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
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
collapse.
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 to18 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
S7N5B4
Phone: 306-966-7093
FAX: 306-966-7174
sue.taylor@usask.ca