Swine influenza A viruses (swIAV) are enzootic in pigs in Europe and cause respiratory disease in pigs. Clinical signs include fever, coughing, nasal discharge, sneezing, laboured breathing and loss of appetite. As pigs are also susceptible to influenza A viruses from birds and humans, they can facilitate the occurrence of co-infections from which influenza viruses may arise with reassorted genomes. These new viruses pose considerable risks if they acquire the ability to cross species barriers and transmit efficiently between humans. However, and in contrast to the data available on viruses in humans, little is known about the circulation of influenza A viruses in pigs. The threat posed by circulating swIAV is increasingly recognised by European and other international health organisations and policymakers as a zoonotic risk after the 2009 influenza A(H1N1) and COVID-19 pandemics. Recent events underline the need to better characterise the circulating influenza A viruses in pigs. First, Eurasian avian-like H1N1 G4 viruses with increased zoonotic potential were detected in pigs in China in 2020. Next, in 2022 and 2023, swIAV infections in humans were reported in European countries, including three patients in the Netherlands and one patient in the United Kingdom (UK). Finally, ongoing outbreaks of highly pathogenic avian influenza (HPAI) H5N1 (clade 2.3.4.4b) in farm animals, including poultry, fur animals, cattle, backyard pigs and recent zoonotic infections in humans in the United States (US), show that similar events in pigs do not seem unlikely, and justify the call for increased influenza A virus surveillance in these animals. The relevance for such surveillance at a national level became clear in 2022 after a Dutch zoonotic H1N2v infection with a swine virus from the 1C Eurasian avian lineage [7]. At that time, relevant questions on cross-species transmission and assessment of public health risks could be addressed through close collaboration between public health and animal health institutions.

To gain insight into the occurrence, potential risk, genomic characteristics and recent evolution of swIAV, a national pilot project was set up in pigs in 2022. The aim of this study with a One Health (OH) focus was to characterise swIAV circulating in the Netherlands and to compare those with relevant human and swine influenza viruses from the Netherlands and other European countries, for instance, with those collected by the European Swine Influenza Network (ESFLU) and World Health Organization (WHO) Global Influenza Surveillance and Response System. Collaboration between animal health and public health professionals is essential in OH surveillance. Therefore, this pilot project was built on collaboration between the National Institute of Public Health (RIVM), Royal GD, Erasmus Medical Center (EMC) and Wageningen Bioveterinary Research (WBVR) using a joint online platform.