Vector Competence of Anopheles kleini and Anopheles sinensis (Diptera: Culicidae) From the Republic of Korea to Vivax Malaria-Infected Blood From Patients From Thailand

Differences in seasonal dynamics and pyrethroid resistance development among Anopheles Hyrcanus group species

BACKGROUND

The Anopheles Hyrcanus group, which transmits Plasmodium vivax, consists of six confirmed species in South Korea. An epidemiological study revealed differences in the seasonal occurrence patterns of each species. Pyrethroid resistance in An. sinensis dates back to the early 2000s, whereas information on pyrethroid resistance in other species is lacking despite their greater significance in malaria epidemiology.

METHODS

Anopheles mosquitoes were collected from two malaria-endemic regions in South Korea for 2 years and their knockdown resistance (kdr) mutations were genotyped. The larval susceptibility to λ-cyhalothrin was compared in six Anopheles species and its seasonal changes in three species were investigated. The full-length sequences of the voltage-sensitive sodium channel (VSSC) were compared across six species to evaluate potential target-site insensitivity. The contribution of the kdr mutation to phenotypic resistance was confirmed by comparing median lethal time (LT50) to λ-cyhalothrin between populations of Anopheles belenrae with distinct genotypes.

RESULTS

The composition and seasonal occurrence of rare species (Anopheles kleini, Anopheles lestri, and Anopheles sineroides) varied considerably, whereas An. sinensis occurs continuously throughout the season. A kdr mutation in the form of heterozygous allele was newly identified in An. belenrae, An. lesteri, An. pullus, and An. sineroides. The baseline susceptibility to λ-cyhalothrin was the highest in An. belenrae, followed by An. lesteri, An. sineroides, An. kleini, An. pullus, and An. sinensis, with median lethal concentration (LC50) values ranging from 6.0- to 73.5-fold higher than that of An. belenrae. The susceptibility of An. sinensis and An. pullus varied by season, whereas that of An. belenrae remained stable. The kdr-heterozygous An. belenare population exhibited 5.1 times higher LT50 than that of the susceptible population. Species-specific VSSC sequence differences were observed among the six species.

CONCLUSIONS

Our findings suggest that the status and extent of pyrethroid resistance vary among Anopheles Hyrcanus group species. While An. sinensis, the predominant species, developed a considerable level of pyrethroid resistance through kdr mutation, the resistance levels of other species appeared to be less pronounced. Large-scale monitoring is crucial to fully understand species-specific seasonal occurrence and resistance status for effective management strategies, considering the ongoing impact of climate change on their vectorial capacity.

Analysis of geometric morphometrics and molecular phylogeny for Anopheles species in the Republic of Korea

Human malaria, transmitted by Anopheles mosquitoes, is the most predominant mosquito-borne disease that is responsible for hundreds of thousands of deaths worldwide each year. In the Republic of Korea (ROK), there are currently several hundred malaria cases annually, mostly near the demilitarized zone (DMZ). Eight species of Anopheles mosquitoes are currently known to be present in the ROK. Similar to other major malaria vectors in Africa and India, it is very challenging to morphologically differentiate Anopheles mosquitoes in the ROK due to their extremely similar morphology. In this study, wing geometric morphometrics (WGM) were used to differentiate the eight Anopheles species collected at six locations near the DMZ, Seoul and Pyeongtaek from April-October 2021. Phylogenetic analysis was also performed using cytochrome c oxidase subunit 1 (COI), internal transcribed spacer 2 (ITS2), and tyrosine hydroxylase (TH) genes for comparison with WGM analysis and to infer evolutionary relationships. The results of cross-validation (overall accuracy = 74.8%) demonstrated that species identification using WGM alone was not possible with a high accuracy for all eight species. While phylogenetic analyses based on the COI region could not clearly distinguish some species, the analysis based on ITS2 and TH was more useful for resolving the phylogenetic correlation of the eight species. Our results may improve Anopheles species identification strategies for effective identification and control of malaria vectors in the ROK.

Species Diversity of Anopheles Mosquitoes and Plasmodium vivax Infection Rates, Gyeonggi Province, Republic of Korea During 2020

There are currently >300 malaria cases reported annually in the Republic of Korea (ROK), with most cases attributed to exposure in northern Gangwon and Gyeonggi provinces near the demilitarized zone (DMZ). The species diversity and malaria infection rate were determined for a sample of Anopheles mosquitoes collected from May to early November 2020 for six sites in a malaria high-risk area in/near the DMZ and two malaria low-risk areas in southern Gyeonggi province using Mosquito Magnet traps in the ROK. A total of 1864 Anopheles spp. were identified to species by PCR. Overall, An. kleini (31.4%, 510/1622) was the most frequently species assayed, followed by An. pullus (25.5%, 413/1622), An. sineroides (23.9%, 387/1622), and An. sinensis (10.2%, 165/1622), while the other four species only accunted for 9.1% (147/1622) collected in/near the DMZ. Only three species, An. pullus, An. sinensis, and An. sineroides were collected at Humphreys US Army Garrison (USAG) (235 individuals), while only An. sinensis was collected at Yongsan USAG (7 individuals). A total of 36 Anopheles specimens belonging to five species collected in/near the DMZ were positive for Plasmodium vivax by PCR. Anopheles kleini (9) was the most frequent species positive for P. vivax, followed by An. belenrae (8), An. pullus (8), An. sinensis (5), An. sineroides (5), and a member of the Anopheles Lindesayi Complex in the ROK (1). This is the first report of P. vivax in a member of the An. Lindesayi Complex in the ROK. These findings can assist in guiding future malaria vector management in the ROK.

Vector Competence and the Susceptibility of Anopheles pullus and Anopheles belenrae to Plasmodium vivax-Infected Blood From Thai Patients

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.

Prediction of species composition ratios in pooled specimens of the Anopheles Hyrcanus group using quantitative sequencing

Background

Plasmodium vivax is transmitted by members of the Anopheles Hyrcanus Group that includes six species in the Republic of Korea: Anopheles sinensis sensu stricto (s.s.), Anopheles pullus, Anopheles kleini, Anopheles belenrae, Anopheles lesteri, and Anopheles sineroides. Individual Anopheles species within the Hyrcanus Group demonstrate differences in their geographical distributions, vector competence and insecticide resistance, making it crucial for accurate species identification. Conventional species identification conducted using individual genotyping (or barcoding) based on species-specific molecular markers requires extensive time commitment and financial resources.

Results

A population-based quantitative sequencing (QS) protocol developed in this study provided a rapid estimate of species composition ratios among pooled mosquitoes as a cost-effective alternative to individual genotyping. This can be accomplished by using species- or group-specific nucleotide sequences of the mitochondrial cytochrome C oxidase subunit I (COI) and the ribosomal RNA internal transcribed spacer 2 (ITS2) region as species identification alleles in a two-step prediction protocol. Standard genomic DNA fragments of COI and ITS2 genes were amplified from each Anopheles species using group-specific universal primer sets. Following sequencing of the COI or ITS2 amplicons generated from sets of standard DNA mixtures, equations were generated via linear regression to predict species-specific nucleotide sequence frequencies at different positions. Species composition ratios between An. sineroides, An. pullus and An. lesteri were estimated from QS of the COI amplicons based on the mC.260A, mC.122C and mC.525C alleles at the first step, followed by the prediction of species composition ratios between An. sinensis, An. kleini and An. belenrae based on QS of the ITS2 amplicons using the rI.370G and rI.389T alleles. The COI copy number was not significantly different between species, suggesting the reliability of COI-based prediction. In contrast, ITS2 showed a slightly but significantly higher copy number in An. belenrae, requiring an adjustment of its predicted composition ratio. A blind test proved that predicted species composition ratios either from pooled DNA specimens or pooled mosquito specimens were not statistically different from the actual values, demonstrating that the QS-based prediction is accurate and reliable.

Conclusions

This two-step prediction protocol will facilitate rapid estimation of the species composition ratios in field-collected Anopheles Hyrcanus Group populations and is particularly useful for studying the vector ecology of Anopheles population and epidemiology of malaria.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12936-021-03868-y.

Optimization of the feeding rate of Anopheles farauti s.s. colony mosquitoes in direct membrane feeding assays

BACKGROUND

Direct membrane feeding assays (DMFA) are an important tool to study parasite transmission to mosquitoes. Mosquito feeding rates in these artificial systems require optimization, as there are a number of factors that potentially influence the feeding rates and there are no standardized methods that apply to all anopheline species.

METHODS

A range of parameters prior to and during direct membrane feeding (DMF) were evaluated for their impact on Anopheles farauti sensu stricto feeding rates, including the starving conditions and duration of starving prior to feeding, membrane type, DMF exposure time, mosquito age, feeding in the light versus the dark, blood volume, mosquito density and temperature of water bath.

RESULTS

The average successful DMFA feeding rate for An. farauti s.s. colony mosquitoes increased from 50 to 85% when assay parameters were varied. Overnight starvation and Baudruche membrane yielded the highest feeding rates but rates were also affected by blood volume in the feeder and the mosquito density in the feeding cups. Availability of water during the pre-feed starvation period did not significantly impact feeding rates, nor did the exposure duration to blood in membrane feeders, the age of mosquitoes (3, 5 and 7 days post-emergence), feeding in the light versus the dark, or the temperature (34 °C, 38 °C, 42 °C and 46 °C) of the water bath.

CONCLUSION

Optimal feeding conditions in An. farauti s.s. DMFA were to offer 50 female mosquitoes in a cup (with a total surface area of ~ 340 cm2 with 1 mosquito/6.8 cm2) that were starved overnight 350-500 µL of blood (collected in heparin-coated Vacutainer tubes) per feeder in feeders with a surface area ~ 5 cm2 (with a maximum capacity of 1.5 mL of blood) via a Baudruche membrane, for at least 10-20 min.

Multiplex PCR assay for the identification of eight Anopheles species belonging to the Hyrcanus, Barbirostris and Lindesayi groups

BACKGROUND

Genus Anopheles mosquitoes are the primary vectors of human malaria, which is a serious threat to public health worldwide. To reduce the spread of malaria and identify the malaria infection rates in mosquitoes, accurate species identification is needed. Malaria re-emerged in 1993 in the Republic of Korea (ROK), with numbers peaking in 2004 before decreasing to current levels. Eight Anopheles species (Anopheles sinensis, Anopheles pullus, Anopheles belenrae, Anopheles lesteri, Anopheles kleini, Anopheles sineroides, Anopheles koreicus, Anopheles lindesayi) are distributed throughout Korea. Members of the Anopheles Hyrcanus group currently cannot be identified morphologically. The other species of Anopheles can be identified morphologically, except when specimens are damaged in traps. The purpose of this study was to develop a rapid and accurate method for simultaneous molecular identification of the eight Anopheles species present in the ROK.

METHODS

Anopheles spp. used in this study were collected near/in the demilitarized zone in ROK, where most malaria cases are reported. DNA from 165 of the Anopheles specimens was used to develop a multiplex PCR assay. The internal transcribed spacer 2 (ITS2) region of each species was sequenced and analysed for molecular identification.

RESULTS

DNA from a total of 165 Anopheles specimens was identified to species using a multiplex diagnostic system. These included: 20 An. sinensis, 21 An. koreicus, 17 An. lindesayi, 25 An. kleini, 11 An. lesteri, 22 An. sineroides, 23 An. belenrae, and 26 An. pullus. Each species was clearly distinguished by electrophoresis as follows: 1,112 bp for An. sinensis; 925 bp for An. koreicus; 650 bp for An. lindesayi; 527 bp for An. kleini; 436 bp for An. lesteri; 315 bp for An. sineroides; 260 bp for An. belenrae; and, 157 bp for An. pullus.

CONCLUSION

A multiplex PCR assay was developed to identify Anopheles spp. distributed in ROK. This method can be used to accurately identify Anopheles species that are difficult to identify morphologically to determine species distributions and malaria infection rates.

Regional and seasonal detection of resistance mutation frequencies in field populations of Anopheles Hyrcanus Group and Culex pipiens complex in Korea

Pyrethroid (PYR) and organophosphate (OP) insecticides have been extensively used for mosquito control for several decades in South Korea, and has resulted in the rapid development of resistance in the field. In this study, quantitative sequencing (QS) protocols were developed for the frequency prediction of insecticide resistance alleles [e.g., the L1014F/C mutation on the voltage sensitive sodium channel as a PYR resistance allele and the G119S mutation on the acetylcholinesterase 1 as OP resistance alleles] in four regional populations of Anopheles Hyrcanus Group and Culex pipiens complex. Both of the L1014F/C and G119S mutations were observed in all examined regional populations of An. Hyrcanus Group, suggesting a wide distribution of both PYR and OP resistance. In contrast, populations of the Cx. pipiens complex were determined to possess almost no G119S mutation, but relatively higher frequencies of the L1014F mutation, showing differential resistance patterns between different mosquito groups. The mutation frequencies were also monitored throughout a mosquito season (May-October) at one collection site to determine the seasonal changes of resistance mutation frequency in mosquito populations. Dramatic decreases of both L1014F/C and G119S mutation frequencies were observed in the An. Hyrcanus Group toward the fall, with no mutations observed in the early spring, suggesting a connection between the fitness costs of overwintering and insecticide resistance. However, no apparent trends were detectable in the Cx. pipiens complex populations due to low or zero mutation frequencies.

Is Genetic Continuity Between Anopheles sinensis (Diptera: Culicidae) and its Sibling Species Due to Gene Introgression or Incomplete Speciation?

The Anopheles mosquito Hyrcanus Group is widely distributed geographically across both Palearctic and Oriental regions and comprises 26 valid species. Although the species Anopheles sinensis Wiedemann (1828) is the most common in China and has a low potential vector rank, it has nevertheless long been thought to be an important natural malaria vector within the middle and lower reaches of the Yangtze River. A number of previous research studies have found evidence to support the occurrence of natural hybridization between An. sinensis and Anopheles kleini Rueda, 2005 (a competent malaria vector). We, therefore, collected a sample series of An. sinensis and morphologically similar species across China and undertook ribosomal and mitochondrial DNA analyses in order to assess genetic differentiation (Fst) and gene flow (Nm) amongst different groups. This enabled us to evaluate divergence times between morphologically similar species using the cytochrome oxidase I (COI) gene. The results of this study reveal significant genetic similarities between An. sinensis, An. kleini, and Anopheles belenrae Rueda, 2005 and therefore imply that correct molecular identifications will require additional molecular markers. As results also reveal the presence of gene flow between these three species, their taxonomic status will require further work. Data suggest that An. kleini is the most basal of the three species, while An. sinensis and An. belenrae share the closest genetic relationship.

Colonization and Maintenance of Anopheles belenrae and Anopheles pullus From the Republic Of Korea

The Anopheles Hyrcanus Group in the Republic of Korea (ROK) consists of 5 morphologically indistinct species that can only be identified with certainty by polymerase chain reaction (PCR). A total of 86 bloodfed Anopheles spp. were collected from a cow barn located in the village of Tongilchon near the demilitarized zone in the ROK on June 13, 2016, and sent to the Armed Forces Research Institute of Medical Sciences in Bangkok, Thailand, where they were identified to species by PCR. The 1st shipment contained 15 An. belenrae and 37 An. pullus females that were used to start the colonies. Parent females that oviposited were identified by PCR for colonization. A higher proportion of F₁-F₄ females of An. belenrae than An. pullus bloodfed when provided both blood meals on human arms and using a membrane feeding system with human blood. Following blood meals, the females were forced mated for colony maintenance. The mean numbers of eggs oviposited per female for An. belenrae was 127.7 ± 19.3 and for An. pullus was 136 ± 23.6. On average, at 25°C (±2°C) An. belenrae and An. pullus took 15.1 and 16.1 days to develop from egg to adult, respectively. A 2nd group of bloodfed Anopheles spp. was collected at the same location in the ROK on June 24, 2017. This group contained 13 An. belenrae and 27 An. pullus. Similarly, eggs were obtained and adults identified by PCR and then reared to adults and subsequent generations forced mated to members of each of the existing colonies to increase genetic diversity. The colonies were established to evaluate their susceptibility to vector vivax malaria, which is essential to better understand the epidemiology of malaria transmission in Korea. This is the 1st report of colonization of both An. belenrae and An. pullus.

Malaria control and chemoprophylaxis policy in the Republic of Korea Armed Forces for the previous 20 years (1997-2016)

Background

Vivax malaria reemerged along the Demilitarized Zone (DMZ), Republic of Korea (ROK), in 1993. While it was hypothesized that vivax malaria would spread throughout the peninsula, nearly all cases were due to exposure near the DMZ. To reduce spillover of vivax malaria to the civilian community, the ROK Ministry of National Defense (MND) initiated malaria prevention policies including a large-scale chemoprophylaxis programme in malaria high-risk areas in 1997. The present study investigated the overall changes in the incidence of malaria among ROK soldiers and the mass chemoprophylaxis program from 1997 to 2016.

Results

Peak numbers of vivax malaria were reported in 2000, with most cases reported near the DMZ, before declining to the current levels. To combat the rapid increase in the number of malaria cases and its expansion throughout the ROK, the MND implemented mosquito control and personal protection programmes. The MND also implemented a large-scale vivax malaria chemoprophylaxis programme using hydroxychloroquine (400 mg weekly) in 1997, and primaquine (15 mg × 14 days) as terminal chemoprophylaxis in 2001. Additionally, an improved medical system enabled the rapid detection and treatment of malaria to reduce morbidity and decrease transmission of malaria from humans to mosquitoes. Following the full implementation of these programmes, the incidence of vivax malaria declined in both ROK Armed Forces and civilian populations. Subsequently, several changes in the ROK Armed Forces chemoprophylaxis programme were implemented, including the reduction of the period of hydroxychloroquine prophylaxis by 2 months (2008) and other changes in the chemoprophylaxis policy, e.g., only ROK Armed Forces personnel in moderate risk groups received terminal primaquine chemoprophylaxis (2011), and in 2016, the discontinuation of terminal primaquine chemoprophylaxis in moderate-risk area.

Conclusions

The resurgence of vivax malaria in the ROK Armed Forces personnel near the DMZ was successfully suppressed through the implementation of a mass malaria chemoprophylaxis programme initiated by the MND in 1997, as well as several other factors that may have contributed to the reduction of malaria transmission since 2000. Given the current malaria situation in the ROK and North Korea, it is necessary to reevaluate the ROK Armed Forces and civilian malaria control policies.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2449-4) contains supplementary material, which is available to authorized users.

Seasonal dynamics of Anopheles species at three locations in the Republic of Korea

Weekly changes in adult Anopheles species were monitored at Camp Humphreys (CH), Ganghwa Island (GH), and Warrior Base (WB), from May-October, 2009-2010 to explore the relationship between Plasmodium vivax development and vector dynamics in the Republic of Korea (ROK). Adult females were trapped and dissected to determine parity for estimating longevity, mortality, and birthrate. A degree-day (DD) method was used to estimate the extrinsic incubation period (EIP) of P. vivax and duration of the gonotrophic cycle and other life stages. Anopheles sinensis was the predominant species, with satellite data showing peak abundance occurring after the period of maximum greenness. Abundance peaks were location dependent, comprised nulliparous and parous females, and timing could not be fully explained by DD estimation. Parity showed synchronicity between locations and years and was highest for September and lowest during maximum greenness. Mosquito longevity was predicted to exceed the EIP (when malaria transmission is possible) during weeks 29, 31, 34, and near the end of the season. Area-wide changes in parity suggest a common cause; information on local larval habitat and agricultural practices may explain location-specific effects. DD estimates of EIP and parity could be used to predict when conditions are suitable for P. vivax transmission.

Comparison of Adult Mosquito Black-Light and Light-Emitting Diode Traps at Three Cowsheds Located in Malaria-Endemic Areas of the Republic of Korea

Adult mosquito surveillance and field trials evaluated selected commercially available ultraviolet black-light (BL) and light-emitting diode (LED) traps at three sites where vivax malaria is endemic from May to October 2015 in northwestern Republic of Korea. Collections totaled 283,929 adult mosquitoes (280,355 [98.74%] females and 3,574 [1.26%] males) comprising 17 species (including six members of the Anopheles Hyrcanus Group) belonging to six genera. The four most predominant female species collected were Aedes vexans nipponii (Theobald) (83.84%), followed by Anopheles Hyrcanus Group (13.66%), Culex pipiens Group (1.67%), and Culex tritaeniorhynchus Giles (0.54%). Overall, LED traps (188,125) collected significantly more female mosquitoes compared with BL traps (92,230; P = 0.0001, P < 0.05). Results from these field evaluations significantly enhance vector and disease surveillance efforts, especially for the primary vectors of malaria (Anopheles Hyrcanus Group) and Japanese encephalitis (Cx. tritaeniorhynchus).