The highly pathogenic H5N1 influenza virus has become endemic in poultry in Southeast Asia since 2003 and causes a major pandemic threat to humans.
In the past, H5N1 virus was found in dead wild birds, usually within flight range of infected poultry farms. On 30 April 2005, a first outbreak was however detected in bar-headed geese as well as brown-headed gulls, great black-headed gulls and great cormorants at Qinghai Lake in western China, which is a protected nature reserve with no poultry farms in the vicinity. Clinical findings included paralysis, unusual head tilt, staggering and neck thrill X all are known features of H5N1 disease in waterfowl. By 4 May, bird mortality was more than 100 a day; by 20 May, the outbreak rapidly spread to other islets, with some 1,500 birds dead.
Researchers at the Joint Influenza Research Centre of Shantou University and the University of Hong Kong put tremendous efforts to unveil disease caused by H5N1 and transmission of the virus among migratory geese population in western China.
The Joint Influenza Research Center was established in 2001 by Shantou University and The University of Hong Kong.
The research findings were published in the top international scientific journal V NATURE Magazine on 7 July 2005. It is believed that this outbreak may help to spread the virus over and beyond the Himalayas and has important implications for developing control strategies.
It was found that 90% of the dead birds were bar-headed geese, with the remainder being brown-headed gulls and great black-headed gulls. The researchers isolated 28 H5N1 viruses from 92 cloacal, tracheal and faecal swabs from the 3 species, and a further 5 viruses from tissue samples from the geese. Genetic analysis methods were adopted.
Research results suggested:
•The H5N1 viruses were almost identical across all gene segments;
•The genes of the Qinghai H5N1viruses were very similar to those isolated from poultry markets in Fujian, Guangdong, Hunan and Yunnan provinces during 2005. However, they were clearly distinguishable from those that have caused human infection in Thailand and Vietnam.
•The virus causing the outbreak at Qinghai Lake was a single introduction, most probably from poultry in southern China.
•H5N1 viruses are now being transmitted between migratory birds at the lake. The viruses might also move to other migratory species that act as carriers, remaining highly pathogenic for domestic chickens, animals and possibly humans after rapid mutations.
There is a danger that the virus might be carried along the birds winter migration routes to densely populated areas in the south Asian subcontinent, a region that seems free of this virus, and spread as new strains along migratory flyways linked to Europe, thereby vastly expanding the geographical distribution of H5N1; therefore, increased surveillance of poultry is called for because previous experience has shown that control measures become almost impossible once the virus is entrenched in poultry populations to prevent large scale outbreak.
To view the full Nature paper, please visit the website at: www.hku.hk/facmed/press.
July 7, 2005