Canine PRA PRA conditions are a known condition to the mastiff breed. P.R.A Progressive Retinal Atrophy | This page gives some general information about progressive retinal atrophy. |
NORMAL VISION | | Here is a very simple explanation of the process by which a dog "sees": | | Light passes through the lens and is directed onto the retina, which contains specialized photoreceptor (light-sensitive) cells called rods and cones. These cells convert the light into electrical nerve signals, which pass along the optic nerve to the brain, where they are "translated" into images. | | ~ rods are responsible for vision in dim light i.e. night vision | | ~ cones are responsible for vision in bright light i.e. daytime and color vision |
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DEFINITION OF PRA | | ~ Progressive | a slowly developing disease process ~ the affected dog will gradually lose its sight and will usually adjust to its handicap | | ~ Retinal | of the retina ~ the light-sensitive area at the back of the eye | | ~ Atrophy | degeneration or deterioration ~ of the specialized light-sensitive cells in the retina |
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 DISEASE PROGRESSION
| | Initially, a dog with PRA will develop "night-blindness" i.e. it will eventually be unable to see in dim light conditions or in the dark. This is due to atrophy, or degeneration, of the rods. The owner may notice that the dog is reluctant to go out in the dark and hesitant to do down stairs in poor light. The dog may also appear to be a little "clumsy" i.e. bumping into things. | | In the later stages of the disease, the cones are affected, and the dog's daytime vision will gradually deteriorate. | | |
EYE TESTING | | The "genetic marker" for PRA has been identified in a number of breeds of dog. A blood sample is sent to a specialist laboratory for testing and the dog can then be classified as normal, a carrier, or affected. Please see OptiGen - prcd-PRA Test - canine genetic testing for a list of the breeds for which blood testing is currently available. | | Unfortunately, many breeds, there is currently no test to determine genetic status for PRA . For the time being, They have to rely on eye testing by a veterinary ophthalmologist. | A "clear" test result simply means that the dog does not have any clinical signs of PRA on the day of the eye examination. | | Eye examination should be carried out by a certified veterinary ophthalmologist. Your dog will be given some eye drops, to dilate the pupils. After about 20 minutes, your dog can be examined. The examination takes place in a darkened room; the vet will look into your dog's eyes using an instrument called an indirect ophthalmoscope. This examination takes just a few minutes, and is a non-invasive and painless procedure for your dog! | | You need to bring your dog's registration documents with you when you have your dog's eyes tested. The veterinary ophthalmologist will provide you with a certificate, valid for one year, detailing his/her findings. |
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INHERITANCE | | Inheritance of PRA in a couple of breeds of dog is known to be sex-linked, and it has a dominant form of inheritance in another couple of breeds. However, in the majority of breeds affected by PRA, the mode of inheritance is known or thought to be autosomal recessive. | | The mode of inheritance of PRA in Glens is believed to be autosomal recessive, which means that the PRA gene has to be inherited from both parents for the dog to be affected. A dog with the PRA gene from only one parent will be a carrier; and a dog with no PRA genes will be normal. | | ~ Affected (pp) | PRA gene from both parents
~ dog will eventually go blind
~ may test "clear" before being tested affected
~ if tested regularly, all affected dogs will eventually be identified | | ~ Carrier (Pp) | PRA gene from only one parent
~ dog will have normal vision and will test "clear" | | ~ Normal (PP) | No PRA genes
~ dog will have normal vision and will test "clear" | | The dominant gene is denoted with a capital letter, in this case P. The recessive gene is then denoted with the same letter, but in lower case. The "dominant" letter goes before the "recessive" letter e.g. Pp represents a carrier i.e. the (dominant) normal gene (P) from one parent and the (recessive) affected gene (p) from the other parent. | | Please see below for: | | ~ Diagram | This demonstrates how to work out the possible outcomes for a carrier x carrier mating. | | ~ Table | This shows all the combinations of outcomes for different matings. | Back to top
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| ~ Diagram demonstrating how to work out the possible outcomes for a carrier x carrier mating |  |
| The P gene from parent one and the P gene from parent two |
| will produce a PP (normal) Glen | | The P gene from parent one and the p gene from parent two |
| will produce a Pp (carrier) Glen | | The p gene from parent one and the P gene from parent two |
| will produce a Pp (carrier) Glen | | The p gene from parent one and the p gene from parent two |
| will produce a pp (affected) Glen |
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Table showing all the combinations of outcomes for different matings |
| Parents | | Progeny (Offspring) | | Each puppy has a ... | | PP x PP | Normal x Normal |  | x | | = | | | | | | 100% chance of being normal | | PP x Pp | Normal x Carrier | | x |  | = | | | | | | 50% chance of being normal
50% chance of being a carrier | | PP x pp | Normal x Affected | | x |  | = | | | | | | 100% chance of being a carrier | | Pp x Pp | Carrier x Carrier | | x | | = | | | | | | 25% chance of being normal
50% chance of being a carrier
25% chance of being affected | | Pp x pp | Carrier x Affected | | x | | = | | | | | | 50% chance of being a carrier
50% chance of being affected | | pp x pp | Affected x Affected | | x | | = | | | | | | 100% chance of being affected |
For the time being, the only "certainties" regarding PRA in Glens are: | | If a Dog is tested affected, its sight will deteriorate and it will eventually go blind. | | If a Dog is tested affected, then both its parents - (if they are not known to be affected) - will be classified as "obligate" carriers. | | If a Dog is tested affected, then all its progeny - (if they are not known to be affected) - will be classified as "obligate" carriers. |
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BREEDING ISSUES | | Breeds with a genetic marker - (autosomal recessive mode of inheritance) | | The advantage of having a genetic marker for PRA is that breeders know the status of the dogs they are breeding, (if they are tested). This means that carriers, and even affected, can be selectively used in the breeding program, without the risk of producing puppies that will eventually go blind as a result of PRA e.g. | | SAFE mating, with known status dogs, are those that produce only normal or carrier puppies i.e. | ~ normal x normal
~ normal x carrier
~ normal x affected | | UNSAFE mating, with known status dogs, are those that can produce affected puppies, which will eventually go blind, i.e. | ~ carrier x carrier
~ carrier x affected
~ affected x affected | | Please see the table above for the combination of outcomes for different mating. | | Breeds without a genetic marker - (autosomal recessive mode of inheritance) | | Breeding decisions are not easy with breeds that do not yet have a genetic marker, | | Breeding considerations would need to include: | | [i] PRA status of parents and grandparents of the chosen dog and bitch and the wider pedigree e.g. siblings of parents/grandparents and their progeny (offspring). | | [ii] Age of first breeding. | | [iii] Use of known carriers in a breeding program. | | |
Special Thanks to the Glens for the help and information |
