Sep 28, 2004 (CIDRAP News) – Thailand"e;s avian influenza outbreak is drawing widespread international attention with the confirmation of two more human cases, one of which opens the possibility of human-to-human transmission.

Thai officials announced yesterday that a 37-year-old woman who remains hospitalized in stable condition has the H5N1 strain that has seen a resurgence in recent months. Confirmation has now come of the disease in the woman"e;s sister, who died Sep 20, according to news service reports.

The latter woman"e;s 11-year-old daughter, the index case in this family cluster, died Sep 8 of pneumonia and is considered a probable case of H5N1; confirmation may not be possible in her case because her body was cremated and only a small sample of lymph tissue was saved, according to a story in the Thai newspaper The Nation. Testing is being carried out on another hospitalized but recovering family member, a 6-year-old cousin of the index patient.

The index patient was living with her aunt, the woman whose case was confirmed yesterday, in Kamphaeng Phet province in north-central Thailand when she became ill. Her mother was away working and living in the Bangkok area but returned to give her daughter bedside care in the hospital.

Home-raised chickens are known to have died at the residence of the aunt and cousin where the girl was living, making bird-to-human transmission the likely cause in those cases. Reports are not clear on whether the girl"e;s mother could have been infected via this route as well. The Nation story and an Agence France-Presse story say she stayed at her sister"e;s home for two nights. Other reports say the mother had no known contact with ill or dead chickens, raising the possibility of human-to-human transmission from the hospitalized daughter. This would be the first such transmission in the H5N1 outbreak that began last fall, ebbed in the spring, and reappeared this past summer.

A World Health Organization (WHO) update on the situation today says that while the new Thai cases do provide evidence of possible human-to-human transmission, they appear to be limited to this family cluster. Kumara Rai, the acting WHO representative in Thailand, and Scott Dowell, director of the International Emerging Infections Program, say the situation should be viewed with concern but not cause public alarm at this point because the human transmission, if it did indeed occur, was among family members with sustained intimate contact, according to an Associated Press story.

The Thai Public Health Ministry has reported no further cases in the village where the index patient, her aunt, and her cousin lived or in the hospital where they were treated, according to the WHO statement. However, surveillance efforts in Thailand have been ramped up and samples from the cluster patients are undergoing genetic and antigenic analysis to determine whether the H5N1 virus is evolving to a form that could be easily transmitted between humans—a process experts fear could spark an influenza pandemic.