Evolution and Pathogenicity of the H1 and H3 Subtypes of Swine Influenza Virus in Mice between 2016 and 2019 in China

Abstract

Pigs are considered a "mixing vessel" that can produce new influenza strains through genetic reassortments, which pose a threat to public health and cause economic losses worldwide. The timely surveillance of the epidemiology of the swine influenza virus is of importance for prophylactic action. In this study, 15 H1N1, one H1N2, and four H3N2 strains were isolated from a total of 4080 nasal swabs which were collected from 20 pig farms in three provinces in China between 2016 and 2019. All the isolates were clustered into four genotypes. A new genotype represented by the H1N2 strain was found, whose fragments came from the triple reassortant H1N2 lineage, classical swine influenza virus (cs-H1N1) lineage, and 2009 H1N1 pandemic virus lineage. A/Sw/HB/HG394/2018(H1N1), which was clustered into the cs-H1N1 lineage, showed a close relationship with the 1918 pandemic virus. Mutations determining the host range specificity were found in the hemagglutinin of all isolates, which indicated that all the isolates had the potential for interspecies transmission. To examine pathogenicity, eight isolates were inoculated into 6-week-old female BALB/c mice. The isolates replicated differently, producing different viral loadings in the mice; A/Swine/HB/HG394/2018(H1N1) replicated the most efficiently. This suggested that the cs-H1N1 reappeared, and more attention should be given to the new pandemic to pigs. These results indicated that new reassortments between the different strains occurred, which may increase potential risks to human health. Continuing surveillance is imperative to monitor swine influenza A virus evolution.

Keywords: genotypes; influenza virus; pathogenicity; subtypes; swine.

Figure 1

Phylogenetic trees constructed using 20 isolates and the H1 HA (n = 515), H3 HA (n = 184), N1 NA (n = 414), N2 NA (n = 240), MP (n = 460), NS (n = 454), PA (n = 432), PB1 (n = 440), and PB2 (n = 437) gene segments available from GenBank. The 20 new swine influenza virus (IAV-S) isolates are marked with red dots.

Figure 2

Pathogenicity of the isolated IAV-S in mice. (A) Mouse body weights were monitored daily for 14 days. The values represent the average scores of overall body weight loss, compared with the initial body weight ± standard deviation (SD). (B) Viral titers in the lungs and nasal turbinate of the infected mice (n = 3) at 3 DPI. (C) Histopathology analysis of lungs. Lungs of the infected mice were fixed with formalin, embedded in paraffin, and stained with hematoxylin and eosin and observed under a microscope with 200× magnification.