Humane Killing

Humane killing methods cause rapid (immediate) unconsciousness and subsequent death with no pain or distress accompanying the procedure.

In science, choice of the most humane method of killing an animal also requires consideration of impacts the method could have on research results.

These principles of euthanasia may help to select a humane killing method:

  • Whenever an animal’s life is to be taken it should be treated with the highest respect
  • The method causing the least animal pain and distress should be used
  • The killing method should:
    • cause rapid loss of consciousness, followed by cardiac or respiratory arrest and ultimate loss of brain function
    • require minimum restraint of the animal
    • be appropriate for the species, age and health of the animal
  • Death must be verified following humane killing
  • Personnel should be trained and competent
  • Human psychological responses should be taken into account, but should not take precedence over animal welfare considerations
  • Animal care committees should be responsible for approval of the method of humane killing

This section has been adapted from the International Council for Laboratory Animal Science International Harmonization of Guidelines on Euthanasia (ICLAS, Nantes, France, 2004), approved in Buenos Aires, Argentina, November 2004, available on Science Online.

Use of Carbon Dioxide

The use of carbon dioxide is no longer considered an acceptable method of humane killing. Carbon dioxide should not be used where other methods are practical for the experiment and the species.

The Newcastle Consensus Meeting on Carbon Dioxide Euthanasia of Laboratory Animals (Hawkins et al., 2006) identified the following problems with use of CO2:

  1. There is no “ideal” way of killing animals with CO2 - both pre-fill and rising concentrations can cause welfare problems.
  2. If animals are placed into a chamber containing a high concentration of CO2 (above 50%), they will experience at least 10-15 seconds of pain in the mucosa of the upper airways before loss of consciousness. This is a serious welfare problem.
  3. If animals are placed into a chamber with a rising concentration of CO2, they will find it aversive at a certain level and may experience “air hunger” or dyspnoea which is unpleasant (in humans, it is reported as highly distressing). This may also be a welfare problem.

In addition, the Newcastle Consensus Meeting identified good practices for CO2 euthanasia:

  • It is more important to avoid or minimize pain or distress than it is to ensure a rapid loss of consciousness; thus a “gentle” death that takes longer is preferable to a more rapid, but more distressing death.
  • The optimum chamber filling rate is uncertain. For rats, use of 100% CO2 at a flow rate of 20% of the chamber volume per minute has been shown to produce loss of consciousness without evidence of pain, but not without evidence of dyspnoea. Reduced flow rates can be increased once the animals have lost consciousness.
  • It is possible that the addition of oxygen (O2) to CO2 may reduce, but not overcome, welfare problems caused by pain or dyspnoea. It is also possible that high concentrations of O2 would prolong consciousness, which may not be desirable. There is currently insufficient information in the literature to reach a clear conclusion on the appropriate level of O2.

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