H9N2 avian flu strain has pandemic potential

New study shows small changes make the subtype more transmissible

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Pandemic planners may have been looking at the wrong avian influenza virus as the source of the next worldwide flu epidemic. A type of avian flu virus known as H9N2 could become transmissible in humans with just a few changes, a new study shows.

“The H9 may be a silent virus that doesn’t get noticed until it’s too late,” says Daniel Perez, a virologist at the University of Maryland, College Park.

Perez and his colleagues analyzed the pandemic potential of the H9 flu viruses in ferrets, a model for human disease transmission. The team found that changing a single chemical building block in the hemagglutinin protein that helps the virus latch onto cells can make the virus more transmissible in ferrets. Mixing the avian virus’s genes with those from human flu viruses also increases transmission and may make the virus more virulent, the researchers report online August 13 in PLoS ONE.

Though the virus did not become airborne in ferrets, it may have the potential to do so in the future.

Much attention has been focused on the H5N1 bird flu virus and H7 avian flu viruses as candidates for the next pandemic, says Chang-Won Lee, a molecular virologist at the OhioStateUniversity in Wooster. But there is plenty of circumstantial evidence that the H9 viruses could also cause a future pandemic, Lee says.

H9 avian influenza is widespread among birds in Asia, the Middle East, Europe and Africa. It doesn’t make birds sick, so it often goes unnoticed. Some people and pigs have also been infected, but the virus causes only a mild illness in people and, so far, has not been known to spread from person to person. But Perez’s results could indicate that the H9 viruses may begin spreading among people with just minor changes.

“Transmission is really the cardinal property a virus has to have in order to cause a pandemic in humans,” says Peter Palese, a virologist at Mount Sinai School of Medicine in New York City. Still airborne transmission is a condition that would be necessary for a human pandemic, so the new results are a “half-full, half-empty glass,” Palese says.

Even if the H9 viruses do acquire the ability to spread in people, at first the infection is likely to cause minor illness. “You’re going to have a bunch of people who don’t feel very well, as opposed to dropping off the face of the Earth,” says Raymond Pickles, a cell biologist at the University of North Carolina at Chapel Hill. But the new data indicate that if the H9 viruses mix with other human viruses, as commonly happens in nature, it could become more potent.

While many pandemic planners put the deadly H5N1 virus at the top of their list of concerns, Robert G. Webster, a leading influenza researcher at St. Jude Children’s ResearchHospital in Memphis, Tenn., says H9N2 tops his “influenza hit list.”

The H9 virus is an “influenza sleeper,” Webster says. “It could cause many more problems than we realize.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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