PRA, also keep in mind that not all retinal disease is PRA and not all PRA is the form currently detectable in your breed. Accurate diagnosis is essential. A dog can test as normal or carrier, yet be affected by a different type of PRA. Although more than one type of retinal degeneration probably occurs in every breed, by far the most common type of PRA for your breed is the type currently being tested by OptiGen.
The Society would like to thank Optigen for allowing us to reproduce this information on PRA that is found on their site.