BANGKOK, Thailand -- A 6-year-old Thai boy became Asia's seventh confirmed bird flu fatality -- making this the deadliest outbreak since 1997 -- and the government said Monday it was awaiting lab results on four other people who died in a northern province.
The World Health Organization said the search for a vaccine had been set back because the virus had mutated. A previous strain detected in Hong Kong in 1997 can no longer be used as the key to producing a vaccine, so an international effort has become necessary, WHO said.
The outbreak seven years ago marked the first time scientists documented that bird flu could be caught by humans.
Scientists believe people get the disease through contact with sick birds. Although there has been no evidence yet of human-to-human transmission, health officials are concerned the disease might mutate further and link with regular influenza to create a form that could trigger the next human flu pandemic.
"This is now spreading too quickly for anybody to ignore it," said WHO spokesman Peter Cordingley in Manila, Philippines.
Officials in Bangkok said they were investigating whether the virus might be carried by migratory birds.
Six people have died in neighboring Vietnam. If all the deaths in Thailand are confirmed as caused by bird flu, the toll would stand at 12.
Pakistan on Monday joined the list of countries affected by the disease that has sparked mass chicken culls across the region.
Pakistan said it detected a form of bird flu in its chicken population that an industry official said had killed up to 3.5 million birds. The commissioner for livestock husbandry said it was not a strain of bird flu that can spread to humans -- something that has happened in other parts of Asia.
"We have confirmed this. The strand that jumps to humans is not in them," commissioner Rafaqat Hussain Raja said.
The reported Pakistani strains differ from the H5N1 strain blamed for the human fatalities in the current outbreak, but similar strains have been known to infect humans, WHO's Web site said.
Laos, meanwhile, fears it might also be hit by the bird flu and is awaiting test results on the nature of an illness killing its fowl, the U.N. Food and Agriculture Organization said.
Other Asian governments frantically slaughtered chicken flocks in a desperate bid to contain the disease, as well as the growing political fallout from accusations that officials in two countries -- Thailand and Indonesia -- initially covered up outbreaks.
Dr. Prasert Phongcharoen, a WHO adviser and viral disease expert, urged caution in the disposal of the chicken carcasses. If infected chickens are thrown in rivers, "the virus could spread to open pig farms and this could result in transmission from pigs to humans," he said.
The virus would pass more easily from pigs to humans because they are genetically closer, as shown by the transplanting of pig organs into humans, he said.
So far, eight countries have reported some strain of bird flu -- Cambodia, Indonesia, Japan, Pakistan, South Korea, Taiwan, Thailand and Vietnam.
Indonesian officials earlier denied the disease's presence, but the country's veterinarian association said independent investigations revealed that bird flu killed millions of chickens over recent months.
The Jakarta Post reported Monday that Indonesian officials may have covered up the outbreak there at the behest of politically connected businessmen who feared it would harm their interests.
A team of agricultural experts said in December they provided the government with test results positively identifying the disease. A team virologist, Dr. Marthen Malelo, said a powerful "businessmen's lobby" prevented officials from making the disease public.
Indonesian officials denied the allegations.
"It's not true. We have zero tolerance for pressure from businessmen. We are talking about the lives of people," Agriculture Department spokesman Hari Priyono said.
Officials said the nation would start culling up to 3.8 million chickens in East Java. Bali already has slaughtered and burned thousands.
The Thai prime minister, Thaksin Shinawatra, faced similar allegations that he covered up his country's outbreak, which has been confirmed in 13 of 76 provinces.
Thaksin said his government suspected "a couple of weeks" ago that bird flu had struck his nation but he did not tell the public because he feared mass panic.
Thailand has killed some 10 million chickens so far.
The outbreak has devastated Thailand's chicken export industry -- the world's fourth-largest. Its two biggest markets -- Japan and the European Union -- have banned Thai imports. Thailand shipped about 500,000 tons of chicken worth $1.3 billion in 2003.
Vietnam has slaughtered more than 3 million chickens.