Even as it crops up in the far corners of Europe and Africa, the virulent bird flu that raised fears of a human pandemic has been largely snuffed out in the parts of Southeast Asia where it claimed its first and most numerous victims.
Vietnam, which has had almost half of the human cases of A(H5N1) flu in the world, has not seen a single case in humans or a single outbreak in poultry this year. Thailand, the second-hardest-hit nation until Indonesia recently passed it, has not had a human case in nearly a year or one in poultry in six months.
Encouraging signs have also come from China, though they are harder to interpret.
These are the second positive signals that officials have seen recently in their struggle to prevent avian flu from igniting a human pandemic. Confounding expectations, birds making the spring migration north from Africa have not carried the virus into Europe.
Dr. Nabarro and other officials warn that it would be highly premature to declare any sort of victory. The virus has moved rapidly across continents and is still rampaging in Myanmar, Indonesia and other countries nearby. It could still hitchhike back in the illegal trade in chicks, fighting cocks or tropical pets, or in migrating birds.
But this sudden success in the former epicenter of the epidemic is proof that aggressive measures like killing infected chickens, inoculating healthy ones, protecting domestic flocks and educating farmers can work, even in very poor countries.
Dr. Nabarro said he was "cautious in interpreting these shifts in patterns" because too little is known about how the disease spreads.
Other officials agreed.
"To say the disease is 'wiped out' there is probably too strong, too positive," said Dr. Wantanee Kalpravidh, chief of flu surveillance in Southeast Asia for the United Nations Food and Agricultural Organization, which fights animal diseases. The governments of Thailand and Vietnam "believe they got rid of it," she said, "but they also believe that it might be coming back at any time."
Very different tactics led to success in the two countries.
While Vietnam began vaccinating all its 220 million chickens last summer, Thailand did not because it has a large poultry export industry, and other nations would have banned its birds indefinitely. (Vaccines can mask the virus instead of killing it.)
Instead, Thailand culled wide areas around infected flocks, compensated farmers generously and deputized a volunteer in every village to report sick chickens.
It vaccinates fighting cocks, which can be worth thousands of dollars, and even issues them passports with their vaccination records so they can travel, Dr. Nabarro said.
Government inspectors sample birds everywhere; in February, Thailand reported that samples from 57,000 birds had come back negative.
According to Dr. Klaus Stöhr, a flu specialist at the World Health Organization, Thailand and Vietnam also delivered the antiviral drug Tamiflu to even the smallest regional hospitals and told doctors to treat all flu patients even before laboratory diagnoses could be made.
Dr. Nabarro particularly praised the leaders of the two countries for ordering high-level officials — deputy prime ministers — to fight the disease, and for making sure that enough cash to entice farmers to hand over their birds for culling flowed down official channels without being siphoned off.
Hints suggest that the disease is also being beaten back in China, the country where it is assumed to have begun. International officials tend to greet official public health reports from China skeptically, in part because it concealed the outbreak of the SARS virus there for months. It did not officially report any bird cases for years, even though many scientists contend the virus incubated there between its first appearance in humans in Hong Kong in 1997 and the current human outbreak, which began in Vietnam in 2003.
Some top Chinese officials have blamed the reluctance of local officials to report bad news to Beijing. Dr. Nabarro said he recently met a vice premier "who made it clear that they are absolutely determined to get the fullest possible cooperation from provincial authorities."
China's reported human cases have remained low: 8 last year and 10 this year.
Perhaps more important, its poultry cases — which lead to human cases and increase the risk of a mutant pandemic strain — seem to be dropping.
According to the World Health Organization, China said it had outbreaks in 16 provinces in 2004. In 2005, it reported outbreaks in only 12 provinces, but one in November was so large that 2.5 million birds were culled to contain it.
After that, the Agriculture Ministry announced that it would vaccinate every domestic bird in China, which raises and consumes 14 billion chickens, ducks and geese each year. The official news agency reported about the same time that a fake flu vaccine, possibly with live virus in it, might have spread the disease.
Dr. Stöhr, who is in charge of W.H.O. flu vaccine efforts, said he was told by Chinese agriculture officials that the country was now producing 46 billion doses of poultry vaccine a year, and was supplying vaccines to Vietnam.
China's most recent monthly reports describe much smaller outbreaks than were previously common: findings of a few dead wild birds and culls of 126,000 birds in one spot and 16,000 in another, for example.
"We are hopeful that China has turned the corner," Dr. Nabarro said.
In Cambodia and Laos, which separate Thailand and Vietnam, the situation is vague.
Laos has reported no human cases and last reported poultry outbreaks two years ago. Cambodia's reported human cases dropped to two this year, from four last year. No poultry outbreaks were reported, but surveillance is so spotty that some must have occurred and gone unnoticed, Dr. Kalpravidh said, because the country's six human victims were infected by poultry.
Cambodia was slow to compensate farmers for their birds because of problems with corruption in a previous cash-for-guns program.
Health specialists generally agree that there is little clear chance of infected birds landing in the United States.
Where the Southeast Asian governments have taken action, however, the risk of the virus returning is ever present, Dr. Nabarro said.
For example, he said, it probably exists in Vietnam in Muscovy ducks, which can harbor the virus but do not get sick, and it has turned up in isolated birds in open-air markets near the Chinese border. (Single birds do not constitute an outbreak.) Since Chinese farmers can get three times as much for a chicken in Vietnam as they can at home, the temptation to smuggle persists.
"Tomorrow, the whole thing could change again," Dr. Nabarro said. "We need to be on the alert at all times."
5 Cases Confirmed in Indonesia
JAKARTA, Indonesia, May 13 (Reuters) — Local tests have confirmed that three Indonesians who died in the past week had avian flu, a Health Ministry official said Saturday.
Authorities have sent blood and swab samples of the three people — all from one family — to a World Health Organization-affiliated laboratory in Hong Kong. Local tests are not considered definitive.
A toddler and a 25-year-old man from the same North Sumatra family also tested positive for bird flu, but they are still alive, said Nyoman Kandun, a director general at the Health Ministry.
He did not say whether they had had any contact with sick fowl, the usual mode of transmission of the virus to humans.