Transmission studies conducted by Kawaoka's group in ferrets animals that, like humans, infect one another through coughing and sneezing and that are a standard model for studies of influenza in mammals showed that one of the H7N9 strains isolated from humans can transmit via respiratory droplets, though not as efficiently as human influenza viruses. The limited aerosol transmission observed in ferrets adds to concerns about the potential threat as avian flu viruses typically lack that ability, Kawaoka notes.
"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," Kawaoka says.
Complicating the H7N9 picture is the fact that the H7N9 virus does not kill poultry, which promises to make surveillance much more difficult. "We cannot simply watch out for sick or dead birds. Rather, tests have to be performed to determine whether or not a bird is infected. Considering the vast number of poultry, this is a daunting task."
The positive news conveyed in the new Nature report is that most of the H7N9 strains tested were somewhat sensitive to antiviral drugs effective against the seasonal flu virus, although one isolate, which appears to be a mix of two variants of the H7N9 virus, seemed to resist neuraminidase inhibitors like Tamiflu.
Further research is needed, Kawaoka argues, to support vaccine development, to assess the risks, and to better understand why the H7N9 viruses infect humans so efficiently.
|Contact: Yoshihiro Kawaoka|
University of Wisconsin-Madison