In March 1999, the Hong Kong Department of Health (HKDH) isolated influenza A viruses from two children that could not be further identified using the usual influenza reagents. The viruses isolated from the children were sent to the National Institute for Medical Research, London and CDC, Atlanta. In April 1999, both laboratories identified the viruses as influenza A H9N2. These are the first confirmed human infections with influenza A H9N2 viruses. Five additional cases were reported from mainland China but have not been confirmed. No additional confirmed or suspected human cases of Influenza A H9N2 infection have been identified in Hong Kong, thus far, and surveillance in Hong Kong has not shown evidence of increased respiratory illness activity. The influenza A H9N2 virus is different than the influenza A H5N1 viruses which caused an outbreak of influenza (the "Avian Flu") in Hong Kong in 1997.
Influenza A H9N2 is an avian flu virus and usually infects birds. However, influenza A H9N2 does not appear to cause high rates of death in poultry as did influenza A H5N1 in Hong Kong. It is not known how the 2 children in Hong Kong became exposed to influenza A H9N2. Both children were hospitalized but recovered fully.
H9N2 is a subtype of influenza A virus. Influenza A H9N2 has been isolated from duck and chicken for many years, and is found in birds, pigs and other animals in Europe and Asia. Infection in humans is rare, and appears to lead to a mild disease. Six human cases of the H9 viruses have previously been reported in Hong Kong since 1999. In 1999 and 2003, an H9N2 influenza strain caused illness in three people, aged one, four and five years old, in Hong Kong. In 2007 an H9N2 influenza strain caused illness in a 9-month old baby in Hong Kong. And the recent case of influenza A/H9N2 in a 3 year old girl was reported on December 23, 2009. There are hundreds of strains of avian influenza viruses, but only four -- H5N1, H7N3, H7N7, and H9N2-are known to have caused human infections. In addition to H5N1, H9N2 is also considered to pose a threat to humanity.
Studies on Molecular characterization of H9N2 influenza viruses by Guan Y and colleagues suggested that H9N2 is a possible source of H5N1. Phylogenetic and antigenic analyses of the H9N2 viruses isolated from Hong Kong markets suggest three distinct sublineages. Among the chicken H9N2 viruses, the PB1 and PB2 genes are closely related to those of the H5N1 viruses. Maryland's Daniel Perez and colleagues found that just a few mutations H9N2 could turn it into a virus that people catch and transmit easily.