| BackgroundNatural focus disease is an important type of infectious diseases that the pathogen infects host animals or people through vectors. Natural foci is a region where natural focus disease exists, and is also a special ecological system consisting of the pathogen, host animals and vectors in a specific natural environment. With the progress of society and expanding scope of human activities following by the increasing chances of exposure to some natural foci, natural focus disease has been thriving gradually in a number of countries and regions in recent years, and most natural focal disease are characterized by strong pathogenicity, serious clinical behavior, high mortality rate and easy to cause outbreaks.Epidemiological methods were mainly applied for the study of natural focus disease previously, the measures of spot investigation and artificial detection were restricted by technical conditions, which have difficult in fully extracting and using spatiotemporal and climate information of diseases as well as in saving and analyzing the large amounts of complex data. "3S" spatial information technology represented by geographic information system(GIS), remote sensing technology(RS) and global positioning system(GPS) is widely used in geography, ecology, agriculture and other fields of research since the 1960 s, and also became successfully used in research of natural focus such as schistosomiasis and Lyme disease. The advantage of spatial information technology in tapping and saving data updating and analyzing and visualization of spatiotemporal distribution have been showed, the difficulties of traditional epidemiology were overcame and many shortcomings were improved. It helped greatly in establishment of disease outbreak surveillance system and improvement of the ability in information processing and decision-making in emergency.Scrub typhus, also called tsutsugamushi disease, is an untypical febrile disease caused by the intracellular pathogen Orientia tsutsugamushi. Scrub typhus is a natural focus disease transmitted to humans after being bitten by chigger mites and its host were mainly rodents especially rats. The clinical manifestations of scrub typhus were characterized by fever, typical eschar or rash accompanied by lymphadenectasis and could be life-threatening due to multiple organ failure. Scrub typhus is primarily endemic in Asia–Pacific regions, it is estimated that there are currently more than 1 billion people living in endemic areas and 1million cases were reported annually. Scrub typhus has a great influence on the combat effectiveness of the armed forces and had caused a large number of non-combat attrition in the past.Scrub typhus remains a serious public health problem in China. Historically, scrub typhus mainly occurred in southern provinces of the Yangtze River before the 1980 s. Recently, case reports and new natural foci of scrub typhus have been identified in the northern provinces of Shandong, Jiangsu, and Tianjin with the intensity of the disease strengthened. Now scrub typhus has become a component of China’s National Notifiable Disease Surveillance System. The present researches of scrub typhus in our country were mainly outbreak investigation aimed at areas of high incidence. There is a big blank in the accurate understanding and risk factors or risk prediction study. Thus, it is necessary to understand the spatiotemporal epidemic characteristics and conduct studies on risk factors and risk prediction of scrub typhus in China to provide a scientific basis for the formulation of disease prevention measuresObjectiveTo understand the epidemiological characteristics of scrub typhus including the temporal, spatial and demographic distributions as well as the clustering areas of scrub typhus occurrence; to explore the correlation between the monthly incidence of scrub typhus and meteorological and environmental factors in different highly epidemic areas; to identify main drivers to the spatial distribution of scrub typhus occurrence, and to predict the high-risk areas of scrub typhus in the future.Methods1. Monthly scrub typhus monitoring data from 2006-2014 were collected at the county level, and descriptive epidemiology were used to analyze the epidemiological characteristics of both scrub typhus cases and deaths; Global spatial autocorrelation analysis and partial autocorrelation analysis(Local indicator of spatial association, LISA) and spatiotemporal cluster analysis were conducted to assess the autocorrelation of scrub typhus occurrence and detected spatiotemporal clusters. SAS 9.2, SPSS 16.0, ArcGIS 9.3, and SaTScan 9.1.1 programmers were used.2. Monitoring data of monthly scrub typhus in epidemic areas with high incidence(Guangzhou, Baoshan, Zhangzhou, Fuyang, Linyi) and data of environmental factors from 2006-2013 were collected. Spearman correlation analyses were performed to explore the relationship between scrub typhus occurrence and meteorological factors. Microsoft Office 2010, ArcGIS 9.3 and SPSS16.0 were used.3. Individual case data from 2006 to 2014 were obtained to geo-referenced to the administrative village level, then meteorological factors(average temperature, minimum temperature, maximum temperature, relative humidity and rainfall), elevation, land cover, normalized difference vegetation index(NDVI) and population density were collected and projected into 1 × 1 km resolution raster images. Ecological niche models based on maximum entropy(ME) were performed to analysis the relationship between environmental variables and scrub typhus occurrence as well as the contribution of each variable to predict the risk of scrub typhus occurrence. Microsoft Office 2010, Arc GIS 9.3 and Max Ent programmer(version 3.3.3k) were used.Results1. A total of 54,558 scrub typhus cases, including 37 deaths, were reported during 2006 to 2014. The annual incidence of scrub typhus increased significantly(Cochran-Armitage trend test, Z = 172.14, P < 0.001).. Scrub typhus cases were distributed in 1,031 counties in 29 provinces in mainland China and the average annual incidence at the county level ranged from 0 to 66.21 per 100,000 during the study period. The majority of cases(77.97%) were reported from five provinces(Guangdong, Yunnan, Anhui, Fujian, and Shandong). The incidence was significantly higher in females than males(χ2= 399.22, P < 0.001). The majority of cases occurred in the age group of 50–59 years, accounted for 22% of the total cases. Farmers had a higher incidence of scrub typhus than non-farmers(χ2= 691.88, P < 0.001). Differences in the seasonal trends of incidence between northern and southern provinces were identified. In Fujian, Guangdong, and Guangxi, scrub typhus occurred mainly in June to July and September to October that demonstrated a bimodal appearance of two peaks; in Yunnan, the number of cases demonstrated a single large peak in July or August. In contrast, scrub typhus cases demonstrated a single large peak in October or November the in northern provinces(Shandong, Anhui, and Jiangsu) each year. Spatial autocorrelation analysis of scrub typhus incidence indicated the cases were clustered each year. LISA analysis showed hotspots(High-High) were primarily located in the southern and southwestern provinces of China with the geographical area expanded annually. Space-time scan statistic demonstrated the primary cluster of scrub typhus cases was shifted between middle-east and southwest areas, and a higher percentage of cases were observed in females and farmers in clustered counties.2. During 2006 to 2013, a total of 3908 cases were reported in Guangzhou with the monthly incidences of 0-1.80/100,000, accounted for 33.45% of all cases in Guangdong province; a total of 2464 cases were reported in Baoshan with the monthly incidences of 0-12.4/100,000, accounted for 27.46% of all cases in Yunnan province. A total of 1406 cases were reported in Zhangzhou with the monthly incidences of 0-1.53/100,000, accounted for 33.45% of all cases in Fujian province. A total of 2116 cases were reported in Fuyang with the monthly incidences of 0-4.02/100,000, accounted for 61.40% of all cases in Anhui province; a total of 692 cases were reported in Linyi with the monthly incidences of 0-1.07/100,000, accounted for 23.33% of all cases in Shandong province. Spearman correlation analysis demonstrated significant associations between meteorological factors and scrub typhus occurrence(except for MEI that only significant in Guangzhou). And the results showed the lag effects of these factors to the monthly incidences of scrub typhus in the five cities were varied.3. The high risk areas predicted by Ecological Niche Model are mainly located at southwestern、middle-east and southern China. Maximum rainfall had the highest contribution(31.6%-45.8%) to the model in 2006-2009; minimum temperature had the highest contribution to the model(39.8 %) in 2010; average temperature(26.8%-32.2%) and average rainfall(29.8%-37.9%) had the highest contributions to the model in 2011-2013; population density had a contribution of 13.7%-31.6% while maximum temperature, minimum relative humidity and elevation had a contribution of 0.7%-6.2%, 0.2%-4.6% and 0.8%-4.3% respectively. Response curve of each factor reflected scrub typhus occurrence initially increased with rainfall, temperature and NDVI before reached a peak,and revealed a large decreased, areas with monthly maximum rainfall of 600, average rainfall of 200, minimum rainfall of 12 mm and maximum temperature of 34℃, average temperature of 22℃, minimum temperature of 12℃ had higher risk of scrub typhus occurrence; areas with monthly NDVI between 100 to 200 and population up to 15000 had higher risk of scrub typhus occurrence; scrub typhus occurrence risk decreased when elevation increased up to 500 m, but reached to a high level when elevation reached to 2000 m. The AUC calculated for each model every year ranged from 0.86 to 0.95, indicating the prediction models performed well. More than 70% of scrub typhus cases occurred in predicted high risk regions, which only covered 2.97%-5.78% of areas in mainland China.ConclusionsThis study described the long-term epidemiologic characteristics and confirmed the seasonal patterns and high risk group of scrub typhus in mainland China. The results demonstrated the spatiotemporal dynamics and the autocorrelation, primary and secondary clusters of areas of scrub typhus in mainland China. The correlation between scrub typhus incidence and meteorological factors in high epidemic areas was explored. A nationwide ecological niche model was built to explore how the environmental factors influence the occurrence of scrub typhus and predict the potential high risk areas. The results of the work enlarged the knowledge of scrub typhus epidemiology in China and could be beneficial for target control and prevention of the disease. |
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