Abstract

Compared to the human primate, our understanding of pain in nonhuman primates is relatively nascent. There are no vigorously validated pain scales available, and current recommendations regarding pain management are mostly empirical or, at least in great apes, extrapolated from human medicine. Signs of pain may be quite obvious or completely absent, depending on factors such as the site and severity of pain or rank within the social hierarchy. Some reported signs of pain include depression, lethargy, inappetance, weight loss, crouched or other abnormal postures, grimacing or facial contortions, teeth clenching, grunting/ moaning/other vocalizations, head pressing or leaning the head against a wall, reduced or absent grooming, withdrawal from interaction with conspecifics, and guarding, holding, touching, or picking at the painful site. Some submissive behaviors (e.g., lying down) may look like signs of pain but can be differentiated from pain by segregating the patient from higher-ranking conspecifics. Providing areas for primates recovering from painful procedures to hide from aggressors after reintroduction to their group may help reduce stress and permit analgesic drugs to work better. Cameras are useful for detecting signs of pain which may not be manifested if the primate knows it is being watched. Analgesic drugs should be given not just after but before surgery (preemptive analgesia), as well as in combination (multimodal analgesia), for maximal efficacy. Not all analgesic regimens will be effective in all cases, and different drugs may need to be tried before an effective one is found for a given patient. Furthermore, some patients will develop side effects, whereas others will not. Acute pain (e.g., from traumatic injuries, biopsies, tooth extractions, etc.) can usually be managed effectively using a combination of opioids, non-steroidal anti-inflammatory drugs (NSAIDs), and when possible, locoregional anesthesia. At least 72 hr of some form of analgesic therapy is recommended following injury or surgery. Opioids can cause sedation, inappetance, pruritus, nausea, ileus/ constipation, and respiratory depression; however, these potential side effects should not preclude the use of these drugs as long as judicious doses are administered and the patient is closely monitored. Buprenorphine, a partial µ agonist, appears to be the most commonly utilized opioid in primates today and seems to provide acceptable analgesia for most mildly to moderately painful conditions encountered in zoos, especially when used as part of multimodal analgesia. In the author’s experience, however, it does not reduce inhalant requirements or the sympathetic outflow caused by surgical stimulation as profoundly as full µ-opioid agonists (e.g., morphine, fentanyl). In the uncommon event a primate undergoes a major surgery, a full µ agonist administered pre- and/or intraoperatively as a bolus(es) or constant rate infusion, as long as the patient’s ventilation can be controlled, can provide excellent cardiovascular stability and preemptive analgesia. Depending on severity of pain, postoperative analgesia can be provided with buprenorphine or a full µ agonist. A topical preparation of fentanyl designed to provide 72 hr of analgesia is being developed and may be an option for post-operative analgesia in the future. Fentanyl lozenges have been used for sedation of great apes, and lower doses in this form for analgesia with less sedation may also prove useful. This author does not recommend the use of opioid patches (e.g., fentanyl, buprenorphine) unless they can be concealed under a jacket of some kind and the patient monitored closely because of the possibility that a primate will remove a patch, ingest it, and overdose. When alimentation is possible, oral opioids are an option. Morphine (available in immediate and sustained-release preparations), methadone, oxycodone, or hydrocodone are commonly sent home with people and might be useful in at least the great apes; however, this author has no experience using these drugs in lower primates. Stool softeners and antiemetic drugs can alleviate constipation and nausea/ vomiting, respectively, caused by opioids if these occur. Tramadol is an analgesic drug with weak µ agonist properties that also inhibits serotonin and norepinephrine reuptake; it causes less sedation and gastrointestinal side effects and provides analgesia comparable to oral NSAIDs in humans. NSAIDs work well in combination with opioids for acute pain; carprofen and meloxicam appear to be the most commonly used. As with opioids, these drugs should be used at judicious doses as part of multimodal analgesia with close patient monitoring for side effects such as acute renal failure or gastrointestinal ulceration. Long-acting local anesthetics such as bupivacaine or ropivacaine can be infiltrated locally or around specific peripheral nerves (e.g., digital nerves for a finger bite wound); if a severely painful, major surgical procedure is to be performed, an epidural or spinal injection or a major peripheral nerve block might be feasible. Icing is a simple, inexpensive, effective way to decrease inflammation around a surgical site and provide analgesia. Chronic pain (e.g., osteoarthritis or degenerative disk disease in a geriatric primate) is usually more challenging to treat as it may not respond as well to traditional analgesic therapy. Nutraceuticals (e.g., glucosamine and chondroitin) can be highly effective in some humans with osteoarthritis. NSAIDs often form the foundation of chronic pain management; chronic NSAID use can often be cut back with time (e.g., a half dose every other day or twice weekly) to reduce side effects while maintaining efficacy, especially if other drugs and non-pharmacologic modalities are used simultaneously. These might include low doses of opioids, tramadol, anticonvulsants (e.g., gabapentin, pregabalin), antidepressants, and acupuncture or laser therapy performed during immobilization, among others (Bourgeois). Intraarticular (e.g., for osteoarthritis) or epidural (e.g., for radiculitis) injection of a local anesthetic plus corticosteroid (e.g., triamcinolone) are performed to provide longer-lasting analgesia in humans and can be highly effective. Finally, some drugs used for immobilization are analgesic drugs as well, notably ketamine and α₂-agonists, and may contribute to analgesia for at least some time after recovery.

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