Contributed by Robert G. Webster, November 26, 2014 (sent for review October 22, 2014)
The emergence of human infection with a novel H7N9 avian influenza reassortant in China raises a pandemic concern. However, it is not fully understood how these H9N2 chicken viruses facilitated the genesis of the novel H7N9 viruses. Here we show that a “fittest” genotype (G57) emerged with changed antigenicity and improved adaptability in chickens. It became predominant in vaccinated farm chickens and caused widespread outbreaks before the H7N9 virus emergence, increasing reassortment between H9N2 and other subtype viruses and finally providing all of their internal genes to the novel H7N9 viruses. The prevalence and variation of H9N2 influenza virus in farmed poultry could provide an important early warning of the emergence of novel reassortants with pandemic potential.
The emergence of human infection with a novel H7N9 influenza virus in China raises a pandemic concern. Chicken H9N2 viruses provided all six of the novel reassortant’s internal genes. However, it is not fully understood how the prevalence and evolution of these H9N2 chicken viruses facilitated the genesis of the novel H7N9 viruses. Here we show that over more than 10 y of cocirculation of multiple H9N2 genotypes, a genotype (G57) emerged that had changed antigenicity and improved adaptability in chickens. It became predominant in vaccinated farm chickens in China, caused widespread outbreaks in 2010–2013 before the H7N9 viruses emerged in humans, and finally provided all of their internal genes to the novel H7N9 viruses. The prevalence and variation of H9N2 influenza virus in farmed poultry could provide an important early warning of the emergence of novel reassortants with pandemic potential.
↵1J.P., S.W., and Y.Y. contributed equally to this work.
Author contributions: J.P., G.Z., J.Z., and J.L. designed research; J.P., Y.Y., G.Z., J.W., G.X., H.S., M.W., C.W., Y.W., D.W., B.Z., Y.J., Q.W., Q.D., M.S., and J.B. performed research; J.P., S.W., G.Z., R.A.C., J.W., M.W., G.L., Y.S., J.Z., H.Z., G.W., J.L., and R.G.W. analyzed data; and J.P., S.D., G.W., J.L., and R.G.W. wrote the paper.
The authors declare no conflict of interest.
Data deposition: The sequences generated in this study have been deposited in the Genbank database (accession nos. are listed in Table S3).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1422456112/-/DCSupplemental.
