Evolution of the H9N2 influenza genotype that facilitated the genesis of the novel H7N9 virus

TLDR

The emergence of human infection with the novel H7N9 avian influenza reassortant in China raises pandemic concerns, yet the role of H9N2 chicken viruses in its genesis remains unclear. The study aims to determine whether the prevalence and genetic variation of H9N2 influenza virus in farmed poultry can serve as an early warning for novel reassortants with pandemic potential. The authors analyze H9N2 prevalence and genetic diversity in poultry farms to assess its contribution to reassortment events. The G57 genotype of H9N2, with altered antigenicity and enhanced chicken adaptability, became dominant in vaccinated farms, drove widespread outbreaks, and facilitated reassortment that supplied all internal genes to the emergent H7N9 virus.

Abstract

Significance 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.