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In 1997, a highly pathogenic avian H5N1 influenza virus was transmitted directly from live commercial poultry to humans in Hong Kong. Of the 18 people infected, six died. The molecular basis for the high virulence of this virus in mice was found to involve an amino acid change in the PB2 protein. To eliminate the source of the pathogenic virus, all birds in the Hong Kong markets were slaughtered. In 1999, another avian influenza virus of H9N2 subtype was transmitted to two children in Hong Kong. In 2000–2002, H5N1 avian viruses reappeared in the poultry markets of Hong Kong, although they have not infected humans. Continued circulation of H5N1 and other avian viruses in Hong Kong raises the possibility of future human influenza outbreaks. Moreover, the acquisition of properties of human viruses by the avian viruses currently circulating in southeast China might result in a pandemic. [ABSTRACT FROM AUTHOR]

Avian influenza viruses rarely infect humans, but the recently emerged avian H7N9 influenza viruses have caused sporadic infections in humans in China, resulting in 440 confirmed cases with 122 fatalities as of 16 May 2014. In addition, epidemiologic surveys suggest that there have been asymptomatic or mild human infections with H7N9 viruses. These viruses replicate efficiently in mammals, show limited transmissibility in ferrets and guinea pigs, and possess mammalian-adapting amino acid changes that likely contribute to their ability to infect mammals. In this review, we summarize the characteristic features of the novel H7N9 viruses and assess their pandemic potential. [ABSTRACT FROM AUTHOR]