May 23, 2006 (CIDRAP News) – All seven confirmed cases of H5N1 avian influenza in the family cluster in Indonesia involved "close and prolonged exposure" to another infected person, suggesting person-to-person transmission, the World Health Organization (WHO) said today.

However, the WHO statement stopped short of a definite conclusion that the virus spread from person to person. In addition, the agency said it has found no evidence of genetic reassortment or mutation in two H5N1 viruses collected from cases in the cluster and no evidence of continuing or efficient human-to-human transmission.

"All confirmed cases in the cluster can be directly linked to close and prolonged exposure to a patient during a phase of severe illness," the WHO said. "Although human-to-human transmission cannot be ruled out, the search for a possible alternative source of exposure is continuing."

The WHO also confirmed today that the 32-year-old man in the family cluster who died yesterday is officially Indonesia"e;s 42nd case and 33rd fatality. He was the father of the 10-year-old boy who died May 13 and the brother of the initial case-patient. "The father was closely involved in caring for his son, and this contact is considered a possible source of infection," the agency said.

Although the WHO stressed that investigation of the outbreak is ongoing, preliminary findings indicate that three of the infected family members had spent the night of April 29 in a small room with the 38-year-old woman who is considered the index case. She had developed symptoms April 27 and was coughing frequently on April 29, according to the WHO.

She died May 4 and was not tested for H5N1 because samples were not obtained before her burial, but her signs and symptoms pointed to avian flu infection.

Those who shared the room with her that night in Kubu Sembelang village were her two sons, both of whom died of avian flu, and her 25-year-old brother, the only surviving member of the cluster. Other infected relatives, according to the WHO, lived in adjacent homes.

As yet, the investigation by officials from the WHO, the US Centers for Disease Control and Prevention, and the Indonesian Ministry of Health has found no evidence of avian flu spreading in the community, the WHO said. Investigators are focusing their search on any additional cases among family members, other close contacts, or area residents, as well as on an alternative source such as infected animals or feces.

Human-to-human transmission of H5N1 probably occurred at least once before, when the mother and an aunt of an infected girl in Thailand fell ill after caring for her in 2004. But no instances of sustained transmission have been documented.

The WHO also reported today that complete genetic sequencing of two H5N1 isolates from the North Sumatra cluster has been completed by WHO reference laboratories in Hong Kong and the United States. The sequencing of all eight gene segments revealed no evidence of genetic reassortment with human or pig influenza virus.

It also showed no evidence of significant mutation. Specifically, the WHO noted, viruses tested showed no mutations associated with resistance to the neuraminidase inhibitors, including oseltamivir.

In addition, viruses in this outbreak are genetically similar to those isolated from poultry in the province during a previous outbreak, the WHO said.

This latest confirmed H5N1 case brings the worldwide total in humans to 218, including 124 deaths. Indonesia"e;s numbers since 2003 are second only to Vietnam, which has reported no new cases since November 2005, according to WHO data. In 2006, Indonesia"e;s WHO-confirmed cases total 25, including 22 deaths, well exceeding other countries.