Comprehensive Analysis of the Impact of Weight Loss Thresholds on Mouse Models of Fatal Viral Infection

Preclinical studies in virological research are pivotal to comprehend mechanisms of viral virulence and pathogenesis and evaluate antiviral therapies or vaccines. Mouse models, through access to various genetic strains and amenable reagents, along with their ease of implementation and cost-effectiveness, remain the gold standard for establishing go/no-go thresholds before advancing to non-human primate or clinical studies. In preclinical mouse studies, standardized weight loss thresholds (WLTs)-which correspond to an established percentage of weight change at which animals are humanely euthanized-are a routine metric to quantitatively evaluate the lethality of a viral pathogen and the effectiveness of antiviral countermeasures in preventing fatal viral disease. While it is recognized that WLTs can significantly impact the assessment of viral virulence, they are often established to meet existing ethical or methodological requirements, rather than being based on a specific scientific rationale. Here, we examine how various experimental variables-including mouse and viral strains and the sex ratio within a mouse cohort-influence the ability of a WLT to support the generation of robust mouse models of fatal viral infection. Using various mouse strains and viral pathogens, we report that variations in experimental conditions in mouse preclinical studies can significantly compromise the performance of a non-adjusted WLT to yield an accurate estimate of viral virulence. Our findings advocate for a robust adjustment of WLT to each experimental framework and associated variables to establish mouse models of fatal viral infection that can generate high-resolution data acquisition while upholding ethical standards. Overall, our study provides methodological insights to enhance the unbiased acquisition and benchmarking of viral virulence and antiviral efficacy data in mouse models.