A highly pathogenic avian influenza virus, H5N1, caused disease outbreaks in poultry in China and seven other east Asian countries between late 2003 and early 2004; the same virus was fatal to humans in Thailand and Vietnam (WHO, 2004a). Here we demonstrate a series of genetic reassortment events traceable to the precursor of the H5N1 viruses that caused the initial human outbreak in Hong Kong in 1997 (Claas et al., 1998; Guan et al., 1999; Xu et al., 1999) and subsequent avian outbreaks in 2001 and 2002 (Guan et al., 2002, 2004). These events gave rise to a dominant H5N1 genotype (Z) in chickens and ducks that was responsible for the regional outbreak in 2003–04. Our findings indicate that domestic ducks in southern China had a central role in the generation and maintenance of this virus, and that wild birds may have contributed to the increasingly wide spread of the virus in Asia. Our results suggest that H5N1 viruses with pandemic potential have become endemic in the region and are not easily eradicable. These developments pose a threat to public and veterinary health in the region and potentially the world, and suggest that long-term control measures are required.

The Asian outbreak of highly pathogenic avian influenza H5N1 disease in poultry in 2003 and 2004 was unprecedented in its geographical extent, and its transmission to humans was an ominous sign (WHO, 2004a). To trace the ecological and genetic origins of these outbreaks, we compared H5N1 viruses recently isolated from poultry in Indonesia, Thailand, and Vietnam as well as from humans in Thailand and Vietnam with 253 H5N1 isolates obtained during prospective surveillance of live poultry markets in Hong Kong and in Guangdong, Hunan, and Yunnan provinces, China, from 2000 to 2004 (Figure 2-1). Results of this surveillance are summarized


The authors declare that they have no competing financial interests.


Joint Influenza Research Centre (SUMC & HKU), Shantou University Medical College, Shantou, Guangdong 515031, China.


Department of Microbiology, The University of Hong Kong, Queen Mary Hospital, Hong Kong SAR, China.


Fakultas Kedokteran Hewan, Universitas Airlangga, Surabaya 60115, Indonesia.


Department of Microbiology, Sriraj Hospital, Bangkok 10700, Thailand.


Department of Livestock Development, National Institute of Animal Health, Bangkok 10900, Thailand.


Department of Virology, National Institute of Veterinary Research, Ministry of Agriculture and Rural Development, Hanoi, Vietnam.


Virology Department, National Institute of Hygiene and Epidemiology, Hanoi, Vietnam.


Virology Division, Department of Infectious Diseases, St. Jude Children’s Research Hospital, Memphis, TN 38105.

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