MOLECULAR ECOLOGY OF CHIGGER MITES (ACARI: TROMBICULIDAE) AND ASSOCIATED BACTERIA IN THAILAND


Chaisiri, K (2016) MOLECULAR ECOLOGY OF CHIGGER MITES (ACARI: TROMBICULIDAE) AND ASSOCIATED BACTERIA IN THAILAND. PhD thesis, University of Liverpool.

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Abstract

Chiggers are the tiny six-legged larval stage of mites in the family Trombiculidae. These mites, particularly the genus Leptotrombidium, act as important vectors of Orientia tsutsugamushi, the causative agent of scrub typhus disease in the Asia-Pacific region (including Thailand). Although the medical impact of these mites has been recognized in the country due to the increasing incidence of the disease in humans, knowledge of the ecology and epidemiological role of these mites is still very limited to date. A systematic review of mite-associated bacteria was conducted from 193 publications (1964 - January 2015) providing a reference database of bacteria found in mites of agricultural, veterinary and medical importance. Approximately 150 bacterial species were reported from 143 mite species with Cardinium, Wolbachia and Orientia as the dominant genera. Nationwide field sampling of small mammals from 13 locations in Thailand revealed a high diversity of chigger mites. From approximately 16,000 mites isolated from 18 host species examined (1,574 individual animals), 38 chigger species were found including three species new to science (i.e., Trombiculindus kosapani n. sp., Helenicula naresuani n. sp. and Walchia chavali n. sp.) and 10 new records for the first time in the country. Brief taxonomic information for the morphological identification of chiggers is provided. A combination of autofluorescent and brightfield microscopy was demonstrated to be a novel approach to study both the morphology and DNA profile of the same individual chigger. Most chigger species showed low host specificity. The diversity of chiggers on hosts was influenced by host intrinsic (i.e., host phylogeny and maturity) and extrinsic factors (i.e., habitat and geographical location). Chigger species richness and host-chigger network connectance were found to be interrelated variables explaining human scrub typhus incidence in Thailand. Chigger-associated bacteria were investigated for the first time using an Illumina MiSeq 16S rRNA amplicon sequencing approach. DNA of O. tsutsugamushi was detected in the chigger population as expected. In addition to O. tsutsugamushi, Borrelia and Mycobacterium were identified aspotential pathogens of human and animals. Potential symbiotic bacteria of arthropods; e.g., Candidatus Cardinium, Pseudonocardia, Rickettsiella and Wolbachia were also discovered for the first time in chiggers. An important technical limitation was that chigger DNA starting quantity (individual specimens versus pooled mites) was found to have a significant impact on the apparent microbiome profile. These outcomes from the studies of chigger taxonomy and the ecology of host-chigger interactions, as well as the composition of the microbiome in chiggers, are of key importance to the chigger research field, providing essential information for disease epidemiology with vector control implications.

Item Type: Thesis (PhD)
Divisions: Faculty of Health and Life Sciences
Depositing User: Symplectic Admin
Date Deposited: 17 Aug 2017 06:55
Last Modified: 19 Jan 2023 07:24
DOI: 10.17638/03004727
Supervisors:
URI: https://livrepository.liverpool.ac.uk/id/eprint/3004727
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