Epidemiology of Influenza A virus in Swiss pig herds: subclinical circulation and associated risk factors

Influenza A viruses (IAVs) are important respiratory pathogens of pigs, impacting both animal health and productivity. However, their epidemiology in Swiss pig herds remains poorly understood. Therefore, we conducted a cross-sectional study encompassing 100 pig herds, combining semi-structured interviews, clinical examinations, and nasal swab sampling analyzed by pan-IAV qPCR. Overall, 35% of herds with mean intra-herd detection rate of 8.5% tested positive for IAV. Among herds without clinical respiratory signs, 34.1% (29/85) were IAV-positive. Exploratory analysis suggested trends toward higher IAV detection in herds with more intensive production or biosecurity shortcomings. Factors such as contact with other animal species or mixing between age groups may facilitate viral circulation. IAV is commonly detected in Swiss pig herds and often occurs without causing clinical signs. Epidemiological patterns are similar to those reported elsewhere in Europe, despite Switzerland’s small-scale and less intensified production sites, generally lower biosecurity standards, and nearly non-existent cross-border live pig transport. Although the clinical impact of IAV was limited, inapparent influences on production performance warrant implementation of IAV control measures. Follow-up studies are needed to better understand circulation dynamics of IAV in Swiss pig herds.