As livestock farms in The Netherlands are nowadays being scaled up in size for economic reasons, the question becomes important how disease introduction risks and risks of onward transmission scale with farm size (i.e. with the number of pigs on the farm). Here we use the epidemic data of the 1997-1998 epidemic of Classical Swine Fever Virus (CSF) in The Netherlands to address this question for CSF risks. This dataset is one of the most powerful ones statistically as in this epidemic a total of 428 pig farms were infected (with the majority of farm sizes ranging between 27 and 1750 pigs). We have extended the earlier models for the transmission risk as a function of between-farm distance, by adding two size dependent factors: these factors describe the farm size dependence of, respectively, the susceptibility of an exposed farm and the infectivity of a source farm. Using the best-fitting model, we show that the size of a farm has a significant influence on both farm-level susceptibility and infectivity for CSF. Although larger farms are both more susceptible to CSF and, when infected, more infectious to other farms than smaller farms, the increase is less than linear. According to the best-fitting model, the higher the farm size, the smaller the effect on the susceptibility and infectivity of a farm of increments in farm size. We show that this implies, based on the pig farming characteristics of 1997-1998 in The Netherlands and assuming that the relative spatial distribution of farm locations does not change, a trend towards less but larger farms would produce a net decrease in between-farm transmission risks.
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