New Study Highlights Risk Factors for Human Infection with H7N9 Bird Flu

A new study by CDC and China CDC experts has identified specific risk factors and exposure risks for human infections with the avian influenza A (H7N9) virus. Key findings showed that exposure to poultry in live bird markets was the primary source of H7N9 infections among people in China. Merely being in a location with poultry was significantly associated with increased risk of H7N9 infection. Many cases were only exposed to a market once in the period when they were likely infected.  However, raising poultry at home and consumption of poultry were not associated with an increased risk of H7N9 illness. Underlying health conditions that were significantly associated with H7N9 infections in China included obesity, chronic obstructive pulmonary disease (COPD), and being on immunosuppressive medications.

This study was conducted from April to June 2013 in eight Chinese provinces and used a case-control design. Researchers analyzed data from 89 patients with laboratory-confirmed H7N9 infection and 339 controls. Controls were people in China that were matched to cases by age, sex and neighborhood.

Interviews and surveys were used to collect the data. Information collected on host factors included demographics, medical history, health behaviors, interactions with backyard domestically-raised poultry, and poultry consumption in the calendar month prior to symptom onset. Information collected also included poultry exposure in the 10 days prior to illness onset. Participants were asked about behaviors during poultry contact, such as wearing protective equipment and hand washing.

Although poultry contact increased the risk of H7N9 infection, slaughtering or processing poultry (indicating contact with internal organs or blood) posed no greater risk than less intense contact. Hand washing was associated with decreased risk of H7N9 infection. Among those with direct poultry contact, controls washed their hands more frequently than cases. Owning birds as pets also was not associated with greater risk of H7N9 infection, nor was poultry contact in other environments outside of live bird markets, such as farms or lakes with waterfowl.

Human infections with the H7N9 virus were first identified in China in spring 2013. Experts believe most of these infections were caused by exposure to infected poultry or contaminated environments, as H7N9 viruses have been found in poultry in China. While some human cases of H7N9 infection have been associated with mild illness, most have had severe respiratory illness, with about one-third resulting in death. No evidence of sustained person-to-person spread of H7N9 has been found, though some evidence points to limited person-to-person spread in rare circumstances. This H7N9 virus has not been detected in people or birds in the United States.

The article is available online from the Clinical Infectious Diseases website at