Scrub typhus ecology: a systematic review of Orientia in vectors and hosts

Abstract

Scrub typhus, caused by Orientia tsutsugamushi, is an important and neglected vector-borne zoonotic disease with an expanding known distribution. The ecology of the disease is complex and poorly understood, impairing discussion of public health interventions. To highlight what we know and the themes of our ignorance, we conducted a systematic review of all studies investigating the pathogen in vectors and non-human hosts. A total of 276 articles in 7 languages were included, with 793 study sites across 30 countries. There was no time restriction for article inclusion, with the oldest published in 1924. Seventy-six potential vector species and 234 vertebrate host species were tested, accounting for over one million trombiculid mites (‘chiggers’) and 83,000 vertebrates. The proportion of O. tsutsugamushi positivity was recorded for different categories of laboratory test and host species. Vector and host collection sites were geocoded and mapped. Ecological data associated with these sites were summarised. A further 145 articles encompassing general themes of scrub typhus ecology were reviewed. These topics range from the life-cycle to transmission, habitats, seasonality and human risks. Important gaps in our understanding are highlighted together with possible tools to begin to unravel these. Many of the data reported are highly variable and inconsistent and minimum data reporting standards are proposed. With more recent reports of human Orientia sp. infection in the Middle East and South America and enormous advances in research technology over recent decades, this comprehensive review provides a detailed summary of work investigating this pathogen in vectors and non-human hosts and updates current understanding of the complex ecology of scrub typhus. A better understanding of scrub typhus ecology has important relevance to ongoing research into improving diagnostics, developing vaccines and identifying useful public health interventions to reduce the burden of the disease.

Marginalized mites: Neglected vectors of neglected diseases

“…This ancient Asian-Pacific disease was unknown in the New World, and its discovery in South America raised many questions, including origin, distribution, and routes of transmission [4]. Speculations included an association with recent worldwide phenomena of globalization and climate change or the migration of animals such as rodents and birds [4,5]. The change of paradigm was supported by the isolation of a new species, Candidatus Orientia chuto, from a patient visiting the Arabian Peninsula and by additional serological and molecular data from countries in Africa, suggesting a much broader distribution of scrub typhus outside the traditional tsutsugamushi triangle [6].…”

“…In sharp contrast, the interest in mites as vectors was low during the past decades. It was not always so; there were reactive paroxysms of mite research during the medical emergency of scrub typhus in the Asia Pacific amongst combatants during World War II [5], but, even though the numbers affected must now be much greater, impetus declined dramatically. Following the logic of our analysis, the body of knowledge regarding mites as vectors in 2019 is at the same surrogate level it was for ticks and mosquitos in the years 1968 and 1973, respectively (1,100 to 1,200 cumulative publications).…”

“…If the search was limited to "Title/ Abstract," the term "chigger OR trombiculid OR Leptotrombidium" scored 596 hits, compared to 15,798, 13,368, and 6,983 for Aedes, Anopheles, and Ixodes, respectively. A recent systematic review of Orientia ecology, the first since 1974 [12], also highlighted the limited knowledge on trombiculid mites; furthermore, many of the included studies were difficult to access and/or were published in Chinese, Japanese, Korean, or Russian [5].…”

“…Within the four conferences organized by the "European Network for Neglected Vectors and Vector-Borne Diseases (EurNegVec)," not a single presentation on mites can be found (https://www.eurnegvec.org/). Similarly, the main reviews on the impact of climate change on vector borne diseases published in 2019 do not mention trombiculid mites [14,15], although climate is a key factor of their distribution [5].…”

Disease Ecology of Rickettsial Species: A Data Science Approach

“…Rickettsial diseases include anaplasmosis, ehrlichiosis, spotted fever, typhus fever, scrub typhus and the understudied trematode-borne neorickettsiasis [3]. Several comprehensive reviews have been published in the recent years on rickettsial diseases [4,5], spotted fever [6], tick-borne rickettsioses [7,8] and scrub typhus [9][10][11][12].…”