FIG. 4.

Protective MNV-1 immunity is influenced by the primary inoculum dose. Groups of 129SvEv mice were mock infected or inoculated perorally with the indicated doses of MNV-1 at 5 to 6 weeks of age. Six weeks later, all groups were infected with 10⁷ PFU MNV-1. (A) Three days after the secondary challenge, stool samples were collected and scored as described in the legend to Fig. 1, except that these fecal samples were scored independently and blindly by three investigators. The majority score was assigned to each mouse. An additional group of mice received a mock inoculum at both the primary and secondary challenges as a negative control (Neg). Data are presented as the frequency of mice receiving a score of 1 or greater. Raw diarrheal scores are presented in Table 2. (B) Two days after the secondary challenge, animals were perfused, organs were harvested, and viral burdens were determined by plaque assay. Each group contained 5 to 15 mice, and the data for all of the mice in each group were averaged. Limits of detection are indicated by dashed lines. The n-fold difference between each secondary challenge group and the primary infection group is shown above each bar for intestines and MLNs.