Mother mice, in contrast to nonbred adult mice, are susceptible to FMDV obtained by passage in primary cultures of calf kidney cells. Factors which might affect the susceptibility of mother mice were investigated in an effort to obtain knowledge on the mechanism of this decrease in natural resistance. Mother mice from another breeding colony were was susceptible as mother mice from the colony at this laboratory. Mother mice with litters of 3, as well as mice without litters, were more resistant

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... than were mothers with litters of 6 or 12 young. As postpartum time increased, the susceptibility of mother mice decreased. After receiving foster litters of young mice at 5- to 7-day intervals to prolong lactation, mother mice were more susceptible at 21 and 28 days postpartum than similar mice with original litters. Mice which received the foster litters, however, were less susceptible than 6-day postpartum mice. Remated mother mice were as susceptible 5 to 8 days following second pregnancy as they were 5 to 8 days after completion of their first pregnancy.