Emerging bird flu strain still far from affecting humans

Scientists have found that avian influenza virus H7N9, which killed several dozen people in China earlier this year, has not yet acquired the changes needed to infect humans easily.

In contrast to some initial studies that had suggested that H7N9 poses an imminent risk of a global pandemic, the new research from the Scripps Research Institute (TSRI), found, based on analyses of virus samples from the Chinese outbreak, that H7N9 is still mainly adapted for infecting birds, not humans.

"Luckily, H7N9 viruses just don't yet seem well adapted for binding to human receptors," Ian A. Wilson, the Hansen Professor of Structural Biology and chair of the Department of Integrative Structural and Computational Biology at TSRI, said.

H7N9 flu viruses infect birds, apparently causing them few or no symptoms. Until this year these strains had never been reported in humans. However, starting in February in two urban areas of eastern China, dozens of people began to come down with H7N9 flu.

Paulson's laboratory evaluated H7N9's ability to bind the sialylated sugar receptors to which flu viruses normally attach on host cells. Wilson's laboratory used X-ray crystallography to determine the atomic structures of the H7N9 hemagglutinin protein bound to these sialic acid receptor molecules.

In Wilson's laboratory, postdoctoral fellow Rui Xu, the study's first author, along with Staff Scientist Xueyong Zhu and Research Assistant Wenli Yu, performed X-ray crystallography studies of the Sh2 HA protein bound to several avian- and human-type receptors. The latter, provided by Paulson's laboratory, were more accurate versions of these receptors than any that had been used in previous H7N9 structural analyses.

The new data highlighted the looseness of the contacts that Sh2 HA makes with human-type receptors, in contrast to the snug couplings it makes with certain avian-type receptors.

The study is published in the journal Science.