Molecular survey of tick microbiome and tick-borne pathogens in ixodid ticks and rodents collected in Sarawak, Malaysian Borneo [an abstract of entire text]

概要

Malaysia is geographically located in the Southeast Asian region and is divided into Peninsular Malaysia and Malaysian Borneo. Malaysian Borneo consists of Sarawak and Sabah states and is located in Borneo Island with two other countries: Brunei and Indonesia. Sarawak is the largest state in Malaysia and has tropical geography and an equatorial climate. These relatively high daily average temperature and all-year-round humidity climatic conditions are ideal for tick survival. Furthermore, Sarawak has undergone massive forest degradation and fragmentation, mainly because of logging activities and oil palm plantations, and land conversion has been significantly related to increasing emerging or re-emerging zoonotic diseases, including vector-borne diseases. 　

Ticks are obligate hematophagous arthropods, feeding on a wide range of animal hosts, including humans. It is established that ticks have high public health importance by harboring various pathogenic agents and play a crucial role as a vector in disease transmission to humans and animals. Tick-borne diseases (TBDs), which afflict humans and animals, are caused by infectious agents transmitted by tick bites. These infectious agents transmitted by ticks are tick-borne pathogens (TBPs), which include bacteria, viruses, and protozoa. For example, ticks are known to harbor medically-important bacterial species from a wide range of genera, including Anaplasma, Ehrlichia, Bartonella, Borrelia, Coxiella, Francisella, and Rickettsia. In addition, Babesia spp. are tick-borne protozoan parasites of the phylum Apicomplexa that infect erythrocytes of mammals and birds. Despite its significant public health and veterinary implications, the tick-borne protozoal diseases caused by Babesia spp. have remained understudied in Malaysia. Currently, reported studies in Malaysia have been limited to bovine and canine babesiosis, with the detection of B. bigemina and B. bovis in cattle, and B. gibsoni and B. vogeli in dogs, as well as from the ticks infesting these animals. 　

In Malaysia, TBDs are regarded to be a potential emerging threat to public health. Thus far, tick bite cases and TBDs were rarely being reported in Malaysia, which could be expected due to a general lack of awareness of TBDs. Based on a survey conducted in 2013 on some farms in Peninsular Malaysia, it was revealed that a large number of farmworkers, including administrative workers, had experienced tick bites. With that remark, the risk of exposure to tick bites may generally be underestimated in Malaysia. 　

Likewise, studies eliciting tick microbiome are limited in Malaysia and not reported from Sarawak, Malaysia Borneo. The tick microbiome consists of largely non-pathogenic microorganisms essential for tick survival, development, and reproduction. Ticks acquire symbiotic microorganisms via the environment, blood meal hosts, and parents by transtandial and transovarial routes. The presence of a high abundance of non-pathogenic organisms in ticks suggests a constant interaction between the tick, pathogen, and endosymbionts. Therefore, understanding the microbial structure in ticks may be crucial to unravel the dynamics of pathogen colonization and transmission. 　

Taken together, the status of TBPs in Malaysia warrants further investigation, especially in Sarawak state that has undergone massive landscape alteration in past decades due to the logging and oil palm plantations. In addition, precise identification of tick species is crucial for epidemiological investigation, prevention, and control of TBDs in general. Obtaining baseline information and insights on tick microbiome and TBPs from Sarawak, Malaysian Borneo, is pivotal for the direction and design of upcoming research from this region, for example, by pinpointing the TBPs that required immediate attention. Therefore, this study aimed to conduct a molecular survey on different tick and rodent species collected from primary forests and an oil palm plantation in Sarawak, Malaysian Borneo. 　

The present thesis consists of five chapters. In chapter Ⅰ, tick species collected from flagging and rodent hosts were identified with morphological and molecular methods. In chapter Ⅱ, rodents and Ixodes ticks were screened for Borrelia spp., followed by species characterization using multilocus sequence analysis. In chapter Ⅲ, high-throughput screening using Next-generation sequencing (NGS) for tick microbiome and tick-borne bacterial pathogens was conducted, followed by the multi-species comparative analysis of tick microbiome. In chapter Ⅳ, a molecular survey of tick-borne bacterial pathogens was conducted in rodents. Finally, in chapter Ⅴ, the tick-borne protozoal screening on Babesia was carried out on rodent and tick samples. 　

Overall, this thesis contributed to the imperative information and insights on tick species, tick microbiome, and tick-borne pathogens in Sarawak, Malaysian Borneo. This data is fundamental and valuable for the upcoming research planning from this region.

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