There is a saying that “Common things occur commonly” and most of the time it is right. The fun starts when the uncommon thing creeps in and you are met with a condition that you have never seen before or one that you last saw 20 years ago.
Rufus was a good example. He was a four-year-old castrated male Labrador X Curly Coated Retriever, apparently healthy on clinical examination but with a history of collapsing on exercise. His owners explained that when he was out for a walk all would be well, he would be chasing his ball and running through the woods but then he would collapse suddenly, losing control of his hindquarters. If he was allowed to rest for 10 to 20 minutes he would recover enough to walk home gently. Apart from these incidents he appeared to be normal. When they realised that Rufus was collapsing repeatedly his owners did their internet research and found that his signs fitted the description of exercise-induced collapse (EIC).
EIC has been studied in Canada and the United States for the last 10 years. Research has discovered that the lack of hindleg function is due to decreased transmission of nerve signals. Ned Patterson of the University of Minnesota explained that this is caused by a mutation of a gene. This is the first time that this mutation has been found in any mammal. It can be tested for so that dogs as young as five to six weeks old can be identified and not put forward for training. Breeders can avoid dogs which carry the gene so that the syndrome is not passed on.
EIC occurs in Retrievers, often in Retriever-cross dogs but also in purebreds such as Labradors, Curly Coated Retrievers and Chesapeake Bay Retrievers. Andrew Jagoe of Bath Referrals said that the dynamin-1 gene mutation which is associated with EIC has variable penetrance. The cases of EIC that he has seen have been in Labradors. Homozygous genes have been seen in Chesapeake Bay and Curly Coated Retrievers, Picardy Spaniels and Pembrokeshire Corgis.
Andrew also said that EIC can be confused with organic acidaemias where hypocarnitaemia due to buffering leads to problems. Also, young animals can present with a pattern similar to EIC that is due to centronuclear myopathy. In Labradors, EIC is most common in dogs bred for field trials, which fits with the observation that it shows most frequently in lean, fit, intensely exercising and highly motivated individuals. About 50 percent of the Labrador population carry the gene, but both parents must be carriers to produce affected puppies. It usually shows from five months of age, in both sexes, most often when the young dog starts intensive training. The animal will be normal in all other respects, including full clinical work-up. Diagnosis is by close observation; a video is very useful as the pattern of collapse is characteristic.
EIC episodes occur when the dog is excited and exercising intensely. Observant owners will notice that the hindquarters begin to wobble until the hind end of the dog collapses. The dog feels no pain and the limbs are not stiff. They may keep trying to play and move forwards, but they need to be made to rest to regain the use of their hindquarters. A few dogs have died but stopping exercise once the dog collapses does lead to recovery without lasting consequences. However, it will all happen again. Affected individuals can have a normal life as pet dogs as long as their exercise is managed to avoid the intensity that triggers a collapse.
There is a similar collapsing syndrome in Border Collies but this does not show up on the DNA test which gives us the diagnosis of EIC. The Border Collie Collapse (BCC) has a slightly different pattern with gradually increasing fatigue followed by loss of mental and optical focus leading to collapse involving forelimbs as well as hindlimbs. Both conditions have an associated transient pyrexia, which is also found in normal dogs in intense activity. This has led to EIC and BCC being mistaken for heat stroke which is a very different condition. Heat stroke follows over-exertion at high ambient temperature. It needs a combination of obsessive behaviour by the dog and lack of perception by the owner as seen in repeated ball-throwing on a hot day.