JAKARTA, Indonesia - The World Health Organization concluded that human-to-human transmission likely occurred among seven relatives who died from bird flu on Indonesia's Sumatra Island, while an animal health expert said the disease was more widespread in poultry than previously thought.

In a report obtained by The Associated Press, WHO experts said the cluster's first case was probably infected by sick birds and spread the disease to six family members living in a remote village. One of those cases, a boy, then likely infected his father, it said.

The U.N. agency stressed the virus had not mutated in any major way and that no cases were detected beyond members of the family, who died last month.

"Six confirmed H5N1 cases likely acquired (the) H5N1 virus through human-to-human transmission from the index case ... during close prolonged contact with her during the late stages of her illness," the report said.

The report was distributed at a closed meeting in Jakarta attended by some of the world's top bird flu experts. The three-day session that wraps up Thursday, was convened after Indonesia asked for international help. The country has recorded the world's highest number of human bird flu cases this year, and 39 of those infected have died.

More outbreaks also are occurring in poultry than earlier thought, said Jeff Mariner, an animal health expert from Tufts University working with the U.N. Food and Agriculture Organization in Jakarta.

He is coordinating a pilot project that involves local surveillance teams conducting field interviews to track backyard poultry that have rapidly died. The teams then use bird flu test kits to identify outbreaks.

In the 12 pilot districts on Java Island, 78 poultry outbreaks were detected from January to May. Birds discovered in those outbreaks were slaughtered to limit the spread of infection.

"We thought there was dramatic underreporting, but we never imagined that it would be so pervasive," Mariner said on the meeting's sidelines. "These numbers of outbreaks only represent, say, a third of the coverage in the district."

The experts were expected to discuss Sumatra's large family cluster during the session. One of the remaining mysteries is why only blood relatives - not spouses - became infected.

The WHO report theorizes the family shared a "common genetic predisposition to infection with H5N1 virus with severe and fatal outcomes." However, there is no evidence to support that.

Keiji Fukuda, WHO's coordinator for the Global Influenza Program in Geneva, said the Indonesian case appears to resemble other family clusters where limited human-to-human transmission occurred following close contact. He said scientists must find out whether anything is different about the way the virus is behaving.

"The really critical factor is why did that cluster develop?" he said. "What's the reason why people in a cluster got infected?"

Bird flu has killed at least 130 people worldwide since it began ravaging Asian poultry stocks in late 2003. Experts fear the virus will mutate into a form that spreads easily among people, potentially sparking a pandemic. So far, it remains hard for people to catch, and most human cases have been traced to contact with infected birds.

Indonesian officials said the country lacks manpower and money to battle the H5N1 virus alone. The government has been saddled with a series of natural disasters, including the 2004 tsunami and an earthquake last month on Java Island.