Scrub typhus in northern Thailand: a seroprevalence study among Thai and highland populations

Background Scrub typhus is a major cause of febrile illness in Southeast Asia, with particularly high burdens reported in northern Thailand. Environmental exposures and use of preventive measures remain incompletely understood. Methods We conducted a population-based cross-sectional serosurvey in Chiang Rai province, Thailand in 2022-2023. Adults aged [&ge;]15 years from 74 villages were selected by two-stage random sampling and interviewed regarding demographic characteristics, agricultural and domestic exposures, and preventive behaviours. IgG antibodies against Orientia tsutsugamushi were measured using a Luminex xMAP Intelliflex(R) platform. Mixed-effects logistic regression models stratified by ethnicity (highland people vs Thai) were used to identify factors associated with seropositivity, with village as a random intercept. Results Among 1,194 participants with valid serology, seroprevalence was higher among highland people than Thai ones (60% vs 13%, p<0.001). Seropositivity increased with age in both groups (p<0.001, and p=0.006), more markedly among highland people. Exposure to forest/high hill (adjusted odds ratio [aOR] 2.87, 95%CI 1.07-7.70) and agroforestry plantations (aOR 1.74, 95%CI 1.04-2.99) were associated with seropositivity among Thai participants, but not among highland people. High-risk occupations were associated with increased odds of seropositivity across both groups, p=0.006 and p=0.043. No clear associations were observed between seropositivity and duration of exposure, domestic/peridomestic risk factors, individual preventive practices, or overall prevention quality. Significant village-level heterogeneity persisted after adjustment, particularly among highland people. Conclusions Scrub typhus exposure is common in rural Chiang Rai and disproportionately affects highland people; whose risk was poorly explained by classical environmental risk factors, suggesting distinct and incompletely understood exposure pathways. Current prevention strategies based on conventional risk messaging may therefore have limited effectiveness in these high-risk communities.