EFFECT OF INDIVIDUAL HOUSING AND OTHER EXPERIMENTAL-DESIGN FACTORS ONTUMOR-INCIDENCE IN B6C3F1 MICE

The effects of individual housing and other experimental design factor s on body weight, survival, and tumor incidence in 72 control groups o f B6C3F1 mice were evaluated. Individually housed males showed a great ly reduced incidence of dermal/subcutaneous tumors and an improved sur vival relative to group-housed animals. However, there were significan t body weight increases in individually housed males and females and a n associated marked increase in liver tumor incidence in both sexes an d a lesser increase in lung neoplasms in males. Body weights of mice a s young as 19 weeks of age were predictive of subsequent liver tumor i ncidence. There were no major differences in tumor rates among the var ious types of control groups, and differences in tumor rates among lab oratories were not significant for most tumors. Differences among anim al suppliers may have contributed to the time-related decreased incide nce of malignant lymphoma observed in control mice, particularly in fe males. Comparisons with earlier control tumor rates suggest that there has been little change in tumor incidence for control groups having a pproximately equivalent body weights. However, control groups with hea vier animals have shown a striking increase in the incidence of liver tumors. The National Toxicology Program recently returned to its earli er practice of group-housing female mice, and this should reduce the i ncidence of liver tumors in this sex-species group. However, if measur es 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 tumor s and possibly also lung neoplasms. (C) 1994 Society of Toxicology.