Wild birds could spread H5N1 virus beyond Asia, reports say

Jul 6, 2005 (CIDRAP News) – The recent outbreak of H5N1 avian influenza among wild waterfowl in western China could provide a launching pad to spread the disease throughout Asia and beyond, according to two reports published by leading science journals today.

Thousands of birds have died in the past 2 months at Qinghai Lake, a wildlife refuge that is an important gathering site for many species of waterfowl. World Health Organization officials have described the outbreak as the first one to kill large numbers of migratory birds.

Two teams of scientists who studied the outbreak report some early findings today in online editions of Nature and Science. Both see a danger that the disease, confined mainly to Southeast Asia and East Asia so far, could vastly expand its range.

"There is a danger that it [the H5N1 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 along migratory flyways linked to Europe," says a report by H. Chen and colleagues in Nature. "This would vastly expand the geographic distribution of H5N1."

In Science, a large team of Chinese authors under the leadership of George F. Gao of Beijing writes, "The occurrence of highly pathogenic H5N1 . . . infection in migratory waterfowl indicates that this virus has the potential to be a global threat: Lake Qinghaihu is a breeding center for migrant birds that congregate from Southeast Asia, Siberia, Australia, and New Zealand."

The Nature report says the illness was killing more than 100 birds daily at Qinghai Lake by early May. Ninety percent of the birds that died were bar-headed geese, and the rest were brown-headed gulls and great black-headed gulls.

Chen and colleagues analyzed the genes of 97 H5N1 viruses isolated from all three species and concluded that they were "clearly distinguishable" from strains that have caused human cases in Thailand and Vietnam. "This indicates that the virus causing the outbreak at Qinghai Lake was a single introduction, most probably from poultry in southern China," the report says.

"Our findings indicate that H5N1 viruses are now being transmitted between migratory birds at the lake," the article continues. "Although the outbreak could burn itself out, the large migratory bird population at Qinghai Lake makes this unlikely." Bar-headed geese fly south from the refuge to Myanmar and India starting in September, the authors say.

"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," the report concludes.

Gao's team writes that the infection caused tremors, spasms, diarrhea, and brain and pancreatic damage in the migratory birds. The team analyzed four viral isolates from the birds and found that they were closely related, but not identical, to a strain found in a peregrine falcon in Hong Kong in 2004.

The investigators tested the virulence of the virus from the birds by exposing eight chickens and eight mice to it. All the chickens died within 20 hours and all the mice died within 4 days. In a study last year, Chen and colleagues found that H5N1 viruses taken from ducks in China were less lethal to mice and chickens.

Neither of the new reports mentions any testing to determine whether healthy birds at Qinghai Lake are silently carrying the virus. Wild waterfowl are known as the natural reservoir for all influenza A viruses (of which H5N1 is one) and commonly carry them without getting sick.

David A. Halvorson, DVM, a veterinarian in avian health at the University of Minnesota in St. Paul, said the two new studies are not the first ones to report wild waterfowl getting sick as a result of H5N1 infection. "But before, they thought that one possibility was that the waterfowl were getting it from the poultry," he told CIDRAP News. "And now this seems to be kind of isolated from poultry, so it looks like the waterfowl are having their own infection, and transmitting it among themselves."

The outbreak clearly signals a risk that the H5N1 virus could spread out of East Asia, Halvorson said. But he added, "The fact of the matter is, the potential for it to be transmitted via flyways has been here all along. The fact it's killing these birds, does it mean there's a greater chance of its being spread or less of a chance? If you have a highly pathogenic virus that's not killing the geese, it seems to me it would be a higher risk of transmission."

Infectious disease expert Michael T. Osterholm, PhD, MPH, commented, "The key message [of the new reports] is that this has potential to spread throughout Asia and Europe." He is director of the University of Minnesota Center for InfectiousDisease Research and Policy, publisher of this Web site.

Osterholm cited the spread of West Nile virus in the United States over the past 6 years as a potential parallel to the current H5N1 situation. The virus spread among birds congregating in southern regions in winter and then traveled with them when they returned north and dispersed across the country in summer, he said.

Chen H, Smith GJD, Zhang SY, et al. H5N1 virus outbreak in migratory waterfowl. Nature 2005 (published online Jul 6) [Full Text]

Liu J, Xiao H, Lei F, et al. Highly pathogenic H5N1 influenza infection in migratory birds. Science 2005 (published online Jul 6) [Abstract]

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