gms | German Medical Science

Background: In preclinical research, a constant assessment of animal well-being is crucial to ensure ethical standards and accordance to guidelines. It is a tough task to define rules within which the well-being is deemed ok, and when to claim that the suffering of the animal exceeds a tolerable burden and thus, the animal needs to be sacrificed. A common and easily accessible indicator for well-being is body weight.

For rodents, a popular rule states that an animal that experiences > 20% body weight loss has to be sacrificed. However, research experiments are of highly various nature [1]. An absolute rule for all of them can lead to unnecessary deaths of lab animals that are still within reasonable limits of well-being, but for various reasons fall below the body weight limit. In food studies, for example, it is usually the case that animals on a restricted diet show very little weight gain but are rather healthy and outlive their companions on a heartier diet by far [1].

An additional challenge is presented by studies on juvenile rodents which are still within their growth phase. Here, a weight loss might not be observable, but a reduced weight gain could indicate complications. One possibility is to monitor their weight gain compared to the weight gain within a control group of animals.

Methods: We investigated a different approach to firstly unify assessment for juvenile and adult animals and to compensate for different conditions within different experiments. If a reasonable control group can be kept within the study design, the body weight within the control group is assumed to be lognormally distributed. Within the interval of mean log body weight plus/minus two standard deviations, about 95.45% of all control animals are expected. Thus, this interval contains acceptable body weights. If the interval is enlarged to mean log body weight plus/minus three standard deviations, 99.73% of all control animals are expected to lie within these limits. In several rodent studies, it is now investigated if the rule “lab animal's body weight lies within the back-transformed limits from the log normal distribution of body weight from control animals”, they are expected to be well enough to stay within the study.

Results: Our data suggests that thus, many animals can stay in the study that would have been falsely sacrificed if the common body weight rule was applied. Thus, the rule is in favor of animal welfare, and it also increases the power of the experiment.

It should be noted that the rule on body weight is only one indicator. If an animal refuses food, gets listless or expresses other visible signs of distress, this animal will still be removed from the study due to ethical reasons.

The authors declare that they have no competing interests.

The authors declare that an ethics committee vote is not required.