Cone Degeneration is an inherited eye disease affecting dogs. Affected dogs develop day blindness (blindness in bright light) and Photophobia (light sensitivity) between 8 to 12 weeks after birth due to degeneration of cells in the eye called cone photoreceptors which are responsible for vision in bright light. Affected dogs have normal vision in low light and structures of the inner eye appear normal on eye exam. Normal cone cell function can be seen on Electroretinogram (ERG) before six weeks of age, but becomes abnormal between 6 to 12 weeks of age and is completely absent in affected adult dogs signifying complete loss of Cone Cells. The cells responsible for vision in low light called Rod photoreceptors are not affected and thus, affected dogs will still be able to see normally in low light throughout life.
Breed-Specific Information for the Australian Shepherd
Australian shepherd is included as a breed susceptible to Cone Degeneration because of its close relatedness to the miniature Australian shepherd breed, which is known to develop this disease due to Mutation of the CNGB3 gene. The frequency of the causal mutation in the general Australian shepherd population is unknown.
Genetic testing of the CNGB3 gene in Australian shepherds will reliably determine whether a dog is a genetic Carrier of cone degeneration. Cone Degeneration 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 CNGB3 gene mutation. Reliable genetic testing is important for determining breeding practices. 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. Australian shepherds that are not carriers of the mutation have no increased risk of having affected pups.
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.
Yeh CY, Goldstein O, Kukekova AV, Holley D, Knollinger AM, Huson HJ, Pearce-Kelling SE, Acland GM, Komaromy AM. Genomic deletion of CNGB3 is identical by descent in multiple canine breeds and causes achromatopsia. BMC Genet. 2013 Apr 20;14(1):27. [Epub ahead of print]