Background.?The majority of humans cases of novel avian influenza A(H7N9), which emerged in China in spring 2013, report 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 eight Chinese provinces. Laboratory-confirmed A(H7N9) cases (n=89) were matched on 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 for the association of A(H7N9) virus infection with potential risk factors.

Results.?Fifty-five percent of cases compared with 31% of controls reported any contact with poultry (matched OR [mOR]: 7.8; 95% confidence interval [CI]: 3.3–18.8). 67% of cases 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.