Susan M. Taylor, DVM, Diplomate ACVIM (Internal Medicine)

Western College of Veterinary Medicine

University of Saskatchewan

Saskatoon, Saskatchewan, Canada

A syndrome of exercise intolerance and collapse is being observed with increasing frequency in young adult Labrador Retrievers. 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. Dogs 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. Nervous system, cardiovascular and musculoskeletal examinations are unremarkable as is routine blood analysis at rest and during an episode of

Affected dogs can tolerate mild to moderate exercise but 5 to 15 minutes of strenuous exercise induces weakness and then collapse. Weakness starts in the rear limbs but then progresses rapidly to the forelimbs, resulting in a generally weak,wobbly gait. Dogs ultimately collapse and are unable to continue exercising. 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.

Affected dogs are less likely to collapse while swimming than when being exercised on land. 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. 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 the genetics of the condition have not been well established.

Body temperature is normal at rest in these dogs but dramatically increased at the time of collapse (temperature >41.5 C, >107.6F). Recently, however, a study performed at the Western College of Veterinary Medicine 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.

Metabolic testing of blood before and after exercise suggests that these dogs may have a defect in the chemical reactions necessary for energy production in their muscles. The specific enzymatic defect has not been identified. Muscle biopsies from affected dogs have been structurally normal but a few dogs have had lower than normal levels of muscle carnitine. Preliminary laboratory testing of muscle has not supported a diagnosis of exercise-induced malignant hyperthermia or of exertional rhabdomyolysis. Serum testing has eliminated myasthenia gravis as a possibility.

This syndrome of Labrador Retriever Exercise-Induced Collapse is distinct from other hereditary muscle disorders which have been identified in Labrador Retrievers including:

  1. Hereditary Labrador Retriever Myopathy (also called type II fiber deficiency or muscular dystrophy)- a condition where puppies (6 weeks to 6 months of age)appear stunted and develop progressive muscle weakness, exercise intolerance, an abnormal gait and muscle atrophy. Signs stabilize in most dogs by 12 to 18 months of age. While the precise undelying defect is still not known, the condition is easily diagnosed by evaluation of muscle biopsy specimens. An autosomal recessive mode of inheritance has been documented in dogs with Type II fiber deficiency.
  2. Dystrophin Deficient Muscular dystrophy -a rare condition affecting very young male dogs, causing severe weakness, progressive muscle atrophy and greatly elevated muscle enzymes (CK). Diagnosis is by detection of the elvated CK in a young pup, identification of pathologic changes in a muscle biopsy characteristic of dystrophy and demonstrating the absence of muscle membrane dystrophin in the muscle biopsy specimens.
  3. Familial Reflex Myoclonus - a rare condition where very young puppies (3 to 6 weeks of age) develop intermittent muscle spasms and then progressive muscle stiffness. Littermates often affected.
  4. Malignant hyperthermia - a rare, hereditary,life-threatening condition where excessive muscle contraction and increased body temperature can be triggered by general anesthesia with certain drugs or by stress and exercise (also called canine stress syndrome). Diagnosis is by rigorous laboratory testing of muscle biopsies or (more recently) through genetic testing.

The exercise-induced collapse syndrome we are describing can only be diagnosed by ruling out all of these other muscle disorders and by observing characteristic clinical features, history and laboratory test results in affected dogs.

Any dog with exercise intolerance should always have a complete veterinary evaluation to rule-out joint diseases, heart rhythm disturbances, respiratory problems,low blood sugar and other systemic disorders.

If the syndrome of Labrador Retriever Exercise-Induced Collapse is suspected,then further metabolic and muscle testing should be performed by a veterinarian in collaboration with a laboratory possessing expertise in metabolic disorders of canine muscle.

Investigators at the Western College of Veterinary Medicine (Saskatoon, Saskatchewan, Canada), in collaboration with Dr. Shelton at the Comparative Neuromuscular Laboratory(University of California, San Diego), are involved in a research program to further investigate and characterize Exercise-Induced Collapse in Labrador Retrievers. We hope to determine the underlying defect in affected dogs and make strides towards accurate diagnosis, treatment and prevention.

There are two groups of dogs being studied:

GROUP 1: Severely affected dogs (dogs who reliably collapse with 10 minutes of retrieving exercise) are being encouraged to come to the WCVM for full evaluation including metabolic, cardiac, respiratory, neurologic and orthopedic evaluation. Dogs are evaluated before, during and after exercise, and exercise is halted at the first sign of weakness or incoordination. Small muscle biopsies are then obtained under general anesthesia. Dogs arrive on a Sunday and go home on Wednesday. All testing is done free of charge and up to $300 US dollars is available to help defray the transportation costs. This phase of the study has been nearly completed - testing has been suspended for the winter, and will resume in April, when the temperatures are more reasonable for travel.

GROUP 2: Less severely affected dogs, and dogs which for some reason cannot travel to Saskatoon for evaluation are also needed for study. We are examining medical histories (obtained by questionnaire), pedigrees , videotapes (when available), and DNA samples (simple blood test) from as many affected dogs as possible. This is to help us to:

  1. describe the condition so that it can be more easily diagnosed by practicing veterinarians
  2. determine the heritable basis (dominant, recessive, etc)of the condition and
  3. develop a simple DNA test that could be used on adult dogs or puppies to determine if they have the disorder.

Please contact Dr. Taylor for further information regarding participation in either phase of this study. All information provided will be kept confidential.

Dr. Susan Taylor, DVM

Diplomate, ACVIM (Internal Medicine)

Department of Small Animal Clinical Sciences

Western College of Veterinary Medicine

University of Saskatchewan

Saskatoon, Saskatchewan, Canada



FAX: 306-966-7174