What is PRA?
PRA is the abbreviation of Progressive Retinal Atrophy, i.e. progressive withering of the retina. The disorder, found in many different dog breeds, is hereditary and slowly makes the dogs almost or completely blind.
Construction of the eye
The retina, 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 color vision).
Course of the disease
PRA usually affects the rods initially, and then cones in later stages of the disease. Early in the disease, affected dogs are night blind, lacking the ability to adjust their vision to dim light. The first signs to be noticed is that the dogs tend to bump into things when walking in the dim light, and that they seem insecure when left alone in darkness.
Later their daytime vision also fails. As their vision deteriorates, affected dogs will adapt to their handicap as long as their environment remains constant, and they are not faced with situations requiring excellent vision. At the same time the pupils of their eyes become increasingly dilated, causing a noticeable "shine" to their eyes; and the lens of their eyes may become cloudy, or opaque, resulting in a cataract.
One can with instruments also see a reduction in the diameter and branching pattern of the retina's blood vessels, and shrinking of the optic nerve head. Usually by the time the affected dog has these changes there is already significant evidence of loss of vision.
Most dogs with PRA sooner or later becomes completely or almost blind. Older and more experienced dogs with a good memory, and keen sense of hearing and smell, can however in a secure and constant environment continue to live a good life in spite of increasing blindness.
Since the progression of disease for affected dogs of the breed Papillon often is very slow, and the dogs often are advanced in years when the sight is deteriorating; the dog-owner might not always notice that the dog is getting blind. He or she often think that it is the old age that is restraining the dog.
How does the dog get PRA?
PRA is a hereditary recessive disorder, which means that to be affected a puppy has to receive one copy of the defective gene from both parents. Thus both parents of an affected puppy must be either carriers or affected themselves. Similarly, because affected dogs have two copies of the defective gene, all their progeny will be at least carriers.
Diagnosis of PRA can be discovery by ophthalmoscopic examination but now we have DNA to test Papillon for PRA.
From Januari 1st 2016 the Papillon-club have new rules regarding testing for PRA. Testing for PRA has to be done with DNA testing. Rules that apply for mating from January 1st 2016:
The dog/dogs have to be tested
before mating or be free from PRA through heritage.