Efficacy of intra-muscular analgesics for acute pain in sheep

The analgesic action of intramuscularly injected buprenorphine, methadone, flunixin meglumine and xylazine was examined in sheep, using algesimetry based on a leg withdrawal response to an electrical stimulus. No analgesic response was detected for buprenorphine, methadone or flunixin meglumine, Only the alpha 2-adrenoceptor agonist, xylazine, produced an analgesic response. The current required to elicit a response increased by 170% (4.5 +/- 0.43 mA to 12.23 +/- 1.14 mA; mean +/- SE) after a dose of 0.05 mg/kg xylazine; by 180% (4.73 +/- 0.3 mA to 13.28 +/- 2.35 mA) after 0.1 mg/kg; and by 510% (4.52 +/- 0.29 mA to 27.63 +/- 3.89 mA) after 0.2 mg/kg. Intramuscular xylazine appears to be an effective analgesic agent for acute pain in the sheep and further investigation into ideal administration regimens and dosage may provide more detailed information on relationships between dose, analgesic and sedative effects. The findings also suggest that some common analgesic agents, and opioids in particular, may be ineffective for the management of acute pain in sheep and that any analgesic should be administered only on the basis of its proven efficacy in that species.