Overview of Canine Progressive Retinal Degeneration
Progressive retinal degeneration or atrophy, commonly abbreviated and referred to as PRD or PRA, represents a group of inherited eye diseases characterized by abnormal development (dysplasia) or premature degeneration (deterioration) of the retina in a dog. In the United Kingdom, a type of retinal degeneration is occasionally seen that affects the retinal layer behind the photoreceptors, and this disease is called central PRA. However, in the United States, PRA is primarily a disease of the light detecting cells of the retina (photoreceptors) and is called generalized PRA.
There are two types of photoreceptors in the retina and these are the light-sensitive rods and cones. They are responsible for detecting light and converting it into an electrical signal that travels to the brain. When the photoreceptor cells deteriorate, vision is lost because the animal has no way to generate an image from the light reaching the retina.
The degenerative form of PRA/PRD in dogs initially affects the rods. The rods are responsible for dim light vision; therefore, the dog loses its nighttime vision first. The disorder is progressive and eventually the cones are affected. Over time, the dog slowly goes completely blind. The disease affects both eyes at the same time.
In the dysplastic forms of the disease the rods of both eyes never develop properly, so the dog is born with poor dim light vision, and the cones rapidly degenerate as the puppy ages. The onset of blindness is much more rapid than with the degenerative forms, and the puppies are usually blind before one year of age.
Over 30 different breeds of dogs are known to have inherited forms of PRA, including the Akita, American cocker spaniel, collie, English cocker spaniel, Irish setter, Labrador retriever, miniature dachshund, miniature schnauzer, all three types of poodle, Norwegian elkhound, papillion, Portuguese water dog and Tibetan terrier. The age of onset varies among the breeds. The onset of clinical signs can be as young as six weeks in dogs with the dysplastic form and as late as six to eight years in dogs with the degenerative form. Most forms of PRD/PRA are inherited as a recessive gene, although a sex-linked form occurs in the Samoyed and Siberian husky, and a dominant form is found in the Mastiff. The disease is uncommon in mixed-breed dogs.
Many dogs are not seen until the late stages of disease and have advanced changes in their retinas because they compensate very well as their vision slowly deteriorates. Sometimes the blindness can appear to be sudden in onset, even though it has been developing for months, because the dog may show almost no clinical signs until the last bit of vision has been lost.
What to Watch For
Diagnosis of Progressive Retinal Degeneration in Dogs
Diagnostic tests are needed to recognize PRD/PRA and exclude other diseases. Your veterinarian will probably take a complete medical history and perform a thorough physical examination.
A complete ophthalmic examination is indicated and involves all of the following tests. Your veterinarian may refer your dog to a veterinary ophthalmologist for completion of some of these tests:
If your veterinarian is concerned that some disease other than PRA is the source of the dog’s blindness, then medical tests to rule out other causes may include the following:
PRA can sometimes be confirmed at the time of retinal examination because it causes characteristic changes in the appearance of the retina. Early stages of the disease can be more difficult to diagnose, and in that instance the disease can be detected with the following test:
Treatment for Dogs with Progressive Retinal Degeneration
No therapy is available to prevent, slow the progression of, or reverse the degenerative changes of PRD/PRA.
Care consists of providing a consistent and safe environment for pets with vision loss. Considerations include:
Preventative Care for Progressive Retinal Degeneration in Dogs
No preventive care is available for an individual because PRA is genetic. Do not breed affected animals.
Genetic testing is available for about 15 breeds of dogs affected with PRA. Testing performed on a blood sample can identify which dogs are affected and which are carriers of the disease. This information can then be used by breeders to decide which dogs may or may not be used for breeding. In addition, dogs can be examined on a yearly basis by a veterinary ophthalmologist and certified to be clinically free of the disease. The certification is valid for a period of one year from the time of examination.
Veterinary care often includes diagnostic tests to confirm the present of PRD and to exclude other diseases. Tests may include:
Diagnosis In-depth for Dogs with Progressive Retinal Degeneration
– Tests to evaluate vision, such as observing the dog navigate an obstacle course in both bright and dim light, and certain neurologic reflex testing
– Pupillary light reflex testing
– A Schirmer tear test and fluorescein staining of the cornea
– Tonometry to measure the pressure within the eye
– Specialized examination of the front chamber of the eye, the iris and lens, the vitreous and the retina.
– A complete blood count (CBC) and serum blood tests
– Blood tests for the tick borne diseases (Lyme disease, ehrlichiosis, etc.), fungal diseases and toxoplasmosis
– A measurement of systemic arterial blood pressure to rule out high blood pressure
– Possibly immune tests
– Possibly chest and abdominal X-rays
– Ultrasonography of the structures behind the eye
– CT scan or MRI of the brain and vision pathways leading to the brain
– Cerebral spinal fluid tap to evaluate the fluid around the brain
Treatment In-depth for Progressive Retinal Degeneration in Dogs
There is no treatment for PRD. Most forms of the disease are inherited as a simple autosomal recessive trait, which allows many dogs to be silent carriers of the disease. This inheritance pattern, combined with the fact that many dogs do not show signs until after they are older, make the disease very difficult to eradicate in some breeds of dogs.
For the 15 breeds of dogs in which genetic testing has been developed (www.optigen.com), blood testing of potential breeding dogs is the best way to identify both carriers and dogs that will show clinical signs. For all other breeds, electroretinography can be performed as a screening tool to detect the disease prior to the onset of clinical signs and retinal abnormalities.
The most common screening test used for PRD/PRA in breeding dogs is the annual examination of their eyes by a veterinary ophthalmologist. Results of this exam are then forwarded to the Canine Eye Registry Foundation (CERF, www.vmdb.org), an organization that keeps data on the eyes of many purebred dogs in the United States. If the dog is found to be free of inherited eye disease, then it is issued a clearance number by CERF that is good for one year
As more CERF examinations are performed and as more genetic tests are developed, hopefully breeders will be able to avoid using affected dogs and carrier dogs in their breeding programs and the incidence of PRA will decrease with time.