Probability distributions of farms’ characteristics in the whole metapopulation (red) and among the secondarily infected farms that transmitted the disease to tertiary cases (blue). Data used are aggregated over the whole period (2005-2013). Intersections between histograms in different populations are in grey. The scenario with 1% of the farms initially infected, uniformly sampled, and with medium levels of within-herd prevalence is shown (blue histograms). (A) Size; (B) In-degree (number of farms from which animals are purchased); (C) Out-degree (number of farms to which animals are sold); (D) In-strength (number of animals purchased); (E) Out-strength (number of animals sold); (F) Polarity (by construction, sellers have values <0, buyers have values >0).