Epidemiology, Evolution, and Pathogenesis of H7N9 Influenza Viruses in Five Epidemic Waves since 2013 in China

H7N9 influenza viruses were first isolated in 2013 and continue to cause human infections. H7N9 infections represent an ongoing public health threat that has resulted in 1344 cases with 511 deaths as of April 9, 2017. This highlights the continued threat posed by the current poultry trade and live poultry market system in China. Until now, there have been five H7N9 influenza epidemic waves in China; however, the steep increase in the number of humans infected with H7N9 viruses observed in the fifth wave, beginning in October 2016, the spread into western provinces, and the emergence of highly pathogenic (HP) H7N9 influenza outbreaks in chickens and infection in humans have caused domestic and international concern. In this review, we summarize and compare the different waves of H7N9 regarding their epidemiology, pathogenesis, evolution, and characteristic features, and speculate on factors behind the recent increase in the number of human cases and sudden outbreaks in chickens. The continuous evolution of the virus poses a long-term threat to public health and the poultry industry, and thus it is imperative to strengthen prevention and control strategies.

Trends

The emergence of highly pathogenic H7N9 avian influenza virus is a potential threat to the poultry industry and to public health.

Elevated numbers of human cases of H7N9 virus infections during the fifth epidemic wave has meant the spread of H7N9 to several western provinces for the first time.

The evolutionary relationships of the HA and NA genes of H7N9 viruses during the five epidemic waves are examined here.

The dual receptor binding capacity of the H7 protein, along with the enhanced receptor affinity of the N9 protein, may contribute to the higher human infectivity of the H7N9 virus.

To reduce the risk of H7N9 human infection, we have to reduce or eliminate infections in poultry. Therefore, prevention and control strategies including strengthened biosecurity and improved outbreak management, as well as the prudent use of vaccines in poultry, should be reviewed and implemented.