Clinical Analgesia in Reptiles

TOPICS IN MEDICINE AND SURGERY CLINICAL ANALGESIA IN REPTILES Kurt K. Sladky, MS, DVM, Dip. ACZM, and Christoph Mans, med. vet.

Abstract Reptile pain and analgesia is only beginning to be understood in veterinary research and clinical medicine. The diversity of the class Reptilia also makes it difficult to extrapolate analgesic efficacy across species. Many veterinary clinicians argue that the administration of analgesic medication is risky to the patient and may mask behavioral signs of pain, which are considered evolutionarily adaptive for survival. However, veterinarians have an ethical obligation to treat painful conditions in all animals, including reptiles, because effective pain management reduces stress-induced disruption to homeostatic mechanisms and also decreases morbidity and mortality associated with trauma or surgery. Nevertheless, several obstacles limit successful analgesic use, including subjectivity of pain assessment, inadequate knowledge regarding analgesic efficacy across species, pharmacokinetics of analgesic drugs, and the unknown relationship between risks and benefits for this class of drugs. The objective of this review is to provide a current perspective on the practical application of analgesic medication in commonly maintained pet reptile species. Copyright 2012 Elsevier Inc. All rights reserved. Key words: analgesia; analgesic; opioid; pain; reptile

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n veterinary medicine, the understanding of pain and analgesia in domestic mammals has grown exponentially during the past 20 years, yet the knowledge obtained is only a fraction of what is required to make appropriate treatment recommendations. Currently for reptile patients, a veterinarian’s ability to describe pain and make logical decisions regarding appropriate analgesic therapy is rudimentary at best. Many veterinary clinicians still argue that the administration of analgesics is risky to the patient and may mask behavioral signs of pain, which are considered evolutionarily adaptive for survival. However, veterinarians have an ethical obligation to treat painful conditions in all animals, because effective pain management reduces stress-induced disruption to homeostatic mechanisms and also decreases morbidity and mortality associated with trauma and/or surgery. The primary objective of this article is to describe and highlight the most current information with respect to clinically relevant analgesic use in reptile species. REPTILE PAIN VERSUS NOCICEPTION Do reptiles feel pain? Perhaps, more importantly, can veterinarians recognize pain in reptiles? Is the perception of pain by a reptile equivalent to that of a mammal? We will never be able to objectively answer whether reptiles feel pain because they simply cannot communicate with humans. However, similar to human infants or mammals, should the inability to verbally communicate dictate whether pain is being perceived or an analgesic drug administered? Most medical

professionals would argue, emphatically, no. Because the word “pain” conjures up anthropocentrism, nociception and antinociception are used when referring to pain and analgesia. This stems from the controversy concerning whether nonmammalian species have central and peripheral nervous system structures and pathways capable of “receiving and processing” noxious stimuli, with a subsequent appropriate response by the animal. In other words, can nonmammals “experience” pain, or are they merely capable of dem-

From the Department of Surgical Sciences, Zoological Medicine, School of Veterinary Medicine, University of Wisconsin, Madison, WI USA. Address correspondence to: Kurt K. Sladky, MS, DVM, Dip. ACZM, Department of Surgical Sciences, Zoological Medicine, School of Veterinary Medicine, University of Wisconsin, 2015 Linden Drive, Madison, WI 53706. E-mail: [email protected]. © 2012 Elsevier Inc. All rights reserved. 1557-5063/12/2102-$30.00 doi:10.1053/j.jepm.2012.02.012

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Journal of Exotic Pet Medicine 21 (2012), pp 158 –167

onstrating a “reflexive” response to a noxious stimulus (nociception)? Like mammals, reptiles have all of the anatomical structures considered critical for the recognition of pain: peripheral nociceptors, appropriate central nervous system structures and pathways, opioid receptors and endogenous opioids, reduction of nociceptive response with analgesics (although data are sparse), pain avoidance learning, and suspension of normal behavior with pain.1-3 Recent research in fish, amphibians, reptiles, and birds has demonstrated that the transmission of peripheral sensory signals, via the spinal cord, to midbrain and forebrain regions is homologous to mammalian cortical and limbic structures.4-7 Thus, the physiologic and anatomical requirements for pain and analgesia appear to be remarkably similar among all vertebrate species. Therefore, as veterinarians, it is our contention that, because of our limited understanding of pain and analgesia in reptiles, we should err on the side of a reptile patient’s well-being and make the assumption that conditions considered painful in humans and other mammals should be assumed to be painful across other vertebrate species. MEASURING NOCICEPTION AND ANTINOCICEPTION IN REPTILE SPECIES Measuring pain in reptiles is the most difficult task when studying analgesic efficacy. In mammals, it is well established that perioperative pain management facilitates recovery and healing, reduces morbidity and mortality, and contributes to a decreased time interval for a return to normal behavior.8,9 An objective understanding of normal behavior of a particular species and the ability to differentiate the presence of abnormal behavior indicative of discomfort are critical to the study of pain and analgesia. In order to be able to discriminate behavior associated with pain, one must first have an understanding of normal species-specific behavior within the environmental context in which that behavior is being displayed. For example, the behavior of a green iguana (Iguana iguana) in its home cage may be different from its behavior in a hospital cage. Methods for assessing and measuring pain in reptiles have been previously described.10 Ideally, a combination of appropriate behavioral and physiologic parameters should be used to measure pain and analgesia in reptiles. Along those same lines, the development of a speciesand context-specific ethogram for each species being evaluated will provide the best method for

distinguishing normal versus abnormal (e.g., painful) behaviors. Often, animal pain, or lack thereof, is assessed before and after surgical procedures. This assessment of animal pain requires the development of a behavioral ethogram, which, in turn, requires the observer to become well versed in subtle behavioral differences through many hours of observation and analysis (videotaped or live observation). For example, in a recent study, our laboratory developed a behavioral ethogram to evaluate preoperative and postoperative behavioral responses to food intake, willingness to swim, and breathing in red-eared slider turtles (Trachemys scripta elegans) after a unilateral orchidectomy (Kinney et al, unpublished data). We were able to compare postoperative behavior in turtles with and without analgesic administration using morphine, butorphanol, and saline solution. Our hypothesis was that preoperative behaviors would have a more rapid return to normal in those turtles receiving a ␮-opioid receptor agonist analgesic agent. We demonstrated that those turtles receiving morphine and undergoing unilateral gonadectomy returned to normal preoperative behavior more quickly than those receiving saline solution or butorphanol. An alternative to studying postsurgical pain is to measure pain under strictly controlled laboratory conditions using established behavioral models during which noxious stimuli (e.g., mechanical, thermal, chemical) are applied to an anatomic location on the reptile subject.11-15 Analgesic drugs can be administered and the response compared with baseline responses. The application of a noxious thermal stimulus provides a well-established behavioral model for assessing pain and analgesia in rodents.16 In our own studies, we have successfully adapted this classic thermal nociception model developed for use in rodents, and determined that reptiles show unambiguous, easily quantifiable withdrawal responses indistinguishable from rodents.3,11-13 The thermal hind limb withdrawal latency model uses a noxious thermal stimulus applied to the plantar surface of the hind limb or the ventral surface of the body (e.g., snake species) of different reptile species (Figs. 1 and 2). Withdrawal latency is automatically determined when the animal withdraws its limb or tail from the noxious stimulus. This model has many advantages over other noxious stimulus paradigms, including rapid application and decay of the noxious stimulus (thereby not causing long-lasting inflammation), instant latency quantification, and unambiguous behavior after stimulus expo-

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cause there is a paucity of analgesic pharmacokinetic and/or pharmacodynamic data collected from reptile species. Whether an analgesic drug is administered intravenously (IV), intramuscularly (IM), subcutaneously (SC), per os (PO), or transcutaneously, its effectiveness is partially dependent on the size and temperament of the individual reptile. Anatomic location is another consideration regarding analgesic administration, because there is pharmacological evidence that drug administration to the hind limbs or tail of a reptile may cause rapid clearance by the renal portal system or, in the case of opioids, the hepatic first-pass effect.10,21 However, whether this more rapid clearance has any implication on the clinical effect of an analgesic drug remains unknown. It was once believed that subcutaneous administration of drugs in reptiles would lead to

FIGURE 1. Red-eared slider turtle (Trachemys scripta elegans) at rest in the Hargreaves apparatus (noxious thermal hind limb withdrawal apparatus) during research testing of analgesic responses to opioid administration. The turtle has the plantar surface of its right hind limb placed directly over the infrared heat source.

sure (either the animal does or does not withdraw its limb). Most importantly, the animal can escape the noxious stimulus by simply withdrawing its limb. Although some might argue that a withdrawal response to a noxious stimulus is not equivalent to pain, there is evidence in a number of animal species that noxious thermal stimulation does cause molecular and cellular changes in the central nervous system.17-19 In addition, some may suggest that reptiles are prone to thermal burns, and therefore it is less likely that they will respond to a noxious thermal stimulus. Although the veterinary community’s understanding of a thermal burn injury in reptiles is inadequate, there is no doubt that all reptile species studied in our laboratory perceive and respond to the focal heat source in an identical manner as a mammal. ANALGESIC DRUGS Practical Methods of Administration In the reptile anesthesia article published in the previous issue, we highlighted routes of drug administration, which are applicable to routes of analgesic drug administration as well.20 It is difficult to describe the most appropriate methods for administering analgesic drugs in reptiles, be1 6 0

FIGURE 2. Bearded dragon (Pogona vitticeps; top image) and corn snake (Pantherophis guttata; bottom image) at rest in the Hargreaves apparatus during research testing of analgesic responses to opioid administration. The bearded dragon has the plantar surface of its right hind limb placed directly over the infrared heat source. Note the circular region at the center of the “X” in the corn snake image, just in front of the snake’s head, which is the focal infrared heat element.

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significantly delayed absorption of that product because of the fact that the subcutaneous space is not well vascularized in reptiles; however, it has been determined that meperidine, an opioid analgesic with rapid absorption and excretion in humans, administered SC in red-eared slider turtles causes measurable behavior changes within 30 minutes of administration.11 The authors routinely administer parenteral analgesics SC in our clinical patients and research subjects. Other routes of analgesic administration are also useful in reptiles. Transdermal fentanyl patches have been applied to 2 reptiles species, and plasma concentrations were detectable in both species.22,23 Oral administration is an uncommonly used method for analgesic administration in reptiles, because there is concern that a potentially slow gastrointestinal transit time will delay onset, peak efficacy, and clearance. However, we have found just the opposite in our laboratory. Tramadol and metabolite plasma concentrations can be detected within hours of oral administration in loggerhead sea turtles (Norton et al, unpublished data), and thermal hind limb withdrawal latencies increased in red-eared slider turtles within 4 hours of oral tramadol administration.3 Table 1 summarizes commonly used analgesic protocols for reptile species. OPIOID AND OPIOID-LIKE DRUGS Opioid drugs are the most effective therapeutic agents for controlling pain in mammals and are classified according to receptor subtypes—␮ (mu), ␬ (kappa), and ␦ (delta). For pain management in mammals, many clinicians prefer administering either a ␮-opioid receptor agonist (e.g., morphine, fentanyl, hydromorphone), a partial ␮-opioid receptor agonist (e.g., buprenorphine), or a mixed-opioid, ␬-receptor agonist–␮receptor antagonist (e.g., butorphanol). Opioid receptors must be present in order for exogenous opioid drugs to have a beneficial analgesic effect in reptiles. The gene family for opioid receptors (␮, ␬, and ␦) is highly conserved across multiple vertebrate orders.24 Two snake species have endogenous brain opiates,25,26 and red-eared slider turtles have both proencephalan-derived peptides and functional ␮- and ␦-opioid receptors in the brain.24,27 Although opioid receptors are expressed in reptiles, the veterinary medical community is just beginning to understand the efficacy of commonly prescribed opioid drugs. Butorphanol tartrate is considered to be the most commonly ad-

ministered opioid analgesic drug in reptiles.28 In a published survey of veterinary medical clinicians, the applied reptile dosage range for butorphanol varied from 0.02 to 25 mg/kg.28 However, there were no clinical data to substantiate that butorphanol was an effective analgesic drug in reptiles. Butorphanol has been used as a primary analgesic in reptiles because it appeared to have a beneficial treatment effect. In 2005, veterinary researchers broached the first question regarding the use of butorphanol in reptile species; how do we know that butorphanol has analgesic properties in reptiles? Using an innovative noxious thermal stimulus paradigm, and through a series of controlled experiments, it was determined that butorphanol had no analgesic efficacy in red-eared slider turtles when administered at dosages of 2.8 and 28 mg/kg SC, or in bearded dragons (Pogona vitticeps) when administered at dosages of 2 and 20 mg/kg SC.11-13 The same butorphanol data obtained in corn snakes were too variable to make any firm conclusions. An unfortunate consequence of that publication13 was that several clinicians decided to administer butorphanol, 20 mg/kg, to debilitated snakes, which resulted in patient fatalities. Consistent with the turtle and bearded dragon data, butorphanol administered IM at a dosage of 1 mg/kg has no analgesic efficacy (determined by use of a thermal noxious stimulus method15) and no isofluranesparing effect in green iguanas.29 In ball pythons (Python regius), butorphanol administered at 5 mg/kg IM had no effect on physiologic parameters compared with saline solution.30 Conversely, one study demonstrated that butorphanol may provide analgesia in green iguanas exposed to a noxious electrical stimulus.14 Morphine sulfate was an effective analgesic agent in bearded dragons (1 and 5 mg/kg) and turtles (1.5 and 6.5 mg/kg) using the thermal noxious stimulus method, but data were not as clear in corn snakes, even when morphine was administered at a dosage of 40 mg/kg.13 Similarly, morphine (5, 7.5, 10, and 20 mg/kg) and pethidine (10, 20, and 50 mg/kg) provided analgesia in Speke’s hinged tortoises (Kinixys spekii) exposed to a noxious formalin stimulus administered into a limb, which was reversible with naloxone treatment.31 To further support the effectiveness of morphine in reptile species, it increased limb withdrawal latencies in crocodiles32,33 and increased tail-flick latencies in anole lizards.34 Fentanyl is a ␮-opioid receptor agonist, with 75 to 100 times the potency of morphine, and may be administered IV or transcutaneously as

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1 6 2 TABLE 1. Analgesic protocols commonly used in reptiles Dose (mg/kg)/Species Opioid drugs Buprenorphine

Butorphanol Fentanyl

Hydromorphone Sladky and Mans/Journal of Exotic Pet Medicine 21 (2012), pp 158 –167

Meperidine (pethidine) Methadone

Route

Frequency

0.075-0.1 0.02-0.1

SC, IM

q24h

1.0-20.0 0.4-8.0 12.5 mcg/h 2.5 mcg/h

SC, IM

NDA

TC

0.5-1.0

SC, IM

One patch for 24-72 hours q24h

1.0-5.0 10.0-50.0 3.0-5.0

SC, IM

q2-4h

SC, IM

q24h

Morphine

1.0-40.0

SC, IM, IT

q24h

Tramadol

5.0-10.0

PO

q48-72h

SC, IM

NDA

Ketoprofen

0.1 1.0-4.0 2.0

SC, IM

⬎q31h

Meloxicam

0.2-0.3

IV, SC, IM, PO

NDA

NSAIDs Carprofen

Local anesthetic drugs Lidocaine (1% or 2%)

1-2 (keep ⬍ 5.0) 4.0

SC, IM, IT

NDA

Bupivicaine (0.5%)

1.0 (keep ⬍ 2.0) 1.0

SC, IM, IT

NDA

Comments No evidence of analgesic efficacy; plasma concentrations equivalent to those effective for analgesia in mammals; respiratory depression not studied Analgesic efficacy questionable; significant respiratory depression in RES (⬎ 10 mg/kg not recommended) Transdermal patch; no analgesic efficacy determined; plasma concentrations detected in ball pythons and prehensile tailed skinks Good analgesic efficacy in RES; respiratory depression not studied Analgesic efficacy short-lived in RES; respiratory depression not studied Good analgesic efficacy in RES; anecdotal evidence in lizards species; respiratory depression not studied Good analgesic efficacy in RES and bearded dragons; unknown efficacy in snakes; significant respiratory depression (⬎ 5 mg/kg not recommended) Good analgesic efficacy with relatively long duration when administered PO in chelonians; less respiratory depression than other opioids in RES

Reference 14,21,35

11-15,29,30 22,23

35 31,33,49 49 11-14,31,32,34

3

No evidence of analgesic efficacy; presumed antiinflammatory efficacy No evidence of analgesic efficacy; presumed antiinflammatory efficacy; plasma concentrations equivalent to those with efficacy in mammals with ⬎ 24-hour duration in plasma No evidence of analgesic efficacy; presumed antiinflammatory efficacy; no physiologic changes in ball pythons administered postoperatively

2,10,50

No systematic data published, but appears to be a good focal nerve block and effective IT in RES No systematic data published, but appears to be a good focal nerve block and effective IT in RES

43,44

47

30,50,51

43

Abbreviations: IM, Intramuscular; IV, intravenous; SC, subcutaneous; PO, oral; TC, transcutaneous; IT, intrathecal; NDA, no data available; Lg, large; RES, red-eared slider turtles.

1.0-2.0 Midazolam

SC, IM

NDA

43,53-56 No evidence of analgesic efficacy; low dose as an analgesic is extrapolated from the mammalian literature; frequently combined with an alpha-2, benzodiazepine, or an opioid in sedation/anesthesia protocols No evidence of analgesic efficacy; frequently combined with an alpha- 54,56-59 2, ketamine, or an opioid; enhances analgesic properties of dexmedetomidine in rats NDA SC, IM, IV 0.1-0.2 2 Ketamine

No evidence of analgesic efficacy; frequently combined with ketamine, 52,53 midazolam, or an opioid in sedation/anesthesia protocols NDA SC, IM

SC 1.0

Mepivicaine (2%) Other Anesthetics Alpha-2 agonists Medetomidine/ Dexmedetomidine

0.05-0.3

42 Used as a mandibular nerve block in alligators NDA

Reference Comments Frequency Route Dose (mg/kg)/ Species TABLE 1. Continued

an impregnated patch. The analgesic efficacy of fentanyl in any reptile species has not been determined. However, 2 separate studies have evaluated the pharmacokinetics of fentanyl administration in 2 reptile species. In ball pythons, fentanyl plasma concentrations reached 1 ng/mL within 4 hours of application of a transdermal fentanyl patch (12.5 ng/h)23 and were detectable by 4 to 6 hours, and for ⬎ 72 hours, in the plasma of prehensile-tailed skinks (Corucia zebra) (fentanyl dose was applied at 10% exposure of total surface area of a 25 ␮cg/h patch for 72 hours).22 It remains unclear whether there is any biological significance to the plasma fentanyl concentrations. Buprenorphine is an effective analgesic drug in many mammalian species and is used extensively in veterinary medicine because of its longer duration of action compared with other opioid agents. Buprenorphine has partial agonist activity at the ␮-opioid receptor, partial or full agonist activity at the ␦-opioid receptor, and antagonist activity at the ␬-opioid receptor. Buprenorphine did not alter responses to a noxious electrical stimulus administered to green iguanas.14 Similarly, in another investigation, buprenorphine provided no analgesic effect in red-eared slider turtles exposed to a noxious thermal stimulus.35 Buprenorphine pharmacokinetic data in a reptile species were determined after SC administration in red-eared slider turtles, with the effective dosages ranging between 0.075 and 0.1 mg/kg. The results of this research study provided plasma concentrations of buprenorphine in red-eared slider turtles that were similar to those associated with analgesic efficacy in humans for approximately 24 hours.21 Although the significance is unknown, plasma concentrations of buprenorphine were reduced by approximately 70% when the drug was administered in the hind limb compared with the forelimb of the red-eared sliders. Tramadol has become a widely used analgesic alternative to other opioid drugs in veterinary medicine, because it is administered orally and is currently not considered a controlled substance. Tramadol and its metabolite, O-desmethyl-tramadol, not only produce analgesia in mammals by activating ␮-opioid receptors, but also by inhibiting central serotonin and norepinephrine reuptake.36,37 Parent tramadol has ␮-opioid activity, whereas O-desmethyl-tramadol, the active metabolite of tramadol, has up to 200 times greater affinity for ␮-opioid receptors.38 Overall, tramadol binds ␮-opioid receptors with 6000 times less affinity than morphine,38 thus having the

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potential for producing fewer ␮-opioid–induced side effects. In fact, tramadol does not appear to alter breathing in humans39 and produces significantly less respiratory depression than morphine in cats and dogs.40,41 In mammals, the analgesic effects of tramadol typically begin within 30 minutes and last for 6 hours. In contrast, tramadol (5.0 mg/kg; PO) administered to turtles significantly increased withdrawal latencies for 12 to 24 hours after drug administration, and 6 to 96 hours after administration of the higher tramadol dosages (10 or 25 mg/kg; PO).3 Tramadol and O-desmethyl-tramadol plasma concentrations can be detected within hours of oral administration in loggerhead sea turtles (Caretta caretta) (Norton et al, unpublished data). With respect to deleterious side effects, respiratory depression associated with tramadol administration in red-eared slider turtles was approximately 50% less than that measured after morphine administration.3 Therefore, tramadol appears to be a promising analgesic alternative to the use of traditional opioid drugs in reptile species. Regardless of which opioid agent is administered to a reptile patient, it is imperative that clinicians continue to monitor resBecause there piration during and after the procehave been no dure.

efficacy studies and few pharmacokinetic investigations with respect to NSAID administration in reptile species, appropriate dosages and frequency of administration can only be extrapolated.

PARENTERAL ANESTHETIC DRUGS Ketamine administered at low dosage levels is commonly used for analgesic purposes in mammals. However, there are no data to substantiate its use in reptiles. As in mammals, combining ketamine with an alpha-2-adrenergic agonist (e.g., medetomidine, dexmedetomidine) increases the level of sedation in reptile species. Although such combinations provide analgesia in mammals, it can only be speculated that the same is true in reptiles, as no data are available. LOCAL ANESTHETIC AGENTS

Drugs administered to provide local anesthetic effect (e.g., lidocaine, bupivacaine, mepivacaine) are commonly used to block peripheral nerve transmission when mammals or reptiles are exposed to procedures

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considered minimally painful. However, only one reptile study has been published, in which a nerve locator identified the mandibular nerve in an alligator so that a mepivacaine block could be performed.42 The limitations associated with local anesthetic administration include a focal nervous block and short duration of action. Local anesthetic agents, as well as opioid drugs (e.g., preservative-free morphine), can be administered intrathecally into the coccygeal region of chelonians.43,44 Intrathecal morphine causes antinociception to thermal noxious stimulation of the hind limbs for ⬎ 24 hours after administration in male red-eared slider turtles.43 NONSTEROIDAL ANTIINFLAMMATORY DRUGS Although not as potent as the opioid class of drugs, nonsteroidal antiinflammatory agents are used widely in reptile clinical practice for their analgesic and antiinflammatory properties. Nonsteroidal antiinflammatory drugs (NSAIDs) provide analgesia in mammals by blocking the binding of arachidonic acid to cyclooxygenase enzyme (COX), preventing the conversion of thromboxane A2 to thromboxane B2, and thus preventing production of prostaglandins, which are potent mediators of inflammation.45 NSAIDs are often classified based on their relative specificity. There are 2 COX enzymes, COX-1 and COX-2, that participate in renal and gastric protection and inflammation, respectively.45 The NSAID ketoprofen is equipotent against both isoenzymes, whereas carprofen is slightly more COX-2 specific and meloxicam is COX-2 specific.45 Therefore, the degree of efficacy and side effects may vary with each of the 3 NSAIDs selected. Although many NSAIDs appear to be relatively safe when used in reptiles, there is only one published study with respect to analgesic efficacy for these species.30 Ball pythons administered meloxicam (0.3 mg/kg, IM) before a surgical placement of an arterial catheter showed no physiologic changes (e.g., heart rate, blood pressure, plasma epinephrine, and cortisol) indicative of analgesia. Concerning published pharmacokinetic information regarding NSAID use in reptile species, plasma concentrations of meloxicam (0.2 mg/kg, PO) administered as a single dose to green iguanas were at levels considered analgesic in mammals, and these levels were measurable to 24 hours after administration.46 Ketoprofen, 2 mg/kg IV, administered to

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green iguanas had a long half-life (31 hours) compared with ketoprofen pharmacokinetics in mammals, but the bioavailability after IM administration was only 78% with a relatively short half-life (8.3 hours).47 Because there have been no efficacy studies and few pharmacokinetic investigations with respect to NSAID administration in reptile species, appropriate dosages and frequency of administration can only be extrapolated. Additionally, clinicians should be aware of the deleterious side effects documented in avian and mammalian species (e.g., renal impairment, gastrointestinal ulceration/ inflammation, hematologic abnormalities), and, therefore, use these drugs with caution. MULTIMODAL ANALGESIC TREATMENTS In reptiles, multimodal drug paradigms may be the best approach for managing pain. Multimodal analgesia refers to the administration of multiple drugs that provide analgesic effect at different levels in the central and peripheral nervous system. For example, opioid drugs will have greatest efficacy at opioid receptors within the central and peripheral nervous system, whereas NSAIDs administered at the same time as the opioid agent will have their greatest effect as antiinflammatory agents at the peripheral tissues.48 In concert, these drugs have the potential to minimize transmission of pain signals to the brain, especially when administered preemptively, before a potentially painful procedure has been performed. CONCLUSIONS Butorphanol tartrate (a ␬-opioid receptor agonist), administered at mammalian-derived dosages, was once believed to be the most effective analgesic drug for use in reptile species. However, recent scientific investigations focusing on pain and analgesia in a variety of reptile species have found that ␮-opioid receptor agonists (e.g., morphine, tramadol) provide effective analgesia, whereas butorphanol appears to be no more effective than saline solution.3,11-13 Extrapolation of analgesic efficacy across reptile orders and species remains a major limitation of analgesic use in these animals, and there is a clear need for evaluating analgesic drugs across a variety of clinical situations, particularly as they apply to postsurgical pain. The authors believe that objectively derived methods for evaluation of pain in animals are critical, but these methods must be species

and context specific. In addition, determining pharmacokinetic parameters, duration of drug efficacy, species-speHowever, recent cific requirements, and deleterious scientific side effects of opioid drugs in different reptile species remains critiinvestigations cal for the field of reptile analgesia focusing on pain to advance. REFERENCES

and analgesia in a variety of reptile species have found that ␮-opioid receptor agonists (e.g., morphine, tramadol) provide effective analgesia, whereas butorphanol appears to be no more effective than saline solution.

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