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

Sporadic human infections with avian or swine influenza A virus (swIAV) have been reported. Zoonotic influenza, including human infections with avian influenza A virus or swIAV, is notifiable in the Netherlands and national and international guidelines state that local and national public health services need to be timely notified of laboratory-confirmed cases allowing source finding and contact tracing.

Swine influenza A viruses are genetically and antigenically distinct for different continents, due to introductions into pig populations from different origins and at different time points. Limited inter-continental spread between pigs or pig farms seems to occur [1-3]. In Europe, four swIAV haemagglutinin (HA) lineages are enzootic: H1 classical swine lineage (clade 1A) including H1pdm09-like viruses, H1 human seasonal lineage (clade 1B), H1 Eurasian avian-like lineage (clade 1C) and European human-like H3 lineage. These HA lineages are combined with four neuraminidase (NA) lineages: N1pdm09-like, avian N1 and two swine N2 lineages. Knowledge about circulating genotypes of swIAV is limited and difficult to obtain due to limited surveillance of swIAV on pig farms in most countries in Europe, including the Netherlands, especially after the European Surveillance Network for Influenza in Pigs (ESNIP 3) was terminated in 2013 [1].

Human infections with swIAV have been detected sporadically in Europe whereas in the United States (US), such infections have been detected more frequently [2,4-6]. These infections occurred mainly after close contact with infected pigs on agricultural fairs [5]. In addition, the A(H1N1)pdm09 pandemic in 2009 was highly likely caused by direct spillover from infected pigs and subsequent human-to-human transmission [7]. In the Netherlands, swIAV was detected in six persons 1986–2019 [4,8-13]. The previous detections of swIAV infections in humans in the Netherlands were mostly coincidental, in hospitalised persons presenting with severe disease [10], as no surveillance system to detect zoonotic spillovers from pigs to humans is in place with the aim to monitor specific risk groups upon exposure. This contrasts with monitoring of individuals exposed to poultry infected with highly pathogenic avian influenza A virus (HPAI) for which passive and active monitoring is in place in the Netherlands.

Here we describe detection of Eurasian swIAV infection in three persons during routine influenza surveillance activities in 2020, 2022 and 2023 in the Netherlands.