FIG 2.

Change in antibody PP values (ΔAb) over the course of infection for farm A. Positive values indicate an increase in the Ab PP value from one time point to the next, and negative values denote a decrease in the Ab PP value compared to that for the previous month. Values from 1 month prior to seroconversion (−1) are considered separately from other seronegative values (−ve), as infection may have been present at this time point, but the animals had not seroconverted. In experimentally infected animals, seroconversion occurs at 2 to 4 weeks postinfection (42).