PRA-prcd

Progressive Retinal Atrophy; progressive rod-cone degeneration

Nearly 1/3 of all poodles tested by Optigen were carriers of a gene that causes blindness on Toy Poodles. Please read the following article carefully.

"A genetic test, offered by OptiGen (www.optigen.com), is used to identify Toys and Miniatures as Pattern A (normal), Pattern B (probably carrier), and Pattern C (probably affected - will go blind at a young age). Within the first 1000 Toys and Miniatures tested by OptiGen, 3-4% are Pattern C and 25-30% are Pattern B. Use of this test for breeding programs is discussed at OptiGen's website. Identification of breeding animals affected with prcd-PRA or carriers of prcd-PRA is essential to avoid producing affected offspring." (The Poodle Club of America)

The Optigen PRA-prcd Test

The OptiGen prcd-PRA test is a DNA-based test that helps you avoid one form of Progressive Retinal Atrophy (PRA). PRA refers to a group of diseases that cause the retina of the eye to degenerate slowly over time. The result is declining vision and eventual blindness. "prcd" stands for "progressive rod-cone degeneration" which is the type of PRA known in several breeds. AFTER reading the information on this page, you can link to information specifically about the breed in which you are interested.

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. 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)

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 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 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. Currently, the test analyzes DNA markers rather than a mutation. (Active research is underway to develop the preferred mutation test.) This means that dogs are identified by a "fingerprint" of markers on chromosome 9 near the prcd gene, rather than by directly detecting the prcd mutation itself. The result of the test is a genotype or pattern - a fingerprint - that allows separation of dogs into three groups: Patterns A, B or C, and for some breeds: Patterns A1, B1 or C1.

The degree of certainty for a test result depends on the specific test developed for a breed. You should read about frequencies of false positive and false negative results at the link to your breed. The possibilities for less than 100% certainty, whether normal, carrier or affected, can be due to frequencies of markers in a specific breed, or due to the estimated rate of recombination of markers or new mutations. While these considerations become fairly technical, we can provide straight forward estimates of "certainty" for each breed. With full information, the reliability of the prcd test in evaluating a particular dog lets you control prcd-PRA in future generations.

Breeding Strategies

This table highlights all the desirable breedings that include at least one parent with Pattern A or A1. All other breedings are at risk of producing pups of Pattern C or C1 with a high probability of developing prcd. 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 dogs with Pattern A or A1 and select against dogs with Pattern C or C1.

Benefits and Limits to All Genetic Testing

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 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.

Additional Points

1. The OptiGen prcd test can be done reliably at any age - even in young pups, and the result will be the same at any age, and will be the same whenever it is repeated. In the future when an improved test is offered that further increases levels of certainty, a repeat with the improved test might be recommended.

2. Since most breeds are affected with other inherited eye diseases, we recommend that yearly eye examinations by a board certified veterinary ophthalmologist be continued, for example, through the CERF program in the U.S. or similar program elsewhere. However, a normal exam in a young dog cannot rule out PRA at a later age. To establish PRA affected status by ERG, it is necessary that a full diagnostic protocol be done with dogs under anesthesia or heavy sedation, and that the rod and cone contributions of the ERG be separately evaluated.

3. Tallies of test results are updated and provided quarterly to national breed clubs, at their request.

A Toy Poodle Is an Investment of About 15 Years.

Please protect this breed by adopting only tested puppies.