Recognition and assessment of pain in lambs

Abstract

Local anaesthetic administered epidurally at the base of the tail, and infiltrated into the testes, spermatic cords and neck of the scrotum eliminated the behavioural and plasma cortisol responses associated with castration and tail-docking, confirming that the pain observed was dependent on afferent activity from the affected areas.

Having established the neural basis of the responses seen, attempts were made to modify the intensity of pain experienced so that measurement of the changes in behaviour and plasma cortisol concentration could be used to determine the usefulness of the behavioural and physiological indices adopted for the recognition and assessment of pain.

Pretreatment with intravenous naloxone (0.2mg/kg I/V), which was anticipated to antagonise an endogenous opioid antinociceptive system activated following castration and tail-docking, provided limited evidence for an increase in pain intensity. 6 procedures which were proposed to represent a range of intensities and duration of noxious stimulation based on the amount and type of tissue involved were ranked in the expected order using the physiological and behavioural indices selected.

A study was undertaken to determine if morphine (16mg) etorphine (10nmol) or xylazine (50μg) by either the epidural or intrathecal route could provide a method by which pain intensity could be reduced in a dose dependant manner, to further improve the indices used for recognising and assessing pain. The results of these studies suggested that the pain produced by castration or castration and tail-docking cannot be eliminated by the doses of the drugs tested and the routes used. To confirm these results preliminary experiments were performed, morphine or xylazine was administered intrathecally to demonstrate active drug gained access to, and had the expected effect on the Sural-semitendinosus reflex in the lamb. Morphine failed to suppress the semitendinosus reflex which is consistent with the results obtained. The suppression of the semitendinosus reflex by xylazine is not consistent with the absence of analgesia in lambs to which either epidural or intrathecal xylazine (50μg) had been administered prior to castration or castration and tail-docking, as the general view is that reflex suppression is highly correlated with the level of analgesia.

The results of this study present data which show. physiological and behavioural indices can be used to recognise and assess a range of pain intensities in this age of lamb originating from the scrotum, testes and tail.