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Tests show that deadly flu could spread among people

New influenza virus transmits through air between ferrets, raising concerns that it could do the same in humans

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A new bird flu that has killed 36 people in China can spread from ferret to ferret through the air. A laboratory test showing airborne transmission of the H7N9 avian influenza virus between the animals has raised fears that the virus is poised to become a human pandemic.

The H7N9 avian influenza virus emerged suddenly at the end of February and has infected 131 people. A few patients may have caught the virus from other infected people, but no evidence has emerged that H7N9 can readily transmit from human to human.

To find out how the virus might spread among people, an international group of researchers infected ferrets, which often stand as proxies for people in influenza studies. Infected ferrets passed the virus to all of the uninfected animals housed in the same cage, indicating that H7N9 spreads through direct contact, the team reports May 23 in Science. One of three uninfected ferrets in neighboring cages also caught the virus, suggesting that it can also transmit through airborne droplets when an infected animal sneezes or coughs. Airborne transmission among people is a prerequisite for a pandemic.

As a comparison, the researchers also exposed ferrets to an airborne flu strain that caused a pandemic in 2009. All of the animals got sick. Because only one of three ferrets that could have contracted the new H7N9 virus through the air actually did, the researchers conclude that airborne H7N9 transmission is inefficient. 

But the experiment was not designed to quantify the efficiency of airborne transmission and ferrets aren’t perfect representations of people, so it may be difficult to gauge a person’s risk of catching H7N9 through airborne droplets, says coauthor Robert Webster of St. Jude Children’s Research Hospital in Memphis. “It shows it can happen. Statistically, who knows what it means?”

There’s no guarantee the virus will spread similarly from person to person, says Ana Fernandez-Sesma, a viral immunologist at Mount Sinai Hospital in New York. In the experiment, ferrets are together for hours with forced airflow under temperature and humidity conditions that favor viral transmission, she says. “I don’t think this is what happens in real life.”

Ram Sasisekharan, an MIT biochemist who studies influenza viruses, agrees that the virus probably is not capable of person-to-person spread through the air. But it could evolve that ability. “With these viruses, you will never know if and when they mutate and if it will acquire mutations that will be a cause of concern,” he says.

That could happen via pigs. The animals can serve as mixing vessels where human and bird viruses swap genes, creating new flu strains. That’s how the 2009 pandemic virus came to be. In the new study, the researchers exposed pigs to the H7N9 virus. The animals became infected but didn’t pass the new flu to other pigs or to ferrets, the researchers found. That result indicates that pigs probably are not a source of the virus and would not pass H7N9 along if they did become infected. Outside the lab, no pigs have been found to carry the virus.

Public health officials have not located the origin of the H7N9 virus, but growing evidence suggests that birds sold at live poultry markets infected some patients. George Gao of the Chinese Center for Disease Control and Prevention in Beijing and colleagues have now traced the source of one woman’s infection to poultry sold at the market where she was a butcher. Those birds already carried the virus when they arrived from a wholesaler. The team recounts the trail May 22 in the New England Journal of Medicine.

No new human cases have been reported since May 8, which Webster attributes partially to the seasons changing (summer weather is not favorable to the flu), but mainly to China’s temporary closing of the live bird markets in Shanghai and other affected areas. The markets have been closed since April 6 and the government has not announced when trading will resume. “We could get it stamped out if China can close the markets for a bit longer,” Webster says.

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