At the end of an animal experiment, the vast majority of animals are killed. A recent study has however, added to the body of evidence that the most commonly used method of killing the animals - carbon dioxide gas - is not the most humane.
Researchers in Canada found that when they gave rats the choice between staying in a dark chamber that was filling up with carbon dioxide gas or a brightly lit chamber filled with normal air, the rats chose the bright room. Because rats are prey animals they usually prefer to keep out of the bright light where they can be seen more easily. This suggests therefore that the rats found the gas aversive. When the researchers switched the option to isoflurane, more of the rats remained in the dark isoflurane chamber until they fell unconscious. Isoflurane is an anaesthetic gas that could be used instead of carbon dioxide but is not, mainly because it is more expensive.
The use of carbon dioxide gas to kill animals used in experiments is extremely common even though studies such as this have shown that the animals find it distressing. Animals have to be gradually exposed to the gas and then checked for signs of life. However, as we found during our investigation at Imperial College London, protocols are not always followed. At Imperial College, animals sometimes took longer to die and were not always checked for signs of life before they were put in plastic bags for disposal. During another investigation, at Wickham Laboratories, we found that one of the methods used involved mice being killed in large numbers inside gas chambers; the distress they experienced can be clearly seen.