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May 24, 2013

Study Identifies Risks of Human Spread of H7N9 Bird Flu

by Jessica Berman

A new study finds that the recently discovered H7N9 bird flu virus, which has been circulating among poultry flocks in eastern China, can be transmitted among ferrets, small mammals that provide a laboratory model for human contagion.  Since the H7N9 virus was first identified in China last February, there have been 132 human infections confirmed in China and one in Taiwan, with a total of 36 deaths.  While there have been concerns that this bird flu could become a global pandemic, the new study suggests that the virus could spread among people.

So far, it appears that people who have been diagnosed with the H7N9 virus have all had close contact with infected birds, primarily in live poultry markets.  To become a pandemic, the avian virus would have to spread easily from human-to-human.

To find out how efficiently the virus can be transmitted among humans, an international team of researchers, led by Chinese scientists, studied its contagion among ferrets. The small mammals can transmit infectious diseases through aerosols produced by sneezing and coughing, much like humans.

Richard Webby is a virologist at St. Jude Children’s Research Hospital in Memphis, Tennessee and a study co-author.  Webby said the work seems to suggest humans would have to be in close contact to transmit this virus. “That’s what our ferret studies would support. That potentially, if you have infected people very close to uninfected people, you might get transmission," he explained. "But it’s not very efficient.”

Chinese investigators swabbed the noses of a group of ferrets infected with the H7N9 virus taken from an infected human patient who had died. Researchers detected evidence of H7 through the nasal samples before the animals developed respiratory symptoms, suggesting more people may be carriers of the virus than is currently believed.  

The disease was also transmitted to uninfected ferrets through direct contact with infected animals in the same cage, simulating what might happen among human family members living in the same home.

But when they were placed in a separate cage and exposed to diseased animals in another enclosure, only 1 of 3 healthy ferrets contracted H7N9.  Researchers also found the virus did not transmit to pigs, another potential reservoir of influenza.

Experts believe both airborne and direct contact transmission is needed to ignite a pandemic.
 
Anthony Fauci is director of the US National Institute of Allergy and Infectious Diseases.  Fauci said the findings of the Chinese study confirm what's already known about H7N9. "That it can be transmitted to mammals rarely and inefficiently, and it's not being transmitted in any way from person to person," he stated.

Meanwhile, in the first human population study of H7N9 influenza, researchers analyzed just over 1,700 blood samples for 5 different avian flu viruses in the H7 subtype, in both urban and rural areas in southern Vietnam, which neighbors China.  

Investigators found low levels of H7 antibodies, markers of the human immune system's ability to defend against pathogens. The researchers say the low levels suggest that international measures to contain the H7N9 influenza, in the event of severe outbreak, will need to be targeted in Asia.

The Chinese transmissibility study recommends that to avoid H7N9 becoming endemic in poultry populations -- a development that would create new opportuinities for human transmission, live poultry markets in the region must be more carefully and strictly managed.

Their research is published in the journal Science Express.