털진드기 유충 (Acari: Trombiculidae)은 쯔쯔가무시증을 전파하는 주요 매개체이다. 털진드기 유충의 발생량 은 가을철에 증가하는 것으로 알려져 있지만, 환경 및 시기에 따라 발생 패턴이 다르게 나타날 수 있어 각 지역에 대한 조사가 필요하다. 이 연구는 충남 예산의 털진드기 발생 양상을 확인하기 위해 2017년부터 2023년까지 36-51 주차 (9-12월)에 걸쳐 현장 조사를 수행하였다. 논, 밭, 수로, 초지에 5m 간격으로 털진드기 트랩을 환경별로 5개씩 설치하여 채집하였다. 그 결과 총 3,257개체로 2017년 1,104마리, 2018년 785마리, 2019년 650마리, 2020년 160마 리, 2021년 139마리, 2022년 233마리, 2023년 186마리 채집되었다. 동정 결과 5속 12종이 확인되었으며 둥근혀털 진드기(Neotrombicula tamiyai)가 1,882개체(57.78%)로 우점도가 가장 높게 나타났다. 이러한 발생 양상에 관한 연구는 매개 질환의 예방 및 관리 전략 수립에 있어 중요한 기초 자료로 활용될 수 있으므로 지속적인 연구와 모니터링이 필요할 것으로 사료된다.

A seasonal chigger-borne disease surveillance program was established at Geoje in March, April, October, and November, 2023. Three species of 45 wild rodents were collected by using Sherman traps, including Apodemus agrarius (Species Ratio; SR 88.9%), Crocidura spp. (SR 8.9%), Micromys minutus (SR 2.2%) in Geoje, 2023. A total of 2,597 chiggers were collected from three species of the rodents in Geoje. The chigger mites were collected from A. agrarius (Chigger Index; CI 64.9) and C. spp. (CI 0.3) for Geoje. In the collection environments, a total number of 734, 659, 172, 520, and 512 chiggers were collected from a reservoir, a ditch, a rice paddy, a field, and a hill, respectively. In the results of the isolation of Orientia tsutsugamushi from the chigger mites, no pathogens were detected from the DNA of the 62 pools of the mites using a Polymerase Chain Reaction method in 2023.

쯔쯔가무시증은 급성 발열성 질환으로 Orientia tsutsugamushi을 보유한 털진드기 유충이 숙주를 흡혈할 때 전파된다고 알려져 있다. 국내에서는 한국전쟁 중에 첫 감염사례가 확인되었고 이후 지속적으로 보고되고 있다. 털진드기 유충이 성장하기 위해선 반드시 동물의 체액을 필요로 하기 때문에, 일시적으로 야생 설치류 등에 기생 하는데 봄철과 가을철에 주로 발생한다. 이처럼 인간에게 피해를 주는 털진드기의 분포와 병원체 유무를 조사하 기 위해, 2022년부터 2023년까지 경상북도 김천시에서 3월, 4월과 10월, 11월 5가지 환경(논, 밭, 수로, 야산, 저수 지)에서 Sherman trap을 이용하여 설치류 채집을 진행하였다. 2022년에는 총 18마리의 설치류가 채집되었고, Apodemus agrarius가 14마리(77.8%)로 가장 많이 채집되었다. 2023년에는 총 25마리의 설치류가 채집되었고, Apodemus agrarius가 19마리(76.0%)로 역시 가장 많이 채집되었다. 채집된 설치류 의 개체수는 2022년 대비 2023 년에 약 39% 증가하였다. 설치류에서 채집된 털진드기의 개체수를 조사한 결과, 2022년에는 총 1,862마리, 2023 년에는 3,243마리가 채집되어 전년 대비 약 74% 증가하였다. Chigger index 값을 살펴보았을 때 2022년에는 Apodemus agrarius에서 122.2, 2023년에는 Apodemus agrarius에서 169.7로 가장 높았다. 설치류에서 채집된 털진 드기의 반수에 대한 병원체 보유 여부를 확인하기 위해 최대 30마리씩 pooling하여 실험한 결과, 모두 음성으로 확인된 2022년과 달리 2023년 10월 3개, 11월 1개의 pool에서 쯔쯔가무시 양성이 확인되었다. 확인된 양성 샘플은 모두 저수지에서 채집된 개체로 해당 지역에 대한 지속적인 감시가 필요할 것으로 생각된다.

Orientia tsutsugamushi 에 의해 발생하는 쯔쯔가무시증은 제3급 법정감염병으로 관리되고 있으며, 설치류에 기생한 털진드기 유충에 의해 주로 매개되는 것으로 알려져 있다. 원인균인 O. tsutsugamushi 는 Gilliam, Karp, Kato와 같은 표준 혈청형 외에도 유행하는 국가에 따라 30종 이상의 다양한 혈청형이 존재하는 것으로 알려져 있어, 각 지역에 대한 지속적인 조사가 필요하다. 우리 연구진은 야생 설치류에 기생하는 털진드기의 O. tsutsugamushi 감염 여부를 확인하기 위해 충남 예산에서 2018년부터 2023년까지 3-4월, 10-11월에 걸쳐 수행하였 다. 총 142마리의 설치류가 포획되었으며, Hanging method로 설치류에서 분리된 털진드기는 총 18,347마리였다. 수거된 털진드기의 샘플 중 1/2은 질병관리청으로 이관하였으며, 나머지 절반을 이용하여 O. tsutsugamushi 감염 여부를 확인한 결과 2018년 5건, 2019년 2건, 2020년 0건, 2021년 9건, 2022년 0건, 2023년 36건이 확인되었으며, 혈청형 분석 결과 Je-cheon 28건, Young-worl 3건, Boryoung 9건, Koto Akita 1건, Gilliam 2건, Karp 9건으로 확인되었 다. 이번 연구를 통해 예산 지역 내 다양한 O. tsutsugamushi 혈청형이 있음을 확인하였으며, 이러한 결과는 향후 지역에 따른 혈청형 차이가 반영된 공중보건학적 전략 수립을 위한 기초자료로 활용될 수 있을 것으로 사료된다.

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

A surveillance of chigger mites was performed to monitor the incidence of scrub typhus vectors at 4 environmental collection points of 6 locations from September to November 2014 in Korea. During the survey period, 420 chigger mites were collected and the dominant species was Leptotrombidium scutellare (42.6%). The first appearance of chigger mite was at 37th week (9.3.-9.10.) and the collected numbers of chigger mites was the highest at 43rd week (10.17.-10.23.). In Goryeong-gun, 299 chigger mites were collected, whereas 5 chigger mites were collected In Yesan-gun. The high environmental collecting rates were recorded at rice field (56%) and waterway (20%). The annually collected numbers (2012-2014) of chigger mites were compared with the average temperatures in August. This result suggests that the average temperature in August might be related with the annual incidence of scrub typhus vectors in Korea. However, the relationship between climate factors and the density of chigger mites needs to be studied by long-term periodical surveillance.

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.

Leptotrombidium pallidum and Leptotrombidium scutellare are the major vector mites for Orientia tsutsugamushi, the causative agent of scrub typhus. Before these organisms can be subjected to whole-genome sequencing, the genome sizes of L. pallidum and L. scutellare were estimated by a method based on quantitative real-time PCR. In addition, k-mer analysis of the genome sequences obtained from Illumina sequencing was conducted to verify the mutual compatibility and reliability of results. The genome sizes estimated by qPCR were 191.3±7 Mb for L. pallidum and 262.1±13 Mb for L. scutellare. The estimated genome sizes based on k-mer analysis were 175.5 Mb for L. pallidum and 286.6 Mb for L. scutellare. The estimates from two independent methods were mutually complementary and in a similar range to those of other Acariform mites. The relatively small genome size would facilitate genome analysis, which could contribute to understanding Arachnida genome evolution and mite vector competence and provide key information for scrub typhus prevention.

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.