The avian influenza that killed 1000 or more migratory birds at Lake Qinghai in western China in mid-May may represent a new and more lethal form of the HN51 virus, Chinese researchers report. The results suggest the virus is evolving and raise the possibility that surviving birds could spread it over an even wider area, endangering more poultry and increasing the chances of further genetic changes that could spark a deadly human pandemic.
The H5N1 virus first appeared in Hong Kong in 1997, causing massive poultry losses and killing six people. Since then, H5N1 has killed 54 people and over 140 million fowl, and it continues to expand its range in China.
Jinhua Liu of the College of Veterinary Medicine in Beijing and colleagues fully sequenced four isolates recovered from various Lake Qinghai bird species and found them all to be very similar to each other but distinct from any H5N1 sequences posted in GenBank. The researchers also tested the pathogenicity of the virus by using it to infect mice, which succumbed more quickly than mice infected with other H5N1 strains. The team publishes its results online today in Science. Similar findings, from other groups, are presented today in Nature.
But some questions remain unresolved, such as how the virus reached this sparsely populated corner of China. Since H5N1 appeared, researchers have debated whether migratory birds can spread it. Some aquatic birds are known to host strains of the virus with no or minimal symptoms. But the United Nations' Food and Agriculture Organization says there is no evidence tying outbreaks in poultry to wild birds.
A more pressing question is where these migratory birds might carry the virus next. A priority, says David Melville, an ornithologist in New Zealand, should be determining if surviving birds are carrying a weakened strain of the virus, or if some species or individual birds are carrying the same variant with minimal health effects.
"These are the important questions," says George Gao, a virologist at the Chinese Academy of Sciences' Institute of Microbiology and the study's corresponding author. To answer them, Gao and colleagues will begin collecting additional samples from healthy birds over the next couple months.
WHO fact sheet on Avian flu