Introduction: Exhaustion Syndrome (ES) in endurance horses is a common clinical
observation, especially in hot and humid conditions. The main cause of ES in horses
undergoing strenuous exercise is inadequate thermoregulation. Insufficient hydration and
inadequate training are contributing factors. An acute form of ES was observed in racing
Thoroughbreds, immediately after competition. The aim of this study is to describe
epidemiologic characteristics of Acute Exhaustion Syndrome (AES) in racehorses.
Material and Methods: All animals (n=29) presented with AES after racing at the
Brazilian Jockey Club (JCB), Rio de Janeiro, between December 2006 and March 2007
were included in this study. Complete clinical examinations were conducted on site and
data from this examinations as well as climate and race characteristics were recorded.
Results: Cloudy weather was observed in 48.3% of the days when there were AES
occurrences. Temperature varied from 17oC to 33oC and relative air humidity from 49%
to 78%. Colts (52%), 3 year olds (41.4%) and horses receiving furosemide before
competition (55.2%) were the majority of animals affected by AES. Clinical signs
included tachypneia (82.8%), tachycardia (89.7%), restlessness (72.4%), anxiety (69%)
and dehydration (65.5%). Two horses collapsed and presented temporary loss of
consciousness and 4 presented diaphragmatic flutter. Conclusion and Discussion: The
observed clinical signs in Thoroughbred racehorses presented with AES are compatible
with those observed in ES in endurance horses. Hot and humid conditions were prevalent
but cloudy days had more cases of AES than sunny days. The fact that colts and 3 year
olds were more affected by AES reflects the general characteristics of the Thoroughbred
population racing at the JCB. Nevertheless, the use of furosemide before competition
appears to predispose to AES, since the majority of animals displaying AES had been
medicated with the diuretic, which does not coincide with the general population. A
greater degree of dehydration due to the use of furosemide might be responsible for that
finding. More studies on AES are required to recommend prophylactic methods.