Elucidating the influence of wild boar density on African swine fever spread in wild boar populations, Italy, 2022–2023

Abstract

Wild boar density has been suggested to play a role in shaping African swine fever (ASF) transmission patterns. To provide quantitative estimates of the influence of wild boar density on ASF spread, a spatially-explicit detection-delay SIR mechanistic model of ASF transmission among density-explicit wild boar habitat was developed and parameterised to observed epidemic data in northern Italy from January 2022 through September 2023. Wild boar density estimates were generated by the ENETWILD consortium. Infectious periods, local prevalence at time of first detection, detection rates, and seasonal recovery rates were estimated directly from surveillance data. Eight models were constructed utilizing static and seasonal transmission rates along with linear relationships between habitat susceptibility/infectivity and wild boar density. Transmission rate, relative susceptibility, and relative infectivity were estimated by fitting each model to the observed epidemic using sequential Monte Carlo approximate Bayesian computation. The model that most closely fit the full data used a seasonal transmission rate but did not support a wild boar density effect on ASF spread across the entire study period. However, further analyses of the model outputs suggest that wild boar density likely played a role in shaping ASF transmission patterns during the second wave only (October 2022 – September 2023). This observation could be due to a lack of power in the first wave, lower surveillance rates in that period, or be from density estimates no longer reflecting the true wild boar density distributions upon the start of the second wave. These results demonstrate that wild boar density impacted ASF propagation in northern Italy. Further investigation by estimating parameters for individual epidemic waves could be beneficial to better characterise the wave-specific impact of wild boar density. The model developed here could be used in other contexts to evaluate if the influence of wild boar density is present across epidemic scenarios.