Quarters were diagnosed to have CNS IMI when at least 2 out of 3 consecutive quarter milk samples were found to be culture-positive for the same CNS species with ≥300 cfu/mL and when consecutive isolates had a similar AFLP type. In all other cases, a quarter was considered to be noninfected or of unknown IMI status when no sample was taken in one of the adjacent months (e.g., first sampling, dry period, culling). An
IMI that was present at only 1 sampling was considered a transient case. All other IMI were considered persistent in nature. Duration of persistent IMI cases was calculated by assuming that infections started at the midpoint between the first detection of IMI and the previous sampling, and ended at the midpoint between the last detection of IMI and the next sampling, respectively. When a cow entered or left the study infected, the extra period was calculated