Electroencephalography during castration in six-month old Bos indicus cattle

Summary

Pain is a sensory and emotional experience and given the difficulties in assessing the emotional component of pain in animals this project aimed to focus on the sensory component (nociception) of the pain experience associated with castration in Bos indicus bull calves. To this end the electroencephalogram (EEG) and the changes in mean arterial blood pressure (MAP) and heart rate (HR) in the immediate period following castration were measured. In addition, post-operative pain assessment including liveweight changes, activity and behaviour were described over the 13 days after surgery.

Forty-two 6-8 month old Bos indicus bull calves were divided into four study groups: The study groups were as follows: anaesthesia without castration (NC) n=6; anaesthesia with castration (C) n=12; anaesthesia and castration with pre-operative lidocaine (CL) n=12; anaesthesia and castration with pre-operative meloxicam (CM) n=12. Animals were randomly allocated to each study group. An open castration technique was employed. In the CL group, 260 mg of lidocaine was injected into the testicles and subcutaneous tissue at the incision site 5 minutes prior to castration. In the CM group, 0.5 mg/kg of meloxicam was administered by subcutaneous injection 30 minutes prior to castration. All the animals were anaesthetised with halothane according to the minimal anaesthesia model (Murrell and Johnson, 2006).

The EEG response to castration indicated that Bos indicus bull calves elicit a nociceptive response to castration without analgesia. The pre-operative administration of lidocaine attenuated, but did not abolish, the EEG response to surgical castration. Moreover, bull calves pre-treated with meloxicam exhibited a diminished EEG response compared to the control group. However, this response was still increased compared to the lidocaine group. Overall these data demonstrate that both lidocaine and meloxicam reduce nociception following surgical castration, however lidocaine does so to a significantly greater extent.

Without analgesia the cardiovascular response to castration was a decrease in MAP and HR. The percentage decrease in MAP and HR in the CL and CM groups was less than in group C. Lidocaine prevented a decrease in MAP and HR associated with the first incision and meloxicam prevented a decrease in MAP in response to the first incision. These data support the EEG findings that both lidocaine and meloxicam interfere with the nociceptive response to castration.

Post-operative assessments were not able to consistently differentiate between study groups with regards to liveweight, activity and behaviour. There were only subtle differences in activity and behaviour in the animals that received meloxicam.