A seasonal chigger-borne disease surveillance program was established at Geoje from March to November of 2018 in Gyeongnam Province, Republic of Korea. Two species of 84 wild rodents were collected by using Sherman traps for Geoje in 2018. Chigger mites were collected from the live-captured rodents to figure out seasonal prevalence of mite species and to identify chigger-borne pathogens. A total of 4,611 chiggers were collected from two species of the rodents in 2018. The chigger mites were collected mainly from A. agrarius (Chigger Index; CI 68.3). A vector of scrub typhus, Leptotrombidium orientale was predominant species (CI 14.9, Species Ratio; SR 27.2%) followed by L. pallidium (CI 14.2, SR 25.9%) and L. scutellare (CI 13.2, SR 24.2%) in 2018. In the results of the isolation of Orientia tsutsugamushi from the chigger mites, no pathogens detected from the DNA of the 171 pools of the mites in 2018, using a Polymerase Chain Reaction method.

The distribution of chiggers and Orientia tsutsugamushi, were investigated at five sites in Seogwipo, Jeju Province in 2017. To accomplish this, 100 Sherman traps were installed to collect the larva of mites from 28 wild rodents and analyze them in April, October and November. A total of 2,560 chiggers were collected, among which 1,283 underwent inspection for Orientia tsutsugamushi and 1,277 were found to contain the organism. Leptotrombidium zetum accounted for 517 of the 1,277 species, or 40.5% of the total, making it the dominant species. Other major species included Leptotrombidium scutellare, which were found in 432 (33.8%) samples, Neotrombicula nagayoi, which was found in 141 (5.5%) samples, and Leptotrombidium orientale, which was found in 72 (2.8%) samples. The majority of the species collected in April were Leptotrombidium zetum, but in fall, the majority were Leptotrombidium scutellare and Leptotrombidium zetum. Additionally, Leptotrombidium scutellare was mainly collected

Seasonal abundance and distribution of mosquitoes and chigger mites are important for human populations. However, factors affecting the abundance and distribution of vector species are not well - understood. Furthermore, global climate change makes it difficult to predict the population dynamics of vector species. Therefore, it is important to monitor this vector species over a long period of time. In 2015, mosquitoes were collected from three potential mosquito habitats using black light trap and BG-Sentinel trap in Gyeongi-do in April through October. Also, we collected chigger mite using sherman live traps in Cheorwon, Gangwon-do in April, September and October. From the survey, we collected a total of 11,649 mosquitoes in three potential mosquito habitats, and a total of 3,656 chigger mites from 54 wild rodents. In the samples, dominant mosquito species were Aedes vexans nipponii (Species Ratio 70%), Culex pipens pallens (SR 9.8%) in Gyeonggi area and dominant chigger mite species was Leptotrombidium pallidium (SR 59%) in Cheorwon. The results of this study can serve as provide fundamental basis to develop effective management strategy under the global climate change environment.

Trombiculid mites are known to be the vector of scrub typhus by transmitting rickettsial pathogen, Orientia tsutsugamushi, to human. In this study, we tried to establish a monitoring system for trombiculid mites using chigger mite collecting traps instead of the conventional rodent-capture method. For selection of collecting points, 10 environmental points were chosen from three regions (Taean, Jinan and Chungju) and two field collections were performed in spring (March-May) and autumn (October-November) seasons from 2013 to 2014. Among 10 environmental points, waterway (37.9%), grass field (28.0%), rice field and field near mountain side (11.4%) and reservoir/wet field (7.3%) showed high collecting rates and they should be included for the representative collecting points for surveillance using chigger mite collecting trap. In order to test the possibility that the dried chigger mites from collecting trap can be used for detection of O. tsutsugamushi, we pooled 30, 10 and 5 chigger mites separately and performed the nested PCR. The infection of O. tsutsugamushi was successfully detected from 5 chigger mites pooling sample. This study shows that chigger mite collecting trap could be an alternative method for monitoring system of scrub typhus vectors.

Leptotrombidium pallidum is the major vector mites for Orientia tsutsugamushi, the causative agent of scrub typhus. To understand the molecular mechanism of L. pallidum, we sequenced the whole genome using Illumina sequencing technology. Totally four genomic libraries with different insert sizes ranging from 280 bp to 8 kb were used to generate 45.1 Gb of genome in the combination of paired-end and mate-pairs sequencing reads. Quality filtering and correction of paired-end reads for very small and/or bad-quality sequences yielded 26.9 Gb of high-quality sequences, which are used to estimate the genome size as 175 Mbusing kmer methods and assembled into a 193.7 Mb genomic sequence scaffolds with N50 length of 92,945 bp. Furthermore, 94% of CEGMA completeness score were obtained from genome scaffold assembly. To facilitate gene annotation, we used a combination of de novo and homology based tools to predict gene models in the chigger mite genome. A combination of evidence-based and de novo approaches predicted 15,842 high-confidence protein-coding genes with an average transcript length of 1,511 bp and 2.4 exons per gene which corresponds to about 12.4% total gene length. Bacterial endosymbiosis are very common in mite species and can range from mutualistic to pathogenic associations. Henceforth, the endosymbionts in L. pallidum were predicted using the NCBI microbial draft genomes and mitochondrial genome. Besides, this L. pallidum draft genome can be used as a significant reference for comparative genomic studies across mite species.

The chigger mite, Leptotrombidium pallidum, is widely distributed throughout South Korea and is a major vector for Orientia tsutsugamushi, the causative agent of scrub typhus. In this study, the genome size of the chigger mite was estimated to determine the necessary coverage level prior to whole genome sequencing. Cloning of EF1α and RpS3 as putative single copy reference genes were conducted and their partial sequences were determined. Using the serially diluted reference genes with known amount as standard templates, the weight of a single copy of the genome was predicted by a method based on quantitative real time PCR. The average genome length estimated from the weight using two methods was 191 ± 7 Mb. When the genome size of other arthropods (Drosophila melanogster, Apis mellifera and Tetranychus urticae), with their genome analysis completed, were estimated using the same method and compared with actual values, the estimation accuracy was 79.8-98.9%, suggesting our current estimation of L. pallidum genome size is reliable. The estimated L. pallidum genome size is in a similar range to other Acariform mites, such as the dust mite and scabie mite, but appoximately 10-fold smaller compared to the deer tick, which belongs to Parasitiform. Our finding provides key information for further genome sequencing and understanding of mite genome evolution.

A surveillance of chigger mites was conducted to clarify the incidence of scrub typhus vectors by new chigger mite collecting traps instead of trapping wild rodents for surveillance from each 4 collection points of 5 regions from September to November 2012 in Korea. During the surveillance period, 2,829 chiggers were collected and 10 species of 4 genera were identified. The first appearance of chigger mite and patients was 39 week (9.19~9.25) and the density of chigger mites had the peak in 43 week (10.17~10.23) and the density of patients had the peak in 44 week (10.24~10.30), respectively. In Goryeong-gun, a total of 1,797 mites representing 4 genera and 6 species were collected and the predominant species were Leptotrombidium scutellare (86.0%), L. pallidum (6.5%) and L. palpale (4.7%) whereas, In Gurye-gun, a total of 89 mites representing 2 genera and 3 species were collected and the predominant species were L. scutellare (46.5%) and L. palpale (36.2%) in similar to the natural environment. The high collecting rates were recorded at rice field (71.8%) and waterway (11.6%). This result shows that the surveillance of scrub typhus vectors by new chigger mite collecting trap is useful as an eco-friendly method.