Preventing a possible flu pandemic of the deadly H5N1 virus may require broader poultry surveillance, Chinese and US scientists reported Monday in a joint study.

The study, appearing in the February 6 issue of US scientific journal Proceedings of National Academy of Sciences, noted the importance of monitoring apparently healthy birds. The first author, Honglin Chen, is a researcher at the Shantou University in China.

From poultry found across Southeast Asia, even in regions without bird flu outbreaks, the researchers identified several different sublineages, or genetic "branches" of the H5N1 virus.

Although the H5N1 virus can be transmitted over long distances in migratory birds, the researchers said, transport of infected poultry appears to be the main means of the virus spreading in Southeast Asia.

By analyzing the gene segments of H5N1 virus strains from different regions, the researchers suggested that migratory birds, which have transferred the virus from Southeast Asia to Mongolia, Russia, and even Europe, may have been first infected by local poultry while over-wintering.

"The migratory ducks could have survived infection with the H5N1 virus and transmitted the virus over long distances during migration," they said.

"Our results demonstrate that long-term endemicity of H5N1 viruses in different regions of Asia has resulted in the establishment of regional sublineages," the researchers wrote in the paper.

"The establishment of regional virus sublineages suggests that H5N1 virus is perpetuated in poultry largely through the movement of poultry and poultry products rather than by continued reintroduction of viruses by migrating birds."

Since 2003, the H5N1 virus has infected about 160 people in Asia and Europe, and killed half of them, according to the World Health Organization (WHO).

For now the virus has only spread from birds to humans, but scientists have warned it could cause a global flu pandemic once it acquires the ability to pass from person to person.

Based on experience gained in Hong Kong, Japan, and South Korea, the researchers said, early detection and large-scale culling of infected poultry, is the most effective method of controlling the bird flu.

But the antigenic diversity of the H5N1 virus shows a single human vaccine may be not enough for human pandemic preparedness, they indicated.

"The choice of candidate viruses for development of human vaccines must reflect the antigenic diversity observed across this wider region," they said.

"Furthermore, antigenic drift observed over time within those H5N1 sublineages highlights the necessity of continually updating the candidate virus chosen for future H5N1 vaccines. These concepts are critical for the control of this pandemic threat."

(Xinhua News Agency February 7, 2006)