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Nature volume 441, page 550 (01 June 2006) | Download Citation

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Researchers have a duty to use the most humane means available of killing laboratory animals.

Scientists who work with animals generally agree that if their subjects have to be killed, the death should be accompanied by as little fear or pain as possible. In practice, many laboratory mice and rats are currently killed with carbon dioxide gas. The method is cheap, easy, is what everyone else does, and seems painless enough. The animals are placed in a box, the gas is turned on, and the researcher can walk away.

Some veterinarians and ethicists are not convinced the method is humane, however. Although the sparse amount of investigation done on the topic is inconclusive, they consider that exposure to slowly rising levels of carbon dioxide engenders in rodents the same feeling it does in humans: blind panic. Alternatives exist and, they say, should be used (see page 570).

“The main alternative to carbon dioxide, where large numbers of animals are involved, is to use high doses of anaesthetics used on humans.”

The main alternative to carbon dioxide, in places where large numbers of animals are involved, is to gas rodents with high doses of anaesthetics used on humans, such as halothane or isoflurane, as this would probably cause the animals little distress. Individual rodents can be dispatched with a manual manoeuvre known, somewhat euphemistically, as cervical dislocation, which breaks the animal's neck. Researchers have been doing this to rats and mice for decades and, done right, it is instantaneous and painless. Skill can be gained by practising on anaesthetized animals. Anaesthesia and cervical dislocation can also be used in combination.

In the United States, the institutional animal use and care committees that oversee animal research are likely to demand a “scientific justification” for breaking rodents' necks rather than gassing them. This is because most of them adhere to guidelines written by the American Veterinary Medical Association (AVMA), which requires such justification, noting that cervical dislocation may be “aesthetically displeasing to personnel” who have to carry it out. This piece of red tape often makes it simpler for US researchers to stick with carbon dioxide.

Some might argue that scientists who kill mice for their research should be willing to do so with their bare hands, if that is the most humane thing to do. But there is no guarantee that it always will be, and sometimes research regulations, or the particulars of the experiment, may call for the use of one method over another. Cost is another consideration, as anaesthetic gas is more expensive, and often less readily to hand, than carbon dioxide. But everyone who makes these final decisions has a duty to carefully examine the options that are available — and to take into account what's best for the rodent, as well as for the researcher.

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https://doi.org/10.1038/441550a

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