| BackgroundHuman infections with avian influenza A(H7N9)virus is acrucial public health problem that seriously threatens human health and socio-economic development in China.Since first reported in Shanghai in February 2013,epidemics of human infections with avian influenza A(H7N9)virus have occurred every winter and spring in mainland China.As of August 2016,China has experienced four waves of human avian influenza A(H7N9)virus infection epidemic.A total of 775 human cases reported and 316 cases died,with a case fatality rate of 41%.The fifth wave occurred from winter of 2016.As of 19 February,2016,there were 379 human cases reported,which was 195.6% of the mean number of cases in the former four waves.It suggested that the epidemic intensity of this wave increased significantly.Is the temporal and spatial distribution pattern of the epidemic in the fifth wave changed or not? The emerge and spread of human avian influenza A(H7N9)virus infection epidemic is affected not only by local natural and social factors,but also by the long-distance transportation of live poultry,the activities of the population.Which factors affect the cross-regional invasion and local transmission of the epidemic in China? What are the natural and social environmental characteristics of the epidemic areas? It is critical to make clear the epidemic trend and its driving factors of these five epidemic waves for effective prevention and control of human avian influenza A(H7N9)virus infections.Therefore,it is the important scientific issues to be solved.ObjectivesTo make clear the epidemic trend and driving factorsof human avian influenza A(H7N9)virus infection epidemic in China through analyzing the temporal and spatial distribution patter and its dynamics of the epidemic,and exploring the influencing factors of the cross-regional invasion and local transmission of the epidemic.To provide basic information and scientific basis for the prevention and control of the human avian influenza A(H7N9)virus infections.MethodsThe histogram was plotted using the onset time(week)as the horizontal axis and the number of cases as the vertical axis,to show the long-term trend of incidence of human infections with avian influenza A(H7N9)virus.The line chart was plotted using the onset time(week)as the horizontal axis and the number of cases as the vertical axis,to show the seasonal fluctuations of the five epidemic waves of human H7N9 cases in China.The heatmap was plotted using the onset time(week)as the horizontal axis and the province as study unit.The numbers of human H7N9 cases of each city in the five epidemic waves and in each epidemic wave were mapped respectively.Each city was marked with different color on the city-level digital map.All provinces in China were divided into several categories according to the number of time that the epidemic occured.The sex ratio and age of each epidemic wave and of each highly endemic province were compared,respectively.The population density,gross domestic roduct(GDP),density of chickens,density of ducks,percentage of urban construction land cover,and percentage of rural settlements around the human H7N9 case in 3km,the distance from the case to the nearest live poultry market,waterbody,freeway,and highway were compared among the five epidemic waves,respectively.Spatial cluster analyses of human cases in each epidemic wave were performed and the areas of high risk in each epidemic wave were mapped.The invation trend surface of epidemics was established,and the spatial invation trend map was drawn based on the spatial trend surface analysis.The Cox proportional hazards regression model was used to analyze the factors influencing the cross-regional invasion of epidemic,using the monthly value of each natural and social environmental factor as independent variables,and the time interval from the time of the first case and the first cases of each city in each epidemic wave as the dependent variable.The Poisson regression model of panel data was used to analyze the factors influencing the epidemic intensity,using the monthly value of each natural and social environmental factor as independent variables,and the number of cases of each city as the dependent variable.ResultsThere was an obvious seasonal fluctuation of the human avian influenza A(H7N9)virus infection epidemic in China.The largest number of cases from the 2nd to the 5th epidemic waves were reported in the fifth week,the fifty-first week,the fourth week and the third week,respectively.The number of cases of the third week in the fifth wave was 94,which was the largest one.There were obvious seasonal fluctuations of the human avian influenza A(H7N9)virus infection epidemic in each province in China.The onset time of the first human H7N9 case and the time period of high risk of the epidemic in each province were not consistent.The human cases in each epidemic wave were first reported in the Yangtze River Delta region,and then spread to the south,north and west areas of China.The human cases were mainly distributed in the southeastern region of China.The cases in the fifth epidemic wave were distributed in the largest spatial range and the largest number of cities.The provinces with the largest cummulative number of cases were Zhejiang Province(267 cases).The provinces with the largest number of cases in each epidemic wave were Zhejiang Province(45 cases),Guangdong Province(111 cases),Guangdong Province(70 cases),Zhejiang Province(34cases)and Jiangsu Province(118 cases).Beijing,Shanghai,Jiangsu,Zhejiang,Anhui,Fujian,Jiangxi,Shandong and Guangdong are relatively stable avianinfluenza A(H7N9)virus endemic areas,which have reported human H7N9 cases in all the four epidemic waves from the second epidemic wave to the fifth one.The proportion of male and female H7N9 cases in China was 70.6% and 29.4% respectively.The proportion of male cases was higher than that of female H7N9 cases in each epidemic wave and each highly endemic province,and that proportion were not different significantly among epidemic waves and provinces(epidemic waves: ?2=1.48,P> 0.05;provinces: ?2=11.81,P>0.05).The median age of human H7N9 cases was 57 years.The age of human cases in each epidemic wave was not different significantly(?2=7.68,P>0.05),while that in each highly endemic province was different significantly(?2=17.84,P<0.05).The population density,density of chickens,density of ducks,percentage of urban construction land cover,percentage of rural settlements around the human case in 3km,the distance from the case to the nearest live poultry market,waterbody,and freeway in the five epidemic waves were different significantly.The spatial distribution of high risk areas of five epidemic waves was not exactly the same,which were mainly concentrated in the Yangtze River Delta and the Pearl River Delta region.The spatial clusters in the fifth epidemic wave were distributed in the largest spatial range and the largest number of cities.The most likely cluster areas of each epidemic wave were moving from south to north areas of China.The general trend of the spread of human avian influenza A(H7N9)virus infection epidemic in China is from east and south areas to north and west areas of China.The areas with the human H7N9 cases are becoming more and more extensive.Population density and the number of live poultry markets were the risk factors of the cross-regional invasion of the epidemic in China,with the HR value(95% confidence interval)of 1.69(1.23,2.34)and 1.15(1.10,1.21).The percentage of dryland cover was the influencing factor of the cross-regional invasion of the epidemic(0.90(0.83,0.97)).The 7-day holiday,3-day holiday,population density and distance to the nearest national highway were the main risk factors of the local transmission of human avian influenza A(H7N9)virus infection epidemic in China,with the IRR value(95% confidence interval)of 2.66(2.33,3.05),3.44(3.02,3.91),5.72(1.91,17.15),and 1.30(0.98,1.72).The monthly mean temperature,monthly mean relative humidity,monthly cumulative precipitation,percentage of grass cover,dryland cover and wetland cover were the influencing factors of the local transmission of the epidemic,with the IRR value(95% confidence interval)of 0.55(0.52,0.59),0.59(0.52,0.67),0.79(0.69,0.91),0.42(0.34,0.53),0.46(0.41,0.52),and 0.06(0.02,0.18).ConclusionThe spread of human avian influenza A(H7N9)virus infection epidemic in China accelated dramatically.The areas affected by the avian influenza A(H7N9)virus is becoming more and more extensive.The areas with high risk of human avian influenza A(H7N9)virus infection were moving from south to north areas of China.The areas affected by the avian influenza A(H7N9)virus is spread from densely populated areas to relatively sparsely populated areas,and from urban to rural areas.Human H7N9 cases are getting more and more away from highways,waterbodies and live poultry markets.The density of chicken and duck around the epidemic areas are getting lower and lower.Meteorological factors play an important role in the local transmission of human avian influenza A(H7N9)virus infection epidemic in China.The live poultry market is an important part in the cross-regional invasion and local transmission of the epidemic.Population density and vacation are important risk factors for the cross-regional invasion and local transmission of the epidemic.The type of live poultry has little effect on the cross-regional invasion and local transmission of the epidemic. |
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