A study led by researchers from Japan's National Institute of Infectious Diseases with colleagues at the University of Wisconsin-Madison examined the genetic sequences of the H7N9 pathogen from four of the human victims as well as samples derived from birds and the environs of a Shanghai market.
"The human isolates, but not the avian and environmental ones, have a protein mutation that allows for efficient growth in human cells and that also allows them to grow at a temperature that corresponds to the upper respiratory tract of humans, which is lower than you find in birds," UW-Madison researcher Yoshihiro Kawaoka said.
Although it is too early to predict its potential to cause a pandemic, Kawaoka said, signs the virus is adapting to mammalian and, in particular, human hosts are unmistakable.
Avian influenza rarely infects humans but can sometimes adapt to people, posing a significant risk to human health, the researchers said.
"These viruses possess several characteristic features of mammalian influenza viruses, which likely contribute to their ability to infect humans and raise concerns regarding their pandemic potential," Kawaoka and his colleagues reported.
The first human cases were reported on March 31 by the Chinese Center for Disease Control and Prevention, and the new virus has sickened at least 33 people, killing nine.