Cortisol, eye temperature explored as potential indicators of Endurance horse welfare

Sixty-one Endurance horses competing at qualifier 40km and 80km rides were used in the study in Portugal.
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Could salivary levels of the stress hormone cortisol and the eye temperature of Endurance horses be useful in monitoring their welfare during competitions?

Researchers in a study in Portugal hypothesised that pre-exercise baseline salivary cortisol and eye temperature, as measured by infrared thermography, would be higher in younger or less experienced horses, and that post-exercise variation from baseline would be higher in the top finishers.

Monica de Mira and her fellow researchers used 61 Endurance horses competing at qualifier 40km and 80km rides.

Baseline cortisol levels were obtained from samples taken at the pre-inspection. Samples collected at the first vet gate revealed a sharp increase, ranging from 93% to 256% over baseline.

Increases at subsequent vet gates were modest, or in some cases had dropped from the levels seen at the first vet gate.

Eye temperature measurements revealed significantly higher levels at the pre-inspection among the less experienced horses participating in the 40km ride than their counterparts in the 80km ride, but not cortisol.

Baseline cortisol levels at the pre-inspection of horses classifying in the top five in the 40km ride category were significantly higher when compared to horses positioned from 10th position on.

Overall, a lower eye temperature in the pre-inspection correlated with better placement, and those who ultimately finished in the top five showed significantly higher variation into the last vet gate.

The authors, writing in the journal BMC Veterinary Research, concluded that pre-exercise baseline eye temperature levels, but not salivary cortisol, were higher in less experienced horses in the 40km ride compared to their counterparts in the 80km ride.

Cortisol and eye temperature showed different indications according to the competition, they said.

“In the 40km ride competition, higher baseline pre-exercise salivary cortisol levels seemed to be linked to a better classification outcome.”

In contrast, in the horses in the 80km ride, the higher eye temperature variation from pre-exercise into the final vet gate was the parameter associated with a better performance.

“A more controlled environment and a larger sample are needed to confirm these results and monitor horse welfare in competitions,” they said.

Discussing their findings, the researchers said elevations of basal cortisol concentrations in response to emotional stress are believed to be harmful to general health, but not necessarily to sport performance.

“Indeed, in the more inexperienced horses of the 40km ride, the higher salivary levels before and during the ride were associated with better performance, reflecting most likely the extra necessary physiological response to effort.”

In the 80km category, cortisol behaved differently. It appears it was not the pre-exercise salivary cortisol level that influenced the results per se, but the magnitude of increase from the pre-inspection to the first vet gate at 30km that was associated with a higher placement group.

They said they could not find a difference or association between eliminated or classified horses and salivary cortisol levels or eye temperature, most likely because of the low number of horses involved.

The study team comprised De Mira, Elsa Lamy, Rute Santos, Mafalda Vaz Pinto and Pedro Martins, all with the University of Évora – Polo Mitra, in Portugal; Jane Williams, with Hartpury University in England; Patrícia Rodrigues with Vasco da Gama University in Portugal; and David Marlin, with David Marlin Consulting in England.

de Mira, M.C., Lamy, E., Santos, R. et al. Salivary cortisol and eye temperature changes during endurance competitions. BMC Vet Res 17, 329 (2021).

The study, published under a Creative Commons License, can be read here

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