BEIJING, China (Reuters) -- Bird flu has infected a farmer in China's first human case in months, killed an Indonesian teenager and spread deeper in Vietnam in a flare-up of infections mirroring past winters.
A second Indonesian bird flu victim, a 37-year-old woman from Banten Province on Java island, was in hospital on Wednesday, the World Health Organization (WHO) said.
Most human victims of bird flu have contracted the virus from infected birds, usually chickens, ducks or geese and there is usually a surge in cases during cooler months when the virus seems to thrive.
The 37-year-old Chinese man from the eastern province of Anhui kept backyard birds, but as in other human bird flu cases in China there was no reported poultry outbreak in the area, raising questions as to how he contracted the virus.
The man developed symptoms of fever and pneumonia early last month and was discharged from hospital on Saturday, the state-run Health News said.
"In China, the challenge is now to identify where this virus is hiding and how it is circulating," Henk Bekedam, the WHO's China representative, told Reuters.
China has reported 22 human cases, including 14 deaths, since 2003 and, with the world's largest poultry population and millions of backyard birds roaming free, it is seen as a center in the fight against the virus.
Bekedam said that as vaccination rates for birds improve in China, detecting avian influenza becomes harder and harder, offering a possible explanation for why there was no reported outbreak where the farmer lived.
Indonesia has the highest human death toll from bird flu of any nation, and on Wednesday that number grew to 58 when a boy, from Tangerang near Jakarta, died, said the head of the Indonesian health ministry's bird flu center Runizar Ruesin.
The boy was admitted to hospital in the capital last week and deaths among poultry in his neighborhood had recently been reported, the WHO said in a statement posted on its Web site.
Muhammad Nadirin, another official at the country's bird flu center, said hospital staff had to take extra care washing the boy's corpse because of concerns the virus might infect them.
"He was washed using special protection methods to prevent contagion," but added he didn't know if it could be transmitted from human-to-human this way.
The H5N1 virus mostly affects birds, but it has infected 263 people in 10 countries since 2003, killing 157 of them.
Scientists fear the virus could mutate and spread rapidly between people, triggering a pandemic that could sweep the globe in weeks and possibly kill millions.
An adviser to the White House said on Monday the number of people that could die in a flu pandemic that matches the 1918-19 outbreak will be "very scary" and far higher than the 62 million deaths forecast by a recent study by Harvard University.
The 1918-19 "Spanish influenza" pandemic killed anywhere from 20 million to 100 million people.
In Vietnam, bird flu has been confirmed in a fourth Vietnamese province after tests on 70 ducks showed they had died from the H5N1 virus, a government report said on Wednesday.
The results of tests on ducks found dead at the weekend prompted health workers to slaughter around 1,800 more ducks in two communes of Kien Giang province in the southern Mekong Delta, the Animal Health Department report said.
The virus that first struck the delta region in late 2003 re-emerged last month. Vietnam has had no human H5N1 cases since November 2005.
