Progressive retinal Atrophy (Shetland sheepdog type) is an adult-onset inherited eye disease affecting dogs. Progressive retinal atrophy (PRA) occurs as a result of degeneration of both Rod and cone type Photoreceptor Cells of the Retina, which are important for vision in dim and bright light, respectively. In general, the rod type cells are affected first and affected dogs will initially have vision deficits in dim light (night blindness) and loss of peripheral vision. Over time affected dogs continue to lose night vision and begin to show visual deficits in bright light. The reported average age of clinical diagnosis in dogs affected with this type of PRA is 5 years of age. However, this PRA type appears to have a wide range in age of onset with reports of clinical diagnosis from 2 to 11 years of age.
Genetic testing of the CNGA1 gene will reliably determine whether a dog is a genetic Carrier of progressive retinal Atrophy (Shetland sheepdog type). Progressive retinal atrophy (Shetland sheepdog 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 inheriting one copy and being a carrier of the CNGA1 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 CNGA1 does not exclude PRA in a pedigree.
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.
Wiik AC, Ropstad EO, Ekesten B, Karlstam L, Wade CM, Lingaas F. Progressive retinal atrophy in Shetland sheepdog is associated with a mutation in the CNGA1 gene. Anim Genet. 2015 Oct;46(5):515-21.