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Pain management in reptile patients (Proceedings)

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Aug 01, 2008

When one speaks of pain management in dogs they are discussing one species with research to back up their statements. Did you realize there are over 7000 species of reptiles? The three major orders are snakes, lizards and turtles. Commonly kept species of snakes range in size from a cornsnake (Elaphe guttata) at 2 feet to a Burmese Python (Python molurus) at 12 or more feet. Not only are they of different in size, but some reptiles are from the rain forest and some are from the desert, some are New World species (from South America for example) and others are Old World species (from Australia or Africa for example). In addition, it is known that different species of reptiles metabolize several drugs differently than other reptiles. For example, turtles die after administration of ivermectin, but most snakes (indigos are an exception) and most lizards (skinks are an exception) do not. It is hypothesized that turtles have GABA receptors in places other than the brain or that the blood:brain barrier in turtles allows ivermectin passage. Urocoan rattlesnakes, milk snakes, tricolor king snakes and indigo snakes all require a much lower dose of metronidazole than other species. There are many unknowns in regards to the metablolism or action of pain relieving drugs in the many species of reptiles. Therefore, when assessing a reptile patient for signs of pain, or deciding on which pain reliever to use, or what dose and how often to administer, it must be taken into account that there is NO GENERIC REPTILE, and one must be familiar with the very limited scientific research that has been conducted regarding pain management in reptiles. Also, each patient must be evaluated and re-evaluated individually and constantly.

Reptiles are poikilothermic, or "cold blooded" animals, therefore, their body temperature is roughly room temperature. Realize though that a reptile can maintain a body temperature for several hours, so if a reptiles is cold to the touch in the examination room that just left its home environment a hour ago, then that reptile should still be warm. Also, "pre-heating" of reptiles prior to surgery or heating as part of therapy is a common and an integral part of reptile medicine, because it is used to increase the metabolic rate. Ideally a reptile should be maintained within its "preferred optimum temperature zone" or POTZ for optimal metabolism of medications.

In general, turtles, with a suspected slower metabolic rate, are administered medications less frequently (for example an antibiotic is given every 3 days) than snakes (same antibiotic may be given every 2 days) than lizards (same antibiotic may be given every day).

What research has been done in reptiles and what is known?

Few studies have evaluated pain perception in reptiles. A 1969 study of indigo snakes (Drymarchon corais couperi), bull snakes (Pituophis catenfir sayi)] and one caiman (Caiman crocodilus) showed that relatively high doses of the opioids, etorphine (up to 5000 ug/kg), oxymorphone (1.5 mg/kg), and meperidine (200 mg/kg) produced no significant outward effects. So, for many years after the 1969 study veterinarians mistakenly assumed that those opioids had no analgesic effect either For comparison purposes, a typical canine dose of butorphanol (an opioid) is 0.2 mg/kg and causes sedation in the dog, whereas iguanas given 8.0 mg/kg IM showed no sedation at all.

Later studies showed more promise at evaluating pain perception in reptiles, or at least antinociception (avoidance of noxious stimuli). Curly-tailed lizards (Leiocephalus carinatus) given morphine (1-3 mg/kg) significantly increased their behaviorly selected body temperatures and this effect could be blocked and reversed with the opioid antagonist naloxone (1 and 10 mg/kg), suggesting opiate control of behavioral thermoregulation in lizards. Crocodiles (Crocodylus niloticus africana) exposed to a hot plate had significantly (P<0.001) increased response latency for "escape" or "lifting the foot" after intramuscular administration of morphine (0.5 and 0.05 mg/kg, respectively) or meperidine (2.0 and 1.0 mg/kg, respectively). Exposure to the hot plate as a stimulus provided a better pain model than did ocular instillation of capsaicin or intramuscular injection of formalin in these crocodiles, since it allowed for the measure of supertheshold "escape" responses, as well as threshold "lifting the foot" responses.