Multifactorial Risk Pathways and Ecological Drivers of African Swine Fever in Domestic Pigs and Wild Boar Populations

Abstract

This thesis investigates risk factors shaping the occurrence of African Swine Fever (ASF) in domestic pigs and wild boar. ASF is a contagious viral disease with severe economic and environmental consequences due to high mortality, trade restrictions, and the costs of control and eradication. The research addresses three overarching questions: (i) which farm-level risk factors for ASF are actionable by farmers; (ii) what explains the consistent seasonal pattern of ASF outbreaks in domestic pigs across Europe; and (iii) whether arthropods may contribute to the mechanical transmission of African Swine Fever virus (ASFV). Chapter 1 introduces ASF, summarising its aetiology, epidemiology, distribution, transmission pathways, seasonal dynamics, and control measures. Chapter 2 presents a systematic literature review identifying variables associated with ASF occurrence. In domestic pigs, reported factors most frequently relate to local infection pressure, socio-economic conditions, production system characteristics, and proximity to wild boar habitats. In wild boar, ASF occurrence is driven by local virus circulation, habitat features, and socio-economic indicators. The review highlights a lack of quantitative observational studies focusing on manageable farm-level biosecurity measures. Chapter 3 examines ASF incursion risks in Romanian pig farms using a matched case–control design. Proximity to infected domestic farms was a key predictor for both commercial and backyard systems. Additional risk factors for backyard farms included larger herd size, higher wild boar abundance, closer proximity to infected wild boar, frequent visits by external professionals, and environmental and management factors such as nearby wild-boar-attracting crops and the use of forage from ASF-affected areas. Chapter 4 extends the analysis to commercial farms in Poland, Romania, and Lithuania, integrating entomological surveillance. ASF risk was again associated with proximity to recent outbreaks and crops attractive to wild boar. Several biosecurity practices influenced risk: off-site carcass collection and sealed carcass storage were protective, whereas machinery sharing and non-routine events increased risk. Farms with insect screens showed reduced risk, while higher numbers of captured Culicoides midges were associated with increased risk. Manure application sourced from other farms also emerged as a risk factor. Chapter 5 presents targeted entomological surveillance assessing the potential role of stable flies (Stomoxys calcitrans) and Culicoides midges in ASFV transmission. Although viable virus was not isolated, ASFV DNA was detected in several Culicoides species and in S. calcitrans. No association was observed between farm outbreak status and viral DNA detection; however, both insect groups were strongly attracted to pig farms, creating opportunities for virus acquisition and transfer. Chapter 6 synthesises the findings, identifying consistent actionable biosecurity gaps, plausible drivers of ASF seasonality, and evidence supporting a potential role for arthropod-mediated mechanical transmission. ASF outbreaks in domestic pigs peak from May to October across Europe, coinciding with increased arthropod activity and ripening crops that attract wild boar. While insect involvement cannot be confirmed conclusively, multiple epidemiological lines of evidence support its plausibility. Overall, this work highlights the importance of strengthening core biosecurity measures—particularly vehicle disinfection, functional disinfection barriers, pest control, visitor management, preventing contact with wild boar—while integrating vector control into ASF prevention strategies.