Canine inherited eye diseases (IEDs) affect over 160 breeds of dog and are caused by variants located in over 60 different genes. Almost half of these genes cause inherited and progressive disorders specific to the retina, a group of diseases known as progressive retinal atrophies, or PRAs. PRA presents at a variable age of onset between and within dog breeds, depending on which mechanisms within the retina are affected. For example, early-onset forms such as in the Irish Setter and Norwegian Eklhound are caused by abnormal differentiation of photoreceptor cells during the developmental stage of the retina. In late-onset forms, such as in the English Shepherd, the cone and rod photoreceptors develop correctly, but some of the mechanisms needed to maintain their structure and/or function is impaired. The primary clinical sign of PRA in the dog is a loss of night vision. This is a consequence of rod photoreceptor cell degeneration that occurs in the initial stages of disease. Degeneration of cone photoreceptors then follows, resulting in a total loss of sight. An electroretinogram which is carried out by a veterinary ophthalmologist is required to ascertain whether a retinal disease is a rod–cone (as in PRA) or a cone–rod type, however, this procedure is not routinely employed when assessing dogs. It is often the owner’s observations of their dog’s behaviour in dim light conditions that are a useful prognosticator for a PRA diagnosis.
There are instances of PRA disease variants being shared across dog breeds, the most notable being PRCD, but in the majority of cases shared variants occur when the breeds have a shared lineage, it is less common when breeds are vastly different from one another.
Our group has been carrying out research into inherited eye diseases including PRA for well over 20 years. Unfortunately, PRA is not a treatable disease, so we strive to identify causal variants so that dog breeders and owners can avoid producing affected puppies by using DNA tests that we develop from our research. We have successfully identified 24 IED variants to date, which is more than any other research group. Eleven of those variants are associated with PRA and we are continuing our efforts in multiple breeds for which the causal variant has yet to be identified. In fact, we have recently had very promising breakthroughs for the Nova Scotia Duck Tolling Retriever and the Miniature Schnauzer dog breeds. We are carrying out validation of the variants we have identified but from the results so far we are hopeful! Unfortunately for the Miniature Schnauzer, the variant we have found is not present in all cases and we shall elaborate more on that soon, but what it does mean is that there is still at least one, and possibly more variants in the breed that need to be found.
We can’t do this work without the help we get from all of the amazing breed groups and dog owners who send us samples and veterinary information on their dogs. So, thank you all and we will keep you updated with our news !
