Behavioural and hormonal responses to acute surgical stress in sheep

Abstract

A combination of plasma cortisol and β-endorphin measurement, behavioural observation and estimation of aversion to humans (by an arena test) was used to assess the response to the modified mules operation in 6–7-month-old Merino wethers. The operation involves the surgical removal of wool and skin from the lamb's breech to protect the sheep against blowfly strike. Ten animals were mulesed and compared with 10 which were handled in the same way except for mulesing. Marked elevation of plasma cortisol and β-endorphin occurred between 5 and 15 min after the operation. The highest levels of both hormones were recorded during the 24 h post-operative handling. At 48 h post-operation there was apparently little hormonal response to handling. An analgesic effect, associated with the release of the β-endorphin, was evident in the sheep behaviour for 1–2 h post-operation. Thereafter, paddock behaviour of mulesed sheep was characterised by abnormal posture and locomotion at first, then by significantly increased time spent standing and significantly reduced time spent lying, grazing and feeding from a trough. After 72 h the mulesed sheep resumed normal behaviour in the paddock. Arena testing at intervals from Day 1 to Day 37 post-operation revealed a pronounced aversion to the human handler (male) who had held the sheep during the operation. At Day 114 this aversion was no longer evident. The regular handler (female) who normally fed the sheep produced significantly less aversion between Days 42 and 114. Mulesing of weaners by contractors (rather than owners) and minimal post-operative handling are both indicated as means of reducing stress from mulesing.