Experimental infection of pigs and ferrets with “pre-pandemic,” human-adapted, and swine-adapted variants of the H1N1pdm09 influenza A virus reveals significant differences in viral dynamics and pathological manifestations

Abstract

Influenza A viruses are RNA viruses that cause epidemics in humans and are enzootic in the pig population globally. In 2009, pig-to-human transmission of a reassortant H1N1 virus (H1N1pdm09) caused the first influenza pandemic of the 21st century. This study investigated the infection dynamics, pathogenesis, and lesions in pigs and ferrets inoculated with natural isolates of swine-adapted, human-adapted, and “pre-pandemic” H1N1pdm09 viruses. Additionally, the direct-contact and aerosol transmission properties of the three H1N1pdm09 isolates were assessed in ferrets. In pigs, inoculated ferrets, and ferrets infected by direct contact with inoculated ferrets, the pre-pandemic H1N1pdm09 virus induced an intermediary viral load, caused the most severe lesions, and had the highest clinical impact. The swine-adapted H1N1pdm09 virus induced the highest viral load, caused intermediary lesions, and had the least clinical impact in pigs. The human-adapted H1N1pdm09 virus induced the highest viral load, caused the mildest lesions, and had the least clinical impact in ferrets infected by direct contact. The discrepancy between viral load and clinical impact presumably reflects the importance of viral host adaptation. Interestingly, the swine-adapted H1N1pdm09 virus was transmitted by aerosols to two-thirds of the ferrets. Further work is needed to assess the risk of human-to-human aerosol transmission of swine-adapted H1N1pdm09 viruses.

The avian influenza A virus receptor SA-α2,3-Gal is expressed in the porcine nasal mucosa sustaining the pig as a mixing vessel for new influenza viruses

“…Some differences between viral loads observed in this study (using IAV IHC) and our previous study (using qPCR) were observed ( Kristensen et al., 2023b ). A possible explanation for this discrepancy is that PCR also measures the presence of IAV in the mucus, whereas the amount of mucus is decreased in IAV IHC due to the decreased mucus preservation when tissues are fixed in 10 % neutral buffered formalin ( Blick, 2019 ).…”

“…Another possible explanation could be that the viral load in these groups was too small to be detected by IAV IHC. Furthermore, no IAV-positive cells were found in the nose of pigs infected with the swine-adapted H3N2, which could be because IAV induces necrotic rhinitis with desquamation of epithelial cells ( Kristensen et al., 2023b ), and therefore, no IAV-positive cells were detected in the nasal mucosa of these pigs. These pigs may still be shedding the virus due to the replication of the virus in other respiratory compartments ( Walther et al., 2013 ; Wong et al., 2019 ).…”

“…Commercial Danish Landrace control pigs (8–9 weeks of age) obtained from previous studies ( Kristensen et al., 2023b ; manuscript in preparation) were used to evaluate the distribution patterns of IAV receptors in the nose, upper and lower part of the trachea, and lung tissues. Furthermore, tissues from pigs (8–9 weeks of age) inoculated intranasally with different strains of IAV circulating in pigs (swine-adapted) or in humans (human-adapted) were included, too.…”

“…Furthermore, tissues from pigs (8–9 weeks of age) inoculated intranasally with different strains of IAV circulating in pigs (swine-adapted) or in humans (human-adapted) were included, too. These IAV-infected tissues were also obtained from previous studies, A/swine/Denmark/3974–2p4/2020 (swine-adapted H1N1pdm09), A/Denmark/238/2020 (human-adapted H1N1pdm09), A/Swine/Mexico/AVX-39/2012 (pre-pandemic H1N1pdm09) ( Kristensen et al., 2023b ; manuscript in preparation), A/swine/Denmark/14348–9/2003 (swine-adapted H3N2), A/Denmark/304/2020 (human-adapted H3N2) and a human-adapted strain isolated from pigs during an outbreak in a Danish herd A/swine/Denmark/S3974–2/2020 (hu/sw H3N2) (manuscript in preparation) were used to investigate potential differences between the tissue and cell tropisms of the different IAVs. The accession numbers of the strains investigated in this study are listed in Table S1.…”