A syndrome of exercise intolerance and collapse (EIC) is being observed with increasing frequency in young adult Labrador Retrievers. It has also been observed in Chesapeake Bay retrievers and curly-coated retrievers. Affected dogs have been found in field-trial, hunt test, conformation, pet, and service lines. Most affected dogs have been from field-trial breedings. Signs become apparent in young dogs as they enter heavy training, usually between 7 months and 2 years of age. Black, yellow and chocolate Labradors of either sex can be affected. Dogs with this condition are always normal at rest and are usually described as being extremely fit, prime athletic specimens of their breed with an excitable temperament and lots of drive. Affected dogs can tolerate mild to moderate exercise but 5 to 15 minutes of strenuous exercise induces weakness and then collapse. The first thing noted is usually a rocking or forced gait. The rear limbs then become weak and unable to support weight. Many affected dogs continue to run while dragging their back legs. Some of the dogs appear to be uncoordinated, especially in the rear limbs, with a wide-based, long, loose stride rather than the short, stiff strides typically associated with muscle weakness. In some dogs, the rear limb collapse progresses to forelimb weakness and occasionally to a total inability to move. Some dogs appear to have a loss of balance and may fall over, particularly as they recover from complete collapse. Most collapsed dogs are totally conscious and alert, still trying to run and retrieve, but affected dogs can appear stunned or disoriented during the episode. It is common for the signs to worsen for three to five minutes even after exercise has been terminated. After 10 to 20 minutes of rest, however, they return to normal. A few affected dogs have died during exercise or while resting after an episode of exercise-induced collapse. Actual ambient temperature does not seem to be a critical factor contributing to collapse, but if the temperature is much warmer than what the dog is accustomed to, collapse may be more likely. Affected dogs are less likely to collapse while swimming than when being exercised on land. Body temperature is normal at rest in dogs with EIC but almost always dramatically increases at the time of collapse. Recently, however, a study showed that clinically normal Labrador Retrievers had similar dramatic elevations in body temperature after 10 minutes of strenuous retrieving exercise. Affected dogs may, however, take longer for their body temperature to return to normal after exercise. Affected dogs are not stiff, sore or painful during the collapse or after recovery. Massage of the muscles or palpation of the joints or spine is not uncomfortable. Nervous system, cardiovascular and musculoskeletal examinations are unremarkable as is routine blood analysis at rest and during an episode of collapse.
Symptomatic dogs are rarely able to continue training or competition. It seems that if affected dogs are removed from training and not exercised excessively the condition will not progress and they will be fine as pets. Littermates and other related dogs are often affected, but depending upon their temperament and lifestyle, they may or may not manifest clinical signs. Some affected dogs will never exhibit signs of EIC; this could be because they do not participate in high excitement strenuous activities or because they have a laid-back temperament. Severely affected dogs may collapse whenever they are exercised to this extent; other dogs only exhibit collapse sporadically and all of the factors important in inducing an episode have not yet been well established. A few affected dogs have died during exercise or while resting immediately after a collapse of exercise-induced collapse so an affected dog’s exercise should ALWAYS be stopped at the first hint of an EIC attack.
The genetics of the condition are only now becoming better established. Recent research has been conducted at the University of Minnesota analyzing DNA harvested from the blood of affected dogs and their relatives to perform a full genome scan in order to identify a genetic marker for EIC, and to find the genetic mutation causing EIC. Because of their efforts, the chromosomal locus (site) of the mutation was found on chromosome 9, and the genetic mutation responsible for susceptibility to EIC was identified. This is a mutation in the gene for dynamin-1 (DNM1), a protein expressed only in the brain and spinal cord, where it plays a key role in forming synaptic vesicles containing neurotransmitters. DNM1 is not required during low-level neurological stimulation, but when a heightened stimulus creates a heavy load on release of CNS neurotransmitters (as with intense exercise, a high level of excitement and perhaps increased body temperature), DNM1 is essential for sustained synaptic transmission in the brain and the spinal cord.
EIC is an autosomal recessive syndrome. To be affected, a dog must have received the mutated version of the EIC gene from both parents . Thirty percent of all tested Labrador Retrievers carry the EIC gene. This is not a problem unless a breeder unknowingly breeds one carrier to another carrier and ends up with affected Labrador Retriever puppies. It can take up to 5 years for symptoms to appear, so it is important that breeders test for this gene. It is also important for buyers to educate themselves and learn about this debilitating problem. Ask your breeder if their dogs have been tested and find a breeder who has done so to prevent problems later on in your dog’s life. The most reliable way to be assured that you are buying an EIC Normal pup is to have the EIC test done on the pup, or to verify that the sire and dam have both been tested and are rated EIC NORMAL. Breeding dogs is a serious process. It affects the lives and well being of both the pups and the families that end up with them. Breeders have an inherent responsibility to protect the comfort and well-being of the dogs they produce. Responsible breeders are and have been responding to the challenge of improving the genetic health of our companions through the encouragement of better breeding practices and education of the public.
The best treatment in most dogs consists of avoiding known trigger activities and activities that involve intensive exercise in conjunction with extreme excitement especially in hot weather. Most dogs that are retired from training/competition or trigger activities live the remainder of their life without any episodes of collapse. Owners/trainers must always keep in mind the importance of ending exercise at the first sign of weakness/wobbliness if it does occur since these dogs are susceptible to collapse and death from EIC.