Bird Flu Virus Is Capable of Human Spread, Ferret Studies Show
The bird flu virus that’s killed 36 people in China is capable of human-to-human spread, scientists found in animal studies that highlight its pandemic potential.
Ferrets experimentally infected with the new H7N9 strain passed it to other ferrets occupying the same cage, indicating the virus’s ability to spread via direct contact, researchers at the Chinese Center for Disease Control and Prevention in Beijing and the University of Hong Kong said. Flu transmission in ferrets is a predictor of patterns in humans, the authors said in the study, published yesterday in Science Express.
The research will help health officials tailor their response to the H7N9 strain, which is known to have infected 131 people since March, mostly through contact with virus-laden poultry. No cases have been reported since May 8, weeks after authorities curbed live poultry sales in the eastern cities of Shanghai, Nanjing and Hangzhou. So far, there is no evidence of sustained human-to-human transmission -- a feature required for flu to spark a pandemic -- the World Health Organization said.
“The emergence of the H7N9 influenza virus in humans in Eastern China has raised concerns that a new influenza pandemic could occur,” Yuelong Shu and colleagues wrote. “Under appropriate conditions, human-to-human transmission of the H7N9 virus may be possible.”
The scientists showed that healthy ferrets caught the virus and began shedding it within three days of being caged with infected ferrets. By the sixth day, they were sneezing, coughing and had runny noses.
In a separate experiment, three ferrets were caged 10 centimeters (4 inches) away from infected ones to test whether they could become infected through the aerosols produced by coughs and sneezes of diseased animals. One of the three ferrets became infected, demonstrating that transmission is possible via direct contact and exposure to aerosols, but “less efficiently,” the Chinese researchers said.
Similar experiments with H7N9-infected pigs didn’t result in transmissions onto ferrets and other pigs sharing the same space or air, according to the study.
By comparison, ferrets infected with the H1N1 swine flu in the study’s control group transmitted that virus to all other ferrets that came in contact directly as well as through the air. H1N1, which originated in pigs and then mixed with human and avian viruses, touched off the first global flu outbreak in more than 40 years and killed about 284,500 people, according to the U.S. Centers for Disease Control and Prevention.
The H7N9 virus can infect the upper and lower respiratory tracts, and lymph nodes and potentially the brain, according to the researchers, who detected the viral RNA in the organs of infected ferrets. The animals also started shedding the virus before most clinical signs appeared, a trait that “has been observed previously for pandemic and seasonal influenza,” they said.
“If this virus acquires the ability to efficiently transmit from human-to-human, extensive spread of this virus may be inevitable, as quarantine measures will lag behind its spread,” the Chinese researchers said.
The findings support the need to reconsider management of live poultry markets, especially in urban areas, in case H7N9 becomes endemic in poultry, increasing the opportunities for the virus to evolve “to acquire human-to-human transmissibility,” the authors said.
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