The virus that decimated poultry flocks throughout Asia and caused several human deaths in Thailand and Vietnam earlier this year poses a long-term threat to both animal and human health, according to a paper in the 8 July issue of Nature. Genetic studies reveal that this lethal virus is now endemic in the region and evolving in a way that may prove more harmful to humans. The researchers predict that the virus, which nearly disappeared with the onset of warmer weather this spring, is likely to come roaring back this fall.
Hong Kong University virologist Yi Guan and his colleagues have been sampling poultry in Hong Kong and southern China since 2000, watching for the re-emergence of what is known as the H5N1 virus. It first appeared in Hong Kong in 1997, causing massive poultry losses and killing six people. Subsequent work traced the virus back to a precursor virus that infects wild geese but is relatively benign. When this precursor virus is picked up by chickens and ducks, however, a subtle change in the way it replicates makes it lethal.
Guan and colleagues analyzed samples of the H5N1 virus gathered from ducks and chickens in China and Hong Kong each year and compared them with samples gathered from Indonesia, Thailand. and Vietnam during the 2003-04 outbreak. The analysis included samples from a number of wild birds felled by disease. They conclude that a specific strain of the virus not seen before 2002, called "genotype Z," has quickly become the dominant form of the virus. Domestic ducks in southern China appear to be playing a key role in hosting this virus as it evolves.
The researchers speculate that genotype Z became more compatible with its fowl hosts, possibly leading to its prevalence among poultry in East Asia. Guan believes the virus is continuing to change rapidly and that its dispersion could touch off a deadly global influenza pandemic. "This virus is dangerous," he says.
Guan's dire warnings are on target, says Erhard Kaleta, a veterinarian at Justus Liebig University, Giessen, Germany, who directs a reference laboratory for the Paris-based World Organization for Animal Health. However, Kaleta says Guan may be putting too much of the blame on ducks in southern China. A similar sampling of poultry in other countries is needed to make that claim, he argues.