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H7N9 bird flu can pass between mammals, researchers find

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Scientists are gaining a better understanding of the H7N9 bird flu that has sickened more than 130 people -- and killed more than 30 -- in China and Taiwan since February.

The latest research into the virus, which before this year had never been detected in humans, was published Thursday (subscription required for full text) in the online edition of the journal Science

Working with ferrets, an animal that is often studied to gain insight into flu transmissibility in people, scientists in China, Canada and the U.S. found that H7N9 could spread from one ferret to another -- suggesting that it could also pass between humans. "Under appropriate conditions human-to-human transmission of the H7N9 virus may be possible," the co-authors wrote. 

But H7N9 only spread efficiently when the ferrets were placed in the same cage and came into direct contact. The virus did not transmit easily between animals in adjacent cages, who couldn't touch but could breathe in the droplets from each others' sneezes and coughs.

"We think for a virus to take off in humans, it has to be efficient at both" forms of transmission, said Richard Webby, a virologist at St. Jude Children's Hospital in Memphis, Tenn. and a coauthor of the paper. But, he added, the H7N9 virus is likely to mutate over time, and could become more transmissible in humans as time goes by.

Webby said that the team's data reflected the epidemiology on the ground: So far, public health officials believe most cases have occurred in people who have had close contact with poultry, and do not think H7N9 spreads easily between humans.  But even though the findings "may seem a little 'duh,' " Webby said, they help scientists answer some of the more subtle questions surrounding H7N9 transmissibility in people.

When people get sick with H7N9, health workers don't know how much of their susceptibility is influenced by underlying heath conditions, cross-reactive immunity, or other external factors. By performing an experiment in ferrets, Webby said, researchers can control for such variables and get a more clear picture of the virus' transmissibility.

The team also performed their experiment in pigs, in an effort to see what role the animals might play in harboring the virus in the wild. H7N9 did not transmit efficiently in the pigs, suggesting that they "probably are not big players in the epidemiology of the disease at the moment," Webby said. 

According to the World Health Organization's latest update on human infections with H7N9, there were no new lab-confirmed cases of H7N9 in humans between May 8 and May 17. Webby said that cases of the illness did seem to be "tailing off," perhaps because live poultry markets have been shut down in the region, or perhaps because it's almost summer.

In the future, the coauthors of the Science paper wrote, as markets reopen, authorities in areas where the virus has taken hold may want to adjust the way they manage poultry markets to prevent further spread of the virus.

A letter published Wednesday in the New England Journal of Medicine also focused on on the importance of poultry markets in the spread of H7N9 among humans. That journal also published new H7N9 research this week, which updated reports of the clinical characteristics of 111 cases of the illness.

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