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All known subtypes of influenza A viruses are maintained in wild waterfowl, the natural reservoir of these viruses. Influenza A viruses are isolated from a variety of animal species with varying morbidity and mortality rates. More importantly, influenza A viruses cause respiratory disease in humans with potentially fatal outcome. Local or global outbreaks in humans are typically characterized by excess hospitalizations and deaths. In 1997, highly pathogenic avian influenza viruses of the H5N1 subtype emerged in Hong Kong that transmitted to humans, resulting in the first documented cases of human death by avian influenza virus infection. A new outbreak started in July 2003 in poultry in Vietnam, Indonesia, and Thailand, and highly pathogenic avian H5N1 influenza viruses have since spread throughout Asia and into Europe and Africa. These viruses continue to infect humans with a high mortality rate and cause worldwide concern of a looming pandemic. Moreover, H5N1 virus outbreaks have had devastating effects on the poultry industries throughout Asia. Since H5N1 virus outbreaks appear to originate from Southern China, we here examine H5N1 influenza viruses in China, with an emphasis on their biological properties. [ABSTRACT FROM AUTHOR]

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]

In order to determine the role of neuraminidase (NA) in host range restriction, we studied a reassortant virus that replicated with poor efficiency in ducks. The reassortant virus, in which NA of an avian virus, A/duck/Hong Kong/278/78 (H2N9), was replaced with that from a human virus, A/England/12/62 (H2N2), was rectally inoculated into ducks. The viruses recovered were then orally inoculated into new ducks and a variant that replicated efficiently in the intestinal tract was isolated. Sequence analysis showed that the variant virus NA contained two amino acid substitutions at positions 165 (Val to Ile) and 431 (Gln to Pro). Differences in low pH resistance of the NAs were also assessed to identify differences in the NAs possibly related to the restriction of viral growth in ducks. The NA activity of the parental reassortant virus almost disappeared after low pH treatment (pH 3.0), while that of the variant was conserved under the same conditions. These results indicate that the amino acids, at positions 165 and/or 431 on the NA molecule, correlate with the ability to support viral growth in ducks, contributing to the low pH stability of NA activity. [Copyright &y& Elsevier]