Plasma levels of beta-endorphin and in vitro lymphocyte proliferation as indicators of welfare in horses in normal or restrained conditions

Plasma β-endorphin levels and lymhocyte proliferation were measured in 14 geldings to evaluate the animals' response to different challenges (application of a lip twitch for five minutes and restraining the animals blindfolded for two hours). The basal β-endorphin levels showed a great variability among the horses, but were consistent for the basal levels measured on different days for each animal. A significant effect of age on β-endorphin concentration was found (p=0.005). The β-endorphin levels showed an increase after the use of the lip twitch (p=0.0001) and returned to the pre treatment values within 30 minutes after the release of the animal. Plasma β-endorphin levels at the end of the blindfold treatment were lower than basal levels (p=0.025) and there was a decrease in the proliferation of lymphocytes (p=0.00001). A negative correlation was found between plasma β-endorphin levels and lymphocyte proliferation for individual horses in samples taken before (p=-0.62; p=0.039) and two hours after the application of the hood (p=-0.59; p=0.049).