Effect of individual housing and other experimental design factors on tumor incidence in B6C3F1 mice.

The effects of individual housing and other experimental design factors on body weight, survival, and tumor incidence in 72 control groups of B6C3F1 mice were evaluated. Individually housed males showed a greatly reduced incidence of dermal/subcutaneous tumors and an improved survival relative to group-housed animals. However, there were significant body weight increases in individually housed males and females and an associated marked increase in liver tumor incidence in both sexes and a lesser increase in lung neoplasms in males. Body weights of mice as young as 19 weeks of age were predictive of subsequent liver tumor incidence. There were no major differences in tumor rates among the various types of control groups, and differences in tumor rates among laboratories were not significant for most tumors. Differences among animal suppliers may have contributed to the time-related decreased incidence of malignant lymphoma observed in control mice, particularly in females. Comparisons with earlier control tumor rates suggest that there has been little change in tumor incidence for control groups having approximately equivalent body weights. However, control groups with heavier animals have shown a striking increase in the incidence of liver tumors. The National Toxicology Program recently returned to its earlier practice of group-housing female mice, and this should reduce the incidence of liver tumors in this sex-species group. However, if measures are not taken to reduce body weights, male mice (which continue to be individually housed because of fighting problems among group-housed animals) will likely continue to show a high incidence of liver tumors and possibly also lung neoplasms.