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Avian influenza A H7N9: Epidemiology, clinical manifestations, and diagnosis

Author
Anna R Thorner, MD
Section Editor
Martin S Hirsch, MD
Deputy Editor
Allyson Bloom, MD

INTRODUCTION

In late March and April 2013, human cases of novel avian influenza A H7N9 infection in China were reported to the World Health Organization [1-7]. The initial wave occurred from February to May 2013, during which 133 cases were detected [8]. The number of new cases peaked in April 2013 and then declined [9], likely related, at least in part, to implementation of control strategies including closure of live bird markets and increased public awareness. A rise in the number of cases occurred in late 2013 and early 2014 as well as in late 2014 and early 2015, coinciding with influenza season [10]. Cases continue to be detected. At least 800 laboratory-confirmed cases have been reported since this virus emerged in humans in 2013 [11].

The epidemiology, clinical manifestations, and diagnosis of avian influenza A H7N9 infections will be reviewed here. The treatment and prevention of avian influenza A H7N9 infections are discussed separately. (See "Avian influenza A H7N9: Treatment and prevention".)

Other avian influenza viruses (eg, H5N1 influenza) and seasonal influenza viruses are also reviewed separately. (See "Epidemiology, transmission, and pathogenesis of avian influenza" and "Clinical manifestations and diagnosis of avian influenza" and "Treatment and prevention of avian influenza" and "Avian influenza vaccines" and "Epidemiology of influenza" and "Clinical manifestations of seasonal influenza in adults" and "Diagnosis of seasonal influenza in adults" and "Seasonal influenza in children: Clinical features and diagnosis" and "Treatment of seasonal influenza in adults" and "Seasonal influenza in children: Prevention and treatment with antiviral drugs" and "Seasonal influenza vaccination in adults" and "Seasonal influenza in children: Prevention with vaccines".)

VIROLOGY

Sources of the reassortant virus — Avian influenza A H7N9 virus appears to have derived from multiple reassortment events of several avian influenza viruses (figure 1) [4,12-17]. One study, which was based on the first four human isolates, showed the following results [13]:

The novel H7N9 avian influenza A virus appears to have derived from at least four different avian influenza viruses. The H7 gene is genetically close to sequences isolated from ducks in Zhejiang province. The NA gene is genetically close to genes from H7N9 viruses isolated from wild ducks in South Korea. The six internal genes are clustered together with H9N2 viruses isolated from poultry and ducks in China. Analyses suggest at least two different origins of H9N2 viruses, one for the NS gene and one for the remaining internal genes.

                          

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Literature review current through: Nov 2016. | This topic last updated: Tue Dec 06 00:00:00 GMT+00:00 2016.
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