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Evolution of H7N9 Virus in China


H7N9 emerged as a zoonotic infection in Guangdong Province during early 2016.  A recent article* reported on molecular analysis of 16 strains of H7N9 virus sequenced during 2015-2017.  It was determined that the isolates could be segregated into three antigenic clusters denoting different lineages.  The difference among strains involved mutations determining the structure of hemagglutinin sites involved in antigenicity.


There is concern that recombinants could arise between H7N9 and other widely distributed avian influenza viruses including H9N2.  This serotype is frequently isolated from live poultry in wet markets especially during seasonal outbreaks of influenza.


Highly pathogenic H7N9 virus has become more pathogenic in mice and demonstrates a higher level of thermal stability compared to low-pathogenicity strains.  Serologic surveys applying hemagglutination inhibition demonstrated that highly pathogenic H7N9 viruses now occur in several provinces in China and are responsible for clinical outbreaks and extensive losses in poultry flocks.


The authors warned of the possibility of widespread dissemination of H7N9 virus through migration of free-living birds similar to the situation following the emergence of H5N1 virus in the early 2000’s. Despite extensive use of vaccine in China, adaptation of the virus is occurring through mutation justifying intensified surveillance applying molecular epidemiology.


It is possible that an H7N9 reassortant may represent the next challenge to poultry industries in North America and the E.U. This presumes maintaining high levels of biosecurity at both the structural and operational levels. Erecting and managing complexes with 1 million or more hens without shower-on/shower-off installations, effective vehicle decontamination, bird-proofing and procedures to limit introduction of pathogens is playing “influenza roulette” especially if farms are situated on a migratory flyway and near an expanse of water.


*Lu, J. et al Molecular Evolution, Diversity, and Adaptation of Influenza A (H7N9) Viruses in China.  Emerging Infectious Diseases 24: 1795-1805