A spate of bird flu cases since the beginning of the year in China has experts watching closely as millions of people and poultry are on the move ahead of the Lunar New Year holiday, the world's largest annual human migration.
China has reported more than 50 H7N9 infections in 2014 after the strain jumped from birds to people for the first time last year. The virus remains hard to catch and most cases have been linked to contact with poultry, but scientists worry that could change if it mutates into a form that allows it to spread easily among people.
For those who track influenza, the holiday, which begins on January 31, is always worrying because it comes during the winter months when flu typically rages.
Add that to hundreds of millions of people - and often birds - crammed together on buses and other forms of transportation going home, and it's always a bit of a gamble. China estimates 3.6 billion trips will be taken over the holiday season.
"This is the first winter we've seen H7N9. We are in uncharted territory," said Gregory Hartl, World Health Organization spokesman in Geneva. "We have seen an upstart in cases, which we are attributing basically to the fact that it's winter. That combined with a lot of movement of people in crowded trains with chickens could give rise to a lot more infections, but we've also seen in past years where it hasn't."
The first H7N9 cases were reported in late March near Shanghai, and more than 200 others have since been identified, including some 50 deaths. A 31-year-old doctor became one of the latest fatalities, raising fears he may have been infected at the hospital where he worked, but none of his patients or other close contacts have reported flu symptoms, according to the Shanghai Municipal Commission of Health and Family Planning.
There have been a handful of family clusters, but WHO says no sustained human-to-human transmission has occurred. However, scientists warn that the H7N9 virus contains genetic markers that could help it infect mammals easier than other avian flus.
Infected birds also do not show symptoms, making it harder to track the disease. Cases declined in China over the summer, which is typically slower for influenza viruses, after some local live poultry markets were temporarily closed.
In Hong Kong, which has logged three cases, officials will test all poultry for the virus beginning later this week. Taiwan has reported two cases.
In past years, it was the H5N1 bird flu virus that spiked during this time of year. That strain, which has killed at least 386 people since 2003, is still circulating widely in poultry stocks and kills about 60 percent of the people it infects.
On Tuesday, Vietnam, which has long battled the virus, confirmed its first H5N1 death in nine months. Earlier in January, the first human case was reported in North America after a person traveling back from a trip to Beijing became ill and died in Canada.
Both bird flus cause high fever and respiratory problems, including pneumonia and shortness of breath. Scientists have repeatedly warned that the viruses cannot be ignored because of their potential to possibly spark a global pandemic. But after years of campaigning in countries where it's common for chickens and pigs to live closely with people, sometimes in the same house, that message is often a hard sell.
"After almost a decade of sitting on the proverbial edge of the H5N1 pandemic cliff and not falling off, people are beginning to think that we never will fall," Michael Osterholm, an infectious disease expert at the University of Minnesota, said by email.
"But the best scientific assessment of microbial genetics tells us we could still fall off of that cliff and if we do, the global consequences could be devastating."
Poultry is a central part of many families' dinner tables during the Lunar New Year festivities, and it's often bought live and killed at home in China and elsewhere across the region. The WHO urges care when slaughtering and preparing birds, including frequent hand washing. However, well-cooked meat and eggs do not pose a threat.
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