Fig 1. Study design.

Five swine-breeding herds (largest rectangles, 1 to 5) in the Midwest were sampled between November 2011 (month 1) and December 2012 (month 14). White rectangles represent sampling visits (12 per farm), and the smallest rectangles (n = 141) indicate the groups of pigs sampled. Groups are colored based on pig subpopulation: yellow (new gilts, n = 21), green (gilts, n = 60), and purple (piglets, n = 60). Missing group-rectangles indicate that there were no new gilts on that visit. Groups were assumed nested within sampling events, and sampling events were assumed nested within farms.