Characteristics of the 2013 Influenza A (H7N9) Virus

Summary and Comment |
July 19, 2013

Characteristics of the 2013 Influenza A (H7N9) Virus

  1. Richard T. Ellison III, MDAU075

A novel avian influenza A (H7N9) virus that causes human disease shows binding to human respiratory receptors, infectivity in multiple species, and respiratory-droplet transmissibility.

  1. Richard T. Ellison III, MDAU075

In March 2013, patients infected with a novel avian influenza A (H7N9) virus were identified in China, and by May 31, 132 cases and 39 deaths had been confirmed. This virus is a reassortant of three avian pathogens — an H7N3 virus, an older H7N9 virus, and an H9N2 virus. Unlike patients with previous H7 infections that manifested predominantly as conjunctivitis and mild illness, most patients with novel H7N9 infection have developed severe pneumonia and acute respiratory distress syndrome. Several multinational groups have now studied the biological features of this virus.

Zhou and colleagues assessed the receptor-binding capacity of three human isolates of the H7N9 virus and found that they bound to both avian-type (α2,3) and human-type (α2,6) glycans. In accordance with this capacity to bind to human-type glycans, the H7N9 strains could infect human lower respiratory tract epithelial cells and type II alveolar pneumocytes in explanted tissue. Testing of serum samples from 90 individuals of various ages revealed no evidence of preexisting immunity to the virus. Antiviral susceptibility studies indicated resistance to amantadine and rimantadine, but not oseltamivir or zanamivir.

Belser and colleagues also identified the ability of the H7N9 virus to bind to α2,6 glycans and then determined that it could infect and cause disease in ferrets, which are similar to humans in their susceptibility to influenza viruses. Experiments involving H7N9-infected and -naive ferrets revealed efficient transmission (8 of 8 animals) through direct contact but less-efficient transmission (3 of 9) via respiratory droplets. Studies with a human bronchial epithelium cell line suggested that the H7N9 virus replicated more efficiently at 37°C than at the lower temperature of 33°C found in the proximal human airways. In contrast, a seasonal H3N2 virus replicated equally well at both temperatures.

Watanabe and colleagues assessed the ability of the H7N9 virus to infect various mammalian and avian species. In mice, the virus was more pathogenic than either a control H7N9 strain or a representative 2009 H1N1 virus. In ferrets, it caused infection and was transmissible between animals via respiratory droplets. Infection and viral replication occurred in both the upper and lower respiratory tracts of macaques. Pigs, chickens, and quail also developed infections, with only mild illness in the pigs and none in the chickens or quail. Again, the virus was found to bind to human-type glycans. Studies of antiviral susceptibility suggested initial susceptibility to neuraminidase inhibitors, but also the presence of a mutation that can induce resistance to these agents.

Zhu and colleagues also showed infectivity and transmissibility of the H7N9 virus in ferrets. In studies with pigs, they found that infected animals shed the virus for 5 to 6 days but did not appear to transmit it to naive pigs by direct contact, or to naive pigs or ferrets through airborne exposure. However, several of these naive animals subsequently developed antibodies to the virus.


The findings of these four studies suggest that the novel H7N9 virus has pandemic potential, should it gain the ability to replicate efficiently at the lower temperatures of the human upper airway. As noted by Zhu and colleagues, if poultry is the source of the virus, continued prevalence of the virus may allow it to become enzootic in poultry, increasing the risk for transmission to humans and pigs and subsequent further mutations. Measures to control its spread in poultry and to develop a human vaccine appear warranted.

Editor Disclosures at Time of Publication

  • Disclosures for Richard T. Ellison III, MD at time of publication Grant / research support NIH-NIAID


Reader Comments (1)

weerakoon W.A.B Resident, Obstetrics/Gynecology, Epidemiology Unit,Sri lanka

I like to know the new developments.

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