Last week a study was published in the journal Clinical Infectious Diseases, that looked at what risk factors were associated with human infection with the H7N9 virus in China in 2013. This study was conducted by an international group of scientists, including researchers from both the Chinese and the US CDC.
Researchers conducted a case control study between April and June of last year (almost immediately after the first clusters were announced) where they compared 89 laboratory-confirmed A(H7N9) cases to 339 controls, matched for age, sex, and neighborhood.
This is the type of study we’ve been hoping to see out of Saudi Arabia on MERS for well over a year now.
Although behind a pay wall, the CDC has published a pretty good summary on the study today. While even casual exposure to poultry in live bird markets was cited as the primary risk factor, people who owned, raised, or slaughtered birds at home, on farms, or in the wild were not found to be at any increased risk.
Which explains the conclusion of the authors:
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.
First, the abstract for the study, which may be viewed at:
Bo Liu1,*, Fiona Havers2,*, Enfu Chen3,*, Zhengan Yuan4,*, Hui Yuan12, Jianming Ou5, Mei Shang6, Kai Kang7, Kaiju Liao1, Fuqiang Liu8, Dan Li1, Hua Ding9, Lei Zhou1, Weiping Zhu10, Fan Ding1, Peng Zhang11, Xiaoye Wang1, Jianyi Yao1, Nijuan Xiang1, Suizan Zhou6, Xiaoqin Liu12, Ying Song6, Hualin Su13, Rui Wang1, Jian Cai3, Yang Cao1, Xianjun Wang14, Tian Bai15, Jianjun Wang16, Zijian Feng1, Yanping Zhang1, Marc-Alain Widdowson17, and Qun Li1
This from the CDC’s Flu News.
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 http://cid.oxfordjournals.org/content/early/2014/06/13/cid.ciu423.abstract. For more information on the H7N9 virus, see Avian Influenza A (H7N9) Virus.