Risk Factors for Influenza A(H7N9) Disease—China, 2013

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Abstract
In this case-control study of 89 subjects infected with novel avian influenza A(H7N9) and 340 age- and sex-matched controls, visiting live bird markets and having poultry contact were independent risk factors for A(H7N9) virus infection. Background. The majority of human cases of novel avian influenza A(H7N9), which emerged in China in spring 2013, include reported exposure to poultry. However, specific host and exposure risk factors for disease are unknown, yet critical to design prevention measures. Methods. In April–June 2013, we conducted a case-control study in 8 Chinese provinces. Patients with laboratory-confirmed A(H7N9) (n = 89) were matched by age, sex, and neighborhood to controls (n = 339). Subjects completed a questionnaire on medical history and potential exposures, including poultry markets and other poultry exposure. We used conditional logistic regression to calculate matched and adjusted odds ratios (ORs) for the association of A(H7N9) virus infection with potential risk factors. Results. Fifty-five percent of patients compared with 31% of controls reported any contact with poultry (matched OR [mOR], 7.8; 95% confidence interval [CI], 3.3–18.8). Sixty-seven percent of patients compared with 35% of controls visited a live poultry market (mOR, 5.4; CI, 3.0–9.7). Visiting live poultry markets increased risk of infection even after adjusting for poultry contact and other confounders (adjusted OR, 3.4; CI, 1.8–6.7). Backyard poultry were not associated with increased risk; 14% of cases did not report any poultry exposure or market visit. Obesity (mOR, 4.7; CI, 1.8–12.4), chronic obstructive pulmonary disease (mOR, 2.7; CI, 1.1–6.9), and immunosuppressive medications (mOR, 9.0; CI, 1.7–47.2) were associated with A(H7N9) disease. Conclusion. Exposures to poultry in markets were associated with A(H7N9) virus infection, even without poultry contact. China should consider permanently closing live poultry markets or aggressively pursuing control measures to prevent spread of this emerging pathogen.

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