Contact structure and Salmonella control in the network of pig movements in France.

Movement restrictions are a key measure to control pathogen transmission in the swine industry due when purchasing animals. Here recorded animal movements between herds in France are used to generate a network model. This network consists of different types of herds together with specific links between them, with some herds being suppliers (of reproductive gilts or piglets) for others. The connectivity between herds within this network exhibits a highly variable number of contacts according to the type of animal moved (8 kg piglets, 25 kg piglets or reproductive gilts). A simulation model is used to assess the impact of control measures aimed at reducing Salmonella prevalence at every level of the production network. In particular we consider restricting animal purchases according to the infectious status of herds (level of prevalence) such that animals do not move from higher to lower herd prevalence; this measure can additionally be combined with infection control at the herd-level. When Salmonella initially invades the production structure, the restriction of animal movements alone is sufficient to significantly decrease the number of herds infected. In contrast, once infection is endemic within the production structure, movement restrictions have to be supplemented by within-herd control applied to a large proportion of herds to reduce the pathogen prevalence. These results demonstrate the important distinction between the control of epidemics and endemic infections and highlight the different impacts of control measures at the within- and at the between-herd levels.