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Number of the secondary cases by year distribution (A–C) and overall month distribution (B–D) from 2010 to 2018. Data are represented by the number of secondary cases defined by nearest-neighbor (A,B) or uniform-smoothed kernel functions (C,D). Statistically significant differences between indoor and outdoor farms are identified by a p-value <0.0001 (***), or p-value between 0.05–0.0001 (**).
