A “more humane” way to kill mice for scientific experiments

Millions of rodents are killed each year for scientific research. A new study in Proceedings of the Royal Society B has proposed a better way to do it than carbon dioxide poisoning.

The mice death rate is unavoidable for much of science: a brand-new drug candidate for a deadly disease, for instance, must be tested on an animal model before trialling in people.

When the research is finished, the mice eventually have to be euthanised.

The most common way of killing mice is by exposing them to carbon dioxide. But researchers at the University of Glasgow and the University of Newcastle upon Tyne, UK, have suggested a more humane method.

They point out that there are several studies questioning whether CO₂ can provide a comfortable death.

“Previous work has amply demonstrated a wide range of negative sensations and experiences that are likely to compromise mouse welfare such as pain, fear, anxiety, respiratory distress and dyspnoea, all of which occur during the conscious phase, and at CO₂ concentrations as low as 10%,” they write in their paper.

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Their proposed alternative is hypobaric hypoxia: gradually decreasing the pressure in a chamber, causing death by lack of oxygen.

This method has already been approved by the European Commission on Food Safety for stunning poultry.

In this mice study, the researchers placed 144 mice in a chamber, one at a time. A third of the mice were killed by CO₂, a third by decompression, and a third were left alive as a control group.

They also gave an analgesic or an anxiolytic (pain and anxiety reducing drugs, respectively) to a subset of the mice, to compare how their behaviours changed.

During each session, the researchers recorded signs of pain and distress in the mice, like ear scratches and head flicking.

They found that, while decompression took longer to knock the mice unconscious and then kill them (more than six minutes to become motionless on average, compared to less than four minutes by CO₂), the rodents displayed fewer signs of pain and anxiety.

While the mice under decompression did get a moderate ear haemorrhage, the researchers believe this happened after they had fallen unconscious.

“Decompression offers a promising avenue to viable and effective refinement,” they write in their paper.

With more research to more closely investigate the mice’s pain, the researchers believe that decompression could be a more ethical way to euthanise them.