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ate last month, a man in northwest China started to feel the telltale symptoms of the flu. Most of us know those signs, which come on suddenly. A sharp headache, a fever, bone-deep fatigue — followed by that sense of dread that something nasty is about to happen.

But the man didn’t have just garden variety influenza. He’d contracted H7N9, a strain of bird flu that kills roughly 4 out of every 10 people it infects. Within a week, he was dead.

The unidentified 35-year-old, from the region of Xinjiang, made his living in what is increasingly being seen as a dangerous occupation. He sold and slaughtered chickens.

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China’s ubiquitous live animal markets, where chickens remain alive until a buyer arrives, have been ground zero for transmission of H7N9 flu. And a unique U.S. study, recently presented at a scientific meeting in Hong Kong, helps to explain why.

Slaughtering and defeathering chickens that are infected with bird flu creates invisible viral clouds that can engulf poultry sellers and buyers, and even passers-by.

Depending on the strain of the virus and the processes used to kill and defeather the poultry, the very air around poultry slaughtering activity can contain both virus-laced droplets of fluids and even viruses that are aerosolized — wafting through the air, waiting to be drawn into nearby lungs.

“It’s very reasonable to say: Hey, in the process of slaughtering infected animals … you do generate large droplets and you generate aerosol particles,” said David Swayne, who led the research. “And they contain the virus in them.”

“So if you are a person in that air space and you breathe that in, you could be exposed and you possibly could be infected.”

The man who died in late June, whose situation was briefly described in a report from Hong Kong’s Center for Health Protection, is one of more than 735 H7N9 infections that China recorded in what has been a massive and unsettling fifth wave of cases caused by this strain of bird flu, which was first detected in early 2013. To date, 268 people infected in this wave have died.

The fifth wave has sharply refocused the attention of influenza scientists on H7N9, which seemed to be on the wane after notching only 224 and 119 cases in 2015 and 2016, respectively. This year there have been more cases recorded than any of the previous years. In fact, nearly half of all H7N9 cases ever diagnosed — 1,533 or so — occurred in this crest of activity.

Adding to the concern is the fact that while in previous years H7N9 activity petered out by early to mid-May, this year infections continued to rack up into June.

“Based on previous waves we would have expected some gradual decline,” said Jacqueline Katz, deputy director of the influenza division at the Centers for Disease Control and Prevention. “It has gone on for a long time.”

A new monthly report from the World Health Organization is due any day. But influenza epidemiologist Wenqing Zhang said in an email that H7N9 cases are decreasing at this point.

That’s to be expected during the warm summer months. Also expected: a resumption of infections when temperatures start to dip in late autumn, early winter.

Although there have been a surprising number of H7N9 cases this year, human infections are still rare. This virus can occasionally infect people, but doesn’t have what it would take to start spreading easily, going from person to person like human flu strains do.

There’s no way to know if it will ever gain that capacity, but given the severity of the illness it causes, this virus tops the CDC’s list of influenza pandemic threats.

Katz and Todd Davis, principal investigator on the CDC team that studies flu viruses that infect other mammals and birds, told STAT in a recent interview that this year’s surge in H7N9 case numbers is likely due to the fact that the virus is infecting poultry in more parts of the country.

More infected birds equals more exposed people.

Swayne’s research, which hasn’t yet been published, points to how that happens. He and colleagues at the Department of Agriculture’s Southeast Poultry Research Laboratory in Athens, Ga., infected chickens, then slaughtered and defeathered them.

The study was done in laboratories with a BSL 3 (enhanced) rating, meaning safety mechanisms were in place to ensure viruses from the experiment could not make their way out of the lab and that the researchers were protected.

But the researchers proved that the viruses could be transmitted through slaughter by placing healthy chickens, and later ferrets, in cages in the laboratory where the experiments were being conducted. The healthy chickens all became infected and most of the ferrets — which are used as stand-ins for people in influenza research — did too.

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The research was done using H5N1 bird flu viruses. And it was aimed at finding ways to lower the risk of home slaughter of H5N1-infected chickens in Egypt, which has had an enormous problem with that type of bird flu. Researchers had concluded that the way women kill and prepare chickens in Egypt explained why cases there were mainly among rural women and young children who were nearby when chickens were killed.

Slaughtering techniques may differ a bit in the Chinese markets, but the general effect is likely similar. In fact, researchers from the University of Hong Kong reported last year that they were able to detect several dangerous bird flu viruses in air samples collected in live poultry markets in the southern Chinese city of Guangzhou, and in Hong Kong.

That article noted the use of a mechanical defeathering device increased the amount of viruses found in the air. Swayne and his team saw that too; part of their study involved simulating mechanical defeathering. “That particular process can put virus in the air,” he said.

Changing the way chickens are slaughtered could help to prevent human cases of both these types of bird flu, and Andrew Clark, a veterinarian and consultant who has worked with officials in Egypt to translate Swayne’s findings into safer practice, said there is interest there.

But the strong preference in China for chicken that is freshly slaughtered may get in the way of major changes in that country, Katz acknowledged. “I think there are still cultural issues there that will be difficult to modify,” she said.

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