Genetic diversity and circulation of influenza A viruses in Italian pig farms: insights from surveillance and vaccination

Background: The genetic diversity of influenza A viruses in swine (IAV-S) in Europe, including key subtypes such as H1N1, H3N2, H1N2, and H1N1pdm09 (which features H1 hemagglutinin genes from the H1A, H1B, and H1C genetic clades), presents challenges for vaccine development and raises concerns about potential swine-to-human transmission. IAV-S also affects the pork industry, requiring management practices such as vaccination and the adoption of biosecurity measures. To evaluate the dynamics of IAV-S infections in large pig herds and assess viral diversity and circulation patterns in pig farms, two Northern Italian farrow-to-finish farms with prior IAV-S infections participated in a 2022 longitudinal study. Sows and their piglets were sampled during farrowing and nursery. In 2023, a follow-up evaluated IAV-S circulation after implementing different vaccination strategies on the two farms, including vaccination of sows and piglets. Results: The circulation of multiple IAV-S strains into the two farms was observed, identifying H1BN2, H1AN1, H1CN1, and H1CN2, characterized by different genetic patterns. We confirmed that maternal immunity was ineffective in preventing virus circulation among piglets. HI tests showed variability in detecting strains, raising concerns about their specificity. After vaccination was implemented on both farms, we detected only H1CN2 strains, but with distinct genetic patterns. Conclusions: The study highlighted the genetic diversity of IAV-S strains circulating in Italian pig farms, including potential incursions from unknown external sources. Vaccination efforts resulted in broad antibody responses; however, as expected, they did not eliminate viral circulation. The diversity of lineage-derived H1C strains, along with the emergence of H1C2.4 strains, will require further investigation.