Progressive retinal Atrophy, progressive Rod-cone degeneration (PRA-prcd) is a late onset, inherited eye disease affecting Cocker Spaniels. PRA-prcd 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. Evidence of retinal disease in Cocker Spaniels can first be seen on an Electroretinogram around 9 months of age, but most affected dogs will not show signs of vision loss until 3 to 5 years of age or later. 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. Other signs of progressive retinal atrophy involve changes in reflectivity and appearance of a structure behind the retina called the Tapetum that can be observed on a veterinary eye exam. Although there is individual and breed variation in the age of onset and the rate of disease progression, the disease eventually progresses to complete blindness in most dogs. Other inherited disorders of the eye can appear similar to PRA-prcd. Genetic testing may help clarify if a dog is affected with PRA-prcd or another inherited condition of the eye.
Breed-Specific Information for the Cocker Spaniel
The Mutation of the PRCD gene associated with progressive retinal Atrophy, progressive Rod-cone degeneration has been identified in the Cocker Spaniel. Though the exact frequency in the overall Cocker Spaniel population is unknown, 14.5% out of 55 Cocker Spaniels from a population from Czech Republic were carriers of the mutation and 1.8% were affected.
Genetic testing of the PRCD gene in Cocker Spaniels will reliably determine whether a dog is a genetic Carrier of PRA-prcd. PRA-prcd 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 PRCD 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. Cocker Spaniels 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 PRCD 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.
Andrade LR, Caceres AM, Trecenti AS, Brandão CVS, Gandolfi MG, Aguiar EV, Andrade DGA, Borges AS, Oliveira-Filho JP. Allele Frequency of the C.5G>A Mutation in the PRCD Gene Responsible for Progressive Retinal Atrophy in English Cocker Spaniel Dogs. Animals (Basel). 2019 Oct 21;9(10):844. doi: 10.3390/ani9100844.
Dostal J, Hrdlicova A, Horak P. Progressive rod-cone degeneration (PRCD) in selected dog breeds and variability in its phenotypic expression. Veterinarni Medicina. 2011 Jun; 56(5):243-47. [Not In PubMed]
Kohyama M1, Tada N, Mitsui H, Tomioka H, Tsutsui T, Yabuki A, Rahman MM, Kushida K, Mizukami K, Yamato O. Real-time PCR genotyping assay for canine progressive rod-cone degeneration and mutant allele frequency in Toy Poodles, Chihuahuas and Miniature Dachshunds in Japan. J Vet Med Sci. 2015 Nov 6.
Moody JA, Famula TR, Sampson RC, Murphy KE. Identification of microsatellite markers linked progressive retinal atrophy in American Eskimo Dogs. Am J Vet Res. 2005 Nov;66(11):1900-2.
Zangerl B, Goldstein O, Philp AR, Lindauer SJ, Pearce-Kelling SE, Mullins RF, Graphodatsky AS, Ripoll D, Felix JS, Stone EM, Acland GM, Aguirre GD. Identical mutation in a novel retinal gene causes progressive rod-cone degeneration in dogs and retinitis pigmentosa in humans. Genomics. 2006 Nov; 88(5):551-63.