Trends in mortality, body weights and tumor incidences of Wistar rats over 20 years.

Historical data of more than 8,000 Wistar rats (designation: WISW SPF Cpb) used as controls in seventy 2-year studies terminated between 1975 and 1994 were analyzed for time trends in food consumption, terminal body weight, mortality, and tumor incidences. In males there was a significant (p < 0.01) time trend towards higher terminal body weight and a tendency (p > 0.05) to lower incidences of pituitary tumors and adrenal pheochromocytomas, while mortality remained stable. Leydig cell tumors showed a significant (p = 0.0005) positive trend. In females, terminal body weight did not increase over time but pituitary and mammary tumors showed a marked and highly significant (p = 0.0001) increase, which explains a significant (p = 0.0045) positive trend in mortality. There was a significant (p = 0.0001) negative time trend for uterine adenomas/carcinomas and a slight tendency (p = 0.4135) towards a decreased incidence of endo-metrial stromal polyps. Since the average food intake data do not indicate a time trend the changes observed might probably not be related to higher caloric intake. In contrast to other authors we could not find a positive correlation between either body weight and incidences of pituitary tumors or body weight and mortality. Certain selection measures at breeding and/or a genetic drift over time might explain the time trends observed. This data does not yet indicate a need for a change in ad-libitum feeding of these animals.