Following four different surgical procedures in lambs 3-5 weeks old, plasma immunoreactive {3- endorphin ({3-EP) and cortisol were assayed at 15 min and 24 h as determinants of post-operative stress. A threefold increase in mean plasma {3-EP levels occurred 15 min after tail docking, and a maximal eight- to tenfold increase occurred in response to castration and/or mulesing with tail docking. Significant increments in mean plasma cortisol levels followed these surgical procedures with the maximal response 15 min after mulesing plus castration with tail docking. The physiologically active 'free' cortisol in plasma represents about 25% of the cortisol, as measured, and the two are highly correlated. At 24 h, {3-EP levels in all. treated groups were similar to controls, although a small elevation in cortisol levels was still present in the lambs subjected to mulesing. Ultrafiltration of plasma extracts showed that peak {3-EP levels contained about 40% immunoreactivity from low molecular weight species (mol. wt the most potent analgesic endorphin, 10% {3-EP l- 27 , and 15% a-N-acetyl-{3-EP. Increased {3-EPI-31 levels may modulate post-operative pain in lambs.

A comparative study has been made in lambs 3 to 6 weeks of age of the behavioural responses and changes in plasma immunoreactive beta-endorphin (ir beta-endorphin) and cortisol after docking or docking plus castration by the application of rubber rings or by surgery. The use of rubber rings on lambs at this age was characterised by very agitated behaviour indicative of considerable distress for a period of up to 1 h. In contrast, surgery was accompanied by some post-operative shock exhibited by reduced exploratory and social behaviour, at least in the lambs exposed to docking plus castration. In the latter group there were highly significant increases in both plasma ir beta-endorphin and cortisol concentrations that may be consistent with the induction of stress-induced analgesia. We conclude that surgery caused less distress than the rubber rings, and is therefore preferable for docking and castration of lambs 3 to 6 weeks of age.

Cutaneous myiasis in sheep arising from the activity of Lucilia cuprina larvae can result in significant physiological changes in susceptible animals. The stress imposed on the pituitary-adrenal axis of the sheep in response to myiasis and acute restraint is the subject of this investigation. Merino wethers were exposed to handling restraint, and blood sampling, during examination for blowfly strike; where necessary, they were treated for cutaneous myiasis. Significant changes in the plasma concentrations of immunoreactive J3-endorphin (J3-EP), ACTH and cortisol were found in sheep with extensive myiasis, as compared with unstruck sheep or those with only localized myiasis. In five susceptible sheep with extensive cutaneous myiasis, mean plasma levels of J3-EP, ACTH and cortisol were 307 ± 71 pg ml-1, 953 ± 58 pg ml-1 and 232 ± 46 nmoll-1 respectively, compared with 818 ± 89 pg ml-1, 641 ± 41 pg ml-1 and 107 ± 17 nmol I-I in six unstruck sheep handled similarly. Whereas significant increases in plasma ACTH and cortisol can result from pituitary-adrenal responses to acute emotional or surgical stress, and are usually accompanied by a concomitant release of J3-EP from the pituitary, the present findings indicate a marked reduction in J3-EP levels and a significant increase in ACTH and cortisol in sheep following blowfly strike and acute handling restraint. This result suggests that cutaneous myiasis in susceptible sheep can alter the pituitaryadrenal response to acute restraint stress, and this could occur either by an alteration of precursor processing in the pituitary or by the selective release of ACTH.

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