1536 JAVMA | DEC 15, 2018 | VOL 253 | NO. 12 C ooling and freezing as methods for anesthetiz- ing or euthanizing amphibians and reptiles have been discussed since at least the late 1980s. 1 Histori- cally, most authors have recommended against the use of these practices 1–7 because of the lack of evi- dence that hypothermia results in analgesia, concerns that the reduced mobility or torpidity associated with hypothermia would result in an inability to react to aversive stimuli, and worries that ice-crystal forma- tion in the tissues could potentially result in pain. 1,4–6 Cooling and freezing are recognized as accept- able methods for performing certain procedures re- quiring local anesthesia and are used for cryosurgi- cal procedures in humans 8–11 and some nonhuman animals. 12 However, this does not imply that these methods are suitable for whole-body anesthesia. Studies 13,14 of adult zebrafish (Danio rerio) suggest that rapid cooling in a laboratory environment is a comparatively humane process for providing anesthe- sia but also indicate that rapid cooling is not recom- mended for invasive procedures. Hypothermia has been suggested for anesthesia of adult Pacific salmon (Oncorhynchus gorbuscha), 15 but the authors of that report cautioned against generalizing their findings to other species. The AVMA Guidelines for the Eu- thanasia of Animals 5 suggest that “[r]eptiles and am- phibians can be euthanized by rapid freezing when it results in immediate death,” but warn that this is applicable only for animals that weigh < 4 g (0.1 oz) and should not be used for species that have adapted freeze-tolerance strategies. Moreover, the guidelines state that many recommendations for amphibians and reptiles are extrapolated from information available for mammals, suggesting that the reliability of the in- formation is limited. In contrast, 2 recent articles 16,17 have revisited the issue of using cooling and freezing (or cooling then freezing) as methods of anesthetizing or euthanizing amphibians and reptiles. Both articles suggested that these methods are humane and convenient in labo- ratory environments. Both also emphasized animals used for research purposes, but the authors’ com- ments have broader implications and challenge eth- ics committees to support the use of hypothermia for Viewpoint Clifford Warwick MedSci George Bates MS, DVM Phillip C. Arena BSc, PhD Catrina Steedman BSc From the Emergent Disease Foundation, Riverside House, River Lawn Rd, Tonbridge TN9 1EP, England (Warwick, Steedman); Wilson College, Chambersburg, PA 17201 (Bates); and the School of Veterinary and Life Sciences, Murdoch University, Mur- doch, WA 6150, Australia (Arena). Address correspondence to Mr. Warwick (cliffordwarwick@gmail.com). anesthetizing or euthanizing amphibians and reptiles, question some assumptions concerning the capacity of amphibians and reptiles to feel pain under certain conditions, and argue that cooling and freezing occur naturally in some species and should be used more widely for anesthetizing or euthanizing amphibians and reptiles. In light of this, we wanted to examine the long-standing guidance and recent information on the use of cooling and freezing of amphibians and reptiles, with a particular emphasis on the precau- tionary principle and avoidance of harm. General Considerations The historical recommendation against using cooling and freezing to anesthetize or euthanize am- phibians and reptiles was not based on dedicated physiologic research but was instead grounded in the precautionary principle. That is, the lack of evidence that cooling and freezing were humane meant that these methods could not be recommended. Shine et al, 16 in a recent review and study involv- ing cane toads (Rhinella marina), concluded that “cooling followed by freezing can offer a humane method of killing cane toads, and may be widely ap- plicable to other ectotherms (especially, small spe- cies that are rarely active at low body temperatures).” In coming to this conclusion, the authors cited the re- actions of mammals and birds to falling ambient tem- peratures (eg, attempts to increase metabolic heat production) as an indication that low temperatures may cause intense discomfort and contrasted these reactions with apparent acceptance of hypothermia in amphibians and reptiles because these ectotherms naturally experience highly variable body tempera- tures. We argue, however, that exposure to falling temperatures and associated body temperature varia- tions do not imply that all such conditions are consis- tently acceptable in ectotherms. Other authors 18 have pointed to the limited current information support- ing hypothermia and the need for further physiologic research to conclude that “[r]apid freezing (without prior cooling) is NOT appropriate for field euthanasia of cane toads” and that “[c]ooling followed by freez- Reevaluating the use of hypothermia for anesthetizing and euthanizing amphibians and reptiles