We investigated the toxicity of (-)-asarinin, α-asarone, methyleugenol, pellitorine, and pentadecane identified in Asarum heterotropoides root to third instar larvae from insecticide-susceptible Culex pipiens pallens (KS-CP strain), Aedes aegypti, and Ochlerotatus togoi as well as field-collected C. p. pallens (DJ-CP colony), identified by polymerase chain reaction. Results were compared with those of two conventional mosquito larvicides: fenthion and temephos. Pellitorine (LC50, 2.08, 2.33, and 2.38 ppm) was 5.5, 10.8, and 25.6 times, 4.5, 11.6, and 24.7 times, and 6.9, 11.1, and 24.6 times more toxic than (-)-asarinin, α-asarone, and methyleugenol against susceptible C. p. pallens, A. aegypti, and O. togoi larvae, respectively. Pentadecane was least toxic. Overall, all the compounds were less toxic than either fenthion or temephos. However, these compounds did not differ in toxicity against larvae from the two Culex strains, even though the DJ-CP larvae exhibited high levels of resistance to fenthion (resistance ratio (RR), 1179), chlorpyrifos (RR, 1174), fenitrothion (RR, 428), deltamethrin (RR, 316), chlorfenapyr (RR, 225), and α-cypermethrin (RR, 94). This finding indicates that the isolated compounds and the pyrethroid, organophosphorus, and pyrrole insecticides do not share a common mode of action or elicit cross-resistance. A. heterotropoides root-derived materials, particularly (-)-asarinin and pellitorine, merit further study as potential mosquito larvicides for the control of insecticide-resistant mosquito populations in light of global efforts to reduce the level of highly toxic synthetic insecticides in the aquatic environment.
Japanese encephalitis virus (JEV) causes significant viral encephalitis and is distributed throughout the Asian countries. The virus is known to be transmitted by Culex tritaeniorhynchus, which mainly breeds in rice paddies in Korea. In this study, we investigated the presence of other mosquito species that can transmit JEV as a second or regional vector. We selected five cities where patients have experienced JE in the last 5 years as mosquito-collecting locations and subdivided them into four collection sites according to the mosquito habitats (cowshed, downtown area, forest, and swamp). Mosquitoes were caught using the BG-Sentinel trap, CDC black-light trap, Fay-Prince trap, and Gravid trap. A total of 993 pools from 22,774 mosquitoes were prepared according to their species, collection date, and site. We performed a SYBR Green 1-based real-time RT-PCR assay to detect JEV from the mosquito pools. A total of six JEV-positive pools were detected from Culex orientalis and Culex pipiens caught in the Gangwon-do and Gyeonngi-do provinces. All the detected JEVs were revealed as genotype V by phylogenetic analysis of the envelope gene. Our findings confirm that a new genotype of JEV was introduced in Korea and suggest that two mosquito species may play a role in JEV transmission.
To clarify the geographical distribution of scrub typhus vectors in Korea, a survey of larval trombiculid mites was conducted from 2005 to 2007 by collecting wild small mammals twice a year (spring and autumn) at 24 sites nationwide. A total of 67,325 mites representing 4 genera and 14 species were collected from 783 trapped rodents, corresponding to a chigger index (number of chigger mites per rodent) of 86.0. The predominant mite species were Leptotrombidium pallidum (52.6%), Leptotrombiduim scutellare (27.1%), Leptotrombidium palpale (8.2%), Leptotrombidium orientale (5.6%), and Neotrombicula tamiyai (1.7%). However, the proportions of L. scutellare in southern areas, including endemic provinces such as Jeollabuk-Do (34.3%), Jeollanam-Do (49.0%), and Gyeongsangnam-Do (88%), 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. L. pallidum could be another vector at all other parts of the Korean peninsula, including the eastern and northern regions that have a low level of scrub typhus incidence.
BackgroundAs a result of the reintroduction of malaria in the Republic of Korea (ROK) in 1993 and the threat to military and civilian populations, the Korea Military National Defense (MND) increased emphasis on vector control in 2012 at ROK Army (ROKA) installations located near the DMZ, while decreasing chemoprophylaxis, fearing potential drug resistance. Mosquito surveillance demonstrated a need for continuous monitoring of disease patterns among ROKA soldiers and vector malaria infection rates to ensure positive outcomes.MethodsAnopheles spp. were collected from May–October at three ROKA installations in three locations near the DMZ. Each of the areas included one installation <2 km and two installations 11–12 km from the DMZ in Paju and Yeoncheon counties, Gyeonggi Province. Anopheles spp. were identified by polymerase chain reaction (PCR) techniques and then assayed for the presence of vivax malaria sporozoites. The ROK MND reported vivax malaria patients monthly to Korea Centers for Disease Control and Prevention. Correlations for the incidence of Plasmodium vivax patients and infected Anopheles species were analysed using the Wilcoxon rank sum test, Pearson correlation test and liner regression analysis.ResultsA total of 4282 Anopheles spp. were collected. Anopheles kleini (69.5 %) was the most commonly collected, followed by Anopheles pullus (17.3 %), Anopheles belenrae (4.5 %), Anopheles sineroides (4.2 %), Anopheles sinensis sensu stricto (2.7 %), and Anopheles lesteri (1.9 %). Overall, 21 malaria patients were reported by the ROK MND. The monthly incidence of the malaria patients correlated with the monthly number of Plasmodium vivax sporozoite positive Anopheles spp. The monthly numbers of An.kleini demonstrated the highest correlations to the numbers of ROKA malaria patients throughout the mosquito season (P < 0.01). Anopheles spp. positive for P. vivax and malaria patients at ROKA installations located <2 km from the DMZ were higher than for ROKA installations located 11–12 km from DMZ.ConclusionThe number of Anopheles spp. positive for P. vivax sporozoites correlated with the monthly number of malaria cases and exposure of ROKA soldiers from May–October to P. vivax malaria infections. Malaria vector surveillance and vector control is warranted as part of an effective malaria management program at ROKA installations located near DMZ.
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