Epidemiology and Ecology of Severe Fever With Thrombocytopenia Syndrome in China, 2010‒2018

Abstract

Background: The growing epidemics of severe fever with thrombocytopenia syndrome (SFTS), an emerging tick-borne disease in East Asia, and its high case fatality rate have raised serious public health concerns.

Methods: Surveillance data on laboratory-confirmed SFTS cases in China were collected. The spatiotemporal dynamics and epidemiological features were explored. The socioeconomic and environmental drivers were identified for SFTS diffusion using survival analysis and for SFTS persistence using a two-stage generalized boosted regression tree model.

Results: During 2010‒2018, a total of 7721 laboratory-confirmed SFTS cases were reported in China, with an overall case fatality rate (CFR) of 10.5%. The average annual incidence increased >20 times and endemic areas expanded from 27 to 1574 townships, whereas the CFR declined from 19% to 10% during this period. Four geographical clusters-the Changbai Mountain area, the Jiaodong Peninsula, the Taishan Mountain area, and the Huaiyangshan Mountain area-were identified. Diffusion and persistence of the disease were both driven by elevation, high coverages of woods, crops, and shrubs, and the vicinity of habitats of migratory birds but had different meteorological drivers. Residents ≥60 years old in rural areas with crop fields and tea farms were at increased risk to SFTS.

Conclusions: Surveillance of SFTS and intervention programs need to be targeted at areas ecologically suitability for vector ticks and in the vicinity of migratory birds to curb the growing epidemic.

Keywords: SFTS; diffusion; ecology; epidemiology; modeling.

Figure 1.

Spatial dispersion of 4 geographic clusters of SFTS cases in China, 2010‒2018. Confirmed SFTS cases were marked as dots with the color indicating the year of reporting. Four clusters (I‒IV) were delineated according to the spatial trend surface analysis and the geographic aggregation of cases. In each cluster, contour lines of the time intervals (in days) from the first case reported in China to the first case reported in each SFTS-affected township reflect the diffusion direction of the disease. Abbreviation: SFTS, severe fever with thrombocytopenia syndrome.

Figure 2.

Temporal trend and seasonality of SFTS epidemics in China from 2010 to 2018. Blue solid lines indicate the trends in the annual incidence of confirmed cases and in total areas of affected townships for the nation (A and F) and the 4 geographic clusters (B‒E and G‒J) fitted by joinpoint regression. Lines change color from blue to green if a joinpoint is identified. AAPCs were estimated based on the slopes of the lines. Red dots indicate observed incidence. Panels K‒O indicate seasonal pattern of confirmed SFTS patients in the nation and clusters I‒IV, respectively. Seasonality is presented as a radar diagram. Circumference is divided into 12 months in a clockwise direction, and the radius represents average monthly incidences over 2010‒2018. Abbreviations: AAPC, average annual percentage changes; SFTS, severe fever with thrombocytopenia syndrome.

Figure 3.

Association between spatial diffusion and influential factors identified by the Cox proportional hazards model by geographic cluster. Spatial diffusion is represented by the trend surface contour plots with darker color indicating earlier importation time of the first case in the township. Columns correspond to the influential factors: elevation (A, D, G), land cover (B, E, H), and distance to the nearest migratory bird habitat (C, F, I); rows are arranged according to geographic clusters: cluster I (A‒C), clusters II and III (D‒F), and cluster IV (G‒I).