Progressive Retinal Atrophy (also known as “PRA”) is a group of inherited diseases that cause degeneration of the retina in dogs and results in permanent blindness. The retina is a thin layer of nervous tissue that lies in the back of the eye and is reponsible for converting light into electrical impulses. The electrical impulse is then transmitted along the optic nerve to the brain, where the electrical impulses are interpretted as an image. The cells within the retina that are directly responsible for the conversion of light to an electrical impulse are called photoreceptors. There are two types of photoreceptors: rods and cones. The rod rods are responsible for dim light vision, and the cones are responsible for bright light and color vision. Progressive Retinal Atrophy begins with degeneration of the rod photoreceptors. This may be noticed by pet owners as night blindness or decreased “confidence” in dimly lit areas. As the degeneration of rods progresses, the cones will begin to degnerate. Therefore, loss of vision in brightly lit environments will occur later in the progression of the disease. As PRA progresses slowly, many dogs will learn to partially accomodate for their visual deficits through their senses of hearing and smell. Because of this accommodation, it is not uncommon for some pet owners notice the visual deficits only after significant degneration of both rods and cones has occured.
At present, researchers at the University of Missouri are studying PRA in English Springer Spaniels. This research is likely to be expanded to other breeds in the near future – please check back for details breeds to be looked at next.
Currently, the most commonly performed diagnositic test for PRA is a complete ophthalmic examination. A veterinary ophthalmologist can detect the later stages of PRA by examining the retina with indirect ophthalmoscopy. Unfortunately, the earliest stages of PRA are not detected in such an exam. As the age of onset of PRA can vary widely from animal to animal, annual examination and eye registration of breeding stock by a board-certified veterinary ophthalmologist through the Canine Eye Registration Foundation (CERF) is recommended.
Another test that can be performed to diagnose PRA is an electroretinogram (ERG). Electroretinography measures the electrical response of the retina that occurs to known amounts of triggered light stimulation. An electroretinogram is capable of detecting PRA much earlier than it can be diagnosed through ophthalmoscopy. Although it has increased sensitivity for early PRA, electroretinography also requires more specialized instrumentation, experience, and general anesthesia for early PRA detection. For these reasons electroretinography is not commonly used as a “screening” tool for PRA, but is instead used to confirm a suspected diagnosis or in research settings where early detection of PRA is required.
Unfortunately, blindness from PRA is permanent and it has no cure.
A New Test for PRA in English Springer Spaniels and Dachshunds
Dr Cathryn Mellersh and colleagues at the Animal Health Trust in England have recently identified and reported a mutation that causes PRA in Miniature Longhaired Dachshunds. Dr. Mellersh has found that the mutation is not restricted to Dachshunds, and at the University of Missouri we have investigated the frequency of this mutation in English Springer Spaniels, primarily from North America.
Considerations for English Springer Spaniel Breeders
The mutation is common among English Springer Spaniels: A DNA test for the PRA mutation was developed at the University of Missouri-Columbia (UMC) and used in a research project funded by the Canine Health Foundation and the English Springer Spaniel Field Trial Association Foundation. Over 1,100 English Springer Spaniels were tested for the mutation. In this survey, 42% tested as “affected” and 38% tested as “carrier” for this mutation. Only 20% tested as “normal.” (“Affected” dogs inherited two mutant genes – one from each parent; “carrier” dogs inherited one mutant gene and one normal gene; and, “normal” dogs inherited two normal genes).
The mutation is a risk factor for the development of PRA in English Springer Spaniels: Most of the dogs that tested as “affected” are considered to have normal eyesight by their owners. Some of these dogs may develop PRA as they get older; however, there are many examples of old English Springer Spaniels that DNA test “affected” but have subtle, if any, visual impairment.
On the other hand, 95% of the English Springer Spaniels with clinically recognized PRA test as genetically “affected.” Erroneous diagnoses or a second rare form of PRA may account for the 5% of English Springer Spaniels with PRA that do not test “affected.”
Our study implies that the likelihood of developing PRA is approximately 20 times higher for English Springer Spaniels testing “affected” than it is for other English Springer Spaniels. This is strong evidence that testing “affected” is a major risk factor for PRA in English Springer Spaniels and indicates that the prevalence of English Springer Spaniel PRA can be reduced by breeding programs that select away from the mutant gene. This can be accomplished by giving highest preference to breeding stock that test “normal,” intermediate preference to dogs that test “carrier,” and lowest preference to dogs that test “affected.”
Recommendations to English Springer Spaniel Breeders: Although we believe that English Springer Spaniel breeders should make efforts to reduce PRA in future generations of their line, we also believe that because the mutation is so common in the breed, overly aggressive elimination from breeding consideration of dogs testing affected or carrier could have an overall detrimental effect on the breed and could devastate successful breeding programs. A realistic approach when considering which English Springer Spaniels to select for breeding would be to consider dogs with the mutation to have a fault, just as lack of working ability, poor topline, or imperfect gait would be considered faults. Dogs that test “affected” with two mutant copies of gene should be considered to have a worse fault than “carriers” with only one mutant copy. English Springer Spaniel breeders could then continue to do what conscientious breeders have always done: make their selections for breeding stock in light of all of the dogs’ good points and all of the dogs’ faults. Using this approach over several generations should substantially reduce the prevalence of PRA while continuing to maintain or improve those qualities that have made English Springer Spaniels so popular.
Ongoing research: One problem with this approach is that the clinical consequences of testing “affected” are, as yet, poorly defined. As a result, it is difficult for breeders of English Springer Spaniels to determine how much priority should be given to selecting away from the mutation. To better understand the clinical consequences, we are continuing to assess CERF reports and other relevant medical records of the dogs we have tested. This is why we are discounting the price of testing a dog for owners that provide us with a recent CERF report along with the test request (see below). In addition, the English Springer Spaniel Field Trial Association Foundation has awarded funds to Dr Kristina Narfstrom at the Laboratory for Comparative Ophthalmology, UMC, to supplement a Canine Health Foundation grant to assess retinal function by electroretinography of selected English Springer Spaniels. When the results of these studies become available, we will post them at this website.
Click here for a pdf version of the bulleted announcement of this test for ESS, which can be used in newsletters and for other informational purposes. If any additional information is desired, please contact us.
Considerations for Dachshund Breeders
We do not know how common the PRA-causing mutation is among Dachshunds in North America; however, a limited survey of 46 randomly selected Dachshund DNA samples in the UMC collection as a result of other research projects had approximately 25% carriers and approximately 2% affected. The mutant form of the gene was found in both standard-sized and miniature Dachshunds, and in shorthaired, longhaired and wirehaired varieties. We are making this DNA test for the PRA mutation available to Dachshund owners as well as the English Springer Spaniel community, effective immediately. From communications with Dr. Mellersh and others with testing experience in Europe, it appears that many of the Dachshunds that test as “affected” are considered to have normal vision by their owners. The above “Considerations for English Springer Spaniel Breeders” section contains an analysis of the consequences of this situation. As we have more information about the frequency of this mutation in North American Dachshunds we will post that information on this website. Additional information about PRA in Dachshunds is available at the Animal Health Trust website.
Testing English Springer Spaniels and Dachshunds:
We are offering a DNA test for the mutantion to owners of English Springer Spaniels and Dachshunds for a fee. The test request form and instructions are available in the “DNA Testing” section of this website. The base cost of the test is $40. Usually we are able to provide test results to you within 3 weeks after we receive the blood sample. Test results will be emailed to you.
Discounts for English Springer Spaniel tests: We have already collected DNA samples from over 2,000 English Springer Spaniels. If you would like the test performed on a DNA sample you have already submitted to us, it will save us the cost of isolating DNA and we will give you a $10 discount on the test.
As indicated above, we are trying to better understand the impact of the mutation on the vision of dogs testing “affected.” To help us in this endeavor, we are asking English Springer Spaniel owners to have their dogs examined by a Board Certified (ACVO) Veterinary Ophthalmologist and to supply us with a copy of the resulting CERF report along with the request for the DNA test. If you send us the results of a CERF examination dated less than one year before the test is ordered, we will deduct $10 from the cost of the test.
If we already have a DNA sample from the dog you want tested, and if you supply us with a copy of a recent CERF report, both discounts will apply.
Progressive Retinal Atrophy Research for English Springer Spaniels
Progressive Retinal Atrophy (PRA) of English Springer Spaniels is an adult-onset (late-onset) degeneration of retina that causes blindness. It is believed to have an autosomal recessive inheritance pattern, but the genetic mutation for PRA in English Springer Spaniels is not known and there is no genetic test for it. It is, unfortunately, most commonly diagnosed later in life through ophthalmic examination, and usually after the affected dog has passed this trait to its offspring. It is our goal to develop a genetic test for PRA in this breed so that affected and carrier animals can be detected before they are committed to breeding programs.
As of March 2007, we have identified 67 individuals in our collection of 2028 English Springer Spaniels that are reported by their owners as affected with PRA, and 230 relatives of these dogs that are reported normal. From this, we have assembled an extended family group of 39 affected dogs and 68 normal relatives to use for mapping the disease. We have also been contacting owners to verify the age at which their dog was diagnosed by an ACVO ophthalmologist as affected with PRA.