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Epidemiology, transmission, and pathogenesis of avian influenza

Iain Stephenson, MD, FRCP
Section Editor
Martin S Hirsch, MD
Deputy Editor
Anna R Thorner, MD


There have been five influenza pandemics during the past 100 years, and each has been caused by the emergence of a novel virus. In the 1957 and 1968 pandemics, the new viruses contained components of previous human, as well as avian, influenza viruses. The origin of the influenza virus responsible for the 1918 pandemic, which killed more people in a single year than the bubonic plague, remains uncertain, but it appears to have been an adapted avian influenza strain. The emergence of a novel H1N1 human-swine-avian reassortant virus in 2009 in North America started a new pandemic. (See "Epidemiology of influenza" and "Epidemiology of pandemic H1N1 influenza ('swine influenza')".)

More than 850 clinical cases of H5N1 influenza infection have been reported in humans since 2003, although the actual number of infections transmitted from birds to humans may be considerably higher [1-9]. Two features of recent avian influenza H5N1 outbreaks are striking: the predominance of children and young adults and the high mortality rate [4,5]. However, seroprevalence studies have found that some exposed individuals may have had a subclinical or mild infection, suggesting that the reported case-fatality rate may be an overestimate. (See 'Subclinical and mild infections' below.)

Highly pathogenic avian H5N1 influenza viruses now appear to be endemic among bird and poultry populations in Eurasia [10]. Sporadic transmission to humans raises concern that the H5N1 virus may mutate or combine with genetic material from coinfecting human influenza viruses to generate a novel strain capable of sustained human-to-human transmission with pandemic potential. The World Health Organization (WHO) has described the potential threat from H5N1 as a "public health crisis."

The epidemiology, transmission, and pathogenesis of avian influenza will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of avian influenza are discussed separately. (See "Clinical manifestations and diagnosis of avian influenza" and "Treatment and prevention of avian influenza" and "Avian influenza vaccines".)

The novel avian influenza A H7N9 virus is also reviewed separately. (See "Avian influenza A H7N9: Epidemiology, clinical manifestations, and diagnosis".)


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