Abstract
Influenza A viruses (IAV) are likely candidates for pandemics. This report summarizes the results of the Swiss national program for surveillance of influenza viruses in pigs by presenting data on their genetic diversity and on transmissions to humans between 2010 and 2022. Challenges and optimization options in the program are discussed. Nasal swabs or lung tissue samples from pigs with influenza-like signs were screened by real-time RT-PCR for swine influenza virus (SIV) genomes and the human seasonal strain A(H1N1)pdm09; positive samples were subtyped for H1, N1, H3 and N2. In parallel, humans with influenza-like symptoms and recent contact with diseased pigs were asked to sample themselves with a nasal swab. Human swabs were tested for IAV, and positive swabs further subtyped to identify potential cross-species transmission between swine and humans. In the pigs, SIV was detected in 375 of 674 farm visits. H1N1 was the only subtype detected in Swiss pigs so far. The human seasonal strain A(H1N1)pdm09 (Hemagglutinin (HA) clade 1A) was detected in seven out of 375 SIV positive farm visits. Phylogenetic analyses from partial HA and neuraminidase (NA) gene sequences indicate that the remaining pigs were infected with the Eurasian avian lineage (HA clade 1C), which is predominant in swine in Europe. The Swiss H1N1 strains form distinct phylogenetic clusters within HA clades 1C.2.1 and 1C.2.2 and seem to evolve comparably slowly. Infection of humans with SIV was identified in five cases. Sequence similarity analysis assigned the five viruses to the Eurasian avian lineage (C), clades 1C.2.1 and 1C.2.2. There was no evidence for sustained human-to-human transmission. Continued surveillance of influenza viruses at the swine-human interface is of major importance to enable early detection of new IAV subtypes and changes in the epidemiological situation. The authors consider the publication of data from the surveillance program to be essential in order to raise awareness of the disease among both veterinarians and the general public and to supplement existing information on circulating influenza viruses in Europe.
