Molecular markers of interspecies transmission of H2N2 and H9N2 avian influenza A viruses

Abstract

Most avian influenza viruses do not replicate or transmit efficiently in mammals. The events that lead to interspecies transmission and host adaptation are unknown. Part one of this project set out to establish quail as an intermediate host of influenza. Our results indicate that adapting a mallard H2N2 virus in quail leads to expanded host range in chickens. The molecular changes, which occur during the adaptation in quail, are crucial for viral replication and transmission in chickens. Further adaptation of this quail-adapted virus in chickens leads to a 27 amino acid-deletion in the stalk region of the NA, changing the tissue tropism and temperature phenotype of the virus.

H9N2 influenza viruses have created in poultry an endemic situation in much of Asia, Europe and the Middle East. This subtype, albeit low pathogenic, carries with it human receptor specificity and the ability to infect humans without prior adaptation. The generation of an influenza pandemic requires interspecies transmission of a novel strain, which can adapt to its new host through either reassortment or point mutations. Given that two previous pandemics were the result of reassortment between low pathogenic avian viruses and human subtypes of that period, and given the endemic situation of avian H9N2 viruses in Eurasia, for part two of this project, we wanted to determine if adaptation of an avian-human H9N2 reassortant in ferrets could support mammalian respiratory droplet transmission. Here we show for the first time that a reassortant virus carrying the HA and NA of an avian H9N2 virus can transmit in respiratory droplets. This is the first report of respiratory droplet transmission of H9N2 influenza, which carries profound implications for pandemic preparedness. The amino acid changes on the HA might identify critical, adaptive mutations necessary for respiratory transmission in subsequent pandemic avian influenza strains. Using reverse genetics we identified key combinations of this adapted reassortant that support respiratory droplet transmission.