Robert G. Webster, PhD and Richard J. Webby, PhD

    Researchers trace evolution of H5N1 virus

    An avian influenza virus that has caused three major outbreaks among poultry and killed several people in East Asia over the past seven years arose through a series of genetic reassortment events with other viruses. This study finding, by scientists from St. Jude, the People’s Republic of China, China’s Special Administrative Region of Hong Kong, Thailand, Vietnam and Indonesia, is published in the July 8 online edition of Nature.

    Reassortment is the swapping of genes when two or more viruses infect the same animal.
    The researchers say that their study of the genetic makeup of H5N1 subtypes collected since 1997 traces the evolution of the virus into a dangerous pathogen through a series of reassortment events. Results of the study indicate that domestic ducks in southern China played a key role in the generation of this virus. The H5N1 virus forced health authorities to slaughter millions of chickens in order to prevent the spread of the disease, which can quickly wipe out poultry in open-air markets and farms and spread to other flocks.

    The investigators warn that outbreaks of H5N1 in East Asian poultry populations must be rapidly and effectively controlled to prevent H5N1 from evolving into a virus that causes a human pandemic, or worldwide epidemic. By cleaning up open-air markets and regularly slaughtering infected birds, Hong Kong remained free of H5N1 outbreaks in poultry during the 2004 influenza crisis, according to Robert Webster, PhD, and Richard Webby, PhD, both of Infectious Diseases.

    “In order to reduce the ability of H5N1 to trigger another poultry epidemic, officials in East Asia must follow Hong Kong’s lead,” Webster said. “Otherwise, H5N1 will likely continue to infect birds and other animals and eventually could evolve into a dangerous human pathogen as well.”

    Webster and Webby are co-authors of the Nature report, which details genetic studies of the evolving H5N1 virus that caused the initial human outbreak in Hong Kong in 1997. The report traces the origins of the outbreaks of highly pathogenic H5N1 disease in Asian poultry that occurred in 2003 and 2004. The report also notes other indications that H5N1 could evolve into a worldwide threat to humans: the outbreak of H5N1 in poultry in Asia in 2003 and 2004 was unprecedented in its geographical range, which showed the wide reach this virus already has.

    “The transmission of H5N1 to even just a relatively few people was an ominous sign that it has the potential to adapt to humans,” Webster said.

    A key question left unanswered by the present study is whether the highly pathogenic H5N1 influenza virus is now being spread by wild migratory birds.

    “Although these deadly viruses were isolated from dead migrating birds, we don’t know if the birds are actually spreading H5N1,” Webby said. “We must do further research to find out, since migratory birds could conceivably spread highly pathogenic H5N1 throughout the rest of Asia and into Europe and the Americas.”

    Last update: November 2004