A surveillance of chigger mites was performed to monitor the incidence of scrub typhus vectors at four environmental collection points in two locations (Sunchang and Haenam) in the Honam region of Korea from August to December 2023. During the surveillance period, 4,174 chigger mites were collected and the predominant species were Leptotrombidium scutellare (94.3%). The density of chigger mites had the peaked at 44 week (10.26~11.1), while the density of patients peaked at 45 week (11.2~11.8) respectively. A positive correlation (r=0.69) observed between scrub typhus patients and vectors. This result suggests that this vector surveillance method will be useful for alarm system of tsutsugamushi disease. However, the relationship between scrub typhus cases and chigger mite density will be studied through long-term periodic surveillance.

Scrub typhus is a zoonotic bacterial disease caused by Orientia tsutsugamushi (Rickettsiales: Rickettsiaceae) and trombiculid mite larvae, also known as chigger, are known vector. Until recently, O. tsutsugamushi is the only species of the genus. However, two new species Candidatus O. chuto and Candidatus O. chiloensis that causes scrub typhus were reported recently in the Middle East, southern Chile, and Africa. In addition, Orientia spp. bacteria detected from field collected free-living Eutrombicula chigger mites in the United States. Despite these trends, research on new species is insufficient in the Republic of Korea. Therefore, we focused on finding the presence of novel species or strains from chiggers. Specimen chiggers harvested from rodents collected in 16 regions in October 2022 collaborating with the Regional Center for Vector Surveillance against Climate Change were selected. A total of 1,249 specimen belonging 4 genera and 14 species were identified by using a fluorescence microscope and 266 pools were produced by pooling up to 10 individuals per species chiggers. To detect Orientia spp., we screened by using a real-time PCR targeting the 16S rRNA gene. Overall minimum infection rate was 0.56% (7 pools/1,249 tested specimen). After screening, conventional nested-PCR for targeting 47-kDa htrA gene was conducted to obtain sequences, and four of the positive pools were amplified. Through phylogenetic analysis, three pools were clustered with O. tsutsugamushi Gilliam and UT221 strain, and the other pool was formed a clade that was distinct from O. tsutsugamushi. These results suggest that novel species of Orientia may exist in the Republic of Korea.

A surveillance of chigger mites was performed to monitor the incidence of scrub typhus vectors at 5 environmental collection points of 3 locations (Muju, Namwon and Gimge) from April 2016 in Jeollabuk-Do, Korea. During the surveillance period, 3,292 chigger mites were collected and the predominant species were Leptotrombidium pallidum (42.9%). The high environmental collecting rates were recorded at reservoir bank (31.6%) and grassfield (29.7%). We detected Orientia tsutsugamushi from the samples collected in Jeollabuk-Do using nested polymerase chain reaction. The specific DNA of O. tsutsugamushi were detected in 11 pools among total 119 pools and its minimum positive rate was 0.35%. In order to clarify the relationship between chigger mites and scrub typhus cases, further survey will be needed.

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.

To clarify the geographical distribution of scrub typhus vectors in Korea, the first survey of chigger mites was conducted from 2005 to 2007 by collecting wild small mammals twice a year (spring and autumn) at 24 sites nationwide. The two predominant mite species were Leptotrombidium pallidum (52.6%) and L. scutellare (27.1%). However, the proportions of L. scutellare in southern areas, including endemic provinces such as Chungcheongnam-Do, Jeollabuk-Do, Jeollanam-Do, and Gyeongsangnam-Do, were relatively higher than in central Korean regions where L. pallidum was predominant. In autumn, the ratio of L. scutellare increased to 42% while the ratio of L. pallidum decreased. The geographical distribution map of the L. scutellare chigger index was identical to the incidence pattern of scrub typhus, whereas those of overall mites and L. pallidum showed no relationship with case incidence patterns. Distribution mapping analysis shows an identical geographical distribution of L. scutellare and epidemic incidence of scrub typhus in South Korea. The second periodical survey was performed from 2011 to 2013. The result suggests that the distribution of L. scutellare has not been changed remarkably in comparison to the first survey.

본 연구는 지속적으로 증가하고 있는 국내 쯔쯔가무시증 발병 분석에 적합한 회귀모형을 선정하고, 이에 영향을 미치는 공간 생태학적 요인을 분석하는 것을 목적으로 한다. 이를 통하여 향후 쯔쯔가무시증 방역사업 및 대상지 선정에 이론적 배경을 제시하고자 한다. 이 연구는 일반선형회귀모형, 공간오차모형, 아이겐벡터 공간필터링 모형 세 모형을 비교하였고, 이 중 아이겐벡터 공간필터링 모형이 가장 뛰어난 것으로 나타났다. 쯔쯔가무시증 발병 요인 분석 결과 가을 기온, 가을 강수량, 가을 습도,가을 NDVI, 논경작인구비율이 유의미한 변수로 나타났다. 한편 일반선형회귀모형의 상대적 중요도 분석 결과 논 경작 인구비율이결정계수 중 가장 큰 비중을 차지하여, 자연환경변수보다 인문환경이 쯔쯔가무시증 발병률에 더 큰 영향을 미치는 것으로 평가되었다.

Leptotrombidium pallidum is the major vector mite for Orientia tsutsugamushi, the causative agent of scrub typhus, in Asian countries, including Korea. The genome size of L. pallidum was previously estimated to be 191 ± 7 Mb (Kim et al., 2014). Genomic DNA (gDNA) was extracted from a single female from a 9-generation inbred L. pallidum colony and used for whole genome amplification (WGA). The resulting amplified gDNA was used for the construction of paired-end and mate-pair libraries and sequenced using Illumina platforms (HiSeq2000 and MiSeq). An unamplified gDNA sample extracted from 20 female mites was also used for sequencing in parallel. More than 45Gb sequence reads from both paired-end and mate-pair libraries of the WGA gDNA were trimmed and then de novo assembled using the CLC Asembly Cell v.4.0 for contig assembly and SSPACE for scaffolding. The assembly generated approximately 6,545 scaffolds with N50 value of 92,945 and total size of ~193Mb, which was in a good agreement with our previous estimation. Repeat analysis showed that about 30% of genome (~58Mb) was masked as repeats, most of which were unclassified novel elements. For gene predictions, generated were the PASA models based on genomic alignments of RNA-seq reads from 4 different chigger mite samples (i.e. male, female, larva, and protonymph) and the GeneWise models based on genomic alignments of protein sequences from 4 closely related species with chigger mite. Independently, ab initio gene predictions were performed with AUGUSTUS and FgeneSH with custom trained matrices optimized for L. pallidum and GENEID with pre-trained matrix for Acyrthopsiphon pisum. By combining all together, 15,842 genes were predicted finally. Manual curation is in progress for various groups of genes, including chemosensory receptor genes, immune-related genes, acaricide target genes, etc.

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.

Diagnosis of scrub typhus is difficult because of its unspecific symptoms. The authors studied the correlation between serum adenosine deaminase (ADA) levels and scrub typhus, with the aim of aiding diagnose of the disease. The medical records of 64 children, who visited Wonkwang University Hospital from 2005 to 2015 for fever of undetermined origin, were analyzed retrospectively. Serum ADA levels were measured and indirect immunofluorescence antibody (IFA) assays were conducted. The patients were divided into two groups based on diagnosis of scrub typhus. Of 64 febrile patients, 39 were confirmed to have scrub typhus. Mean serum ADA level in the scrub typhus group (58.54±2.82 IU/L) was significantly higher than that in the non-scrub typhus group (29.30±4.03 IU/L, p<0.05). In the 25 non-scrub typhus patients, mean serum ADA level was significantly higher (50.24±8.52 IU/L) in patients with Mycoplasma infection than that in those with other diseases (19.46±1.39 IU/L, p<0.05). The study shows that in patients with only fever who are difficult to diagnose, serum ADA levels substantially aid the diagnosis of scrub typhus. Ed:- please consider carefully