Staphylococcus aureus intramammary infections (IMI) on two farms over 13 months. Total prevalence (data points on line) expressed as number of days infected per 10,000 quarter days (right axis). Number new IMI from naïve susceptible quarters (S) and number new IMI from recovered susceptible quarters (R) per 10,000 quarter days at risk (left axis). In treatment pens an initial observation period of months 1-3 was followed by the intervention period during months 4-13 when cases of subclinical IMI received 8-day therapy, while control pens were observed with no intervention applied for the entire 13 months. Treatment pens top panels, Control pens bottom panels, Farm 1 left panels, Farm 2 right panels.