Epidemiologic features and environmental risk factors of severe fever with thrombocytopenia syndrome, Xinyang, China

Abstract

Background: Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease discovered in rural areas of Central China in 2009, caused by a novel bunyavirus, SFTS virus (SFTSV). The disease usually presents as fever, thrombocytopenia, and leukocytopenia, with case-fatality rates ranging from 2.5% to 30%. Haemaphysalis longicornis was suspected to be the most likely vector of SFTSV. By the end of 2012, the disease had expanded to 13 provinces of China. SFTS patients have been reported in Japan and South Korea, and a disease similar to SFTS has been reported in the United States.

Methodology/principal findings: We characterized the epidemiologic features of 504 confirmed SFTS cases in Xinyang Region, the most severely SFTS-afflicted region in China from 2011 to 2012, and assessed the environmental risk factors. All cases occurred during March to November, with the epidemic peaking from May to July. The patients' ages ranged from 7 to 87 years (median 61 years), and the annual incidence increased with age (χ2 test for trend, P<0.001). The female-to-male ratio of cases was 1.58, and 97.0% of the cases were farmers who resided in the southern and western parts of the region. The Poisson regression analysis revealed that the spatial variations of SFTS incidence were significantly associated with the shrub, forest, and rain-fed cropland areas.

Conclusions: The distribution of SFTS showed highly significant temporal and spatial heterogeneity in Xinyang Region, with the majority of SFTS cases being elderly farmers who resided in the southern and western parts of the region, mostly acquiring infection between May and July when H. longicornis is highly active. The shrub, rain-fed, and rain-fed cropland areas were associated with high risk for this disease.

Figure 1. Temporal distribution of the confirmed SFTS cases in Xinyang Region, 2011–2012.

The black histogram represents the number of SFTS cases per period of ten days, and the red line represents the monthly number of SFTS cases.

Figure 2. Age and gender distribution of the confirmed SFTS cases in Xinyang Region, 2011–2012.

The black histogram represents the number of male cases and the white histogram represents the number of female cases over 5-year age groups. The line represents the annual average incidence (1/100,000) over age groups.

Figure 3. The annual incidence of confirmed SFTS for each township in Xinyang Region, 2011–2012.

The top five incidences displayed in the figure were 64.94, 56.93, 43.32, 38.84, and 38.00 per 100,000 persons in the townships of Gaoliangdian, Wanggang, Guanmiao, Hefengqiao, and Yanghe, respectively.

Figure 4. Spatial distribution of the confirmed SFTS cases overlapping the map of land cover in Xinyang Region, 2011–2012.

Seven land cover types have been categorized in the study area: irrigated cropland, rainfed cropland, orchard, forest, shrub, built-up land, and water body. Irrigated cropland comprises fields under irrigation for planting crops. Rainfed cropland represents areas with rainfed herbaceous crops. Orchard includes cultivated and managed terrestrial areas. Forest areas comprise broad-leaved forest and coniferous forest. Shrub areas included shrubland, grassland, and isolated trees. Built-up land areas concern surfaces with buildings and associated areas. Water body areas comprise lakes, reservoirs and ponds.