Avian Influenza Virus Shows Pandemic Potential

Several qualities that characterize the Chinese H7N9 avian influenza virus suggest to epidemiologists that this strain has the potential to trigger a worldwide flu pandemic.

The recent sporadic infections of humans in China with a previously unrecognized avian Influenza A virus of the H7N9 subtype have caused concern owing to the appreciable case fatality rate associated with these infections (more than 25%), potential instances of human-to-human transmission, and the lack of pre-existing immunity among humans to viruses of this subtype.

Investigators at the University of Wisconsin (Madison, USA) and their colleagues at the University of Tokyo (Japan) characterized two early human H7N9 isolates, A/Anhui/1/2013 and A/Shanghai/1/2013.

In mice, Anhui/1 and Shanghai/1 were more pathogenic than a control avian H7N9 virus (A/duck/Gunma/466/2011) and a representative pandemic 2009 H1N1 virus (A/California/4/2009). Anhui/1, Shanghai/1 and Dk/GM466 replicated well in the nasal turbinates (scrolled spongy bones of the nasal passage) of ferrets. Anhui/1 transmitted through respiratory droplets in one of three pairs of ferrets. Ferrets infect one another with influenza virus through coughing and sneezing and are a standard model for studies of influenza in mammals.

In nonhuman primates, Anhui/1 and Dk/GM466 replicated efficiently in the upper and lower respiratory tracts, whereas the replication of conventional human influenza viruses is typically restricted to the upper respiratory tract of infected primates. Anhui/1 was found to be less sensitive in mice to neuraminidase inhibitors than a pandemic H1N1 2009 virus, although both viruses were equally susceptible to an experimental antiviral polymerase inhibitor.

The findings obtained during this study, especially the ability of the virus to replicate in mice, ferrets, and nonhuman primates and the limited transmissibility in ferrets suggest that H7N9 viruses may have pandemic potential.

"H7N9 viruses have several features typically associated with human influenza viruses and therefore possess pandemic potential and need to be monitored closely," said senior author Dr. Yoshihiro Kawaoka, professor of virology at the University of Wisconsin. "H7N9 viruses combine several features of pandemic influenza viruses, that is, their ability to bind to and replicate in human cells and the ability to transmit via respiratory droplets. If H7N9 viruses acquire the ability to transmit efficiently from person to person, a worldwide outbreak is almost certain since humans lack protective immune responses to these types of viruses."

The study was published in the July 10, 2013, online edition of the journal Nature.

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