Jan 24, 2005 (CIDRAP News) – Researchers who analyzed a family cluster of three cases of H5N1 avian influenza in Thailand say in an article published today that two family members probably acquired the disease from the third.
The mother and an aunt of an 11-year-old girl who died of probable avian influenza last September most likely caught the disease from the girl when they cared for her in a hospital, according to the report published online by the New England Journal of Medicine.
The findings appear to be the strongest evidence so far of person-to-person transmission of H5N1 avian flu since the disease became widespread in Southeast Asia last year. But the authors say they found no sign of any further transmission and no evidence of mutations that could enable the H5N1 virus to spread easily among people. Disease experts fear that such mutations could lead to a flu pandemic.
The 11-year-old girl, who lived with her 32-year-old aunt, died of pneumonia on Sep 8, 2004, according to the report by Kumnuan Ungchusak, MD, MPH, of the Thai Ministry of Public Health, and colleagues from Thailand and the US Centers for Disease Control and Prevention. The girl had fallen ill 3 or 4 days after having contact with dying chickens.
While the girl was in a hospital, her 26-year-old mother, who worked in a distant city, came to be at her bedside, providing "16 to 18 hours of unprotected nursing care," the report says. The mother, who had no known exposure to poultry, became ill with a fever Sep 11 and died Sep 20.
The girl's aunt also helped care for her in the hospital, spending about 12 hours at her bedside on Sep 7. The aunt in turn fell ill with fever Sep 16 and eventually experienced pneumonia, but she gradually recovered and was discharged Oct 7.
Because no adequate tissue samples were preserved, investigators could not confirm that the girl had avian flu, but her case had all the cardinal features of previous cases, the report says. Laboratory tests (reverse-transcriptase polymerase chain reaction) confirmed the disease in her mother and aunt.
The mother made a 10-minute visit to her daughter's household Sep 7 but had no known exposure to poultry before she became ill on Sep 11, the article says. The aunt's last known exposure to poultry was 17 days before she fell ill, a period longer than the typical incubation period of 2 to 10 days.
"We believe that the most likely explanation for the family clustering of these three cases of avian influenza is that the virus was transmitted directly from the infected index patient to her mother and to her aunt," the report states.
Epidemiologic investigation revealed no other human cases associated with the family cluster. In addition, RT-PCR analysis and phylogenetic analysis of the virus showed no changes that could equip it for efficient person-to-person transmission.
The report says avian flu has spread from person to person before. For example, in Hong Kong's 1997 outbreak of H5N1, some healthcare workers were found to have antibodies to the virus, and one reported mild symptoms. However, "The current family cluster is unique in that the secondary infections resulted in severe disease and death and in that the epidemiologic circumstances and laboratory findings made it possible to rule out transmission from poultry."
The lack of further transmission in the family cluster should not be an excuse for complacency, the authors write. If H5N1 avian flu remains endemic for years in the affected countries, "it is likely that such clusters will occur again, and it will be necessary to investigate each one rapidly and thoroughly to determine whether a critical change in the virus has occurred," they conclude.
Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A (H5N1). N Engl J Med 3005;352(4):333-40 [Full text]