Can closure of live poultry markets halt the spread of H7N9?

After isolation of avian infl uenza A H7N9 virus from live poultry markets (LPMs), and reports that several people who were infected with the virus had visited such markets a few days before disease onset, LPMs were suspected to be a main source of human exposure to H7N9 in China. After LPM closure was enforced in several Chinese cities, the incidence of H7N9 human cases rapidly reduced. In The Lancet, Hongjie Yu and colleagues quantify the risk of human infections before and after LPM closure in the Chinese cities of Nanjing, Shanghai, Hangzhou, and Huzhou. The investigators used information about laboratory-confi rmed cases of avian infl uenza A H7N9 virus infection reported in the four cities by June 7, 2013, to fi t a statistical model to assess the daily probability of infection for each susceptible individual before and after LPM closure. They report that closure of LPMs reduced the mean daily number of infections by 99% (95% credibility interval 93–100%) in Shanghai, 99% (92–100%) in Hangzhou, 97% (68–100%) in Huzhou, and 97% (81–100%) in Nanjing. They conclude that LPM closure should be sustained in areas with high risk of disease spread, and implemented in any urban areas where avian infl uenza A H7N9 virus appears in the future. LPMs are known to be a high-risk environment for pathogen transmission between birds and for zoonotic transfer to people. If birds spend a suffi cient amount of time in LPMs to become infected and transmit the virus to other susceptible birds, sustained virus circulation in LPMs can occur. LPMs can then become a permanent source of infection for poultry fl ocks and for people who are in close contact with infected poultry and contaminated fomites. LPM closure would therefore be expected to stop human infections caused by a zoonotic pathogen transmitted within markets. Although LPM closure seems to have had an important eff ect on incidence of avian infl uenza A H7N9 virus infection in people, the underlying epidemiological and socioeconomic system is complex. LPM closure might not have been the most important cause of a reduction in incidence. Indeed, soon after LPM closures in Nanjing, Shanghai, Hangzhou, and Huzhou, human infections were also interrupted in other cities where this measure was not implemented. An example of the complex chain of interacting socioeconomic and epidemiological factors is that reports that poultry is a source of avian infl uenza A H7N9 virus triggered a drop in demand for poultry meat and consequently in poultry prices. Some producers were even forced to destroy their fl ocks because poultry rearing became cost prohibitive, which probably reduced the absolute number of poultry substantially in aff ected areas and reduced viral circulation, irrespective of LPM closure. Strong consumer reactions to zoonotic health hazards are not uncommon. Fears of avian infl uenza A H5N1 virus led to reduced consumption of poultry products in Asia in early 2004. However, these reductions were temporary: consumers’ anxiety decreased rapidly after the fi rst wave of the outbreak. Consumption of poultry products resumed within a few months, sometimes exceeding levels recorded before the outbreak, even though the virus had become endemic and human cases were reported regularly. Reduced consumer demand could have increased the compliance of poultry production stakeholders with LPM closures in China, because informal alternative ways to sell poultry were unavailable. Avian infl uenza A H7N9 virus is probably still circulating in Chinese poultry fl ocks, as suggested by new human cases reported in August, 2013. If poultry consumption resumes, the seasonal increase in poultry production and trading activities that usually occurs in the months preceding Chinese New Year could amplify virus circulation, resulting in a surge in human cases. If this increase occurs and consumer reactions are not as strong as during the spring of 2013, Published Online October 31, 2013 http://dx.doi.org/10.1016/ S0140-6736(13)62109-1