Thermal Response of Laboratory Rats (Rattus norvegicus) during the Application of Six Methods of Euthanasia Assessed by Infrared Thermography

SIMPLE SUMMARY: The present study aimed to assess the infrared thermal response of laboratory rats (Rattus norvegicus) during the application of six euthanasia methods (injectable, inhalational, and physical) to determine the method that prevents or diminishes the stress response. The surface temperature was assessed in four thermal windows: ocular (T°(ocu)), auricular (T°(ear)), interscapular (T°(dor)), and caudal (T°(tai)). The results showed that inhalant methods (CO(2) and isoflurane) had temperature alterations that could be suggestive of a marked stress response, in contrast to other methods such as pentobarbital, decapitation, and xylazine + ketamine. In conclusion, according to the thermal response of the rats, it is suggested that CO(2) and isoflurane might cause distress and this needs to be considered when selecting these techniques as the method of euthanasia for laboratory rats. ABSTRACT: Refinement is one of the principles aiming to promote welfare in research animals. The techniques used during an experimental protocol, including euthanasia selection, must prevent and minimize suffering. Although the current euthanasia methods applied to laboratory rodents are accepted, the controversial findings regarding the potential stress/distress they can cause is a field of research. The objective was to assess the thermal response of Wistar rats during various euthanasia methods using infrared thermography (IRT) to determine the method that prevents or diminishes the stress response and prolonged suffering. Pentobarbital (G(1)), CO(2) (G(2)), decapitation (G(3)), isoflurane (G(4)), ketamine + xylazine (G(5)), and ketamine + CO(2) (G(6)) were evaluated at five evaluation times with IRT to identify changes in the surface temperature of four anatomical regions: ocular (T°(ocu)), auricular (T°(ear)), interscapular (T°(dor)), and caudal (T°(tai)). Significant differences (p < 0.05) were found in G(2) and G(4), registering temperature increases from the administration of the drug to the cessation of respiratory rate and heart rate. Particularly, isoflurane showed a marked thermal response in T°(ocu), T°(ear), T°(dor), and T°(tai), suggesting that, in general, inhalant euthanasia methods induce stress in rats and that isoflurane might potentially cause distress, an effect that must be considered when deciding humane euthanasia methods in laboratory rodents.