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Progressive Retinal Atrophy
Progressive Retinal Atrophy (PRA) is a collective term comprising a group of hereditary degenerative lesions of the retina. Hereditary diseases of the eye in dogs are common in the dog. They can be divided into congenital conditions: those present at birth, and those occurring later in life.
Here is some introduction to the anatomy of the eye and how it works. The retina is a layer of nervous tissue which covers the back of the eyeball where the sensation of vision occurs. The whole eye is just a container for this tissue that supplies the eye with the necessary nutrition and focuses light on the retina. When light enters the eye, it passes through the lens and is refracted, focusing an image onto the retina. Through complex connections of retinal nerves, the information is passed to the visual centers of the brain where it is interpreted.
Forms of Progressive Retinal Atrophy
Generalized Progressive Retinal Atrophy
In this condition, the light sensitive membrane at the back of the eye called retina degenerates. The condition always affects both eyes. The first sign noticed by the owner is night blindness or poor vision in subdued light which progresses over months or years to total blindness. There is no treatment that will either halt or reverse the retinal degeneration.
Central Progressive Retinal Atrophy
Central progressive retinal atrophy, also called RPE dystrophy, is characterized by accumulations of pigment in the layer of pigmented lining of the retina, which results in day blindness and eventually terminates in total blindness. The rate of vision loss is much slower than with generalized PRA, and not all dogs become totally blind. Breeds affected include the Miniature and Toy Poodle, Lhasa Apso, Tibetan Terrier, Tibetan Spaniel, English and American Cocker Spaniels, Labrador and Golden Retrievers and Irish Setter amongst others.
Prevention of Progressive Retinal Atrophy
Although some hereditary eye diseases can be treated, it is much better to control these diseases and ultimately eradicate them by breeding from sound dogs. With this in mind, eye certification schemes for the control of hereditary eye diseases are to be recommended. Breeding animals are examined by a specialist veterinary ophthalmologist and certified free from hereditary eye diseases which affect their breed. This has lead to the reduction of hereditary eye diseases in some breeds, examples being cataracts in the Golden Retriever and Afghan Hound, and collie eye anomaly in the Shetland Sheepdog. However, most hereditary eye diseases are due to recessive genes and the carrier state cannot be diagnosed by ophthalmoscopic and slit lamp examination.
PRA in Lhasa Apsos
This breed shows a late-onset form of PRA, although the age of onset is variable, differing by several years among affected dogs. Even though the number of affected animals is low, the proportion of carriers in some countries is estimated to be as high as 25%. Given the absence of a genetic test, carriers may only be identified when one of its parents develops the disease, or when one or more of its descendants shows signs of being affected. In any case, the identification of the carrier is too late, since it may have many half-siblings that may also be carriers, and the carrier, as well as its full and half siblings has been mated, usually more than once. The fact that signs of the disease appear several years after the dog has reached its sexual maturity and has been mated, causes the mutated gene to be passed on to the next generation and to be maintained in the population.
PRA in Irish Wolfhounds
In this breed, PRA is of the early-onset type. The fact that the disease manifests itself very early allows owners to exclude affected animals from the breeding population. However, there is no way of identifying carriers so, when these carriers are mated, this ensures that the mutated allele remains in the population.
If the unknown carrier is a popular dog, the mutated allele may even increase in frequency. In addition to excluding affected animals from the breeding population, their siblings, and in general all suspected carriers, may also be excluded. However, this may mean removing a significant number of individuals having highly valuable characteristics, and some of them would not be carriers. The only way of identifying carriers is through genetic testing and, for this to be possible, it is necessary to identify the gene associated with the disease. A number of studies have been done to uncover the mutated gene underlying PRA in this breed. Irish Wolfhounds do not have the PDEB or PDEA gene mutations that cause PRA in Irish setters and Cardigan Welsh corgis, respectively.
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Canine Eye Diseases and Vision Disorders
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