The Role of Emotional Contagion in the Distress Exhibited by Grouped Mice Exposed to CO₂.

The 2013 AVMA Guidelines for the Euthanasia of Animals recommends a chamber volume displacement rate of 10% to 30% per minute (v/min) when euthanizing small laboratory rodents with CO₂. Group euthanasia of mice is a common practice, and grouping strangers is often avoided to minimize distress; however, emotional contagion, which occurs between familiar animals but not strangers, has not been studied in the context of group CO₂ euthanasia. This study examined cagemate- and stranger-grouped mice exposed to 10%, 30%, or 50% v/min CO₂ to determine whether emotional contagion plays a role in this context and whether that role is influenced by CO₂ flow rate. Videos of adult male C57BL/6J mice exposed to different CO₂ flow rates were scored for durations of dyspnea, ataxia, and consciousness as well as the numbers of face pawing and jump behaviors. Blood was collected at time of unconsciousness and assayed for ACTH. Cagemates experienced significantly longer durations of conscious dyspnea and ataxia with 10% v/min CO₂ compared with 30% and 50% v/min. Similarly, strangers experienced significantly longer duration of conscious dyspnea with 10% v/min CO₂ compared with 30% and 50% v/min and significantly longer duration of ataxia with 10% compared with 50% v/min. Cagemates showed significantly more jumps with 10% v/min CO₂ compared with 30% and 50% v/min, whereas jumping was unaffected by CO₂ flow rate in strangers. We conclude that more potential for distress exists when cagemate and stranger mice are exposed to a 10% v/min CO₂ flow rate and that emotional contagion may contribute to distress in cagemates at this flow rate. Therefore, we propose that 30% v/min CO₂ should be used for euthanasia of mice, and that 50% v/min should also be considered humane

Sex- and Strain-related Differences in the Stress Response of Mice to CO₂ Euthanasia.

The 2013 edition of the AVMA Guidelines for the Euthanasia of Animals recommends a 10% to 30% volume displacement rate (VDR) per minute for CO2 euthanasia of rodents. Here we sought to evaluate behavior and plasma catecholamine levels in multiple strains of male and female mice, euthanized individually or in a group, with CO2 VDR of 10% to 100%. Behavioral observations included ataxia, labored breathing, time to recumbency, time to surgical plane of anesthesia, and the number of jumps or paws at the face during the euthanasia process. Behaviors did not differ significantly between male and female mice at any of the VDR, but interstrain differences occurred. Slower VDR resulted in longer periods of ataxia and labored breathing regardless of euthanasia as a group or as an individual. In addition, mice jumped and pawed at the face more often with slower VDR than higher. At all VDR, mice euthanized as a group had lower catecholamine levels than mice euthanized individually, but there were no significant differences between VDR. Time to recumbency and time to surgical plane anesthesia were longer with slower displacement rates; in addition, these parameters were prolonged for mice euthanized as a group compared with individually. Overall, faster VDR do not appear to be more distressful than slower rates. In fact, faster VDR shorten the time during which mice might experience distress prior to recumbency

Review of CO₂ as a Euthanasia Agent for Laboratory Rats and Mice.

Selecting an appropriate, effective euthanasia agent is controversial. Several recent publications provide clarity on the use of CO2 in laboratory rats and mice. This review examines previous studies on CO2 euthanasia and presents the current body of knowledge on the subject. Potential areas for further investigation and recommendations are provided. J Am Assoc Lab Anim Sci 2017 Sep 1; 56(5):491-499