The H7N9 influenza virus did not emerge alone. Researchers have traced the evolution of the deadly avian flu currently spreading in China, and have found evidence that it developed in parallel with a similar bird flu, H7N7, which can infect mammals.

Although there is no evidence that this H7N7 strain will infect humans, the authors of a study published today in Nature1 say that their finding reinforces the idea that H7 avian viruses are constantly mixing and exchanging genetic material — a process known as reassortment — in Asian poultry markets. This raises the threat that H7N7 will reassort and become able to spread to humans.

“H7 is out there in China and not just in the form of this H7N9,” says Richard Webby, a co-author of the study and an influenza specialist at St. Jude Children’s Research Hospital in Memphis, Tennessee.

Ducks, in particular, act as living mixing bowls for avian viruses. Domestic species encounter a large catalogue of wild-bird viruses, which swap genes to form versions that can spread to chickens and to humans.

Better surveillance of Chinese bird populations is needed to monitor the emergence of dangerous viruses such as H7N9, says lead author Yi Guan, an influenza specialist at the University of Hong Kong. In China, the virus has infected 135 people and resulted in 44 deaths since February. “This is a very different influenza ecosystem from other countries,” says Guan.

Tracking changes
Guan’s team sampled wild birds and poultry markets around Shanghai in April, weeks after the H7N9 outbreak began there. The researchers collected throat and intestinal swabs from 1,341 birds, including chickens, ducks, geese, pigeons, partridges and quails, plus 1,006 water and faecal samples from bird markets. About 10% of samples tested positive for an influenza virus; of those, 15% were an H7 virus.

When the team sequenced the two viruses’ genomes and compared them to other bird-flu strains, they found H7N9 and H7N7 to be hybrids of wild Eurasian waterfowl strains, such as H7N3 and H11N9. The scientists think that those viruses swapped genes in domestic ducks before spreading to chickens, where they traded genes with a common chicken virus, H9N2. That improved the viruses’ ability to spread in chickens, which live in close contact with humans.

So far, the latest H7N7 strain has not infected a human. But Guan and his team found that ferrets could become infected with the virus, suggesting that a spread to humans is possible.

“It really shows that the emergence of these types of viruses can happen at any time,” says Camille Lebarbenchon, a viral ecologist at the University of Reunion Island in St Denis, France, who has also studied the evolution of H7N9 using archived viral sequences2.

David Morens, an influenza researcher and senior adviser at the US National Institutes of Health in Bethesda, Maryland, says that the evolutionary pathway that the viruses followed suggests that more surveillance and better sanitation practices at poultry markets are crucial to monitoring risks to human health.

But Ian Lipkin, an epidemiologist at Columbia University in New York City, says that surveillance is not a foolproof solution. “It’s inevitable that something is going to slip through the cracks.”