Do wild boar movements drive the spread of African Swine Fever?

The spatial behaviour of hosts can seriously affect the transmission of pathogens and spatial spread of diseases. Understanding the relationship between host movements and disease dynamics is of prime importance for optimizing disease control efforts. African swine fever (ASF), a devastating disease of wild and domestic suids, has been spreading continuously through eastern Europe since 2007. The wild boar (Sus scrofa) has been implicated in the epidemiology of this disease, but the role of wild boar movements in ASF dynamics and spread has not been studied and remains largely speculative. Here, we examined whether monthly parameters of wild boar movements (dispersal distance of yearlings, home range size of adult males and females) can explain variation in the spatio-temporal dynamics of the ASF outbreak in the wild boar population in north-eastern Poland, 2014-2015. We expected to observe a positive relationship between host mobility and disease spread. Contrary to our expectations, we found that movements of wild boar, despite their seasonal variation, were poor predictors of ASF dynamics in space and time. During the 2 years of the study, ASF spread gradually at a steady pace of 1.5 km/month without significant changes across seasons. None of the analysed movement parameters explained variation in the measures of ASF occurrence and spread (i.e., number of cases, prevalence, size and expansion rate of the outbreak area). We believe that the factor limiting the influence of host movements on ASF dynamics is the severity of the disease, which quickly hampers extensive movements and restricts disease transmission to only the most immediate individuals. Three natural factors constrain direct disease transmission: wild boar social structure, the short duration of low-level virus shedding and high virus-induced lethality, followed by indirect transmission through infected carcasses. These most likely shape the gradual spread of ASF in space and its persistence in already infected areas.