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Report: H7N9 Flu Virus Quiet for Now, But Threat Remains

Virus has potential to become ‘human-adapted’ (July 9)

Although the H7N9 avian flu strain that emerged in China earlier this year has subsided for now, the virus has several unusual traits that paint a disquieting picture of a pathogen that may yet lead to a pandemic, according to scientists at the National Institute of Allergy and Infectious Diseases.

In a paper published in mBio, the online journal of the American Society for Microbiology (ASM), the authors describe the history of H7 viruses in animal and human disease and point out that H7 influenza has a tendency to become established in bird, horse, and swine populations and may “spill over” repeatedly into humans.

The outbreak of H7N9 earlier this year led China to temporarily close scores of live poultry markets in an effort to limit the spread of the virus. Although this previously unrecognized strain of avian influenza A has now been associated with 132 confirmed human infections and 39 related deaths (as of June 14), the rate at which new cases are recognized has dwindled in recent weeks.

In their review, the authors point out that, despite this apparent hiatus, viruses such as H7N9, which have subtype-7 hemagglutinin, are a cause for concern because of several unusual characteristics. First, H7 viruses have repeatedly been involved in numerous poultry outbreaks, including incidents in New York, Canada, Mexico, the Netherlands, and Italy, and in almost all of these cases the virus eventually spilled over into humans. Moreover, H7 viruses have the ability to mutate from a low pathogenicity form to a high pathogenicity form in birds — a scenario that can lead to large-scale culling and ultimately to human exposure to the virus among poultry workers.

The authors report that many H7 viruses have adapted to infect mammals, including horses and pigs, which raises the possibility that H7N9 could adapt in a similar fashion. The potential for H7N9 to infect pigs is particularly troubling, as swine are considered a “mixing vessel” for viruses — a breeding ground for novel viral reassortments, such as the 2009 H1N1 pandemic influenza strain commonly known as “swine flu.”

Although avian influenza viruses have not caused widespread human transmission in 94 years of surveillance, there have been numerous instances of avian influenza spill-over, and H7N9 “might arguably be more likely than other avian viruses to become human-adapted,” the authors say.

Source: ASM; July 9, 2013.

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