Heung-Chul Kim

Background: Culex tritaeniorhynchus, a major vector of Japanese encephalitis virus (JEV), is found across a broad geographical range, including Africa, Asia, Australia and Europe. Understanding the population structure and genetic diversity of pathogen vectors is increasingly seen as important for effective disease control. In China and Japan, two countries in close proximity to the Republic of Korea (ROK), Cx. tritaeniorhynchus has been categorized into two clades based on the DNA barcoding region of mitochondrial cytochrome c oxidase subunit I (COI), suggesting the presence of cryptic species. No comprehensive analysis of the genetic diversity in Cx. tritaeniorhynchus has been conducted in the ROK. To address this gap, we investigated the population structure of Cx. tritaeniorhynchus in the ROK. Methods: In Daegu, mosquito collections were conducted over a 2-year period from 2022 to 2023. For all other regions, Cx. tritaeniorhynchus specimens collected in 2023 were used. The COI barcoding region was analyzed to determine the genetic structure of the populations, supplemented with data from the 28S ribosomal DNA region. Each population was also examined for the eventual presence of Wolbachia infection. Finally, a back trajectory analysis was conducted to assess the possibility of international introduction of Cx. tritaeniorhynchus into the ROK. Results: The analysis of the COI region revealed the presence of two distinct clades within Cx. tritaeniorhynchus; these clades were the same as Cx. tritaeniorhynchus continental type (Ct-C) and C. tritaeniorhynchus Japanese type (Ct-J) previously reported. In contrast, the nuclear 28S region showed no significant genetic differentiation between these clades. Wolbachia infection was confirmed in some populations, but there was no evidence of an association with Wolbachia in Ct-C and Ct-J. It was also confirmed that the ROK is currently dominated by the Ct-J clade, with a possible introduction of Ct-C via air currents. Conclusions: Determining the presence of cryptic species is important for preventing vector-borne diseases. The results of this study confirm the existence of two clades of Cx. tritaeniorhynchus in the ROK, with Ct-J being the dominant clade. Our findings enhance current understanding of the genetic diversity within Cx. tritaeniorhynchus and provide valuable insights for the prevention of JEV outbreaks and the effective management of Cx. tritaeniorhynchus populations in East Asia. Graphical

Members of the Anopheles Hyrcanus Group, Culex pipiens complex, and Culex tritaeniorhynchus are prevalent vector species in the Republic of Korea (ROK), transmitting Plasmodium vivax and various arboviruses. Extensive use of insecticides to control these mosquitoes has led to insecticide resistance. In this study, we monitored 3 target site mutations associated with insecticide resistance (kdr for pyrethroid resistance, ace1 for organophosphate resistance, and rdl for phenylpyrazole resistance) in these mosquito groups over four consecutive years to understand the seasonal dynamics of resistance in different areas with distinct ecological characteristics. In the Anopheles Hyrcanus Group, the frequencies of kdr and ace1 mutations exhibited seasonal fluctuations in an urban-rural complex area (Humphreys US Army Garrison) (hereafter Humphreys), suggesting an overwintering fitness cost, whereas the rdl mutation frequencies remained constant at nearly saturated levels. These patterns were less clear in rural areas (the demilitarized zone separating Korea), indicating area-specific profiles related to different insecticide usage patterns. The kdr and rdl mutation frequencies associated with the Cx. pipiens complex were relatively constant, but varied by the collection area, with higher rdl frequency in Humphreys and higher kdr frequency in Yongsan, a metropolitan area, suggesting different selection pressures. Overall resistance mutation frequencies were highest in Cx. tritaeniorhynchus, with ace1 and rdl mutations being seasonally saturated, while the kdr mutation frequency varied over time. Our findings demonstrate species- or group-specific seasonal and regional dynamic patterns of insecticide resistance, presenting the need for targeted control strategies and further improving the management of mosquito-borne diseases in the ROK.

The global resurgence of bed bug infestations, exacerbated by increasing international travel, trade, and insecticide resistance, has significantly impacted Korea. This study identified the bed bug species and performed pyrethroid resistance genotyping of recently resurgent bed bugs in Korea. Thirty-one regional bed bug samples were collected from 5 administrative regions: Gyeonggi-do (n=14), Seoul (n=13), Busan (n=2), Jeonllanam-do (n=1), and Chungcheongbuk-do (n=1). The samples underwent morphological and molecular identification. Twenty-four regional samples (77.4%) were identified as the tropical bed bug, Cimex hemipterus, and the remaining 7 regional samples (22.6%) were identified as the common bed bug, Cimex lectularius. The C. hemipterus regional samples carried at least three mutations associated with knockdown resistance (kdr), including 2 super-kdr mutations. The 7 C. lectularius regional samples possessed at least one of the 3 kdr-related mutations associated with pyrethroid resistance. This study confirms that the prevalent bed bug species recently in Korea is C. hemipterus, replacing the previously endemic C. lectularius. Additionally, the rise in bed bug populations with pyrethroid resistance underscores the necessity of introducing alternative insecticides.

When free-ranging birds are accidentally killed or die, there may be greater potential for their associated ticks to detach, seek alternate hosts, and become established. We examined 711 carcasses of 95 avian species for ticks at a stopover island of migratory birds in the Republic of Korea where only Ixodes nipponensis and I. persulcatus were previously reported from local mammals and vegetation. A total of 16 ticks, I. turdus and Haemaphysalis flava, were collected from 8 fresh carcasses belonging to 5 avian species. Despite their known abundance on migratory birds and mainland Korea, these species had not colonized the isolated insular ecosystem possibly due to the low abundance and diversity of local hosts. The results imply that increasing human impact, such as the anthropogenic mortality of migratory birds and the introduction of non-native mammalian hosts, will increase the potential invasion and colonization risk of ticks. This finding also suggests that tick surveillance consisting of fresh carcasses of dead migratory birds may provide additional information, often ignored in surveillance of ticks on live birds, for the potential introduction of non-native ticks and associated pathogens affecting animal and human health.

There are eight Anopheles spp. present in the Republic of Korea (ROK), including five members of the Anopheles Hyrcanus Group that cannot be identified using current morphological methods. The vector competence of only Anopheles sinensis s.s., An. lesteri, and An. kleini have been investigated. As the geographical distribution of Anopheles spp. varies in the ROK, determining the relative vector competence of the Anopheles spp. provides a basis for delineating malaria risks to Korean populations and U.S. military/civilian populations deployed to the ROK. Anopheles belenrae and An. pullus, collected from a malaria high-risk area in the ROK, were evaluated for vector competence of P. vivax. A total of 1,000 each of An. dirus (Thai strain), and Korean strains of An. pullus and An. belenrae were fed on P. vivax infected blood collected from Thai patients via artificial membrane feeding. The overall oocyst infection rates for An. dirus, An. pullus, and An. belenrae dissected on days 8–9 postfeed were 64.1, 12.0, and 11.6%, respectively. The overall sporozoite infection rates for those species dissected on days 14–15 postfeed were 84.5, 3.4, and 5.1% respectively. The salivary gland sporozoite indices for positive females with +4 (>1,000 sporozoites) were observed in An. dirus (72.8%), but not observed for either An. pullus or An. belenrae. Most sporozoite-positive An. pullus (83.3%) and An. belenrae (71.4%) females were observed with only +1 (1–10 sporozoites) salivary glands. These data indicate that both An. belenrae and An. pullus are very poor vectors of P. vivax.

Tick-borne pathogens are contributing factors for the increased incidence of vector-borne diseases throughout the world, including Lyme borreliosis, one of the most prevalent spirochetes belonging to the Borrelia burgdorferi sensu lato group. The present study focused on the detection of Borrelia species from hard ticks collected at U.S. Army Garrison Humphreys, Republic of Korea (ROK), using molecular and genotypic analyses. Tick-borne disease surveillance was conducted from January to December, 2018–2019. A total of 24,281 ticks (2 genera and 5 species) were collected from road-killed Korean Water deer (KWD) and by tick drag. Haemaphysalis longicornis (92.0%) was the most commonly collected species, followed by Haemaphysalis flava (4.9%), Ixodes nipponensis (3.1%), Haemaphysalis phasiana (0.07%), and Haemaphysalis japonica (ospA gene sequences of Borrelia afzelii were detected in 12/529 pools of I. nipponensis. Three and one pools were positive for B. afzelii and Borrelia miyamotoi, respectively, using the 16s rRNA gene. None of the pools of Haemaphysalis ticks collected from KWD or by tick drag were positive for Borrelia species. I. nipponensis was collected throughout the year from KWD and from February to November by tick drag, suggesting that they were active throughout the year, and expanding the risk period for acquiring Lyme borreliosis and Borrelia relapsing fever in the ROK. This study assessed disease risk factors associated with the prevalence of Lyme disease in ticks collected from KWD and by tick drag using molecular analysis. These results provide an understanding and awareness into the prevalence and molecular characteristics of Borrelia species in the ROK.

Paramyxoviruses, negative-sense single-stranded RNA viruses, pose a critical threat to human public health. Currently, 78 species, 17 genera, and 4 subfamilies of paramyxoviruses are harbored by multiple natural reservoirs, including rodents, bats, birds, reptiles, and fish. Henipaviruses are critical zoonotic pathogens that cause severe acute respiratory distress and neurological diseases in humans. Using reverse transcription-polymerase chain reaction, 115 Crocidura species individuals were examined for the prevalence of paramyxovirus infections. Paramyxovirus RNA was observed in 26 (22.6%) shrews collected at five trapping sites, Republic of Korea. Herein, we report two genetically distinct novel paramyxoviruses (genus: Henipavirus): Gamak virus (GAKV) and Daeryong virus (DARV) isolated from C. lasiura and C. shantungensis, respectively. Two GAKVs and one DARV were nearly completely sequenced using next-generation sequencing. GAKV and DARV contain six genes (3′-N-P-M-F-G-L-5′) with genome sizes of 18,460 nucleotides and 19,471 nucleotides, respectively. The phylogenetic inference demonstrated that GAKV and DARV form independent genetic lineages of Henipavirus in Crocidura species. GAKV-infected human lung epithelial cells elicited the induction of type I/III interferons, interferon-stimulated genes, and proinflammatory cytokines. In conclusion, this study contributes further understandings of the molecular prevalence, genetic characteristics and diversity, and zoonotic potential of novel paramyxoviruses in shrews.

The horse industry has grown rapidly as a leisure industry in the Republic of Korea (ROK) in parallel with an increased demand for equestrian activities. As a result, there has been an increase in horse breeding and equestrian population and potential exposure to ticks and their associated pathogens. To provide a better understanding of the potential disease risks of veterinary and medical importance, a study was conducted to determine the geographical distribution and diversity of ticks collected from horses and vegetation associated with horse racetracks/ranches throughout the ROK. This included a survey of five associated common pathogens, Anaplasma phagocytophilum, Ehrlichia chaffeensis, Borrelia spp., Babesia caballi, and Theileria equi. A total 9220 ticks were collected from horses and associated pastures. Ticks were identified to species, stage of development, and sex. Two species of ticks, Haemaphysalis longicornis (99.9%) and Ixodes nipponensis (0.1%) were identified. Two of the target pathogens, A. phagocytophilum and Borrelia spp., were detected in 5/1409 tick pools (0.35%) and 4/1409 pools (0.28%) of H. longicornis, respectively, both of which are zoonotic pathogens of medical importance. The results of 16S rRNA phylogenetic analysis of A. phagocytophilum showed a close relationship to strains distributed in China, USA, Germany, Italy, Turkey, and Poland. Borrelia spp. showed a close relationship, based on 16S rRNA gene, to the strains reported from the USA (B. burgdorferi and B. americana) and Japan (B. tanukii and B. garinii). These results provide information about the potential risks of veterinary and medical importance and the development of mitigation strategies for disease prevention.