Prevalence, evolution and interspecies transmission potential of avian H1 subtype influenza A viruses in China

Abstract

H1 subtype of influenza A viruses were the causative agents for three pandemics in the last 100 years, and have been circulating in humans and pigs for decades. The natural reservoirs for H1 subtype of avian influenza viruses (AIVs) are aquatic birds, where highly diverse variants might be produced with pandemic potential. In light of the active influenza activity in China during the recent years, we sought to find out the prevalence, evolution and interspecies transmission potential for H1 AIVs in this region. Based on our long-term surveillance conducted in seven provinces in southern China during the 1970s and from 2001 to 2013, we identified 401 H1 isolates out of 674,428 samples (0.06%) from farms, live poultry markets and wild habitats. H1 isolates only accounted for 0.62% of all the AIVs and 84% of them were isolated from domestic ducks. Molecular analysis on 148 genomes of H1 AIVs revealed few mammalian adaptation markers. Phylogenetic analysis showed that the majority of these H1 AIVs belonged to the Eurasian avian gene pool. Reassortments of H1 viruses were frequent and no stable lineages were formed, with 61 genotypes identified for the 148 strains. Pigs are susceptible to both avian and human influenza viruses, and could serve as a vector host for viruses with pandemic potential. To assess the mammalian infectivity of H1 AIVs, representative strains (n=17~46) of AIVs of nine HA subtypes (H1 to H10, except H8) were examined in an ex vivo pig lung infection model. Results indicated a generally higher replication ability of H1 AIVs than most of the other subtypes, especially H2, H3, H4, H6 and H9. Chickens are an important intermediate host for emerging human infections of AIVs in China, as exemplified by the recent H7N9 outbreaks. Surveillance evidence also suggested that H1 AIVs might take a similar evolutionary pathway to that of the H7N9 viruses. To assess such a possibility, infectivity and transmissibility of wild-type H1 AIVs, reverse genetically reconstructed wild-type H1 AIVs, and their reassortant counterparts with internal gene cassette from a predominant H9N2 genotype, were evaluated in chickens. Results demonstrated a low infectivity and transmissibility for both wild-type and reassortant H1 AIVs in chickens. However, the incorporation of pre-adapted internal genes did enhance the infection and transmission of H1 AIVs in chickens. Combining both phylogenetic and experimental data, this study provided a systematic evaluation on the evolution and risks for interspecies transmissions of H1 AIVs in China. Although a minor subtype in waterfowl, H1 could pose a continuous threat for mammalian species including humans, as this subtype displayed a relatively higher mammalian replication capacity than most of the other subtypes, which might have contributed to its historical lineage establishments in humans and pigs. The H1 AIVs might be able to reassort with other viruses in the live poultry market system in China and generate advantageous variants to cause human infections. Early identification of emerging H1 AIVs with interspecies transmission potential requires continuous surveillance and monitor on the evolutionary dynamics of these viruses.