EXERCISE INDUCED COLLAPSE IN LABRADOR RETRIEVERS
Susan M. Taylor, DVM, Diplomate ACVIM (Internal Medicine)
Western College of Veterinary Medicine
University of Saskatchewan
Saskatoon, Saskatchewan, Canada

Affected dogs can tolerate mild 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 agenerally 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 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, 107.6F).
Recently, however, astudy performed last year 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.
 

  This disorder is distinct from other hereditary muscle disorders
which have been identified in Labrador Retrievers including:

(1) Labrador Retriever Hereditary Myopathy (also called type II muscle
fiber deficiency)- a condition where puppies (6 weeks to 6 months of age)
develop progressive muscle weakness, exercise intolerance, an abnormal
gait and muscle atrophy. Signs stabilize in most dogs by 12 to 18 months
of age. This conditionis easily diagnosed using muscle biopsies. Autosomal
recessive inheritance.

(2) Labrador Retriever Muscular dystrophy -a rare condition affecting very
young male dogs, causing severe weakness and progressive muscle atrophy.
Diagnosis by muscle biopsy, dystrophin analysis.

(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 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. 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.

Dr. Susan Taylor, DVM
Diplomate, ACVIM (Internal Medicine)
Phone:306-966-7093
FAX: 306-966-7174
e-mail: sue.taylor@usask.ca