Abstract

BackgroundScrub typhus is an important neglected vector-borne zoonotic disease across the Asia–Pacific region, with an expanding known distribution. The disease ecology is poorly understood, despite the large global burden of disease. The key determinants of high-risk areas of transmission to humans are unknown.MethodsSmall mammals and chiggers were collected over an 18-month period at three sites of differing ecological profiles with high scrub typhus transmission in Chiang Rai Province, northern Thailand. Field samples were identified and tested for Orientia tsutsugamushi by real-time PCR. The rates and dynamics of infection were recorded, and positive and negative individuals were mapped over time at the scale of single villages. Ecological analyses were performed to describe the species richness, community structure and interactions between infected and uninfected species and habitats. Generalised linear modelling (GLM) was applied to examine these interactions.ResultsThe site with the highest rates of human infection was associated with the highest number of infected chigger pools (41%), individual chiggers (16%), proportion of the known vector species Leptotrombidium deliense (71%) and chigger index (151). Chigger species diversity was lowest (Shannon diversity index H′: 1.77) and rodent density appeared to be high. There were no consistent discrete foci of infection identified at any of the study sites. The small mammals Rattus tanezumi and Bandicota indica and the chiggers L. deliense and Walchia kritochaeta emerged as central nodes in the network analysis. In the GLM, the end of the dry season, and to a lesser extent the end of the wet season, was associated with O. tsutsugamushi-infected small mammals and chiggers. A clear positive association was seen between O. tsutsugamushi-positive chigger pools and the combination of O. tsutsugamushi-positive chigger pools and O. tsutsugamushi-positive small mammals with lowland habitats.ConclusionsThese findings begin to reveal some of the factors that may determine high-risk foci of scrub typhus at a fine local scale. Understanding these factors may allow practical public health interventions to reduce disease risk. Further studies are needed in areas with diverse ecology.Graphical abstract

Highlights

  • Scrub typhus is an important neglected vector-borne zoonotic disease across the Asia–Pacific region, with an expanding known distribution

  • Scrub typhus has a complex and poorly understood ecology, with much existing knowledge dating back to research conducted around World War II, and only in the past two decades has there been a resurgence in research interest in this illness [6]

  • Study site selection and climate description Three villages were selected for detailed investigations of chiggers and small mammals: Ban Thoet Thai (BTT) (20.24°N, 99.64°E), Mae Fahluang district; Ban Song Kwair (BSK) (20.02°N, 99.75°E) and Ban Mae Mon (BMM) (19.85°N, 99.61°E), Mueang district in Chiang Rai Province (Fig. 1)

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Summary

Introduction

Scrub typhus is an important neglected vector-borne zoonotic disease across the Asia–Pacific region, with an expanding known distribution. Scrub typhus is a potentially fatal febrile illness with an expanding known distribution, caused by Orientia tsutsugamushi, an obligate intracellular bacterium. Elliott et al Parasites Vectors (2021) 14:540 transmitted by the larval stage of trombiculid mites (chiggers) [1]. Scrub typhus has a complex and poorly understood ecology, with much existing knowledge dating back to research conducted around World War II, and only in the past two decades has there been a resurgence in research interest in this illness [6]. Orientia tsutsugamushi is transmitted efficiently transovarially and transstadially, suggesting trombiculid mites act as both vector and host [7, 8]. The pathogen is present in a large variety of habitats, ranging from semi-urban parks and gardens [5] to plantations [9], sandy beaches [10], forest [11] and alpine meadows at 3200 m a.s.l. [12]

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