Progressive Retinal Atrophy (PRA)
Progressive Retinal Atrophy, or PRA, is a general term referring to several different genetic diseases. These diseases have one thing in common: they affect a dog's vision by gradual destruction of the retina of the dog's eye. The retina is a membrane that covers the back portion of the eye. It collects light focused through the eye's lens, creates images, and sends them to the brain for interpretation. Think of the retina as a large movie screen, and the lens as a projector. If there are holes and gaps in the screen, the image will not be complete, no matter how good the projector is. As more and more holes and gaps appear in the screen, less of the image is visible. This is what happens with PRA. As the retina deteriorates over time, the dog gradually goes blind.
The retina has two main cell types associated with it: rods, and cones. Each collects light in a different way. Cones respond to bright light, and are valuable for daylight vision. Rods are rod-shaped cells in the retina that are valuable for collecting images in dim light, and for detecting movement. Before domestication, our dogs were hunters, active primarily during the twilight hours of dawn and dusk. They needed to be able to see better during gray, low-light hours, and also at night. Ability to detect motion is also critical in a predator. Because of this specialized need, dog eyes have more rod cells than cones, so any disease which affects rod cells will affect dog vision in a profound way.
Most forms of PRA are specific to rod cells, or affect rod cells first, so "night blindness" may be one of the first signs a dog has PRA. Night blindness occurs when a dog has difficulty getting around in dim light or at night, but still has vision in daylight hours. prcd (Progressive Rod-Cone Degeneration) PRA is the most common form of PRA in Chesapeake Bay Retrievers, and affects both rod cells and cone cells, eventually leading to complete blindness.
Known forms of PRA for which there is a genetic test:
CD - Cone Degeneration Disease is found in German Shorthaired Pointers, Alaskan Malamutes, and Australian Shepherds
Dominant PRA - found in Mastiffs and Bullmastiffs
rcd3 PRA - found in Cardigan Welsh Corgis
rcd2 - found in Collies
GR_PRA1 - PRA found in Golden Retrievers
rcd-1 PRA - early onset PRA found in Irish Setters and Irish Red-And-White Setters
Type A PRA - Miniature Schnauzers
XL-PRA - found in Siberian Huskies and Samoyeds
rcd1a - form of PRA found in Sloughis
prcd-PRA - found in many breeds, including Chesapeake Bay Retrievers
As we can see by this list, while there are several forms of PRA already identified (and some still being researched), there has been only one identified in Chesapeake Bay Retrievers, to date. While the prcd form of PRA is known to occur in the Chessie breed, there is a chance that one or more other forms of PRA may also occur rarely in the breed. For this reason, it is important for breeders to continue to have regular CERF exams done of all breeding stock, even if they have tested Normal with the OptiGen prcd-PRA test.
prcd-PRA is found in several breeds, including several of the gundog breeds. Labrador Retrievers, Golden Retrievers, American & English Cocker Spaniels, and all varieties of Poodle are affected, as well as the Chesapeake Bay Retriever and Nova Scotia Duck Tolling Retriever. prcd-PRA is caused by a simple autosomal recessive gene, meaning that it is not sex-linked, and is caused by only one gene, not several. Click here for a review of basic single-gene genetic disease inheritance.
A test was developed to identify markers for the disease, and released commercially in 1999. Shortly after that time, a revised, more specific marker test was developed. Since these tests did not identify the actual mutation itself, but rather unused "junk" DNA that was believed to be associated with the actual mutation, dogs tested with these first, early versions of the DNA-based PRA test are identified separately from those tested later, with the actual gene-mutation test. Once the gene-mutation test was developed, identification of actual Clear, Carrier, or Affected animals was certain, so terminology was changed to reflect the new status. Dogs in the database are identified as follows A-dog tested as normal with the first version of the marker test. A1-dog tested clear with the second version, and Normal-dog tested with the gene-mutation test. B-dogs tested as carriers with the first marker test, B1-dogs tested as carrier with the second version, and Carrier for those tested with the gene-mutation test. C-dogs tested as affected with the first test, C1-tested with the second test, and Affected-tested with the gene-mutation test.
Most of the dogs tested with the first version of the marker test were re-tested with the improved marker test, and those results were updated in the ACC PRA database. With the advent of the gene-mutation test, dogs are now being directly tested to see if they have the actual mutation that causes prcd-PRA, so no further testing for prcd-PRA is necessary. However, there have been a very few (rare) cases of dogs developing blindness similar to PRA, but that do not have prcd-PRA. This suggests that there may be another, rarer form of PRA in our breed. Research into PRA is ongoing, and testing for any dog that has been given a PRA diagnosis via a CERF exam or an ERG test is free through OptiGen. If you have a dog that has been diagnosed with PRA, please contact OptiGen for information on what type of sample they need, and where to send it.