April 25, 2013 at 9:12 AM ET
Poultry markets where birds are sold live and slaughtered on the spot are the likely source of the new H7N9 bird flu that’s killed more than 20 people in China, researchers said Thursday.
Genetic analysis shows the strain that infected some of the first patients is very similar to a strain found in chickens and pigeons at so-called “wet” markets, the team of Chinese researchers said. Their report was rushed into publication by the Lancet medical journal.
Global disease experts have been unsettled by the rapid rise of H7N9 in China. It’s infected at least 109 people and killed 23 of them since it first seems to have emerged in February. The good news is that it doesn’t easily pass from one person to another, but they have been unclear about just where people have been getting it from.
The findings are good news, if they hold out. They suggest the virus can be controlled by slaughtering poultry and disinfecting markets, says Kwok-Yung Yuen of the University of Hong Kong.
“Overall, the evidence, in terms of epidemiology and virology, suggests that it is a pure poultry to human transmission, and that controlling [the epidemic in humans] will therefore depend on controlling the epidemic in poultry,” Kwok said in a statement.
Hong Kong officials did this in 1997, during the first human outbreak of H5N1 bird flu, during which 18 people were infected and six died. They closed all the bird markets in the territory, and the virus disappeared -- although it re-emerged later in South Korea in 2003, and has been causing outbreaks ever since. It’s killed 60 percent of the more than 600 people infected in 15 countries.
H5N1 has been fairly easy to track because it kills chickens in large numbers, and quickly. The H7N9 virus doesn’t seem to. The Chinese researchers tested poultry and found the virus in chickens and pigeons, but not in ducks and quail, being sold at the markets.
“Aggressive intervention to block further animal-to-person transmission in live poultry markets, as has previously been done in Hong Kong, should be considered,” the researchers wrote.
“Temporary closure of live bird markets and comprehensive programs of surveillance, culling, improved biosecurity, segregation of different poultry species, and possibly vaccination programs to control H7N9 virus infection in poultry seem necessary to halt evolution of the virus into a pandemic agent.”
On Wednesday a joint team of doctors from China and the World Health Organization reached a similar conclusion. They said 77 percent of the documented cases were in people who had either visited live animal markets, or had some sort of contact.
"This raises the possibility of zoonotic H7N9 virus transmission from healthy-appearing swine or poultry to humans through direct or close contact or through exposure to environments that are contaminated with infected swine or poultry," they wrote in the New England Journal of Medicine.
For Thursday's report, the researchers looked at the genetic sequence of four patients from China’s eastern Zhejiang province. Two of the men died. One was a chef who prepared chicken, another had slaughtered poultry from one of the markets and two had bought live birds from wet markets.
The genetic sequences from the men very closely resembled virus taken from poultry at the markets they had visited or prepared chickens from. But there were exceptions -- including a mutation that makes the virus more comfortable in mammals, such as humans, than in birds.
“The key concerns about the current outbreak of influenza A H7N9 virus are how the virus crosses the species barrier and whether it will further adapt to enable efficient person-to-person transmission,” they wrote.
The researchers also noted that the virus doesn’t act like seasonal flu in people. Seasonal flu causes upper respiratory symptoms -- cough and congestion. The H7N9 flu goes straight to the lower lung, causing high fever, pneumonia and, sometimes, a strong immune system reaction called a “cytokine storm” that can be deadly.
Health experts point out that any animal flu can mutate or recombine into a form that easily infects people. They also note that any new virus is only a plane ride away from the rest of the world. A Taiwanese man who visited China has been diagnosed with H7N9 and doctors are watching three of the health care workers who cared for him and who have developed flu-like symptoms.
Severe acute respiratory syndrome (SARS) was carried around the world by travelers, infected about 8,000 people and killing around 800 of them before it was stopped.