Measuring transboundary disease spread - ASF in wild boars straddling Piedmont and Liguria

Abstract

The aim of this paper is to describe the impact, dynamics and risk factors of the incursion of the African Swine Fever (ASF) virus detected in early 2022 in the wild boar population straddling Piedmont and Liguria, 24 months after its emergence.

A study area of 4,162.2 km²s, comprising 234 municipalities in Liguria and Piedmont (north-western Italy), along with an external buffer zone, was considered. The epidemiological units were wild boars tested for ASF using real-time PCR between late December 2021 and December 31, 2023. The data were obtained from passive and active surveillance. Conventional methodologies were applied in the analysis of time series of tests, cases, or prevalence rates of positive animals. Maps were used to visualise and compare the monitoring activities, the location of cases and municipal standardised prevalence ratios. Clustering of high and low trends was studied by semester using a space-time permutation model. Risk factors analysis was based on multivariate Poisson regression modelling.

Over the 2-year study period, the epidemic's spread was closely monitored by testing 10,412 wild boars (25 % of them from passive surveillance). Probability of disease detection was 10.9 times higher among found dead animals compared with animals tested in active surveillance. Difficulties in carcass searching led to heterogeneities in surveillance sensitivity achieved locally and inherent uncertainties. A total of 1,165 wild boars that tested positive for ASF have been detected in 125 municipalities across three provinces. The outbreak has expanded over time and exhibited an increasing trend, with an epidemic doubling time of 10.7 months. There was seasonality, with an increasing trend from summer to spring. Prevalence rates followed the same temporal pattern. Significant low and high clusters respectively indicated endemic episodes in the interior areas of virus circulation and an ongoing invasion of surrounding regions.

The characteristics of this outbreak align with the cycle of wild boar habitat transmission observed in other outbreaks across Europe. The measures implemented were not sufficient to stop the epidemic: however, targeted interventions such as depopulation campaigns, reinforcement of physical barriers and increased biosecurity measures have prevented local spillover to domestic pigs and partially hindered the advance of the disease. In addition, the shown seasonality of the disease, likely can enhance control measures. The ongoing studies of the local wild boar populations and the evolution the epidemic along with the lessons learned so far will improve the efficiency and effectiveness of efforts to limit and ultimately eradicate the disease.