David B. Morton
Centre for Biomedical Ethics and BMSU, University of Birmingham, UK
To date most methods of assessing the humaneness of a killing method have relied on the interpretation of an animal's behaviour, and rarely with additional information on physiological and hormonal measures. Three important factors emerge when assessing humaneness: the time to loss of consciousness whereafter it cannot suffer; secondly, the effect of the administration of the agent in terms of manual skills or systems involved; and thirdly, the aversiveness of the agent to the animal. We have collected data on this latter aspect using mixtures of gases that are commonly used, or have the potential to be used, as a method to kill animals. Animals are exposed to these gases and their mixtures but at all times the animal has the choice of whether to enter and stay, or to leave. Response measures include the time taken to enter the gas mixture, the withdrawal time, the number of re-entries, and the total dwelling time. From these data we have strong reason to carbon dioxide as a humane agent to kill laboratory rats or mice. When exposed to lethal concentrations (i.e. between 35 and 95%), animals showed the most rapid withdrawal time, the shortest dwelling time, and the fewest re-entries. We found that this effect was not mitigated through humidification or by mixing with other agents such as argon or oxygen. Argon, on the other hand, was more humane according to these measures. Mixtures of the gaseous anaesthetic gases such as the fluorinated hydrocarbons with oxygen appeared to be the most humane of those tested. Other gases such as carbon monoxide have also been evaluated. We recommend, as a practical method of euthanasia, the induction of anaesthesia and hence a loss of consciousness with a fluorinated hydrocarbon (such as halothane or enflurane), followed by carbon dioxide to kill the animal.
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