Further studies of the mammary tumor agent of strain riii mice

Abstract

The establishment of an inbred colony of high-mammary-tumor strain RIII mice and their incidence of mammary tumors for the first 44 generations of inbreeding are presented. The results, thus far, suggest that the agent is gradually losing its effectiveness and has either disappeared or lost its effectiveness in 3 instances; descendants of these 3 mice for 12, 17, and 18 generations of inbreeding have displayed low incidences of mammary tumors. The agent has not reappeared suddenly in any of the 3 lines, but 1 line is offering suggestive evidence that the agent, if present, is increasing in activity. Nine families of mice have been established by foster-nursing, presumably agent-free, strain C3H females on high-tumor strain RIII females. Two of these families have been propagated for 17 generations of inbreeding and 7 families for 14 generations. The families show a wide variation in their responses to the RIII agent, ranging between a mammary tumor incidence of 3 percent at a mean age of 20 months and an incidence of 44 percent at a mean age of 15 months. The results show that the RIII agent is less active than the C3H agent and, of more importance, only 1 family has yielded evidence that the RIII agent has increased in activity during passage through strain C3H mice by the natural route of transmission. Two families of mice have been established by foster-nursing, presumably agent-free, strain RIII mice on high-tumor strain C3H females. Through 17 generations of inbreeding, 1 family has shown a mammary tumor incidence of 99 percent at an average age of 6.4 months in breeding mice and an incidence of 94 percent at an average age of 8.1 months in 76 virgins of the F14, F15, and F16 generations. The other line, through 15 generations of inbreeding, has shown an incidence of 8 percent at an average age of 17.8 months. This line is presenting evidence that the C3H agent can also disappear from, or persist as an inapparent infection in, strain RIII mice. © Oxford University Press.