A new variant of the bird flu virus H5N1 emerged in late 2005 and replaced most of the previous variants across a large part of southern China, despite an ongoing program to vaccinate poultry, according to researchers at the University of Hong Kong in collaboration with scientists at St. Jude Children's Research Hospital.
The new virus, called Fujian-like (FL), appears to be responsible for the increased occurrence of H5N1 poultry infections since October 2005, as well as recent human cases in China, the researchers said. FL has now also been transmitted to Hong Kong, Laos, Malaysia, and Thailand, resulting in a new bird flu outbreak wave in Southeast Asia that has caused human infections as well, according to the Hong Kong/St. Jude team.
The investigators also warned that it is possible that this new H5N1 variant will spread further through Asia and into Europe, as it evolves to form other sublineages that vary from place to place. This evolution into different sublineages also occurred during the previous two waves of H5N1 transmission that occurred during the past several years, according to the investigators. A report on these findings appears in the November online edition of the Proceedings of the National Academy of Sciences (PNAS).
The findings are significant because experts believe that H5N1 is the most likely virus to trigger a human influenza pandemic (worldwide epidemic). Moreover, the increasing number of transmissions from birds to humans in the past year supports this opinion, said Robert G. Webster, Ph.D., a co-author of the PNAS paper. Webster is a member of the Infectious Diseases department and holder of the Rose Marie Thomas Chair at St. Jude.
Based on their study of vaccinated poultry the Hong Kong/St. Jude team suggested that the vaccination itself might have facilitated emergence of this new variant.
This emergence and rapid distribution of FL, despite the vaccination program that was started in September 2005, also suggests that the current H5N1 control measures are still inadequate, Webster said.
Moreover, since November 2005, some of the 22 H5N1 human infections reported from 14 provinces in China were from infected residents of metropolitan areas such as Shangai, Wuhan and Guangzhou, which are remote from poultry farms.
"We don't know yet whether the people in those metropolitan areas were infected locally by contact with poultry or by contact with other humans," Webster said, "but we suspect from the studies they are being infected by contact with poultry."
The researchers found the virus in samples taken from infected chickens in 11 of the last 12 months of the present study, compared with only four months during 2004-05. This indicates an increase in the incidence of H5N1 infection in 2005-2006 compared with previous years, which suggests that H5N1 viruses have not been effectively contained.
The investigators also conducted genetic studies of 390 H5N1 viruses isolated from poultry in the current study (30 percent of the total found in southern China) and found that 68 percent were of the FL sublineage.
The emergence of FL-like viruses and their success in replacing other H5N1 variants in such a short time demonstrates how difficult it is to control H5N1 in China, Webster said.
The other authors of this paper are Gavin Smith, X. H. Fan, J. Wang, K. S. Li, K. Qin, J.X. Zhang, D. Vijaykrishna, C.L. Cheung, K. Huang, Marik Peiris, Honglin Chen and Yi Guan (University of Hong Kong), and J.M. Rayner (formerly of St. Jude).
This work was supported in part by the Li Ka Shing Foundation, the National Institute of Allergy and Infectious Diseases and ALSAC.
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