Which external symptoms predict impaired welfare in aged mice? Validation of a live scoring system against necropsy findings

Health monitoring of aged animal cohorts is challenging, and validated health assessment systems are rare. In this study, we developed a live score for aged laboratory mice based on external symptoms and applied it to untreated mice of advanced age (18–40 months) from three different C57BL/6-background strains. We then validated this system against necropsy findings obtained from the same mice to distinguish external symptoms that accurately predict internal pathologies from those that merely represent by-products of normal ageing without relevance to animal well-being. Live and necropsy score values showed a strong positive association, indicating that the live score reflects internal disease burden with reasonable accuracy overall. Abdominal palpable masses, a body condition score of 2, severe neurological/motoric symptoms and abnormal posture or gait showed the highest predictive value for high necropsy scores. Abdominal palpable masses identified internal tumours >1 cm with high accuracy but showed low sensitivity for smaller tumours. In contrast, frequently occurring symptoms such as impaired fur or skin appearance and mild neurological or motoric impairments showed no apparent association with internal pathologies. Based on these results, we propose a standardized and feasible examination scheme for routine monitoring of aged mice. This data-driven framework supports objective humane endpoint decisions and is likely to improve the detection of clinically relevant diseases, particularly tumours, while acknowledging the limitations of early-stage tumour detection.