Most animal facilities schedule regular, time-dependent cage changes to manage the microenvironment within mouse cages. A 2-week cage-change frequency is accepted as the industry standard for individually ventilated cages (IVCs). Experiments involving certain enteric viruses may call for longer intervals between full changes to sustain persistent infection. Cage-change frequency is tied to both increases in animal stress and labor costs. Using 8-week-old wild-type C57BL/6J mice housed in IVCs (76.03 sq. in. with approximately 75 air changes per hour), we assessed the ability to extend the full cage-change interval for cages housing either 2 males or 2 females per cage. As an alternative to full cage changes, we also assessed the effect of changing half the bedding every 14 days for cages containing either 2 female, 4 female, or 2 male mice. We monitored microenvironmental parameters (in-cage ammonia, humidity, and temperature) along with visual cleanliness scores, daily health assessments, and histopathology of nasal tissue at study endpoint to determine the maximum acceptable time between full cage changes. We found that both pair-housed males and females could be housed without bedding change for 42 days, reaching exclusion due to soiled bedding accumulation instead of ammonia threshold or animal health concern. Cages with 2 male or 2 female mice that underwent biweekly partial bedding changes could be housed without a complete cage change for 84 days. Cages with 4 female mice on biweekly partial bedding changes maintained appropriate microenvironmental parameters for 62 days. No mice, in any group, exhibited severe histopathologic evidence of ammonia-induced injury or other disease. Our results indicate that, when cage density is reduced or partial bedding changes can be completed, extending the time before full cage change did not exceed predefined action thresholds and did not produce clinically apparent illness or histologic evidence consistent with ammonia-related injury.
Lamacchia et al. (Sun,) studied this question.