Mapping the global distribution of and environmental suitability for scrub typhus

Scrub typhus, an acute febrile illness caused by Orientia tsutsugamushi , has emerged as a significant public health concern, expanding beyond its traditional endemic region, the "tsutsugamushi triangle" in the Asia-Pacific. Despite its increasingly global distribution, comprehensive spatial assessments of scrub typhus risk remain sparse. An exhaustive assembly of 56,093 unique human scrub typhus occurrence records worldwide was undertaken from published literature and national surveillance datasets. Covering 27 countries/regions, these records were combined with 28 climatic, geographic, and socio-economic covariates environmental covariates using an ensemble machine learning modelling approach, capturing possible nonlinear effects and complex interactions, to map the probability of occurrence at 5×5 km resolution globally. This approach involved stacking of three sub-models (generalized additive models, boosted regression trees and random forest). Environmental suitability for scrub typhus was found to be highest in moderate to tropical climates, notably extending beyond the classic "tsutsugamushi triangle" into large sections of Central and South America, Central and West Africa. Approximately 2.5 billion people (95% CI: 2.43–2.69 billion) are estimated to be currently living in environmentally suitable areas within countries or regions where human cases of scrub typhus have already been confirmed. This number increases to 4.4 billion people (95% CI: 3.86–4.90 billion) if countries without confirmed cases are included. This data assembly and modelled environmental suitability surface provide novel insights into the potential public health impact of scrub typhus. This may serve as a catalyst for broader discussions regarding the neglected global impact of this disease, the need to improve public awareness, drug, and vector control methods, and lead to further burden assessment. The study highlights key data gaps, particularly in regions with limited surveillance and accessibility of healthcare facilities, and emphasizes the need for future research in the context of ongoing climate and environmental changes, which may further alter the global distribution of scrub typhus.