Longitudinal Surveillance of Influenza A Virus Exposure in Wild Boars (Sus scrofa) in Spain (2015-2023): Serologic and Virologic Evidence of Subtype Infections and H5N1 Spillover Risk.

Influenza A viruses (IAVs) are responsible for respiratory infections in a wide range of species, including birds, swine and humans. The role of wild boar (Sus scrofa) in IAV epidemiology remains underexplored. Here, we present a longitudinal serologic and virologic surveillance study of wild boars in Spain from 2015 to 2023. A total of 1643 nasal exudates and 2932 serum samples were analysed using quantitative RT-PCR, ELISA and haemagglutination inhibition (HI) assays to detect IAV positive samples and IAV targeted antibodies to characterise circulating viral subtypes. In addition, in the context of highly pathogenic avian influenza H5N1 outbreaks, we explored the potential transmission of avian IAV to wild boar. In summary, 6% of the serum samples tested positive and one IAV H3N1 was isolated. The seroprevalence remained stable from 2015 to 2018, undetected in 2019 and increased significantly from 2020 to 2023. The most frequently detected subtype was Eurasian avian-like H1 (clade 1C) while pandemic H1 (clade 1A) and human-like H1 (clade 1B) were less common. Human seasonal-like H3 strains from the 2000s (2000s-like H3) emerged in 2017 and have become more seroprevalent in recent years. A subset of wild boar sera from areas overlapping with H5N1 HPAI outbreaks in poultry and wild birds tested positive for recombinant H5 by ELISA, although H5N1 HI assays were negative. The monitoring of IAV in wild boar population allowed the identification of the temporal and spatial trends and shifts in the prevalence and characterisation of the infecting IAV strains. Our data suggest potential spillover events from human or other sources and support the inclusion of integrated monitoring of the wild suids as IAV reassortment-prone hosts in influenza surveillance programs.