Abstract
Recent intensive surveillance in Hong Kong resulted in the detection of two novel viruses in children during 1999—two cases of infection by an H9N2 avian virus in March and a single case of infection by an H3N2 swine virus in September. The two human H9N2 isolates are similar in antigenic and genetic characteristics to an H9N2 virus isolated from a quail in late 1997, but are antigenically distinct from other lineages of H9N2 viruses circulating in other species of birds as well as those isolated from pigs during 1998. It is likely, therefore, that as for the H5N1 viruses in 1997, these infections are the result of direct avian-to-human transmission. A striking similarity between the six internal genes of the H9N2 and H5N1 human isolates indicates that they are related by reassortment. The H3N2 virus A/HK/1774/99 is distinct from contemporary human H3N2 viruses and is similar in antigenic and genetic properties to H3N2 viruses recently circulating in pigs in Europe. Although the source of infection was not traced, the likely presence of similar viruses in pigs in Hong Kong has subsequently been confirmed. These two instances provide further evidence for the direct transfer of avian and swine viruses in causing human disease and highlights the potential for emergence, either by adaptation or genetic reassortment with a circulating human strain, of a novel human pathogen.
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