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Exercise-Induced Collapse of Labrador
Retrievers: Survey Results and Preliminary
Investigation of Heritability
Completed surveys were obtained from owners of 225 Labrador retrievers affected by the syn-
drome of exercise-induced collapse. Questions addressed signalment, age of onset, descrip-
tion of collapse episodes, and owner perception of activities and factors associated with
collapse. Most dogs were young (mean 12 months) when collapse episodes began. Retrieving
was the activity most commonly associated with collapse. Owners felt that excitement
(187/225; 83%) and high environmental temperatures (71/225; 31%) increased the likelihood
of collapse. Analysis of pedigrees collected from 169 affected dogs was most consistent with
an autosomal recessive mode of inheritance. J Am Anim Hosp Assoc 2008;44:295-301.
Susan M. Taylor, DVM,
Diplomate ACVIM (Internal Medicine)
Cindy L. Shmon, DVM, DVSc,
Diplomate ACVS (Surgery)
G. Diane Shelton, DVM, PhD,
Diplomate ACVIM (Internal Medicine)
Edward (Ned) E. Patterson, DVM,
PhD, Diplomate ACVIM
(Internal Medicine)
Katie Minor, BS
James R. Mickelson, PhD
Introduction
A syndrome of exercise intolerance and collapse in young adult Labrador
retrievers was first described in the veterinary literature in 1993.1
Elevated rectal temperatures and increased plasma lactate concentrations
after exercise, and normal findings on examination of muscle biopsies
were documented in a few affected dogs.1Recently this syndrome has
been shown to be an important cause of exercise intolerance, interfering
with performance in otherwise healthy, young adult, competitive field
trial and hunting Labrador retrievers.2-4 Veterinarians evaluating affected
dogs have attributed collapse to heat intolerance, hypoglycemia, a car-
diac rhythm disturbance, or a possible metabolic myopathy, but no con-
sistent abnormalities have been identified in affected dogs.1-4 The
syndrome has come to be called exercise-induced collapse (EIC) of
Labrador retrievers.4,5 Affected dogs sometimes develop an abnormal
gait or collapse when subjected to strenuous exercise, but factors impor-
tant in inducing episodes of collapse have not been well established.
This study reports results of surveys completed by owners of 225
Labrador retrievers with presumed EIC. Also presented are preliminary
results of an investigation into the heritability of this condition. This is
the first large-scale descriptive study of EIC, providing veterinarians with
the information needed to recognize the syndrome and make informed
decisions regarding management and prognosis of affected dogs.
Materials and Methods
Descriptive Study
A survey was developed to obtain demographic and descriptive clinical
histories on dogs with presumed EIC [see Appendix]. The survey was
distributed to owners of Labrador retrievers with a collapse syndrome
determined by their veterinarian to most likely be EIC. Owners were
identified when they contacted investigators after learning about the
study through their veterinarian or magazine articles targeting owners of
working retrievers.2,3 Between the years of 2000 and 2006, 361 owner-
completed surveys were received.
In order to be included in the descriptive study, dogs had to meet
three criteria: (1) be a purebred Labrador retriever registered with the
Canadian Kennel Club (CKC) or American Kennel Club (AKC); (2)
O
JOURNAL of the American Animal Hospital Association 295
From the Department of Small Animal
Clinical Sciences (Taylor, Shmon),
Western College of Veterinary Medicine,
University of Saskatchewan, Saskatoon,
Saskatchewan S7N 5B4 Canada;
the Comparative Neuromuscular Laboratory
(Shelton), Department of Pathology,
School of Medicine,
University of California-San Diego,
La Jolla, California 92093-0709;
and the Departments of Veterinary
Clinical Sciences (Patterson) and
Biomedical Sciences (Minor, Mickelson),
College of Veterinary Medicine,
University of Minnesota,
St. Paul, Minnesota 55108.
Funding provided by the Morris Animal
Foundation and the Western College of
Veterinary Medicine’s Companion
Animal Health Fund.
have experienced at least three observed episodes of exer-
cise-induced collapse; and (3) veterinary evaluation must
have ruled out commonly recognized systemic and cardiac
causes of collapse. Minimum requirements for veterinary
evaluation included physical and neurological examina-
tions, complete blood count and serum biochemical profile,
and thoracic radiographs. Normal results were reported for
some dogs on additional tests including electrocardio-
grams, cardiac echocardiography, adrenocorticotropic hor-
mone stimulation tests, and muscle biopsies. Completed
surveys were obtained from 223 owners describing 225
dogs meeting all of these criteria, and those results were
tabulated and analyzed. Descriptive statistics were calcu-
lated and are reported as median and minimum and maxi-
mum values for all data.
Heritability
Pedigrees and whole blood were collected from 169 affect-
ed Labradors and 157 unaffected, related dogs. Affected
dogs were determined by their veterinarians to have EIC
based on at least two episodes of collapse or incoordination
typical of EIC and failure to identify a systemic, cardiac, or
neurological reason for the collapse. Many of these affect-
ed dogs had at least three episodes of collapse, so they
qualified for and participated in the descriptive study as
well. Dogs were considered unaffected if they had never
experienced weakness or collapse in spite of rigorous field
training. Pedigrees were assembled for analysis using
Pedigraph software,aand they were examined for evidence
of heritability.6,7
Results
Descriptive Study
Completed surveys were obtained from the owners of 225
Labrador retrievers that met the study criteria [see
Appendix]. Male (53.8%) and female (46.7%) dogs were
approximately equally represented, and all of the dogs were
sexually intact when their episodes of collapse began. Black
(53.8%), yellow (36.9%), and chocolate (9.3%) dogs were
affected. Age at the first episode of collapse ranged from 5
to 60 months (median 12 months). When asked to comment
on their dog’s body condition, level of fitness, and tempera-
ment, all owners reported that their dogs were in good phys-
ical condition; dogs were commonly described as extremely
fit, muscular, and excitable.
Dogs had experienced from three to 34 (a median of five)
observed episodes of collapse at the time the survey was
completed. Twenty-two (10%) dogs had experienced >25
episodes. Seven (3%) of the dogs for which surveys were
completed had died during their last episode of collapse;
these dogs had experienced three to 10 (a median of four)
previous episodes of collapse. Three of the seven dogs that
died during collapse were observed to experience what the
owners reported as a short, generalized seizure just before
death.
Owners described in their own words the clinical find-
ings during each collapse episode, and they completed
checklists [Table 1]. Most (78%) owners reported that their
dog’s rear legs were floppy and unable to support weight
during episodes and that many continued to run while drag-
ging their rear legs. A wobbly, incoordinated, base-wide or
“loose” gait was described in 135 (60%) dogs. Although
rear limbs alone were most often affected, all four limbs
were abnormal during at least one episode of collapse in
18% of dogs. Staggering and falling to the side or difficul-
ty maintaining balance was reported in 68% of the dogs
during at least one episode, but no head tilt or nystagmus
was noted.
When dogs were recumbent during an episode, 18% of
owners noticed forelimb extensor rigidity. Most (77%) dogs
were reported by their owners to be mentally normal during
every episode, but 52 (23%) dogs had experienced one or
more episodes where the owners felt they lost concentration
or appeared dazed or confused. Excessive or loud panting
was noted during at least one episode in 43 (19%) dogs, and
an abnormally enlarged tongue and ropey saliva were
reported in 18 (8%) dogs. Seven (3%) dogs experienced a
short, generalized seizure during a single episode of col-
lapse, and three of these dogs died. Body temperature was
measured during at least one episode of collapse in 82 dogs
and ranged from 39.4˚C to 42.8˚C (median 41.1˚C). Owners
described recoveries from episodes as gradual, with the time
required for return to a normal gait estimated at 5 to 45 min-
utes (median 10 minutes).
Activities most often associated with collapse are pre-
sented in Table 2. Not all dogs participated in all activities
listed, making it impossible to assess the comparative risk
associated with each activity. Commonly cited trigger activ-
ities included hand-thrown “fun” retrieves of balls, toys, or
296 JOURNAL of the American Animal Hospital Association November/December 2008, Vol. 44
Table 1
Owner Description of Collapse Episodes in
225 Labrador Retrievers With Exercise-
Induced Collapse
Description % of Dogs
Rear limbs floppy/dragged 78
Wobbly, incoordinated 60
Falling to side/balance problem ≥1 episode 68
Rear limbs only affected 82
All four limbs affected ≥1 episode 18
Forelimb rigidity ≥1 episode 18
Dazed/disoriented ≥1 episode 23
Loud/excessive panting ≥1 episode 19
Generalized seizure during 1 episode 3
bumpers (a soft, plastic tube or retrieving dummy), espe-
cially in competition with other dogs; retrieving bumpers or
birds on land as part of a training exercise or field trial;
excited play with other dogs; and hunting on land for pheas-
ant, dove, or quail (upland hunting). Collapse during trained
swimming retrieves or waterfowl hunting were less com-
mon, and many owners commented that collapse was less
likely to occur when the dog was swimming. Individual col-
lapse episodes were also reported in dogs chasing horses
(one dog), deer (three dogs), and rabbits (two dogs).
Owners were asked to speculate on possible factors con-
tributing to collapse in their dog [Table 3]. Excitement asso-
ciated with a trigger activity was the most commonly
reported factor (187 dogs, 83%). Some owners reported
they “could tell” that their dog was going to have an
episode, because the dog would bark or be excessively
excited when starting the trigger activity. The use of live
birds in retriever training, chasing crippled birds during
hunting, and stress during formal retriever training (e.g.,
difficulty finding a marked retrieve, electronic collar cor-
rection, repeating an unsuccessful marked retrieve) were all
cited as factors leading to collapse.
Competition with other dogs during play or retrieving
was also considered to be a factor increasing the level of
excitement and likelihood of collapse. All owners reported
that they believed their dogs could engage indefinitely in
moderately strenuous activity not associated with excite-
ment, such as jogging or running alongside a bicycle, with-
out exhibiting signs of collapse.
Some owners (71 dogs, 31%) felt that excessive envi-
ronmental heat and humidity increased the likelihood of
collapse in their dogs. Owners were asked to report the
environmental conditions during observed episodes of col-
lapse, and their estimates of ambient temperature were cat-
egorized as hot (>21˚C), moderate or cool (5˚C to 21˚C), or
cold (-20˚C to 5˚C). Ambient temperatures were estimated
for 834 episodes of collapse in the 225 dogs. Forty-two per-
cent of the individual collapse episodes were reported to
occur in hot weather; 55% were reported to occur in mod-
erate or cool weather; and 3% were reported to occur in cold
weather.
Most owners were unable to state with certainty whether
other dogs related to theirs were affected with EIC, but 29
(13%) dogs were known to have one or multiple littermates
affected with a collapse syndrome. Occasional observations
were reported of other closely related dogs being affected
and a few instances of sudden exercise- or heat-associated
death in related dogs.
Owners were asked to assess the impact that EIC had on
their dog’s life and to describe the responses to treatments
they had tried. Seven (3%) dogs had died during an episode
of collapse. Twenty (9%) dogs were euthanized or placed in
a pet home because they could no longer be bred or inten-
sively trained for field competition. The response to treat-
ment or management was difficult to determine from the
questionnaires. Most owners who had tried restricting their
dog’s participation in trigger activities (135/138) reported
either a dramatic decrease in the number of collapse
episodes or a complete resolution of the clinical syndrome.
Five owners commented that if they observed their dog
carefully and stopped exercise at the first sign of an abnor-
mal gait, they could prevent collapse. Owners of 10 dogs
(eight males, two females) concluded that neutering had
resulted in clinical improvement; they reported no further
episodes of collapse with moderately restricted exercise.
Four competitive field trial dogs treated with phenobarbital
(2 mg/kg once or twice a day) had a dramatic decrease in
episodes or complete resolution of the syndrome, despite
continued participation in training and field trial competi-
tion. None of the dogs with EIC developed unusual sys-
November/December 2008, Vol. 44 Exercise-Induced Collapse of Labrador Retrievers 297
Table 2
Owner-Reported Activities Associated With
Collapse in 225 Labrador Retrievers With
Exercise-Induced Collapse
Activity % of Dogs
Fun retrieves 46
Training retrieves on land 43
Upland hunting (pheasant, dove, quail) 25
Excited play with other dogs 22
Training retrieves in water 12
Waterfowl hunting 2
Table 3
Factors Reported by Owners as
Contributing to Collapse in 225 Labrador
Retrievers With Exercise-Induced Collapse
Factor % of Dogs
Excitement 83
Heat and humidity 31
Live birds in training or hunting 25
Stress during training 13
Competition with other dogs 9
temic or neurological abnormalities during the follow-up
period of 3 months to 13 years (median 1.5 years) between
the first collapse episode and completion of the survey.
Heritability
Examination of the pedigrees established that many affect-
ed dogs were closely related. All EIC dogs for which the
authors had a sufficient amount of generational information
could be assembled into one large kindred, indicating a
familial basis for the condition. Eight more manageable
pedigrees containing three to 49 affected individuals were
assembled for analysis. Within these eight pedigrees, 16
matings resulted in two or more affected offspring. Males
and females were equally represented (51%:49%), exclud-
ing an X-linked mode of inheritance. In the authors’ sample
set of 326 dogs, only nine of 169 affected dogs were known
to have an affected parent. Six known affected parents
appear in these pedigrees. From these affected parents, one
family with full phenotypic information (in which one
affected parent mated to an unaffected parent) produced two
affected and two unaffected offspring. In three families, an
affected dog produced multiple (in one case 15) affected
second- and third-generation offspring. The pedigree analy-
sis was most consistent with an autosomal recessive mode
of inheritance, although a dominant disorder with partial
penetrance or a polygenic disorder could not be excluded.
Discussion
The syndrome of EIC is a relatively common cause of exer-
cise intolerance, limiting performance in working Labrador
retrievers; but detailed descriptions of the syndrome are not
available in the veterinary literature. No test for EIC cur-
rently exists, so presumptive diagnosis requires recognition
of the syndrome and elimination of other causes of exercise
intolerance and collapse. This report describes owner sur-
vey responses of the demographic and clinical features of
EIC as well as the types of activities most likely to lead to
collapse in affected dogs.
Dogs with EIC often continued to run while dragging
their crouched rear legs during episodes of collapse.
Incoordination was noted in the rear legs or, occasionally, in
all four legs; falling to the side suggested a problem with
balance in >50% of the dogs. These findings are more sug-
gestive of a neurological disorder than a condition affecting
skeletal muscle.8-10 Dogs with weakness due to an abnor-
mality of muscle, peripheral nerve, or the neuromuscular
junction are normally reluctant to continue exercising. They
typically develop a short-strided, stiff gait rather than the
“wobbly,” base-wide, or incoordinated gait described in
these dogs.10-13 Dogs with EIC, upon recovery, exhibited
no evidence of muscle pain or stiffness suggesting overex-
ertion.
Some owners reported that once they learned to recog-
nize early signs of incoordination in their dog, they could
prevent collapse by halting activity. Recovery after collapse
was gradual, occurring over 5 to 45 minutes. The progres-
sive nature of the collapse and the gradual recovery make it
unlikely that EIC is a manifestation of a paroxysmal disor-
der like epilepsy or cataplexy.14,15
Most affected dogs were mentally normal during EIC
episodes, but 23% were reported to be dazed or confused
during at least one episode. Seven dogs died during collapse;
in three of these seven, a short, generalized seizure was
observed terminally. Veterinarians must recognize that EIC
can be fatal, and owners and trainers must be counseled to
halt activity in their dog when signs of incoordination or gait
abnormality first become apparent. The authors know of sev-
eral instances where dogs with EIC have died during an
episode of collapse induced for the purpose of veterinary
evaluation.
Exercise-induced collapse has been speculated to be a
heat-related disease.9,12,16 Body temperature recorded dur-
ing collapse was extremely elevated in some dogs of this
study; body temperature may have been underestimated if it
was measured as dogs were recovering rather than at the
onset of collapse. Body temperatures have, however, been
shown to be similarly increased in normal exercise-tolerant
Labrador retrievers immediately after 10 minutes of inten-
sive retrieving exercise (mean 41.8˚C) and following com-
pletion of a 3.5-minute land retrieve test (mean 40.8˚C)
during an AKC retriever field trial.17,18 Other breeds of
dogs engaging in less intensive or shorter-duration exercise
have been reported to experience body temperatures
between 40.1˚C and 42˚C.19-22 Owners of dogs with EIC
commonly reported that excessive environmental heat and
humidity increased the likelihood of collapse, and many
individual collapse episodes occurred in hot weather. It was
apparent, however, that extreme heat was not absolutely
necessary for collapse to occur, as the majority of the col-
lapse episodes for which ambient temperature was reported
occurred during moderate or cool weather (≤21˚C), and
occasional episodes of collapse occurred in snow or while
dogs were swimming in cold water or breaking through ice.
Nonetheless, recommendations are that dogs with EIC
should avoid trigger activities and intensive exercise, espe-
cially when the ambient temperature is hot.
Dogs with EIC seem to tolerate mild to moderate exer-
cise and can engage normally in activities such as jogging,
hiking, or swimming. Activities with continuous, intense
exercise accompanied by a high level of excitement or anx-
iety most commonly caused collapse. Competition with
other dogs, hunting for live birds, and the stress of formal
training were all cited as factors contributing to collapse in
these dogs. Genetically affected dogs not being trained for
hunting or retriever competition may go unrecognized if
they never experience the necessary combination of activi-
ty and excitement to induce an episode of collapse.
The consequences of a dog having EIC were variable in
this population of dogs and largely related to owner expec-
tations. Dogs with EIC were rarely able to continue training
or competition without some restrictions, and some of the
dogs in this study were euthanized if their primary purpose
was for competition or for breeding. Limiting trigger activ-
ities and excitement allowed some dogs to continue as
298 JOURNAL of the American Animal Hospital Association November/December 2008, Vol. 44
working dogs or pets. Exercise-induced collapse does not
seem to be a progressive disorder, and most affected dogs
are able to live a normal life if trigger activities are avoided.
Although some owners reported that neutering or the
administration of phenobarbital may decrease the likelihood
of episodes of collapse and allow some dogs to participate
in trigger activities, further investigation into potential treat-
ments for EIC is necessary.
The occurrence of EIC in littermates and the estab-
lished tendency for dogs from specific pedigree lines to be
affected strongly suggest that EIC is hereditary, but the
precise mode of inheritance has not been determined.
Pedigree analysis was most consistent with an autosomal
recessive mode of inheritance.7Thus, owners and breeders
must be aware that mating apparently unaffected dogs that
are carriers of an EIC susceptibility gene can produce
affected puppies. By using pedigrees and deoxyribonucle-
ic acid collected from affected dogs in this study, investi-
gators recently have identified multiple genetic markers
linking EIC to a particular chromosomal locus and have
identified a genetic mutation that is highly associated with
EIC susceptibility.b
Conclusion
At the present time, EIC is a presumptive diagnosis made by
ruling out other disorders causing exercise intolerance and
by observing characteristic clinical features in affected
Labrador retrievers. This descriptive study of EIC should
help practicing veterinarians recognize the syndrome and
counsel owners of affected dogs regarding management
strategies and prognosis.
Footnotes
aPedigraph software; University of Minnesota, St. Paul, MN 55108
bMickelson, personal communication, 2008
Acknowledgments
The authors acknowledge the substantial contributions that
Monica Roberts, a PhD student at the University of
Minnesota, made to their understanding of the genetics of
exercise-induced collapse. Monica was a major collaborator
in this research until her untimely death in 2006.
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300 JOURNAL of the American Animal Hospital Association November/December 2008, Vol. 44
Appendix
Survey Completed by Owners of Labrador Retrievers With Exercise-Induced Collapse
Dog name: Color: Sex:
11. What is the birth date of your dog?
12. Has your dog ever had any medical problems except for collapse episodes?
13. Is your dog on any medications?
14. When was the first collapse noted?
What was the dog doing at the time?
What was the weather like?
What did you see first?
What did you do and what happened next?
How long did the collapse last?
Could your dog walk?
How long until your dog was normal?
15. How many episodes has your dog had?
16. Have all episodes been the same as the first, or were there differences?
17. What have been the circumstances (activity and weather) for each collapse?
18. Has there been anything abnormal that you note before, during, or after an episode?
19. Has there been anything that you think seems to make your dog more or less likely to have an episode on a
given day?
10. Does it seem to you that it has gotten easier to induce an episode, or is it harder to induce or unchanged?
11. Have you tried any treatments or changed management? If so, do you think this has helped?
12. Is your dog always completely conscious and aware during the collapse?
13. Are just rear legs affected, or all four legs?
14. During collapse and recovery, which of the following best describes your dog:
(circle all that apply)
a. Legs seem floppier than normal, don’t seem to be able to hold up my dog’s weight
b. My dog seems to have trouble maintaining balance and will fall over to the side
c. Legs seem to be stiffer than normal, and held straight
15. Have you taken your dog’s temperature during an episode of collapse? What was it?
(Continued on next page)
November/December 2008, Vol. 44 Exercise-Induced Collapse of Labrador Retrievers 301
Appendix (cont’d)
Survey Completed by Owners of Labrador Retrievers With Exercise-Induced Collapse
16. Do you know of any other related dogs that are affected?
17. Have other dogs from this dog’s litter been used for competitive retrieving events?
18. What do you feed your dog?
19. When was the last time you observed an episode of collapse?
20. How would you judge your dog’s body condition right now?
a. Fat, out of shape
b. A little heavy, but in good shape
c. In perfect condition
d. Very thin
