The spearmint extract L-carvone as a euthanasia agent in sheep

L-carvone is a natural monoterpene found in spearmint and other foods that can be administered intravenously to euthanize rats. We hypothesized intravenous L-carvone rapidly alters consciousness and produces death in sheep, resulting in tissue L-carvone concentrations that are commonly found in foods. Jugular catheters were placed in five adult Hampshire-Suffolk ewes, and a carotid arterial catheter was inadvertently placed in a sixth ewe. Animals were restrained in a seated position and instrumented with surface electroencephalographic (EEG) and electromyographic electrodes and a two-lead electrocardiogram (ECG) accompanied by pulse and respiratory rate and behavioral monitoring. A 50% solution of food-grade L-carvone administered as a rapid intravenous bolus at a dose of 0.5 mL/kg was followed by bilateral ocular strabismus, loss of responsiveness, shallow breathing, and a 50% reduction in heart rate within 6 ± 4 s (mean ± SD). In 4 sheep, myoclonus and increased muscle tone was present for 15-54 s after injection. Pulses were undetectable within 20 ± 9 s, and animals were apneic in 48 ± 14 s and without cardiac electrical activity 46 ± 19 s after injection. During this time, the EEG spectral edge frequency rapidly decreased, indicative of progressively altered consciousness and deepening anesthesia. Intermittent, large amplitude polyspike waves were observed on the EEGs of most sheep, consistent with seizure activity. Intra-arterial L-carvone injection yielded quantitatively similar results except for persistence of ECG electromechanical dissociation for several minutes after death. Sheep tissues collected after euthanasia were analyzed for L-carvone by liquid chromatography-mass spectroscopy. For intravenous doses, L-carvone concentrations ranged from 283 ± 155 in the kidney to 2.2 ± 1.6 in the semitendinosus muscle. Intravascular L-carvone produces rapid onset of unconsciousness and irreversible death in sheep without contaminating the carcass with non-edible residues. However, increased muscle tone and EEG seizure activity make L-carvone imperfect for euthanasia when administered as a sole agent.