Anatomy of the eye
The eye is a very delicate, yet surprisingly durable organ. It consists of several layers. The cornea is a transparent layer that covers the front
of the eye. The iris is the colored part of the eye and it is responsible for letting in more or less light. The lens gathers and 'bends' light in order to focus it on the retina. In between the cornea and lens is an area of fluid which bathes the lens and helps it focus. The retina lines the inside of the eye and converts light into signals which travel down the optic nerve to the brain. A large area between the lens and the retina contains a jelly-like fluid called 'vitreous.' The vitreous gives the eye its form and shape, provides nutrients, and removes waste products.
The retina is the structure affected in PRA. This important part of the eye receives the light gathered and focused by the other eye structures. It takes the light and essentially converts it into electrical nerve signals that the brain, via the optic nerve, interprets as vision. The retina contains photoreceptors, called rods and cones, which help the animal see in darkness (rods) and see certain colors (cones).
What is PRA?
Normally, the photoreceptors in the retinas develop after birth to about 8 weeks of age. In PRA, the photoreceptors develop in the puppies, but as the dog ages, the receptors degenerate. Progressive rod-cone degeneration (PRCD) is the most common form of PRA in Mastiffs, and starts with night blindness and progresses to total blindness at 3 to 5 years of age. The late onset of clinical signs in PRCD is particularly devastating to breeding programs because dogs may have already been bred prior to the onset of symptoms.
What are the signs of PRA?
PRA is non painful and outward appearance of the eye is often normal, i.e.; no redness, excess tearing, or squinting. Owners may notice a change in personality of their dog such as a reluctance to go down stairs or down a dark hallway. This is characteristic of night blindness, in which vision may appear to improve during the daytime. As the disease progresses, owners can observe a dilation of the pupils and the reflection of light from the back of the eye. If the blindness is progressing slowly, the owner may not notice any signs until the dog is in unfamiliar surroundings and the lack of vision is more apparent. In some animals, the lens of their eyes may become opaque or cloudy.
How is PRA diagnosed?
Depending on the form of PRA, characteristic changes in the retina and other parts of the eye may be observed through an ophthalmic examination by a veterinary opthalmologist. More sophisticated tests such as electroretinography may also be used. Both tests are painless and the animal does not have to be anesthetized.
How is PRA treated?
Unfortunately, there is no treatment for PRA, nor a way to slow the progression of the disease. Animals with PRA usually become blind. Cats are remarkably adaptable to progressive blindness, and can often seem to perform normally in their customary environments. Evidence of the blindness is more pronounced if the furniture is rearranged or the animals are in unfamiliar surroundings.
Can PRA be prevented?
Abolutely! Dominant-PRA has has a genetic marker that can be found by a simple blood test. Puppies from parents who have tested "Normal" for PRA will never get the disease. Puppies from "Affected" parents have a100% chance of getting PRA(& going blind) generally within the first 5 years of their life. 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. If your dog develops PRA, please... notify the breeder if at all possible!
Read all about the struggle to find the genetic marker for PRA & the effect it has had on the Mastiff community
Read what Optigen has to say about PRA here