Limited human-to-human transmission of bird flu might have occurred in an Indonesian family but there is no evidence the virus has mutated to allow it to pass easily among people, the WHO said.
Concern has been growing about the case in north Sumatra in which seven family members from a village died this month. The case is the largest family cluster known to date, the WHO has said.
The WHO and Indonesian health officials are baffled over the source of the infection but genetic sequencing has shown the H5N1 bird flu virus has not mutated, the UN agency said on its Web site on Tuesday. Nor was there any sign of the virus spreading among other villagers.
"To date, the investigation has found no evidence of spread within the general community and no evidence that efficient human-to-human transmission has occurred, the WHO said.
"Sequencing of all eight gene segments found no evidence of genetic reassortment with human or pig influenza viruses and no evidence of significant mutations," the WHO statement read.
"The human viruses from this cluster are genetically similar to viruses isolated from poultry in North Sumatra during a previous outbreak," it said.
Sick poultry have been the source of bird flu infections for the majority of human cases worldwide. The virus also infects pigs.
Clusters are looked on with far more suspicion than isolated infections because they raise the possibility the virus might have mutated to transmit efficiently among humans.
That could spark a pandemic, killing millions of people.
Financial markets have become worried after the WHO said one of the family members, a 32-year-old father, died on Monday after caring for his ailing son, who also died.
The agency said such close contact was considered a possible source of infection.
"This is the first time that we've been completely stumped about possible single-source infection," Peter Cordingley, spokesman for the Western Pacific region of the WHO, said yesterday, describing the infections as "the mother of all clusters."
Isolated cases of very limited human-to-human transmission have been documented -- including one in Thailand involving a mother and child -- but such cases do not mean a pandemic flu strain has emerged.
Still, the scenario worries scientists.
"No matter what's going on at this stage, it's a limited transmission between members of the same family," Cordingley said.
"What we are looking out for is any sign of this virus going outside of this family cluster into the general community, that would be very worrying. We haven't seen any signs of that yet," he added.
Bird flu has killed 124 people worldwide, more than a quarter of them in Indonesia. So far, most human cases have been traced to contact with infected poultry.
Steven Bjorge, the WHO team leader in the village of Kubu Sembelang, said the virus that infected the family members was genetically the same as the one found circulating in the area earlier.
"We can't find sick animals in this community and that worries us," he added.
Bird flu has killed 124 people in 10 countries since it re-emerged in Asia in 2003. It remains essentially a disease in birds and has spread to dozens of countries in wild birds and poultry.