JUNE SPECIAL FEATURE - NEW STUDY IN LABRADOR RETRIEVERS
EXERCISE INDUCED COLLAPSE IN LABRADOR RETRIEVERS

Exercise induced collapse in Labrador Retrievers
Contributed by 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. Although the problem has been recognized more in male dogs, 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 collapse.

Affected dogs can tolerate mild exercise (See video clip 1) 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 (See video clip 2). 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 but ambient temperature does not seem to be a critical factor contributing to 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 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, >107.6 F). 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 (Changes in rectal temperature and hematologic, biochemical, blood gas and acid-base values in healthy Labrador retrievers before and after strenuous 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 yet been identified. Muscle biopsies from affected dogs have been structurally normal, but a few dogs tested 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.

This disorder 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) - This is 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 underlying defect(s) is still not known, this condition is easily diagnosed with evaluation of muscle biopsy specimens. An autosomal recessive mode of inheritance has been documented in dogs with the type II fiber deficiency.

(2)    Dystrophin Deficient Muscular Dystrophy - This is a rare condition affecting very young male dogs resulting in severe weakness, progressive muscle atrophy, and markedly elevated serum creatine kinase (CK) concentrations. Diagnosis is made by detection of markedly elevated serum CK in a young puppy, pathological changes within a muscle biopsy specimen consistent with dystrophy, and demonstration of absence of muscle membrane dystrophin within a muscle biopsy specimen.

(3)    Familial Reflex Myoclonus - This is a rare condition where very young puppies (3 to 6 weeks of age) develop intermittent muscle spasms and then progressive muscle stiffness. Littermates are often affected.

(4)    Malignant hyperthermia - This is 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 specimens 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 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 interest and expertise in metabolic disorders of canine muscle.

Investigators at the Western College of Veterinary Medicine (Saskatoon, Saskatchewan, Canada), in collaboration with Dr. Diane Shelton at the Comparative Neuromuscular Laboratory (University of California, San Diego) have recently received funding 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. Contact Dr. Taylor for further information regarding participation in this study or to provide videotapes and pedigrees of affected dogs for analysis.