In many animal experiments scientists and local authorities define a body-weight reduction of 20% or more as severe suffering and thereby as a potential parameter for humane endpoint decisions. In this study, we evaluated distinct animal experiments in multiple research facilities, and assessed whether 20% body-weight reduction is a valid humane endpoint criterion in rodents. In most experiments (restraint stress, distinct models for epilepsy, pancreatic resection, liver resection, caloric restrictive feeding and a mouse model for Dravet syndrome) the animals lost less than 20% of their original body weight. In a glioma model, a fast deterioration in body weight of less than 20% was observed as a reliable predictor for clinical deterioration. In contrast, after induction of chronic diabetes or acute colitis some animals lost more than 20% of their body weight without exhibiting major signs of distress. In these two animal models an exclusive application of the 20% weight loss criterion for euthanasia might therefore result in an unnecessary loss of animals. However, we also confirmed that this criterion can be a valid parameter for defining the humane endpoint in other animal models, especially when it is combined with additional criteria for evaluating distress. In conclusion, our findings strongly suggest that experiment and model specific considerations are necessary for the rational integration of the parameter ‘weight loss’ in severity assessment schemes and humane endpoint criteria. A flexible implementation tailored to the experiment or intervention by scientists and authorities is therefore highly recommended.

Objective Ethical approval of experiments in chronic epilepsy models requires a careful balancing of the expected gain‐in‐knowledge with the level of distress. Thus recommendations for evidence‐based severity assessment and classification are urgently needed for preclinical epilepsy research. Methods Therefore, we have completed a comprehensive analysis of alterations in behavioral, biochemical, and physiological parameters in a rat electrical post‐status epilepticus model. Selected parameters were repeatedly analyzed during different experimental phases to obtain information about the level of distress throughout the course of the model. Results Behavioral patterns comprised an increase in activity along with a reduction in risk assessment behavior, active social interaction, saccharin preference as well as nonessential, but evolutionary‐determined behavior such as nest building and burrowing. Among the biochemical parameters, fecal corticosterone metabolites proved to be increased in different phases of the experiment. In the early post‐insult phase, this increase was reflected by elevated serum corticosterone concentrations. Telemetric recordings demonstrated increases in home cage activity and heart rate in selected experimental phases but argued against relevant changes in heart rate variability. Comparison between animals with tethered or telemetric recordings including a principal component analysis revealed differences between both groups. Significance The present findings further confirm that burrowing behavior and saccharin preference might serve as valid parameters for severity assessment in chronic epilepsy models. Considering the course of alterations providing evidence for a more pronounced level of distress in the early phase following status epilepticus (SE), we suggest a classification of the electrical post‐SE model as severe. This suggestion may serve as a guidance for laboratory‐specific evaluations. Comparison between data from animals with tethered and telemetric recordings indicated an impact of the mode of recordings. However, further research is necessary to analyze the validity of telemetry as a putative refinement measure.

Objective Considering the complexity of neuronal circuits and their epilepsy‐associated alterations, epilepsy models cannot be completely replaced by in vitro experimental approaches. Decisions about ethical approval of in vivo studies require a thorough weighing of the animal's burden and the benefit regarding the expected gain in knowledge. Methods Based on combined behavioral, biochemical, and physiological analyses, we assessed the impact on animal well‐being and condition in different phases of the pilocarpine post–status epilepticus (SE) model in rats. Results As a consequence of SE, increased levels of impairment were evident in the early postinsult phase and late chronic phase, whereas only mild impairment was observed in the interim phase. Parameters that stood out as sensitive indicators of animal distress include burrowing, which proved to be affected throughout all experimental phases, saccharin preference, fecal corticosterone metabolites, heart rate, and heart rate variability. Significance The cumulative burden with temporary but not long‐lasting phases of more pronounced impairment suggests a classification of severe as a basis for laboratory‐specific prospective and retrospective evaluation. Among the parameters analyzed, burrowing behavior and saccharin preference stand out as candidate parameters that seem to be well suited to obtain information about animal distress in epileptogenesis models.

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Comparative severity assessment of animal models and experimental interventions is of utmost relevance for harm-benefit analysis during ethical evaluation, an animal welfarebased model prioritization as well as the validation of refinement measures. Unfortunately, there is a lack of evidence-based approaches to grade an animal's burden in a sensitive, robust, precise, and objective manner. Particular challenges need to be considered in the