Masters Alternatives to carbon dioxide euthanasia for laboratory rats

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ALTERNATIVES TO CARBON DIOXIDE EUTHANASIA FOR LABORATORY RATS
Abstract
The most commonly used method of euthanasia of laboratory rodents is exposure to
carbon dioxide (CO2), but recent studies have shown that rodents find this gas aversive. The
aim of my thesis was to evaluate rat aversion to inhalant agents that could be used as
humane alternatives to CO2. The first study used approach-avoidance testing to examine rat
responses to argon-induced hypoxia when argon was introduced at flow rates of 40-239% of
the test cage volume per min. Rats never remained in the test cage long enough to lose
consciousness when tested with argon. They consumed fewer reward items, stopped eating
sooner, and left the test cage more quickly than when tested with air. Rats stopped eating
and left the test cage when the oxygen (O2) concentration had dropped to about 7.7 and
6.8%, respectively, but these O2 concentrations are too high to cause unconsciousness.
Although humans exposed to hypoxia report only subtle symptoms that include cognitive
impairments and light headedness, rats are burrowing rodents and could therefore be more
sensitive to these effects. I conclude that argon is not a humane alternative to CO2. The
second study used approach-avoidance testing to evaluate rat responses to different
concentrations of the inhalant anaesthetics halothane and isoflurane introduced with
vaporizers or from soaked cotton balls. On the first day of exposure to anaesthetics, most
rats remained in the test cage until they were ataxic and showing difficulty returning to the
home cage. On subsequent days of testing most rats left the test cage within seconds, but if
given the option, all promptly returned and stayed until they were ataxic, indicating that the
learned aversion is transient. Rats were likely sedated by the time they chose to leave,
suggesting that forced exposure from the onset of aversion until loss of consciousness is less
of a welfare concern than forced exposure to non-sedating agents. I suggest that the use of
inhalant anaesthetics for inducing unconsciousness prior to euthanasia is a more humane
method than the commonly used CO2.
Pages:76

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