Health experts say the wide range of the bird flu striking Asia's poultry boosts chances that the virus could mutate into a global menace for people, but say the disease is "nowhere close" to being declared a pandemic.
Most human cases have been traced directly to contact with sick birds, and although human-to-human transmission has not been ruled out in the case of one Vietnamese family, the experts say there is no sign of a new strain that can easily infect many people.
However, a WHO official acknowledged that the race against the disease's spread was tough.
"The virus is faster than we are," said Peter Cordingley, the agency's regional spokesman in Manila, Philippines. "Until the surveillance systems are working properly, we're going to have trouble keeping up with the virus."
New deaths were announced Wednesday in Vietnam, where a 16-year-old girl became the country's 10th bird flu fatality, and Thailand, where a 6-year-old boy died, bringing its toll to five. Human cases of bird flu have not been reported in any other countries.
China said Wednesday it has no human infections, but that officials were investigating cases in poultry in 12 of its 31 regions — with the provinces of Gansu and in Xi'an added overnight to regions with suspected cases.
A new National Bird Flu Prevention Headquarters has opened in Beijing to oversee regional efforts to kill all sick birds and keep close watch on people who handled them, though it was unclear how the center would overcome the typical communications obstacles between local governments and Beijing.
"The whole of China is trying to prevent bird flu," Foreign Ministry spokeswoman Zhang Qiyue said.
Health experts say the best policy is to destroy infected poultry while ensuring that people carrying out the culls are not exposed to the virus.
On an optimistic note, Thailand's deputy prime minister said Wednesday that officials there believe they'll eradicate the virus from the country's farms by week's end. The virus has struck more than half of the country's provinces.
After the World Health Organization announced Sunday that human-to-human transmission could not be ruled out in the case of the Vietnamese family — raising alarms globally — the organization has sought in recent days to play down concerns.
WHO officials say contact with sick birds also can't be ruled out in the infections, and that anyway the case doesn't seem to represent a newly dangerous strain of the illness.
"I think it's very important at this stage that we remain calm about worst-case scenarios," Mike Ryan, WHO's chief of global epidemic response, said Tuesday in Geneva. "We have a strain of influenza with the potential to pick up human genes, and we're nowhere close to declaring a pandemic."
What experts fear most is a new virus hybrid combining the contagiousness of human flu with the deadliness of bird flu — and there's been no evidence of that yet.
Illustrating the deadliness of bird flu, only two of the 14 confirmed cases of bird flu in humans in Vietnam have recovered, while 10 have died and two remain hospitalized.
Governments in Cambodia, Laos, Indonesia, Japan, Pakistan, South Korea and Taiwan also are battling the disease, and at least 45 million chickens have been slaughtered in government-ordered culls. However, health officials say the strain of bird flu striking Taiwan and Pakistan is milder and is not a serious threat to humans.
Many countries still free of the virus have banned poultry imports from affected countries and tightened disease surveillance.
In Singapore, the city-state asked residents to hand over any pet chickens to eliminate possible reservoirs of the virus. The government said Wednesday that residents gave up 57 pets for slaughter so far.