Orientia tsutsugamushi dynamics in vectors and hosts: ecology and risk factors for foci of scrub typhus transmission in northern Thailand

Abstract

Background Scrub 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. Methods Small 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. Results The 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. Conclusions These 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

Not Only Leptotrombidium spp. an Annotated Checklist of Chigger Mites (Actinotrichida: Trombiculidae) Associated with Bacterial Pathogens

“…), the Savile’s bandicoot rat Bandicota savilei Thomas, 1916, the greater bandicoot rat B. indica (Bechstein, 1800) (Mammalia: Rodentia), the northern treeshrew Tupaia belangeri (Wagner, 1841) and the common treeshrew T. glis Diard and Duvaucel, 1820 (Mammalia: Scadentia). Reports come from India, Thailand and Vietnam [ 23 , 30 , 46 , 47 , 48 , 49 , 50 ].…”

“…Due to the significance of the illness, involving lethality without treatment, and its high prevalence in the endemic areas of SE Asia, the relation Leptotrombidium spp.— O. tsutsugamushi is most well explored in literature. Issues raised so far concern scrupulously investigated ecological, epidemiological, molecular and geographical aspects of the chigger-borne rickettsiosis occurrence in general as well as in particular areas of China, South Korea, Japan, Russia, Taiwan, Thailand and Asia–Australia–Pacific region, which together constitute the so called ‘tsutsugamushi triangle’ [ 10 , 14 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. Recent studies, however, indicate that scrub typhus is no longer limited to the above-mentioned zone, as cases from Africa, Middle East and South America have been also reported [ 36 , 37 , 38 , 39 ].…”

Encephalitis: diagnosis, management and recent advances in the field of encephalitides

“…28 Deforestation, increased land use in agriculture and greater rainfall during monsoon seasons increase chigger numbers. 30 These changes, along with the successful JE vaccination programme in India, have led to scrub typhus replacing JE to become the leading cause of acute encephalitis in certain regions of India. 29 If this pattern continues, we can expect an increase in arthropodborne encephalitis globally, and more research into the epidemiology and management of these diseases is vital in the face of a changing global climate.…”

“…After first emerging in 1940s in north-eastern region of India, the region experienced no cases for decades until it re-emerged in 2010 28. Deforestation, increased land use in agriculture and greater rainfall during monsoon seasons increase chigger numbers 30. These changes, along with the successful JE vaccination programme in India, have led to scrub typhus replacing JE to become the leading cause of acute encephalitis in certain regions of India 29.…”

The incidence of Orientia tsutsugamushi infection in rural South India

“…The risk of human infection depends on the exposure of humans to chiggers. Exposure in turn depends on environmental factors influencing chigger abundance such as humidity, temperature and vegetation [ 9 ], as well as human behavioural factors related to the risk of chigger infestation, such as agricultural activities [ 10 , 11 ]. As a consequence, the infection shows a marked seasonality, in South India in the form of a gradual increase during the rainy season and a gradual decline during the hot and dry months, with little inter-annual variation [ 12 ], likely reflecting seasonal changes in chigger abundance and human behaviour [ 11 ].…”