African swine fever in wild boar: density drives spread more than persistence

A new EFSA-supported modelling study suggests that wild boar density plays a stronger role in the geographical spread of African swine fever (ASF) than in determining whether the virus occurs or persists once established. Local environmental and epidemiological conditions may become more important after introduction.

Researchers analysed ASF in wild boar across Estonia, Germany, Hungary, Italy, Latvia, Lithuania and Slovakia, using 10 × 10 km grid cells. They modelled both occurrence based on the proportion of ASF-positive wild boar carcasses, and persistence, measured as the proportion of positive months over two- and five-year periods.

Density alone does not explain where ASF persists

The analysis found very limited evidence that available estimates of wild boar density were associated with ASF occurrence or persistence across the seven countries.

Instead, several environmental variables were associated with infection patterns, although their effects differed between countries. These included annual temperature, precipitation, forest cover, forest fragmentation and the human footprint index.

This means that once ASF becomes established, local ecology may shape transmission more strongly than simple estimates of the number of wild boar present.

Italy shows a different role for density

The researchers then used a more detailed mechanistic model for the epidemic in northern Italy, following four successive waves between January 2022 and November 2025 at a spatial resolution of 2 × 2 km. Here the result was much clearer. Areas with higher expected wild boar density were more likely to be susceptible to infection and to transmit ASF into neighbouring areas. The study therefore distinguishes between two epidemiological processes. High wild boar density appears particularly important for the outward expansion of ASF into previously unaffected territory. But after the virus has arrived, persistence within an infected area may depend on a more complex combination of habitat, climate, carcass-mediated transmission and local epidemiological conditions.

Implications for ASF control

The findings challenge the idea that reducing wild boar density alone can explain or control ASF persistence. Population management may remain particularly relevant at the epidemic front, where lowering host density could reduce the probability and speed of geographical spread. But in established infected zones, control strategies also need to consider habitat structure, carcass detection and removal, climate, human activity and surveillance intensity.

The authors also stress that available wild boar density estimates remain imperfect, which may weaken statistical associations at continental scale.

The study therefore supports a more spatially targeted approach to ASF management: wild boar density matters, but its importance changes according to whether the objective is preventing expansion or eliminating persistent infection.