Continued circulation in China of highly pathogenic avian influenza viruses encoding the hemagglutinin gene associated with the 1997 H5N1 outbreak inpoultry and humans

Since the outbreak in humans of an H5N1 avian influenza virus in Hong Kong in 1997, poultry entering the live-bird markets of Hong Kong have been clos ely monitored for infection with avian influenza. In March 1999, this monit oring system detected geese that were serologically positive for H5N1 avian influenza virus, but the birds were marketed before they could be sampled for virus. However, viral isolates were obtained by swabbing the cages that housed the geese. These samples, known collectively as A/Environment/Hong Kong/437/99 (A/Env/HK/437/99), contained four viral isolates, which were co mpared to the 1997 H5N1 Hong Kong isolates. Analysis of A/Env/HK/437/99 vir uses revealed that the four isolates are nearly identical genetically and a re most closely related to A/Goose/Guangdong/1/96. These isolates and the 1 997 H5N1 Hong Kong viruses encode common hemagglutinin (H5) genes that have identical hemagglutinin cleavage sites. Thus, the pathogenicity of the A/E nv/HK/437/99 viruses was compared in chickens and in mice to evaluate the p otential for disease outbreaks in poultry and humans. The A/Env/HK/437/99 i solates were highly pathogenic in chickens but caused a longer mean death t ime and had altered cell tropism compared to A/Hong Kong/156/97 (A/HK/156/9 7). Like A/HK/156/97, the A/Env/HK/437/99 viruses replicated in mice and re mained localized to the respiratory tract. However, the A/Env/HK/437/99 iso lates caused only mild pathological lesions in these tissues and no clinica l signs of disease or death. As a measure of the immune response to these v iruses, transforming growth factor beta levels were determined in the serum of infected mice and showed elevated levels for the A/Env/HK/437/99 viruse s compared to the A/HK/156/97 viruses. This study is the first to character ize the A/Env/HK/437/99 viruses in both avian and mammalian species, evalua ting the H5 gene from the 1997 Hong Kong H5N1 isolates in a different genet ic background. Our findings reveal that at least one of the avian influenza virus genes encoded by the 1997 H5N1 Hong Kong viruses continues to circul ate in mainland China and that this gene is important for pathogenesis in c hickens but is not the sole determinant of pathogenicity in mice. There is evidence that H9N2 viruses, which have internal genes in common with the 19 97 H5N1 Hong Kong isolates, are still circulating in Hong Kong and China as well, providing a heterogeneous gene pool for viral reassortment. The impl ications of these findings for the potential for human disease are discusse d.