Use and Efficacy of Analgesic Agents in Sheep (Ovis aries) Used in Biomedical Research

Sheep (Ovis aries) are widely used as large animal models in biomedical research. However, current literature on the use of analgesics in sheep generally focuses on an industry or farm level of use. This structured review evaluates use and efficacy of analgesics administered to sheep in a biomedical research setting. Electronic databases were searched with terms related to analgesia in research sheep. After application of exclusion criteria, 29 peer-reviewed publications were evaluated from 1995 to 2018. Drugs used for analgesia in sheep include opioids, alpha(2) agonists, NSAID, local anesthetics, NMDA receptor antagonists, and calcium channel blockers. Opioid agonists have previously been considered short acting and of questionable efficacy in sheep, but newer modalities may provide effective analgesia. NSAID may exhibit an analgesic effect only when inflammatory pain is present and may not be beneficial for use in acute pain models. alpha 2 agonists provide effective yet short-lived analgesia; however, side effects are of concern. Local anesthetics were previously widely used as stand-alone agents, as alternatives to the use of general anesthetics in sheep. These agents have since fallen out of favor as sole agents. Despite this, they provide a valuable analgesic effect when used as adjuncts to general anesthetic regimes. The NMDA antagonist ketamine provided good analgesia and is likely underutilized as an analgesic agent in sheep. Future controlled studies should further evaluate the analgesic properties of ketamine in sheep.