Risk Factors Associated with Subclinical Human Infection with Avian Influenza A (H5N1) Virus-Cambodia, 2006

Background. We conducted investigations in 2 villages in Cambodia where outbreaks of influenza H5N1 occurred among humans and poultry to determine the frequency of and risk factors for H5N1 virus transmission. Methods. During May 2006, similar to 7 weeks after outbreaks of influenza H5N1 among poultry occurred, villagers living near households of 2 patients with influenza H5N1 were interviewed about potential H5N1 exposures and had blood samples obtained for H5N1 serological testing by microneutralization assay. A seropositive result was defined as an influenza H5N1 neutralizing antibody titer of >= 1:80, with confirmation by Western blot assay. A case-control study was conducted to identify risk factors for influenza H5N1 virus infection. Control subjects, who had seronegative results of tests, were matched with H5N1-seropositive persons by village residence, households with an influenza H5N1-infected poultry flock, sex, and age. Results. Seven (1.0%) of 674 villagers tested seropositive for influenza H5N1 antibodies and did not report severe illness; 6 (85.7%) were male. The 7 H5N1-seropositive persons, all of whom were aged <= 18 years, were younger than participants who tested seronegative for H5N1 antibodies (median age, 12.0 years vs. 27.4 years; P = .03) and were more likely than were the 24 control subjects to report bathing or swimming in household ponds (71.4% vs. 20.8%; matched odds ratio, 11.3; P = .03). Conclusions. Avian-to-human transmission of influenza H5N1 virus remains low, despite extensive poultry contact. Exposure to a potentially contaminated environment was a risk factor for human infection.