Progressive Retinal Atrophy (PRA)
PRA is a disease of the retina. This tissue, located inside the back of the eye, contains specialized cells called photoreceptors that absorb the light focused on them by the eye’s lens, and converts that light, through a series of chemical reactions into electrical nerve signals. The nerve signals from the retina are passed by the optic nerve to the brain where they are perceived as vision. The retinal photoreceptors are specialized into rods, for vision in dim light (night vision), and cones for vision in bright light (day and colour vision). PRA usually affects the rods initially, and then cones in later stages of the disease.
There are multiple forms of PRA which differ in the age of onset and rate of progression of the disease. Some breeds experience an earlier onset than others; other breeds do not develop PRA until later in life.
Normally, the photoreceptors in the retinas develop after birth to about 8 weeks of age. The retinas of dogs with PRA either have arrested development (retinal dysplasia) or early degeneration of the photoreceptors. Retinal dysplastic dogs are usually affected within two months of birth and may be completely blind by one year. Dogs with retinal degeneration are affected from one year to eight years of age and the symptoms progress slowly.
The term progressive retinal atrophy covers several types of inherited degeneration (deterioration) of the retina. Sub-classifications of PRA are based on the age at which dogs show signs of the disease and the type of retinal cell which is affected.
Generalised PRA: These diseases affect primarily the photoreceptor cells. Both eyes are similarly affected and dogs eventually become totally blind.
- Early onset photoreceptor dysplasias: In these conditions, the photoreceptor cells develop abnormally in the first few weeks after birth, and then degenerate along with the inner layers of the retina.
- Later onset photoreceptor degeneration (progressive rod-cone degeneration): Here the retina matures and functions apparently normally for varying periods of time before degenerating. Dogs are not usually clinically affected until 1 year of age or more, although abnormalities can be seen in the eye and on the electroretinogram (ERG) long before owners notice signs of visually impairment.
Progressive rod-cone degeneration has similarities to retinitis pigmentosa in people.
Central PRA: (also called RPE dystrophy) The abnormality is in the retinal pigmented epithelium (RPE). The photoreceptor cells will also degenerate eventually. The rate of vision loss is much slower than with generalized PRA, and not all dogs become totally blind.
What breeds are affected by progressive retinal atrophy?
Many breeds are affected by one or sometimes more than one form of PRA. Progressive Retinal Atrophy (PRA) and Central Progressive Retinal Atrophy (CPRA) are among the most common eye problems in German Shepherd Dogs.
- Generalised PRA - early onset: Cardigan Welsh corgi, collie (rod-cone dysplasia type II), Cairn terrier, Gordon setter, Great Dane, Irish setter (rod-cone dysplasia type I), miniature schnauzer (photoreceptor dysplasia), Norwegian elkhound (rod dysplasia, also early retinal degeneration), Tibetan terrier (progressive rod degeneration causing night blindness only) Alaskan malamute - progressive cone degeneration causing hemeralopia (day blindness) - this condition occurs rarely.
- Generalised PRA (progressive rod-cone degeneration) - later onset (usually older than 1 year): Akita, Australian cattle dog, Australian shepherd, American and English cocker spaniel, Basenji, beagle, Belgian sheepdog, Briard, Brittany spaniel, Chesapeake Bay retriever, collie (rough and smooth), dachshund, English springer spaniel, German shepherd, German short-haired pointer, golden retriever, greyhound (without typical initial night blindness), Irish setter, Labrador retriever, mastiff, Nova Scotia duck tolling retriever, old English sheepdog, papillon, pekingese, poodle (miniature and toy), Portuguese water dog, Rottweiler, samoyed, Shetland sheepdog, Shih tzu, Siberian husky, Tibetan spaniel, Tibetan terrier, Welsh springer spaniel, Yorkshire terrier
- Central PRA - retinal pigment epithelial dystrophy (RPED): This disease occurs mostly in dogs in the United Kingdom, of the following breeds: border collie, Cardigan Welsh corgi, English cocker spaniel, English springer spaniel, golden retriever, Labrador retriever, rough and smooth collie, Shetland sheepdog
What are some of the Signs of PRA?
The disease causes gradual blindness, first affecting vision in dim light (night vision) and eventually, resulting in complete blindness many months or years later. Towards the end of the disease the affected eyes develop cataracts.
The first sign noticed by owners of pets is poor vision in dim light. Their dog may be reluctant to go outside in poor light, may seek out brighter light and may actually bump into things at night. Some dogs may show an increased green reflection of their eyes in artificial light
With time the day vision begins to deteriorate and cataracts start to develop. These are seen as a white opacity in the centre of the pupil. The cataracts exacerbate the loss of vision because they prevent light passing through the eye to the retina.
Since the vision loss is so gradual, most owners don’t notice a problem until the blindness has progressed to a severe end stage.
Diagnosis
If you suspect that your dog has impaired vision, your veterinarian will look for abnormalities with an ophthalmoscope. PRA may also be detected by electroretinogram (ERG) before your dog has any apparent visual difficulties. Electroretinography, which measures electrical patterns in the retina, is usually only available in specialty veterinary centres. Generalized PRA can often be detected by electroretinography at least a year before clinical signs are apparent.
Genetic testing is quickly becoming available for different forms of PRA in different breeds. The advantage of such testing is that it can identify dogs whose sight is unaffected, but who are carriers of the disorder (heterozygotes). Unfortunately at this time, there is no PRA genetic testing for German Shepherds, hence White Shepherds.
Prevention
Puppies from parents who have no history of the disease and have been certified free of PRA will have less risk of developing the disease. Affected animals should not be bred and should be spayed or neutered. The littermates or parents of animals with PRA should also not be bred as they are considered carriers, unless they are bred to a dog who is known to be clear of PRA and not a carrier. If your dog develops PRA, notify the breeder, if possible.
This article was first published on Fellowship of White Shepherds.