What is it?
Progressive retinal atrophy (PRA) is a hereditary disease of the eye that causes blindness. The retina is the tissue lining the back wall of the inside of the eye and is composed of two classes of photoreceptor cells called rods and cones; the rods function in dim light, and the cones in bright light. A dog affected with PRA begins to have difficulty seeing in dim light, then gradually loses the ability to see in bright light, eventually becoming completely blind. As the vision fails, the pupils become increasingly dilated, and may take on a shiny or iridescent quality. When properly trained and managed most dogs can adjust to blindness well.
PRA is a blanket term for many types of retinal diseases, all of which result in blindness. There are two groupings of PRA – early onset, and late onset. In early onset PRA poor vision in low light may be detectable shortly after birth, with total blindness occurring from 1 to 5 years. In late onset forms, night blindness usually occurs from 1 to 5 years, progressing to total blindness anytime after 3 years of age.
Symptoms and Diagnosis
Clinical signs of the different types of PRA are often similar. The disease always affects both eyes, and the generalised form always leads to blindness. However, dogs are remarkably good at adapting and the blindness may not be noticed until the owners move or change the position of the furniture in the house. The first sign is usually deterioration of vision in dim light and darkness (night blindness). Changes in the retina will be visible to a veterinarian by examining the interior of the eye with an ophthalmoscope. The pupils are dilated with eyedrops and the eyes examined and this takes less than five minutes. If an eye exam diagnoses retinal changes potentially associated with PRA, a more sensitive test may be needed in order to confirm that the dog has PRA. The electroretinograph (ERG) measures the response of the rods and cones to specific colors of light. Some ophthalmologists will do the ERG procedure without anaesthetising the dog. When properly performed, an ERG provides the most definitive diagnosis, and is so sensitive that it can detect the presence of disease long before it can be seen by clinical examination. As the disease progresses, colour changes in the back of the eye will become more marked. The optic disc (where the optic nerve enters the retina) will eventually become pale in colour and lose its distinct edges. The pupil progressively loses its ability to respond to light, resulting in a dilated pupil even in brighter lights; and in the later stages of the disease, a secondary cataract generally forms.
How is it treated?
Unfortunately there is no treatment to prevent, treat, or cure PRA. Although a number of vitamin therapies have apparently been suggested, there is no more information on what these may be and certainly no conclusive evidence that they help in any way.
Can it be Prevented?
PRA is hereditary and is always assumed to be an autosomal recessive trait until proven otherwise. A recessive trait requires two copies of the defective gene. An autosomal recessive trait is one in which a recessive trait is carried on a chromosome pair other than the XY sex pair. An affected dog must have two copies of the defective gene. A dog with only one copy of the gene is a carrier and will never have PRA, but will be able to pass that defective gene on to approximately half its offspring. Specifically, the laws of inheritance for autosomal recessive inheritance tell us the following about breeding:
- Two dogs affected with PRA bred with each other will result in 100% affected offspring.
- One affected dog bred with a carrier will result in 50% affected offspring and 50% carriers.
- One affected dog bred with a PRA clear dog will result in 100% carriers.
- One carrier bred with another carrier will result in 50% carriers, 25% affected with PRA, and 25% PRA clear offspring.
- One carrier bred with a PRA clear dog will result in 50% carriers and 50% PRA clear offspring.
- One PRA clear dog bred with another PRA clear will result in 100% PRA clear offspring.
PRA has been found in Papillons, albeit it appears to be more common overseas than in the UK. It has been shown to be late-onset, not occuring before at least 3 years old.
At the present time, the only way to determine if a dog is a carrier is if one of its offspring is affected. Identifying carriers is imperative in order for breeders to make informed decisions and minimise the risk of producing PRA affected dogs. It is therefore a good idea to have regular eye examinations, with the results made public – silence in this case will increase the chances of producing dogs with PRA.