Trends in seroprevalence of influenza A virus infections in pigs in France (2008-2022)

Background Swine influenza A viruses (swIAV) are highly contagious zoonotic pathogens that cause an acute respiratory infection in pigs, presenting substantial economic and health risks. This drives the pig industry and stakeholders in animal health to monitor swIAV in livestock. Prior to the 2009 flu pandemic, H1 _av N1 (HA-1C.2.1) and H1 _hu N2 (HA-1B.1.2.3) circulated in pig herds in France. The H1N1 _pdm (HA-1A.2.3.3) lineage became enzootic after its introduction. In 2020, a new H1 _av N2 genotype (HA-1C.2.4) emerged, altering the frequencies of enzootic swIAV lineages. To support our knowledge built on event-based surveillance and secure the exhaustiveness of the information, serological studies were conducted. Three independent surveys were performed nationally in 2008 and in 2018 (before and after the A/H1N1pdm09 pandemic, respectively), and in North West in 2022 (after the H1 _av N2 emergence in Brittany area). These surveys aimed to estimate swIAV prevalence in livestock using ELISA on fattening pigs and hemagglutination inhibition (HI) tests to determine the relative frequencies of different swIAV lineages. Results The national seroprevalence was 91% [83–96] _CI95, in 2008, and 87% [81–92] _CI95 in 2018. In 2022, seroprevalence in the North-West reached 91% [73–97] _CI95 . At each period, antibodies against several subtypes were detected simultaneously in a quarter of positives batches. In 2008, anti-HA-1C.2.1 antibodies were widespread, while anti-HA-1B.1.2.3 antibodies were found in most regions except in the South-West. H3N2 seroprevalence was very low, restricted to the North-East. By 2018, anti-HA-1C.2.1 antibodies remained the most prevalent and anti-H3 antibodies the weakest, but anti-HA-1B.1.2.3 prevalence had strongly decreased, while anti-HA-1A.3.3.2 antibodies were detected nationwide. In 2022, the North-West showed higher seroprevalence for H1 _av N2 (HA-1C.2.4) than H1N1 _pdm and H1 _hu N2, though still lower than H1 _av N1. Conclusions These surveys reveal high, sustained swIAV seroprevalence, particularly in dense herd areas. They also highlight changing in anti-HA antibodies relative frequencies, reflecting viral emergence and dynamics. Despite challenges in interpreting HI test results, the surveys provided valuable data, uncovering rare events, potential undiagnosed cases, and co-circulating viruses, which may lead to genomic reassortments and new virus emergence.