Cortisol Rhythm and Colic in Horses

Cortisol is a hormone naturally produced by horses. Cortisol is one measure of stress in horses, with higher blood levels indicating that the horse is increasingly stressed by health, environmental, or performance factors. 

Under normal circumstances, cortisol levels are higher in the morning and as much as 30% lower in the afternoon. This variation is known as circadian rhythm, and is triggered by sunlight and other environmental cues.

In a research project conducted in Brazil, police horses were monitored to see if unusual cortisol circadian rhythms (CCR) could be correlated to colic cases. Four groups of horses were studied. Horses in one group were stalled 24 hours a day in an urban environment. Those in the second group were stalled in an urban environment for one-half day and turned out in paddocks the rest of each day. Those in the third group were stalled except when they were used for therapy work or recreational riding. Those in the last group were turned out in a pasture at all times and did no work.

For all horses, blood was drawn early in the morning and again in early afternoon. Cortisol levels were determined for each sample, and ratios of morning to afternoon levels were calculated. In the group of pastured horses, almost two-thirds of the horses had normal CCR numbers, while CCR was normal for fewer than a quarter of the horses that were stabled full-time in an urban environment. This would tend to indicate that some aspects of living in stalls in an urban setting generally lead to elevated stress levels, though some horses in the stalled group had normal CCR and therefore seemed not to be particularly stressed by their management.

Next, the researchers examined health records for all horses in the study during the six months prior to the study, and also tracked health for six months following the study. They found that horses with abnormal CCR numbers were two to three times more likely to colic than horses with normal CCR ratios. Though many factors can contribute to a higher risk of colic in horses, it was suggested that horses with abnormal CCR might have trouble coping with their management practices and therefore might be more susceptible to gastrointestinal problems. If testing shows a particular horse to have abnormal CCR, its owner might consider changes in environment, a different exercise plan, or more social contact to lower its colic risk level.