The effect of three anaesthetic protocols on the stress response in cane toads (Rhinella marina)

Objective Three anaesthetics (MS222, clove oil and a mixture of ketamine/diazepam) were administered to cane toads to determine their effect on the hypothalamic-pituitary-adrenal (HPA) axis. Time to induction and recovery and any adverse events were also evaluated. Study design Prospective randomized experimental trial. Animals Thirty adult male cane toads (Rhinella marina) with body mass ranging between 130 and 250 g were captured from the field. Methods Three groups of 10 toads were anaesthetized with ketamine (200 mg kg-1) and diazepam (0.2 mg kg-1) by intramuscular injection, MS222 (3 g L-1) or clove-oil (0.3 mL L-1) both by immersion. Blood samples were collected to determine plasma corticosterone concentrations. Induction and recovery time were recorded in each treatment. After full recovery animals were euthanized and a complete post-mortem examination was performed. Results Significant differences were found in the activation of the HPA axis and in the times of induction and recovery between treatments (p < 0.001). Animals anaesthetized with clove-oil had the highest levels of corticosterone in plasma (42.5 +/- 21.6 ng mL-1). No differences were found between ketamine/diazepam (15.0 +/- 13.3 ng mL-1) and MS222 (22.0 +/- 13.6 ng mL-1) groups. The mean +/- SD induction (minutes) and recovery (hours) times respectively were; ketamine/diazepam 66.5 +/- 11 and 8 +/- 3, clove oil 39 +/- 12 and 7.6 +/- 3, and MS222 42.5 +/- 11 and 1.5 +/- 0.5. Clove oil exposure had 30% mortality. Death followed a period of respiratory distress with changes consistent with non-cardiogenic oedema observed at post-mortem examination. Conclusions and Clinical relevance Based on shorter induction and recovery times and minimal activation of HPA, MS222 is the anaesthetic of choice in cane toads. If it is not possible to use immersion methods of anaesthesia, ketamine/diazepam can be used but induction and recovery times are prolonged. Clove oil had unacceptable mortality in this study and should be used with extreme caution