Common Symptoms
Progressive Retinal Atrophy (Swedish vallhund type) is an inherited eye disease affected dogs. Progressive Retinal Atrophy (Swedish vallhund type) occurs due to degeneration of both Rod and cone type Photoreceptor Cells of the Retina, which are important for vision in variable light. Evidence of retinal disease in affected dogs can first be seen on veterinary eye exam as early as 2 months of age. However, first signs of vision loss occur later and have been reported from 1 to 12 years of age. Rod type cells are affected first and dogs will initially have vision deficits in dim light (night blindness). Signs of progressive retinal atrophy on eye exam involve changes in reflectivity and appearance of a structure behind the retina called the Tapetum. The disease follows a variable progression, but most dogs will eventually progress to complete blindness as early as 9 years of age.
Breed-Specific Information for the Swedish Vallhund
The Mutation of the MERTK gene associated with Progressive Retinal Atrophy has been identified in the Swedish vallhund, although its overall frequency in this breed is unknown. Swedish vallhunds with two copies of the MERTK mutation are reportedly 20 times more likely to develop Progressive Retinal Atrophy than dogs without the mutation.
Testing Tips
Genetic testing of the MERTK gene will reliably determine whether a dog is a genetic Carrier of Progressive Retinal Atrophy (Swedish vallhund type). Progressive Retinal Atrophy (Swedish vallhund type) is inherited in an Autosomal Recessive manner in dogs meaning that they must receive two copies of the mutated gene (one from each parent) to develop the disease. In general, carrier dogs do not have features of the disease but when bred with another carrier of the same Mutation, there is a risk of having affected pups. Each pup that is born to this pairing has a 25% chance of inheriting the disease and a 50% chance of being a carrier of the MERTK gene mutation. Reliable genetic testing is important for determining breeding practices. Because symptoms do not appear until adulthood, genetic testing should be performed before breeding. In order to eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding of known carriers to each other is not recommended. Dogs that are not carriers of the mutation have no increased risk of having affected pups. However, because there are multiple types of PRA caused by mutations in other genes, a normal result in MERTK does not exclude PRA in a pedigree. In addition, some dogs with two copies of this mutation, but without PRA, have been identified. However, it is not yet clear if these dogs will eventually develop PRA or if this represents Variable Expressivity or Incomplete Penetrance.
There may be other causes of this condition in dogs and a normal result does not exclude a different mutation in this gene or any other gene that may result in a similar genetic disease or trait.
References
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Ahonen S. Canine Models of Human Vision Disorders: Identification of New Loci and Genes for Glaucoma and Retinal Degeneration [published doctoral dissertation]. 2014. Univ. of Helsinki. http://urn.fi/URN:ISBN:978-951-51-0303-1
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Everson R, Pettit L, Forman OP, Dower-Tylee O, McLaughlin B, Ahonen S, Kaukonen M, Komaromy AM, Lohi H, Mellersh CS, Sansom J, Ricketts SL. An intronic LINE-1 insertion is MERTK is strongly associated with retinopathy in Swedish Vallhund dogs. PLoS One. 2017 Aug 16;12(8):e0183021. doi: 10.1371/journal.pone.0183021. eCollection 2017.
[PubMed: 28813472]
