Progressive Retinal Atrophy
Progressive retinal atrophy or degeneration (PRA or PRD) is the
name for several diseases that are progressive and lead to blindness.
First recognized at the beginning of the 20th century in Gordon
Setters, this inherited condition has been documented in over
100 breeds, and mixed breed animals as well. PRA is not very common
in cats, although the Abyssinian breed seems to have a predilection.
In cats, a deficiency of the amino acid taurine can result in
PRA. This is one reason why some feline nutritional supplements
contain taurine.
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
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?
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 do; 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 (termed retinal dysplasia) or early degeneration (deterioration) of the photoreceptors. Retinal dysplastic dogs are usually affected within two months of birth and may be completely blind by one year. Retinal degenerative dogs are affected from one year to eight years of age and the symptoms progress slowly.
PRA worsens over time. The affected animal experiences night blindness initially because the rods are affected first. The condition progresses to failed daytime vision.
What are the signs of PRA?
Signs may vary depending on the type of PRA and its rate of progression.
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 pet 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.
If no abnormalities are found during the exam by a board certified
veterinary ophthalmologist, the dog can be certified free of heritable
eye disease through the Canine Eye Registration Foundation (CERF).
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. Dogs and 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?
PRA has been shown to have a genetic component. Kittens and puppies
from parents who have no history of the disease and have been
certified free of PRA (in the case of dogs) 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. If your dog develops
PRA, notify the breeder, if possible.
In the last several years, DNA is being used to identify which genes are responsible for PRA. In one form of PRA called 'rod cone dysplasia 1' (rcd1), which affects Irish Setters, the gene mutation has been identified.
Progressive rod-cone degeneration (prcd) is the most widespread
form of PRA and affects many breeds including Poodles, American
and English Cocker Spaniels, Labrador Retrievers and Portuguese
Water Dogs. Prcd 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 many dogs have already been bred prior to the onset of
symptoms. Thus the development of a genetic test for this disease
which could identify both affected animals and those that just
carry the gene would be particularly useful. The researchers at
the James A. Baker Institute have found a set of genetic markers
that usually indicate the presence of the gene mutation that causes
prcd in English Cockers, Labrador Retrievers, Chesapeake Bay Retrievers,
and Portuguese Water Dogs. Although the exact gene mutation that
causes prcd has not been found, every dog that is affected with
prcd has two copies of the genetic marker and every dog that does
not have the marker is clear of prcd. Unfortunately, this test
for the genetic marker is not as accurate in diagnosing prcd as
it would be if the actual gene mutation was found. This is because
dogs that are not affected by prcd may still have the genetic
marker present and have false positive test results. Thus dogs
that are positive for the gene marker may be either false positives
or may be carriers of the disease or may be affected by it. The
marker test may be done at a very early age, so potential breeding
animals may be selected when they are still puppies.
Dog Breeds Most Commonly Affected by PRA, and/or Have DNA Testing
Available
American Cocker Spaniel
Briard*
Cardigan Welsh Corgi*
Chesapeake Bay Retriever*
Collie*
Dachsunds
Doberman Pinshcers
English Cocker Spaniel*
Irish Setter*
Labrador Retriever*
Miniature Schnauzer*
Norwegian Elkhound
Poodle
Portuguese Water Dog*
Siberian Husky*
Samoyed*
* Indicates DNA test available
The information on this page was obtained from the site www.peteducation.com