Scrub typhus islands in the Taiwan Area and the association between scrub typhus disease and forest land use and farm worker density: Geographically weighted regression

Background: The Taiwan Area includes the main island of Taiwan and several small associated islands, which are located off the coast of Southern China. The eastern two-thirds of Taiwan’s main island are characterized by rugged mountains covered with tropical and subtropical vegetation. Western Taiwan’s main island is characterized by flat to gently rolling plains. The geographical distribution of the Taiwan Area exhibits diversity in ecology and environment, which are both under the threat of scrub typhus. This study investigates the effects of seasonal and meteorological factors on scrub typhus infection in 10 local climate regions. This study also investigates the spatial distribution of scrub typhus incidence in correlation with forest land uses and farm workers on the main island of Taiwan. Methods: This study uses Pearson’s product moment correlation to describe the correlation between confirmed cases of scrub typhus and meteorological factors in 10 local climate regions. This study also uses geographically weighted regression (GWR), a local version of spatial regression that generates parameters disaggregated by the spatial units of analysis, to describe and map each regression point for the response variable of standardized incidence ratio (SIR)-district scrub typhus. This study also uses GWR to describe the explanatory variables of various forest land uses and farm worker densities on the main island of Taiwan in 2005. Results: Scrub typhus endemic areas in the Taiwan Area are located in southeastern Taiwan, mountainous township areas, Pescadore Islands, Kinmen Islands, and Matou Islands. The age-adjusted standard incidence rates range from 15.6 to 185.5 per 100 000 people for males and from 9.3 to 40.4 per 100 000 people for females. These rates are higher than those for the low incidence areas in Northern Taiwan, Northwestern Taiwan, West-Central Taiwan, Southwestern Taiwan, and Northeastern Taiwan, which have age-adjusted standard incidence rates ranging from 0.5 to 1.6 per 100 000 people for males, and 0.3 to 1 per 100 000 people for females. In the Taiwan Area, the 20-24 year-old male group and 60-64 year-old female group have the highest age-specific incidence rates. The seasonal incidence of scrub typhus was 12.7% in spring (468 cases), 40.2% in summer (1488 cases), 30.2% in autumn (1115 cases), and 16.9% in winter (626 cases) showing significant seasonal differences (χ 2 = 704.7, p< 0.001). In addition, the number of infectious cases in the warm season (2540 cases, 68.7%) was significantly higher than that in the cold season (1157 cases, 31.3%) (χ 2 = 517.4, p< 0.001). A high infectious peak in summer was reflected in 472 cases in June and 638 cases in July, respectively. However, the high-risk groups varied in age and seasonal incidence among the 10 local climate regions. The Pescadores, Kinmen, and Matou islands, along with low-incidence areas in west-central Taiwan and Southwestern Taiwan, were significantly correlated to air temperature, but showed no significance in endemic areas (e.g., Southeastern Taiwan and the mountainous township areas). In the GWR models, the response variable of SIR-district scrub typhus was positively and significantly associated with the farm worker and timber management area explanatory variables. In addition, the explanatory variables of recreational forest area, natural reserve area, and other purpose area showed positive and negative signs for parameter estimates in various locations of Taiwan’s main island. Negative signs of parameter estimates only appeared in the explanatory variables of the national protective area, plantation area, and clearcut area. Conclusion: The results of this study indicate that the scrub-typhus type in the Taiwan Area is an island type of scrub typhus rather than the summer type of scrub typhus found in southern China. Results also show that higher SIR values for scrub typhus in the central and southern portions of the mountainous township area and Southeastern Taiwan are associated with the variables of farm worker density, timber management area, recreational forest area, natural reserve area, and other purpose area.