Human infections with Eurasian avian-like swine influenza virus detected by coincidence via routine respiratory surveillance systems, the Netherlands, 2020 to 2023.

BackgroundZoonotic influenza, including infections with avian and swine influenza A viruses (swIAV), is a notifiable disease in the Netherlands. Human cases infected with swIAV have previously been rarely detected in the Netherlands.AimWe aimed to describe detection and characterisation of Eurasian avian-like swIAV infections in humans in the Netherlands 2020-2023.MethodsThe Dutch National Influenza Center coordinates different activities to monitor respiratory infections and circulating human influenza viruses. This monitoring includes sentinel surveillance in general practitioner practices, community participatory surveillance and characterisation of influenza viruses received from diagnostic laboratories. A subset of the specimens positive for influenza A virus from the monitoring activities are sent for further characterisation. We characterised swIAV from human patients using whole genome sequencing, tested the viruses for antiviral susceptibility and in haemagglutination inhibition assays for antigenic characterisation and compared them with previous detections from humans and pigs.ResultsAvian-like swine influenza virus was detected in three persons presenting with mild respiratory symptoms, and all recovered fully. Only one patient had close contact with pigs shortly before the start of symptoms. Sequence analyses of the viruses showed clustering with swAIV from pigs in a recently initiated surveillance system on pig farms.ConclusionsThese human cases show that swIAV viruses with zoonotic potential are enzootic in the Netherlands. Finding them by coincidence suggests human infections might occur more frequently than noticed.