Research Article: The Diversity and Geographical Structure of Orientia tsutsugamushi Strains from Scrub Typhus Patients in Laos

Date Published: August 28, 2015

Publisher: Public Library of Science

Author(s): Rattanaphone Phetsouvanh, Piengchan Sonthayanon, Sasithon Pukrittayakamee, Daniel H. Paris, Paul N. Newton, Edward J. Feil, Nicholas P. J. Day, Ruifu Yang.

Abstract: Orientia tsutsugamushi is the causative agent of scrub typhus, a disease transmitted by Leptotrombidium mites which is responsible for a severe and under-reported public health burden throughout Southeast Asia. Here we use multilocus sequence typing (MLST) to characterize 74 clinical isolates from three geographic locations in the Lao PDR (Laos), and compare them with isolates described from Udon Thani, northeast Thailand. The data confirm high levels of diversity and recombination within the natural O. tsutsugamushi population, and a rate of mixed infection of ~8%. We compared the relationships and geographical structuring of the strains and populations using allele based approaches (eBURST), phylogenetic approaches, and by calculating F-statistics (FST). These analyses all point towards low levels of population differentiation between isolates from Vientiane and Udon Thani, cities which straddle the Mekong River which defines the Lao/Thai border, but with a very distinct population in Salavan, southern Laos. These data highlight how land use, as well as the movement of hosts and vectors, may impact on the epidemiology of zoonotic infections.

Partial Text: Scrub typhus, caused by the Gram negative obligate intracellular coccobacillus Orientia tsutsugamushi, is an important cause of acute febrile illness in Asia responsible for up to 23% of cases of undifferentiated fever [1]. The infection represents a major disease burden throughout a region ranging from northern Japan to Pakistan, to Russia in the north and northern Australia in the south. Over 55% of the world’s population lives in this densely populated endemic area [2]. It can affect patients of all ages, with at least one billion people living in rural areas at risk, and perhaps approximately a million patients needing medical attention every year [3]. Scrub typhus is transmitted to humans through the bite of infected larval trombiculid mites [4]. The clinical manifestations range from fever, headache, muscle pain, cough, and gastrointestinal symptoms, to coma, multi-organ failure and death [5].

This study represents the first investigation into the diversity and phylogeography of O. tsutsugamushi in Laos. As the isolates were characterized using the same MLST scheme, it was possible to combine these data with those from a previous study focusing on strains from Udon Thani in Northeast Thailand. Our results reveal a highly diverse and recombining population in Laos, as evidenced by the high diversity index (0.98) with high number of STs per isolate (50 STs in 74 isolates, 0.68 STs per isolate). This is consistent with the previous Thai study (0.95 STs per isolate) [9], although the diversity in the current Lao sample set might be expected to be slightly higher, as it represents three distinct geographic sources. The ecological implications of the very high rate of recombination are unclear, but this may reflect co-colonisation of either the mites or the rodents. It is possible that high rates of recombination might also reflect a mechanism for diversification and host adaptation in O. tsutsugamushi [25]. The O. tsutsugamushi genome displays a massive proliferation of mobile elements and repeat sequences [26] which are thought to facilitate horizontal gene transfer.



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