Background: The verified human cases of highly pathogenic avian influenza in Vietnam may represent only a selection of the most severely ill patients. The study objective was to analyze the association between flulike illness, defined as cough and fever, and exposure to sick or dead poultry.
Methods: A population-based study was performed from April 1 to June 30, 2004, in FilaBavi, a rural Vietnamese demographic surveillance site with confirmed outbreaks of highly pathogenic avian influenza among poultry. We included 45 478 randomly selected (cluster sampling) inhabitants. Household representatives were asked screening questions about exposure to poultry and flulike illness during the preceding months; individuals with a history of disease and/or exposure were interviewed in person.
Results: A total of 8149 individuals (17.9%) reported flulike illness, 38,373 persons (84.4%) lived in households keeping poultry, and 11,755 (25.9%) resided in households reporting sick or dead poultry. A dose-response relationship between poultry exposure and flulike illness was noted: poultry in the household (odds ratio, 1.04; 95% confidence interval, 0.96-1.12), sick or dead poultry in the household but with no direct contact (odds ratio, 1.14; 95% confidence interval, 1.06-1.23), and direct contact with sick poultry (odds ratio, 1.73; 95% confidence interval, 1.58-1.89). The flulike illness attributed to direct contact with sick or dead poultry was estimated to be 650 to 750 cases.
Conclusions: Our epidemiological data are consistent with transmission of mild, highly pathogenic avian influenza to humans and suggest that transmission could be more common than anticipated, though close contact seems required. Further microbiological studies are needed to validate these findings.