South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
and limited data in equine endurance athletes, suggests that the magnitude of body fluid loss,
typically reflected by weight loss, may be less important than previously thought. Rather, factors
leading to a decrease in effective circulating volume, including maintenance of sympathetic tone
and tissue perfusion, may be more important in the development of fatigue and possible organ
dysfunction. This latter statement is supported by multiple anecdotal descriptions of marked
clinical improvement after exhausted athletes are treated with simple manipulations (i.e., a few
minutes of rest in the supine position with the pelvic limbs elevated for people) or administration
of volumes of intravenous fluids that are less than would be required to replace sweat fluid losses.
In addition to apparent loss of effective circulating volume and tissue perfusion, duration of
exercise and decreased perfusion appears to be an important factor determining whether transient
tissue dysfunction will be largely reversible or may result in either immediate or delayed organ
failure. In horses, organ dysfunction and failure may clinically appear to affect one organ system
primarily (e.g., gastrointestinal tract, kidneys, or central nervous system) but can also be
manifested as multiple organ failure leading to death despite aggressive supportive care. Perhaps
the most important, but as of yet unanswered, question is what is (are) the key factor(s) that
differentiate whether a moderate degree of tissue hypoperfusion will lead to reversible organ
dysfunction or irreversible organ failure. Further, it is unclear why some metabolic disorders,
typically muscle cramping and rhabdomyolysis, occur relatively early in the course of endurance
exercise while other equine athletes can successfully complete the endurance competition with
similar, dramatic elevations in circulating muscle enzyme activities.
Although “metabolic failure” often has more dramatic clinical consequences (Figure 2), equine
endurance athletes more commonly fail to finish endurance competitions due to development of
musculoskeletal disorders. In fact, lameness is the most common cause for elimination of equine
athletes from endurance competition. Occasionally, musculoskeletal failures are dramatic (i.e.,
when a horse crashes over a solid cross country fence or when an endurance horse sustains a
serio