Knockdown resistance in Anopheles vagus, An. sinensis, An. paraliae and An. peditaeniatus populations of the Mekong region

Insights into the mitochondrial cytochrome oxidase I (mt-COI) gene and wing morphometrics of Anopheles baimaii (Diptera: Culicidae) in malaria-endemic islands of Thailand

Anopheles baimaii (Diptera: Culicidae) significantly contributes to the transmission of parasites causing malaria in Southeast Asia and South Asia. This study examined the morphological (wing shape) and molecular (mitochondrial gene) variations of An. baimaii in four of Thailand's border islands, and also investigated the presence of Plasmodium parasites in these mosquitoes. No Plasmodium infections were detected in the samples. Significant differences in wing shape were observed in most island populations (p < 0.05). A single-linkage tree, constructed using Mahalanobis distances, clustered the populations into two groups based on geographical locations. Genetic variation in An. baimaii was also analyzed through cytochrome c oxidase subunit I (COI) gene sequences. This analysis identified 22 segregating sites and a low nucleotide diversity of 0.004. Furthermore, 18 distinct haplotypes were identified, indicating a high haplotype diversity of 0.825. Neutrality tests for the overall population revealed a significantly negative Fu's Fs value (-5.029), indicating a population expansion. In contrast, Tajima's D yielded a negative value (-1.028) that did not reach statistical significance. The mismatch distribution analysis exhibited a bimodal pattern, and the raggedness index was 0.068, showing no significant discrepancy (p = 0.485) between observed and expected distributions. Pairwise genetic differentiation assessments demonstrated significant differences between all populations (p < 0.05). These findings provide valuable insights into the COI gene and wing morphometric variations in An. baimaii across Thailand's islands, offering critical information for understanding the adaptations of this malaria vector and guiding future comprehensive research.

Genomic complexity of parasites and vectors challenges malaria control in Southeast Asia

Southeast Asia is a uniquely complex region of malaria transmission that maintains an astounding level of species diversity among potential malaria vectors and also generates drug-resistant and quickly diverging populations of malaria parasites. All five human malaria species circulate in Southeast Asia with over 50 Anopheles species that vary in their ability to transmit these pathogens. The intricate relationships of these parasites and vectors are not well-understood. Human activity in Southeast Asian countries has created an increasingly fragmented landscape, bringing humans and mosquitoes into more frequent contact, sustaining malaria transmission in a region where few control tools are effective. Genomic shifts at the species, population, and individual level in parasites and vectors introduce variation that has produced drug- and insecticide resistance. The goal of this review is to highlight genomic studies of Southeast Asian malaria parasites and vectors that demonstrate how diversity in these organisms presents unique challenges and opportunities for global malaria control and eradication efforts.

Functional Characterization of Knockdown Resistance Mutation L1014S in the German Cockroach, Blattella germanica (Linnaeus)

The effectiveness of pyrethroid insecticides is seriously threatened by knockdown resistance (kdr), which is induced in insects by inherited single-nucleotide polymorphisms in the voltage-gated sodium channel (VGSC) gene. VGSC's L1014F substitution results in the classic kdr mutation, which is found in many pest species. Other substitutions of the L1014 locus, such as L1014S, L1014C, L1014W, and L1014H, were also reported. In 2022, a new amino acid substitute L1014S of Blattella germanica was first discovered in China. We modified the BgNa_v1-1 sodium channel from cockroaches with the L1014S mutation to study how pyrethroid sensitivity and channel gating were affected in Xenopus oocytes. The L1014S mutation reduced the half-maximal activation voltage (V_1/2,act) from -19.0 (wild type) to -15.5 mV while maintaining the voltage dependency of activation. Moreover, the voltage dependence of inactivation in the hyperpolarizing shifts from -48.3 (wild type) to -50.9 mV. However, compared with wild type, the mutation L1014S did not cause a significant shift in the half activation voltage (V_1/2,act). Notably, the voltage dependency of activation was unaffected greatly by the L1014S mutation. Tail currents are induced by two types of pyrethroids (1 μM): type I (permethrin, bifenthrin) and type II (deltamethrin, λ-cyhalothrin). All four pyrethroids produced tail currents, and significant differences were found in the percentages of channel modifications between variants and wild types. Further computer modeling showed that the L1014S mutation allosterically modifies pyrethroid binding and action on B. germanica VGSC, with some residues playing a critical role in pyrethroid binding. This study elucidated the pyrethroid resistance mechanism of B. germanica and predicted the residues that may confer the risk of pyrethroid resistance, providing a molecular basis for understanding the resistance mechanisms conferred by mutations at the 1014 site in VGSC.

Population genetic structure and evolutionary genetics of Anopheles sinensis based on knockdown resistance (kdr) mutations and mtDNA-COII gene in China-Laos, Thailand-Laos, and Cambodia-Laos borders

BACKGROUND

Vector control is still a pivotal method for preventing malaria, and its potency is weakened by the increasing resistance of vectors to chemical insecticides. As the most abundant and vital malaria vector in Southeast Asia, the chemical insecticide resistance status in Anopheles sinensis remains elusive in Laos, which makes it imperative to evaluate the true nature of chemical insecticide resistance-associated genetic mutations in An. sinensis in Laos.

METHODS

Adult An. sinensis were collected from three border regions in Laos. DNA was extracted from individual mosquitoes. PCR amplification and DNA sequencing of a fragment containing codon 1014 of the voltage-gated sodium channel (vgsc) gene were completed to study the kdr allele frequency distribution, kdr intron polymorphism, population genetic diversity, and the evolutionary status of the kdr codon. The mitochondrial cytochrome c oxidase subunit II gene (COII) was amplified and sequenced to examine population variations, genetic differentiation, spatial population structure, population expansion, and gene flow patterns.

RESULTS

Nine wild kdr haplotypes of the vgsc gene were detected in this study, and eight of them, namely 1014L1, 1014L2, 1014L4, 1014L7, 1014L9, 1014L10, 1014L11, and 1014L21, were discovered in the China-Laos border (northern Laos), while 1014L3 was only detected in the Thailand-Laos border (northwestern Laos) and Cambodia-Laos border (southern Laos). The newly identified haplotype, 1014L21, was uniquely distributed in the China-Laos border and was not identified in other countries. Based on sequence analysis of the mitochondrial COII genes, significant genetic differentiation and limited gene flow were detected between the China-Laos and Cambodia-Laos An. sinensis populations, which suggested that those two regions were genetically isolated. The distinct distribution of the kdr haplotype frequencies is probably the result of geographical isolation in mosquito populations.

CONCLUSIONS

Lack of kdr mutations in the vgsc gene was probably due to genetic isolation and the absence of intense selection pressure in the three border regions of Laos. This study reveals that pyrethroid-based chemical insecticides are still appropriate for battling An. sinensis in parts of Laos, and routine monitoring of chemical insecticide resistance should be continuously implemented and focused on more restricted areas as part of chemical insecticide resistance management.

Community structure and insecticide resistance of malaria vectors in northern-central Myanmar

BACKGROUND

Myanmar is one of the six countries in the Greater Mekong Subregion (GMS) of Southeast Asia. Malaria vectors comprise many Anopheles species, which vary in abundance and importance in malaria transmission among different geographical locations in the GMS. Information about the species composition, abundance, and insecticide resistance status of vectorial systems in Myanmar is scarce, hindering our efforts to effectively control malaria vectors in this region.

METHODS

During October and November 2019, larvae and adult females of Anopheles mosquitoes were collected in three sentinel villages of Banmauk township in northern Myanmar. Adult female mosquitoes collected by cow-baited tent collection (CBTC) and adults reared from field-collected larvae (RFCL) were used to determine mortality rates and knockdown resistance (kdr) against deltamethrin using the standard WHO susceptibility test. Molecular species identification was performed by multiplex PCR and ITS2 PCR, followed by DNA sequencing. The kdr mutation at position 1014 of the voltage-gated sodium channel gene was genotyped by DNA sequencing for all Anopheles species tested.

RESULTS

A total of 1596 Anopheles mosquitoes from seven morphologically identified species groups were bioassayed. Confirmed resistance to deltamethrin was detected in the populations of An. barbirostris (s.l.), An. hyrcanus (s.l.), and An. vagus, while possible resistance was detected in An. annularis (s.l.), An. minimus, and An. tessellatus. Anopheles kochi was found susceptible to deltamethrin. Compared to adults collected by CBTC, female adults from RFCL had significantly lower mortality rates in the four species complexes. A total of 1638 individuals from 22 Anopheles species were molecularly identified, with the four most common species being An. dissidens (20.5%) of the Barbirostris group, An. peditaeniatus (19.4%) of the Hyrcanus group, An. aconitus (13.4%) of the Funestus group, and An. nivipes (11.5%) of the Annularis group. The kdr mutation L1014F was only detected in the homozygous state in two An. subpictus (s.l.) specimens and in a heterozygous state in one An. culicifacies (s.l.) specimen.

CONCLUSIONS

This study provides updated information about malaria vector species composition and insecticide resistance status in northern Myanmar. The confirmed deltamethrin resistance in multiple species groups constitutes a significant threat to malaria vector control. The lack or low frequency of target-site resistance mutations suggests that other mechanisms are involved in resistance. Continual monitoring of the insecticide resistance of malaria vectors is required for effective vector control and insecticide resistance management.

First Report of the L993S Mutation in the Voltage-Gated Sodium Channel in Field Populations of the German Cockroach Blattella germanica

The long-term and frequent use of pyrethroid insecticides has led to the development of pyrethroid resistance in many insect populations around the world. Specific mutations in the voltage-gated sodium channel (VGSC) have been well documented to be responsible for knockdown resistance (kdr) to pyrethroids and dichlorodiphenyltrichloroethane (DDT) in a variety of arthropods. However, reports regarding naturally occurring kdr mutation in field populations of the German cockroach Blattella germanica (Linnaeus) (Dictyoptera: Blattellidae) in China have remained scarce. In this study, a survey was conducted to detect the presence and frequency of kdr mutations in field populations of B. germanica across Neijiang, Sichuan province of China. In addition to the previously reported L993F mutation, a new amino acid substitution L993S was discovered for the first time. Overall, the classical 993F was the dominant allele with frequencies ranging from 61.8 to 89.6%, while the frequencies of the novel L993S mutation were in the range between 2.5 and 15.0%. Notably, high frequencies (50.0-79.2%) of resistant homozygotes were detected in our samples, indicating high levels of pyrethroid resistance in these B. germanica populations. The results suggest that alternative insecticides with a mode of action different from pyrethroids should be considered in the control of German cockroaches in these regions.

Status of insecticide susceptibility in Anopheles arabiensis and detection of the knockdown resistance mutation (kdr) concerning agricultural practices from Northern Sudan state, Sudan

BACKGROUND

Chemical control has been the most efficient method in mosquito control, the development of insecticide resistance in target populations has a significant impact on vector control. The use of agricultural pesticides may have a profound impact on the development of resistance in the field populations of malaria vectors. Our study focused on insecticide resistance and knockdown resistance (kdr) of Anopheles arabiensis populations from Northern Sudan, related to agricultural pesticide usage.

RESULTS

Anopheles arabiensis from urban and rural localities (Merowe and Al-hamadab) were fully susceptible to bendiocarb 0.1% and permethrin 0.75% insecticides while resistant to DDT 4% and malathion 5%. The population of laboratory reference colony F189 from Dongola showed a mortality of 91% to DDT (4%) and fully susceptible to others. GLM analysis indicated that insecticides, sites, site type, and their interaction were determinant factors on mortality rates (P < 0.01). Except for malathion, mortality rates of all insecticides were not significant (P > 0.05) according to sites. Mortality rates of malathion and DDT were varied significantly (P < 0.0001 and P < 0.05 respectively) by site types, while mortality rates of bendiocarb and permethrin were not significant (P >0.05). The West African kdr mutation (L1014F) was found in urban and rural sites. Even though, the low-moderate frequency of kdr (L1014F) mutation was observed. The findings presented here for An. arabiensis showed no correlation between the resistant phenotype as ascertained by bioassay and the presence of the kdr mutation, with all individuals tested except the Merowe site which showed a moderate association with DDT (OR= 6 in allelic test), suggesting that kdr genotype would be a poor indicator of phenotypic resistance.

CONCLUSION

The results provide critical pieces of information regarding the insecticide susceptibility status of An. arabiensis in northern Sudan. The usage of the same pesticides in agricultural areas seemed to affect the Anopheles susceptibility when they are exposed to those insecticides in the field. The kdr mutation might have a less role than normally expected in pyrethroids resistance; however, other resistance genes should be in focus. These pieces of information will help to improve the surveillance system and The implication of different vector control programs employing any of these insecticides either in the treatment of bed nets or for indoor residual spraying would achieve satisfactory success rates.

The first implementation of allele-specific primers for detecting the knockdown and acetylcholinesterase target site mutations in malaria vector, Anopheles sacharovi

Anopheles sacharovi, a primer malaria vector species of Turkey, have a significant public health importance. It is aimed to determine the insecticide resistance status in Anopheles sacharovi populations in the Aegean and Mediterranean regions of Turkey. A total of 1638 individuals were analysed from 15 populations. Bioassay results indicated all An. sacharovi populations were resistant to DDT, malathion, fenitrothion, bendiocarb, propoxur. Many populations have begun to have resistance against permethrin and deltamethrin. Biochemical analyses results revealed that glutathione-S-transferases and P450 monooxygenases might be responsible from the mechanisms of DDT resistance; esterases and acetylcholinesterase might be responsible for organophosphate and carbamate resistance; P450 monooxygenases and esterases might be responsible for pyrethroid resistance into populations sampled from the study area. Allele-specific primers detected L1014F and L1014S mutations that provide kdr resistance against pyrethroids and DDT. Increased acetylcholinesterase insensitivity was detected while Ace-1 G119S mutations were not detected in An. sacharovi populations by using allele-specific primers. Overall results indicate the presence of multiple resistance mechanisms in Turkish An. sacharovi field populations suggesting that populations might gain resistance against all possible insecticide in the future. Therefore, insecticide resistance management strategies are urgently needed for effective vector control implementation.

Species diversity and insecticide resistance within the Anopheles hyrcanus group in Ubon Ratchathani Province, Thailand

Background

Members of the Anopheles hyrcanus group have been incriminated as important malaria vectors. This study aims to identify the species and explore the insecticide susceptibility profile within the Anopheles hyrcanus group in Ubon Ratchathani Province, northeastern Thailand where increasing numbers of malaria cases were reported in 2014.

Methods

Between 2013 and 2015, five rounds of mosquito collections were conducted using human landing and cattle bait techniques during both the rainy and dry seasons. Anopheles mosquitoes were morphologically identified and their insecticide susceptibility status was investigated. Synergist bioassays were carried out with An. hyrcanus (s.l.) due to their resistance to all insecticides. An ITS2-PCR assay was conducted to identify to species the Hyrcanus group specimens.

Results

Out of 10,361 Anopheles females collected, representing 18 taxa in 2 subgenera, 71.8% were morphologically identified as belonging to the Hyrcanus Group (subgenus Anopheles), followed by An. barbirostris group (7.9%), An. nivipes (6.5%), An. philippinensis (5.9%) and the other 14 Anopheles species. Specimens of the Hyrcanus Group were more prevalent during the rainy season and were found to be highly zoophilic. Anopheles hyrcanus (s.l.) was active throughout the night, with an early peak of activity between 18:00 h and 21:00 h. ITS2-PCR assay conducted on 603 DNA samples from specimens within the Hyrcanus Group showed the presence of five sisters species. Anopheles peditaeniatus was the most abundant species (90.5%, n = 546), followed by An. nitidus (4.5%, n = 27), An. nigerrimus (4.3%, n = 26), An. argyropus (0.5%, n = 3), and An. sinensis (0.2%, n = 1). All An. hyrcanus (s.l.) specimens that were found resistant to insecticides (deltamethrin 0.05%, permethrin 0.75% and DDT 4% and synergist tests) belonged to An. peditaeniatus. The degree of resistance in An. peditaeniatus to each of these three insecticides was approximately 50%. Addition of PBO (Piperonyl butoxide), but not DEF (S.S.S-tributyl phosphotritioate), seemed to restore susceptibility, indicating a potential role of oxidases as a detoxifying enzyme resistance mechanism.

Conclusions

A better understanding of mosquito diversity related to host preference, biting activity and insecticide resistance status will facilitate the implementation of locally adapted vector control strategies.

Intermittent Preventive Treatment for Malaria in Pregnancy: Optimization of Target Concentrations of Dihydroartemisinin-Piperaquine

Background

Dihydroartemisinin-piperaquine (DHA-PQ) is highly efficacious as intermittent preventive therapy for malaria during pregnancy (IPTp). Determining associations between piperaquine (PQ) exposure, malaria risk, and adverse birth outcomes informs optimal dosing strategies.

Methods

Human immunodeficiency virus-uninfected pregnant women (n = 300) were enrolled in a placebo-controlled trial of IPTp at 12-20 weeks' gestation and randomized to sulfadoxine-pyrimethamine every 8 weeks, DHA-PQ every 8 weeks, or DHA-PQ every 4 weeks during pregnancy. Pharmacokinetic sampling for PQ was performed every 4 weeks, and an intensive pharmacokinetic substudy was performed in 30 women at 28 weeks' gestation. Concentration-effect relationships were assessed between exposure to PQ; the prevalence of Plasmodium falciparum infection during pregnancy; outcomes at delivery including placental malaria, low birth weight, and preterm birth; and risks for toxicity. Simulations of new dosing scenarios were performed.

Results

Model-defined PQ target venous plasma concentrations of 13.9 ng/mL provided 99% protection from P. falciparum infection during pregnancy. Each 10-day increase in time above target PQ concentrations was associated with reduced odds of placental parasitemia, preterm birth, and low birth weight, though increases in PQ concentrations were associated with QT interval prolongation. Modeling suggests that daily or weekly administration of lower dosages of PQ, compared to standard dosing, will maintain PQ trough levels above target concentrations with reduced PQ peak levels, potentially limiting toxicity.

Conclusions

The protective efficacy of IPTp with DHA-PQ was strongly associated with higher drug exposure. Studies of the efficacy and safety of alternative DHA-PQ IPTp dosing strategies are warranted.

Clinical Trials Registration

NCT02163447.

Resistance to pyrethroid and organophosphate insecticides, and the geographical distribution and polymorphisms of target-site mutations in voltage-gated sodium channel and acetylcholinesterase 1 genes in Anopheles sinensis populations in Shanghai, China

Background

In the final phase of China’s national programme to eliminate malaria by 2020, it is vitally important to monitor the resistance of malaria vectors for developing effective vector control strategies. In 2017 Shanghai declared that it had eliminated malaria; however, the insecticide resistance status of the primary malaria vector Anopheles sinensis remains unknown.

Methods

We examined the pyrethroid and organophosphate resistance of An. sinensis via a bioassay of two populations from the Chongming District of Shanghai. The voltage-gated sodium channel (VGSC) and acetylcholinesterase 1 (ace-1) genes were partially sequenced to examine the association between resistance phenotype and target site genotype. In addition, the geographical distribution, polymorphism and genotype frequencies of insecticide resistance genes were examined using samples collected during routine mosquito surveillance in 2016 and 2017 from Chongming, Songjiang, Jiading and Qingpu Districts.

Results

In Chongming District, the An. sinensis population near Dongtan National Nature Reserve showed resistance to pyrethroids, sensitivity to organophosphate, no knockdown resistance (kdr) mutations in the VGSC gene, and a low frequency (1.71%) of the ace-1 119S allele. An An. sinensis population near the Chongming central area (CM-Xinhe population) showed high resistance to pyrethroids and organophosphates and high frequencies of kdr 1014F and 1014C (80.73%) and ace-1 119S (85.98%) alleles. A significant association was detected between the homozygous kdr mutation 1014F/1014F and pyrethroid resistance in the CM-Xinhe population, indicating that the kdr mutation is probably recessive. Eight kdr genotypes with 1014F and 1014C substitutions were detected in the four surveyed districts of Shanghai. TTT and GGC/AGC were the dominant kdr allele and ace-1 genotype, respectively, and were prevalent in most Shanghai An. sinensis populations.

Conclusions

On the basis of our assessment of insecticide resistance gene mutations in Shanghai, we identified a kdr mutation-free population in Chongming Dongtan. However, high frequencies of target-site mutations of insecticide resistance genes were observed in most areas of Shanghai. Bioassays of An. sinensis populations in the central Chongming District indicated the high insecticide resistance status of An. sinensis populations in Shanghai. We accordingly recommend a restriction on insecticide usage and development of effective integrated pest/vector management interventions to support disease control efforts.

Insecticide Resistance Status and Mechanisms of Anopheles sinensis (Diptera: Culicidae) in Wenzhou, an Important Coastal Port City in China

Although scaled-up interventions and effective control efforts have drastically reduced malaria morbidity and mortality, malaria remains a serious threat to public health worldwide. Anopheles sinensis Wiedemann 1828 is a historically important vector of Plasmodium vivax (Haemosporida: Plasmodiidae) malaria in China. Insecticide resistance has become a major obstacle to vector-borne disease control. However, little is known about the insecticide resistance of An. sinensis in Wenzhou, an important coastal port city in Zhejiang province, China. The aim of this study was to examine insecticide resistance and mechanisms in An. sinensis field mosquito populations. Evidence of multiple insecticide resistance was found in An. sinensis adult female populations. Medium to high frequencies of target site kdr together with fixed ace-1 mutations was detected in both the Ruian and Yongjia populations. Both populations showed an association between kdr L1014 mutation and resistance phenotype when tested against deltamethrin and DDT. Significantly different metabolic enzyme activities were found between the susceptible laboratory strain and field-collected mosquitoes from both Ruian and Yongjia. Both field collected An. sinensis populations exhibited significantly higher P450 enzyme activity compared with the laboratory strain, while the field-collected resistant mosquitoes exhibited various GST and COE enzyme activities. These results indicate multiple resistance mechanisms in An. sinensis field populations. Effective implementation of insecticide resistance management strategies is urgently needed. The data collected in this study will be valuable for modeling insecticide resistance spread and vector-control interventions.

Trends in insecticide resistance in Culex pipiens pallens over 20 years in Shandong, China

Background

Culex pipiens pallens is the most abundant Culex mosquito species in northern China and is an important vector of bancroftian filariasis and, potentially, West Nile virus. Insecticides, particularly pyrethroids, are widely used for adult mosquito control. Insecticide resistance has become common in several mosquito species, and vector control is the main method currently available to prevent disease transmission. The voltage-gated sodium channel (Vgsc) gene is the target site of pyrethroids, and mutations in this gene cause knockdown resistance (kdr).

Methods

Culex pipiens pallens larvae were collected from May to November over two decades, from 1992 to 2018, in four cities in Shandong Province, China. The World Health Organization (WHO) standard resistance bioassay was applied to test the resistance levels of Cx. p. pallens larvae to five different insecticides and to test deltamethrin resistance in adults, using the F1 generation. Mutations at Vgsc codon 1014 were also screened in 471 adult samples collected in 2014 to determine the association between kdr mutations and phenotypic resistance.

Results

Larval resistance against deltamethrin showed an increasing trend from the 1990s until 2018, which was statistically significant in all populations; resistance to cypermethrin increased significantly in mosquitoes from the Zaozhuang population. However, larval resistance to other insecticides remained relatively stable. Larval resistance against deltamethrin was consistent with adult bioassays in 2014, in which all tested populations were highly resistant, with mortality rates ranging from 39.4 to 55.23%. The L1014S and L1014F mutations were both observed in five Cx. p. pallens populations, with L1014F significantly associated with deltamethrin resistance.

Conclusions

The long-term dataset from Shandong demonstrates major increases in pyrethroid resistance over a 20-year period. The L1014F kdr mutation may be considered a viable molecular marker for monitoring pyrethroid resistance in Cx. p. pallens.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3416-9) contains supplementary material, which is available to authorized users.

UDP-glycosyltransferase genes and their association and mutations associated with pyrethroid resistance in Anopheles sinensis (Diptera: Culicidae)

BACKGROUND

UDP-glycosyltransferase (UGT) is an important biotransformation superfamily of enzymes. They catalyze the transfer of glycosyl residues from activated nucleotide sugars to acceptor hydrophobic molecules, and function in several physiological processes, including detoxification, olfaction, cuticle formation, pigmentation. The diversity, classification, scaffold location, characteristics, phylogenetics, and evolution of the superfamily of genes at whole genome level, and their association and mutations associated with pyrethroid resistance are still little known.

METHODS

The present study identified UGT genes in Anopheles sinensis genome, classified UGT genes in An. sinensis, Anopheles gambiae, Aedes aegypti and Drosophila melanogaster genomes, and analysed the scaffold location, characteristics, phylogenetics, and evolution of An. sinensis UGT genes using bioinformatics methods. The present study also identified the UGTs associated with pyrethroid resistance using three field pyrethroid-resistant populations with RNA-seq and RT-qPCR, and the mutations associated with pyrethroid resistance with genome re-sequencing in An. sinensis.

RESULTS

There are 30 putative UGTs in An. sinensis genome, which are classified into 12 families (UGT301, UGT302, UGT306, UGT308, UGT309, UGT310, UGT313, UGT314, UGT315, UGT36, UGT49, UGT50) and further into 23 sub-families. The UGT308 is significantly expanded in gene number compared with other families. A total of 119 UGTs from An. sinensis, An. gambiae, Aedes aegypti and Drosophila melanogaster genomes are classified into 19 families, of which seven are specific for three mosquito species and seven are specific for Drosophila melanogaster. The UGT308 and UGT302 are proposed to main families involved in pyrethroid resistance. The AsUGT308D3 is proposed to be the essential UGT gene for the participation in biotransformation in pyrethroid detoxification process, which is possibly regulated by eight SNPs in its 3' flanking region. The UGT302A3 is also associated with pyrethroid resistance, and four amino acid mutations in its coding sequences might enhance its catalytic activity and further result in higher insecticide resistance.

CONCLUSIONS

This study provides the diversity, phylogenetics and evolution of UGT genes, and potential UGT members and mutations involved in pyrethroid resistance in An. sinensis, and lays an important basis for the better understanding and further research on UGT function in defense against insecticide stress.

Target-site mutations (AChE-G119S and kdr) in Guangxi Anopheles sinensis populations along the China-Vietnam border

Background

In South Asia, the epidemiology of malaria is complex, and transmission mainly occurs in remote areas near international borders. Vector control has been implemented as a key strategy in malaria prevention for decades. A rising threat to the efficacy of vector control efforts is the development of insecticide resistance, thus it is important to monitor the type and frequency of insecticide resistant alleles in the disease vectors such as An. sinensis along the China-Vietnam border. Such information is needed to synthesize effective malaria vector control strategies.

Methods

A total of 208 adults of An. sinensis, collected from seven sites in southwest Guangxi along the China-Vietnam border, were inspected for the resistance-conferring G119S mutation in acetylcholinesterase (AChE) by PCR-RFLP (polymerase chain reaction restriction fragment length polymorphism) and kdr mutations in the voltage-gated sodium channel (VGSC) by sequencing. In addition, the evolutionary origin of An. sinensis vgsc gene haplotypes was analyzed using Network 5.0.

Results

The frequencies of mutant 119S of AChE were between 0.61–0.85 in the seven An. sinensis populations. No susceptible homozygote (119GG) was detected in three of the seven sites (DXEC, LZSK and FCGDX). Very low frequencies of kdr (0.00–0.01) were detected in the seven populations, with most individuals being susceptible homozygote (1014LL). The 1014F mutation was detected only in the southeast part (FCGDX) at a low frequency of 0.03. The 1014S mutation was distributed in six of the seven populations with frequencies ranging from 0.04 to 0.08, but absent in JXXW. Diverse haplotypes of 1014L and 1014S were found in An. sinensis along the China-Vietnam border, while only one 1014F haplotype was detected in this study. Consistent with a previous report, resistant 1014S haplotypes did not have a single origin.

Conclusions

The G119S mutation of AChE was present at high frequencies (0.61–0.85) in the An. sinensis populations along the China-Vietnam border, suggesting that the vector control authorities should be cautious when considering carbamates and organophosphates as chemicals for vector control. The low frequencies (0.00–0.11) of kdr in these populations suggest that pyrethroids remain suitable for use against An. sinensis in these regions.

Pyrethroid resistance in the dengue vector Aedes aegypti in Southeast Asia: present situation and prospects for management

Human arboviral diseases transmitted by Aedes aegypti such as dengue, Zika, chikungunya and yellow fever remain global public health threats to date. Of these diseases, dengue fever is particularly prevalent in Southeast Asia. Relentless vector control efforts are performed to curtail disease transmissions through which pyrethroid insecticides are broadly used as the first line of defense to control Ae. aegypti, especially in the course of disease outbreaks. Here, we compile the largest contemporary database for susceptibility profiles and underlying mechanisms involved in Ae. aegypti resistant to pyrethroids in Southeast Asia. The extensive use of pyrethroids inevitably elicit different levels of resistance to numerous populations despite the presence of geographical isolation. The most common mechanisms of pyrethroid resistance that have been identified in Ae. aegypti includes mutations in the voltage sensitive sodium channel gene (Vssc gene) and metabolic-mediated insecticide resistance. Aedes aegypti develops resistance to pyrethroids by acquisition of one or several amino acid substitution(s) in this Vssc gene. Enzymes involved in metabolic-mediated detoxification (i.e. monooxygenases, glutathione-S-transferases and esterases) have been reported to be related to pyrethroid resistance but many specific contributory enzymes are not completely studied. An inadequate amount of data from some countries indicates an urgent need for further study to fill the knowledge gaps. Perspectives and future research needs are also discussed.

First report of target site insensitivity to pyrethroids in human flea, Pulex irritans (Siphonaptera: Pulicidae)

The human flea, Pulex irritans, is the most important ectoparasite of humans. Intensive use of pyrethroids for its control has led to insecticide resistance. Monitoring pyrethroid resistance and its underlying mechanisms is essential for flea control. The aims of this study were to identify the susceptibility status of human flea to permethrin and to detect the presence of knockdown resistance (kdr) mutation and its frequency in populations of P. irritans. Adults of P. irritans were collected from Zanjan Province, northwest of Iran, during 2013-2017. Different populations of this flea were exposed to permethrin 0.75% for one and 8 h and then the mortality rate, as well as KD₅₀ and KD₉₅ times were calculated. Total RNA and gDNA of samples were extracted, and the fragments of cDNA encoding the partial voltage-gated sodium channel (VGSC) peptides were amplified using degenerated primers. Specific PCR and TaqMan real-time assays were conducted to characterize the vgsc gene and to detect the presence of mutation and genotyping of the populations. Mortality rates were in the range from 32% to 67% for one-hour and 73% to 90% for eight-hour exposure to permethrin 0.75%. KD₅₀ and KD₉₅ times varied in a range from 46 to 241 and 177 to 899 min, respectively. Sequencing of 70 amplified fragments of gDNA resulted in a 578-bp product. These fragments contained two introns (92 and 63 bp) and three exons (141, 189, and 92 bp) encoding 138 amino acids that encompassed IIS4-IIS6 and the partial linker between domains II and III of VGSC. All the studied populations showed L1014F mutation, substitution of CTT for TTT at the 1014 allele. The result of TaqMan assay for 624 samples showed 96.6% homogenous and 3.36% heterozygous mutant. The development of permethrin resistance and the presence of the L1014F mutation at high frequency in flea populations indicate that pyrethroids are likely ineffective in controlling human flea. Therefore, novel alternative control methods are needed to combat this human ectoparasite.

Malaria Vectors Insecticides Resistance in Different Agroecosystems in Western Kenya

Background

Malaria vector control efforts have taken malaria related cases down to appreciable number per annum after large scale of intervention tools. Insecticides-based tools remain the major control option for malaria vectors in Kenya and, therefore, the potential of such programs to be compromised by the reported insecticide resistance is of major concern. The objective of this study was to evaluate the status of insecticide resistance in malaria vectors in different agro ecosystems from western Kenya.

Methods

The study was carried out in the lowlands and highlands of western Kenya namely; Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu, and Kabula. World Health Organization tube bioassays was conducted using standard diagnostic dosages of Lambdacyhalothrin, Deltamethrin, Permethrin, DDT, Bendiocarb, and Malathion tested on Anopheles mosquitoes collected from seven sites; Ahero, Kisian, Chulaimbo, Emutete, Emakakha, Iguhu, and Kabula. Biochemical assays, where the enzymatic activity of three enzymes (monooxygenases, esterases, and glutathione S-transferases) were performed on susceptible and resistant mosquito populations. Wild mosquito populations were identified to species level using polymerase chain reaction (PCR). The species of the wild mosquito populations were identified to species level using PCR. Real-time PCR was performed on the susceptible and resistant mosquitoes after the WHO tube bioassays to determine the presence of knockdown resistance (kdr) allele.

Results

WHO susceptibility tests indicated that Anopheles gambiae showed resistance to Pyrethroids and DDT in all the study sites, to Bendiocarb in Iguhu and Kabula and susceptible to Malathion (100% mortality) in all the study sites. There was an elevation of monooxygenases and esterases enzymatic activities in resistant An. gambiae mosquito populations exposed to Lambdacyhalothrin, Permethrin, Deltamethrin and DDT but no elevation in glutathione S-transferases. A high frequency of L1014S allele was detected in An. gambiae s.s. population, but there was no kdr allele found in Anopheles arabiensis mosquitoes.

Conclusion

An. gambiae mosquitoes from western Kenya have developed phenotypic resistance to pyrethroids and DDT. Therefore, there is a need for further research covering different climatic zones with different agroeconomic activities for detailed report on current status of insecticide resistance in malaria vectors.

A spatial agent-based model of Anopheles vagus for malaria epidemiology: examining the impact of vector control interventions

BACKGROUND

Malaria, being a mosquito-borne infectious disease, is still one of the most devastating global health issues. The malaria vector Anopheles vagus is widely distributed in Asia and a dominant vector in Bandarban, Bangladesh. However, despite its wide distribution, no agent based model (ABM) of An. vagus has yet been developed. Additionally, its response to combined vector control interventions has not been examined.

METHODS

A spatial ABM, denoted as ABM[Formula: see text], was designed and implemented based on the biological attributes of An. vagus by modifying an established, existing ABM of Anopheles gambiae. Environmental factors such as temperature and rainfall were incorporated into ABM[Formula: see text] using daily weather profiles. Real-life field data of Bandarban were used to generate landscapes which were used in the simulations. ABM[Formula: see text] was verified and validated using several standard techniques and against real-life field data. Using artificial landscapes, the individual and combined efficacies of existing vector control interventions are modeled, applied, and examined.

RESULTS

Simulated female abundance curves generated by ABM[Formula: see text] closely follow the patterns observed in the field. Due to the use of daily temperature and rainfall data, ABM[Formula: see text] was able to generate seasonal patterns for a particular area. When two interventions were applied with parameters set to mid-ranges, ITNs/LLINs with IRS produced better results compared to the other cases. Moreover, any intervention combined with ITNs/LLINs yielded better results. Not surprisingly, three interventions applied in combination generate best results compared to any two interventions applied in combination.

CONCLUSIONS

Output of ABM[Formula: see text] showed high sensitivity to real-life field data of the environmental factors and the landscape of a particular area. Hence, it is recommended to use the model for a given area in connection to its local field data. For applying combined interventions, three interventions altogether are highly recommended whenever possible. It is also suggested that ITNs/LLINs with IRS can be applied when three interventions are not available.

Cypermethrin resistance conferred by increased target insensitivity and metabolic detoxification in Culex pipiens pallens Coq

In order to elucidate the molecular mechanisms of cypermethrin resistance in Culex pipiens pallens Coq, the susceptible strain (SS strain) and cypermethrin resistant strain (CR strain) of Cx. p. pallens were investigated in this paper. The cypermethrin resistance ratio of CR strain to SS strain was measured by biological assays method, the cDNA sequence of sodium channel was cloned and analyzed. Real-time quantitative RT-PCR was used to detect the expression levels of the detoxification-related genes across between CR strain and SS strain of Cx. p. pallens. Bioassays indicated that CR strain was 283.06 and 80.68-fold resistance to cypermethrin and permethrin as compared to the susceptible strain, respectively. The sequence variability analysis of sodium channel gene between SS strain and CR strain shows that 4 point mutations (R954Q, L1023F, S1775G and A1989E) appear on the amino acid sequence of sodium channel of CR strain. The transcriptional levels of CYP6Z10, CYP9M10, CPGSTd1 and CPGSTd2 in the resistant strain are significantly higher than it is in the susceptible. The transcripts of CYP4H34 and E4 esterase have no significant difference between the CR strain and SS strain. The results indicated that sodium channel mutations, combined with elevated levels of P450s and GSTs, are associated with cypermethrin resistance in CR strain.

Status of insecticide resistance in Anopheles mosquitoes in Ubon Ratchathani province, Northeastern Thailand

Background

Malaria is common in hilly, forested areas along national borders in Southeast Asia. Insecticide resistance in malaria vectors has been detected in a few countries in the Greater Mekong sub-region (GMS), representing a threat to malaria control and prevention. This study aims to determine the insecticide resistance status of Anopheles mosquitoes in Ubon Ratchathani province, northeastern Thailand, where increasing number of malaria cases were reported recently.

Methods

Mosquitoes were collected in 2013–2015 using human landing and cattle bait collections in six sites during both the rainy and dry seasons. Mosquitoes were first morphologically identified to species and their susceptibility status to deltamethrin (0.05%), permethrin (0.75%) and DDT (4%) investigated, according to WHO guidelines. Bioassays with the synergists PBO and DEF were carried out to address the role of detoxifying enzymes in insecticide resistance. DNA sequencing of a fragment of the voltage-gated sodium channel gene was carried out to detect knock-down resistance (kdr) substitutions at position 1014 in resistant species.

Results

Due to low vector abundance, complete bioassays (n ≥ 100 mosquitoes) were only achieved for Anopheles hyrcanus s.l., which was resistant to all insecticides tested (mortality ranged from 45 to 87%). Suspected resistance to DDT was found in Anopheles barbirostris s.l. (mortality 69%), but it was susceptible to deltamethrin (mortality 97–100%) and permethrin (mortality 100%). Although insufficient number of primary vectors were collected, results showed that Anopheles dirus s.l. and Anopheles maculatus s.l. were susceptible to deltamethrin (mortality 100%). Anopheles nivipes and Anopheles philippinensis were susceptible to all three insecticides. PBO significantly increased mortality to deltamethrin and permethrin in pyrethroid-resistant An. hyrcanus s.l. None of the sequenced specimens presented the L1014F or L1014S mutation.

Discussion

This study shows that insecticide resistance is present in potential malaria vectors in northeastern Thailand. The absence of kdr mutations in all Anopheles species tested suggests that metabolic resistance is the main mechanism of pyrethroid resistance. This study provides new findings about insecticide susceptibility status of potential malaria vectors in northeastern Thailand that are deemed important to guide malaria vector control.

Insecticide resistance status of malaria vectors in Lao PDR

Knowledge on insecticide resistance in Anopheles species is a basic requirement to guide malaria vector control programs. In Lao PDR, vector control relies on insecticide residual spraying (IRS) and impregnated bed-nets (ITNs) with the use of pyrethroids. Here, the susceptibility of Anopheles species, including several malaria vectors (An. maculatus and An. minimus), to various insecticides was investigated in ten provinces of Lao PDR through a north-south transect. Bioassays were performed on field caught female mosquitoes using the standard WHO susceptibility tests with DDT (4%), deltamethrin (0.05%) and permethrin (0.75%). In addition, the DIIS6 region of the para-type sodium channel gene was amplified and sequenced to identify knockdown resistance mutations (kdr). Resistance to DDT and permethrin was detected in suspected malaria vectors, such as An. nivipes and An. philippinensis in Lao PDR. Resistance to the formerly used DDT was found in a population of An. maculatus s.l. from Luang Prabang province. No resistance to pyrethroids was found in primary vectors, indicating that these insecticides are still adequate for malaria vector control. However, high resistance levels to pyrethroids was found in-vector species and reduced susceptibility to permethrin in An. minimus and An. maculatus was reported in specific localities which raises concerns for pyrethroid-based control in the future. No kdr mutation was found in any of the resistant populations tested hence suggesting a probable role detoxification enzymes in resistance. This study highlights the necessity to continue the monitoring of insecticide susceptibility to early detect potential occurrence and/or migration of insecticide resistance in malaria vectors in Lao PDR.

Insecticide resistance in malaria vectors along the Thailand-Myanmar border

BACKGROUND

There is a paucity of data about the susceptibility status of malaria vectors to Public Health insecticides along the Thailand-Myanmar border. This lack of data is a limitation to guide malaria vector-control in this region. The aim of this study was to assess the susceptibility status of malaria vectors to deltamethrin, permethrin and DDT and to validate a simple molecular assay for the detection of knock-down resistance (kdr) mutations in the study area.

METHODS

Anopheles mosquitoes were collected in four sentinel villages during August and November 2014 and July 2015 using human landing catch and cow bait collection methods. WHO susceptibility tests were carried out to measure the mortality and knock-down rates of female mosquitoes to deltamethrin (0.05%), permethrin (0.75%) and DDT (4%). DNA sequencing of a fragment of the voltage-gated sodium channel gene was carried out to identify knock-down resistance (kdr) mutations at position 1014 in mosquitoes surviving exposure to insecticides.

RESULTS

A total of 6295 Anopheles belonging to ten different species were bioassayed. Resistance or suspected resistance to pyrethroids was detected in An. barbirostris (s.l.) (72 and 84% mortality to deltamethrin (n = 504) and permethrin (n = 493) respectively), An. hyrcanus (s.l.) (33 and 48% mortality to deltamethrin (n = 172) and permethrin (n = 154), respectively), An. jamesii (87% mortality to deltamethrin, n = 111), An. maculatus (s.l.) (85 and 97% mortality to deltamethrin (n = 280) and permethrin (n = 264), respectively), An. minimus (s.l.) (92% mortality, n = 370) and An. vagus (75 and 95% mortality to deltamethrin (n =148) and permethrin (n = 178), respectively). Resistance or suspected resistance to DDT was detected in An. barbirostris (s.l.) (74% mortality, n = 435), An. hyrcanus (s.l.) (57% mortality, n = 91) and An. vagus (97% mortality, n = 133). The L1014S kdr mutation at both heterozygous and homozygous state was detected only in An. peditaeniatus (Hyrcanus Group).

CONCLUSION

Resistance to pyrethroids is present along the Thailand-Myanmar border, and it represents a threat for malaria vector control. Further investigations are needed to better understand the molecular basis of insecticide resistance in malaria vectors in this area.

Diversity of the Bacterial Microbiota of Anopheles Mosquitoes from Binh Phuoc Province, Vietnam

The naturally acquired microbiota of Anopheles can influence vector’s susceptibility to Plasmodium and its capacity to transmit them. Microbiota modification is a new challenge to limit disease transmission but it still needs advanced knowledges on bacterial community in Anopheles, especially in wild and infected specimens from diverse origin and species. Bacterial culture and 16S rRNA gene-PCR associated to Temporal Temperature Gradient Electrophoresis (TTGE) were applied to explore the bacterial diversity in the abdomen of 100 wild specimens (eight Anopheles species) collected in the Binh Phuoc Province, Vietnam. Culture and PCR-TTGE were complementary. The bacterial richness of the mosquito collection encompassed 105 genera belonging to seven phyla, mostly Proteobacteria, Firmicutes, and Actinobacteria. Staphylococcus, Clostridium, and Bacillus in Firmicutes were the most prevalent genera. However, Proteobacteria represented by 57 genera was the most diversified phylum in Anopheles microbiota. The high overall of Anopheles-associated bacteria is confirmed with, to our knowledge, 51 genera described for the first time in Anopheles microbiota. However, the diversity per specimen was low with average diversity index and the average Shannon–Wiener score (H) of 4.843 and 5.569, respectively. The most represented bacterial genera were present in <30% of the specimens. Consequently, the core microbiota share by Anopheles from Binh Phuoc was very narrow, suggesting that Anopheles microbiota was greatly influenced by local environments. The repertory of bacterial genera in two specimens of An. dirus and An. pampanai naturally infected by Plasmodium vivax was also described as preliminary results. Finally, this study completed the repertory of bacteria associated to wild Anopheles. Anopheles associated-bacteria appeared specimen-dependent rather than mosquitoe species- or group-dependent. Their origin and the existence of Anopheles-specific bacterial taxa are discussed.

Insecticide susceptibility of Anopheles stephensi to DDT and current insecticides in an elimination area in Iran

BACKGROUND

Iran has recently initiated a malaria elimination program with emphasis on vector control strategies which are heavily reliant on indoor residual spraying and long-lasting insecticidal nets. Insecticide resistance seriously threatens the efficacy of vector control strategies. This study was conducted to determine the insecticide susceptibility of Anopheles stephensi to DDT and current insecticides in Jask county as an active malaria focus in southeastern Iran.

METHODS

In this study, the anopheline larvae were collected from different aquatic habitats in Jask county and transported to insectarium, fed with sugar and then 3-day-old adults were used for susceptibility tests. WHO insecticide susceptibility tests were performed with DDT (4 %), malathion (5 %), lambda-cyhalothrin (0.05 %), deltamethrin (0.05 %) and permethrin (0.75 %).

RESULTS

The field strain of An. stephensi was found resistant to DDT and lambda-cyhalothrin. The LT50 values for DDT and lambda-cyhalothrin in this species were 130.25, and 37.71 min, respectively. Moreover, An. stephensi was completely susceptible to malathion and permethrin and tolerant to deltamethrin.

CONCLUSION

The present study results confirm the resistance of the major malaria vector, An. stephensi, to DDT and lambda-cyhalothrin, and tolerance to deltamethrin, which could gradually increase and spread into other malaria endemic areas. Thus, there is a need for regular monitoring of insecticide resistance in order to select suitable insecticides for vector control interventions towards malaria elimination.

Sodium Channel Mutations and Pyrethroid Resistance in Aedes aegypti

Pyrethroid insecticides are widely used to control insect pests and human disease vectors. Voltage-gated sodium channels are the primary targets of pyrethroid insecticides. Mutations in the sodium channel have been shown to be responsible for pyrethroid resistance, known as knockdown resistance (kdr), in various insects including mosquitoes. In Aedes aegypti mosquitoes, the principal urban vectors of dengue, zika, and yellow fever viruses, multiple single nucleotide polymorphisms in the sodium channel gene have been found in pyrethroid-resistant populations and some of them have been functionally confirmed to be responsible for kdr in an in vitro expression system, Xenopus oocytes. This mini-review aims to provide an update on the identification and functional characterization of pyrethroid resistance-associated sodium channel mutations from Aedes aegypti. The collection of kdr mutations not only helped us develop molecular markers for resistance monitoring, but also provided valuable information for computational molecular modeling of pyrethroid receptor sites on the sodium channel.

Reductions in malaria in pregnancy and adverse birth outcomes following indoor residual spraying of insecticide in Uganda

Background

Indoor residual spraying of insecticide (IRS) is a key intervention for reducing the burden of malaria in Africa. However, data on the impact of IRS on malaria in pregnancy and birth outcomes is limited.

Methods

An observational study was conducted within a trial of intermittent preventive therapy during pregnancy in Tororo, Uganda. Women were enrolled at 12–20 weeks of gestation between June and October 2014, provided with insecticide-treated bed nets, and followed through delivery. From December 2014 to February 2015, carbamate-containing IRS was implemented in Tororo district for the first time. Exact spray dates were collected for each household. The exposure of interest was the proportion of time during a woman’s pregnancy under protection of IRS, with three categories of protection defined: no IRS protection, >0–20 % IRS protection, and 20–43 % IRS protection. Outcomes assessed included malaria incidence and parasite prevalence during pregnancy, placental malaria, low birth weight (LBW), pre-term delivery, and fetal/neonatal deaths.

Results

Of 289 women followed, 134 had no IRS protection during pregnancy, 90 had >0–20 % IRS protection, and 65 had >20–43 % protection. During pregnancy, malaria incidence (0.49 vs 0.10 episodes ppy, P = 0.02) and parasite prevalence (20.0 vs 8.9 %, P < 0.001) were both significantly lower after IRS. At the time of delivery, the prevalence of placental parasitaemia was significantly higher in women with no IRS protection (16.8 %) compared to women with 0–20 % (1.1 %, P = 0.001) or >20–43 % IRS protection (1.6 %, P = 0.006). Compared to women with no IRS protection, those with >20–43 % IRS protection had a lower risk of LBW (20.9 vs 3.1 %, P = 0.002), pre-term birth (17.2 vs 1.5 %, P = 0.006), and fetal/neonatal deaths (7.5 vs 0 %, P = 0.03).

Conclusion

In this setting, IRS was temporally associated with lower malaria parasite prevalence during pregnancy and at delivery, and improved birth outcomes. IRS may represent an important tool for combating malaria in pregnancy and for improving birth outcomes in malaria-endemic settings.

Trial Registration Current Controlled Trials Identifier NCT02163447

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1489-x) contains supplementary material, which is available to authorized users.

Landscape genetic structure and evolutionary genetics of insecticide resistance gene mutations in Anopheles sinensis

Background

Anopheles sinensis is one of the most abundant vectors of malaria and other diseases in Asia. Vector control through the use of insecticides is the front line control method of vector-borne diseases. Pyrethroids are the most commonly used insecticides due to their low toxicity to vertebrates and low repellency. However, the extensive use of insecticides has imposed strong selection pressure on mosquito populations for resistance. High levels of resistance to pyrethroid insecticides and various mutations and haplotypes in the para sodium channel gene that confers knockdown resistance (kdr) have been detected in An. sinensis. Despite the importance of kdr mutations in pyrethroid resistance, the evolutionary origin of the kdr mutations is unknown. This study aims to examine the evolutionary genetics of kdr mutations in relation to spatial population genetic structure of An. sinensis.

Methods

Adults or larvae of Anopheles sinensis were collected from various geographic locations in China. DNA was extracted from individual mosquitoes. PCR amplification and DNA sequencing of the para-type sodium channel gene were conducted to analyse kdr allele frequency distribution, kdr codon upstream and downstream intron polymorphism, population genetic diversity and kdr codon evolution. The mitochondrial cytochrome c oxidase COI and COII genes were amplified and sequenced to examine population variations, genetic differentiation, spatial population structure, population expansion and gene flow patterns.

Results

Three non-synonymous mutations (L1014F, L1014C, and L1014S) were detected at the kdr codon L1014 of para-type sodium channel gene. A patchy distribution of kdr mutation allele frequencies from southern to central China was found. Near fixation of kdr mutation was detected in populations from central China, but no kdr mutations were found in populations from southwestern China. More than eight independent mutation events were detected in the three kdr alleles, and at least one of them evolved multiple times subsequent to their first divergence. Based on sequence analysis of the mitochondrial COI and COII genes, significant and large genetic differentiation was detected between populations from southwestern China and central China. The patchy distribution of kdr mutation frequencies is likely a consequence of geographic isolation in the mosquito populations and the long-term insecticide selection.

Conclusion

Our results indicate multiple origins of the kdr insecticide-resistant alleles in An. sinensis from southern and central China. Local selection related to intense and prolonged use of insecticide for agricultural purposes, as well as frequent migrations among populations are likely the explanations for the patchy distribution of kdr mutations in China. On the contrary, the lack of kdr mutations in Yunnan and Sichuan is likely a consequence of genetic isolation and absence of strong selection pressure. The present study compares the genetic patterns revealed by a functional gene with a neutral marker and demonstrates the combined impact of demographic and selection factors on population structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1513-6) contains supplementary material, which is available to authorized users.

Pesticide knowledge and practice among horticultural workers in the Lâm Đồng region, Vietnam: A case study of chrysanthemum and strawberries

BACKGROUND

In Vietnam, large amounts of pesticides are being used in agriculture. Next to benefits for agriculture, pesticides impose a huge threat to the environment when they are used in the wrong way. The objective of this work was to determine the level of knowledge and awareness of the smallholder farmers towards the use of pesticides in Vietnam, Dà Lat region. Based on the collected data, an occupational and environmental risk assessment was performed.

RESULTS

The results indicate that the majority of the pesticide operators in the strawberry and chrysanthemum crops have a rather high education. Pesticide knowledge, on the other hand, is usually gained through experience with pests and diseases. Only 30% of the farmers consulted a pesticide specialist or government stewardship for information on (new) pesticide products. Pesticide usage is rather high with application frequencies up to once every three days during the wet season. Pesticide packages are stored to be incinerated (51%) or to be thrown away with the garbage/taken to the landfill (37%). Only a small percentage disposes the packages into the local river (2%). The use of personal protection equipment is well established. Occupational risk assessment showed that the re-entry worker is exposed to a high risk.

CONCLUSION

While a general awareness of the hazard of pesticides to human health and the environment is present, practical implementation of this awareness, however, is still limited in strawberry and chrysanthemum crop. The environmental risk evaluation indicated plant protection products of which the use should be limited.

Evaluation of Insecticides Susceptibility and Malaria Vector Potential of Anopheles annularis s.l. and Anopheles vagus in Assam, India

During the recent past, development of DDT resistance and reduction to pyrethroid susceptibility among the malaria vectors has posed a serious challenge in many Southeast Asian countries including India. Current study presents the insecticide susceptibility and knock-down data of field collected Anopheles annularis sensu lato and An. vagus mosquito species from endemic areas of Assam in northeast India. Anopheles annularis s.l. and An. vagus adult females were collected from four randomly selected sentinel sites in Orang primary health centre (OPHC) and Balipara primary health centre (BPHC) areas, and used for testing susceptibility to DDT, malathion, deltamethrin and lambda-cyhalothrin. After insecticide susceptibility tests, mosquitoes were subjected to VectorTest™ assay kits to detect the presence of malaria sporozoite in the mosquitoes. An. annularis s.l. was completely susceptible to deltamethrin, lambda-cyhalothrin and malathion in both the study areas. An. vagus was highly susceptible to deltamethrin in both the areas, but exhibited reduced susceptibility to lambda-cyhalothrin in BPHC. Both the species were resistant to DDT and showed very high KDT50 and KDT99 values for DDT. Probit model used to calculate the KDT50 and KDT99 values did not display normal distribution of percent knock-down with time for malathion in both the mosquito species in OPHC (p<0.05) and An. vagus in BPHC (χ2 = 25.3; p = 0.0), and also for deltamethrin to An. vagus in BPHC area (χ2 = 15.4; p = 0.004). Minimum infection rate (MIR) of Plasmodium sporozoite for An. vagus was 0.56 in OPHC and 0.13 in BPHC, while for An. annularis MIR was found to be 0.22 in OPHC. Resistance management strategies should be identified to delay the expansion of resistance. Testing of field caught Anopheles vectors from different endemic areas for the presence of malaria sporozoite may be useful to ensure their role in malaria transmission.

Diversity of the Bacterial Microbiota of Anopheles Mosquitoes from Binh Phuoc Province, Vietnam

The naturally acquired microbiota of Anopheles can influence vector's susceptibility to Plasmodium and its capacity to transmit them. Microbiota modification is a new challenge to limit disease transmission but it still needs advanced knowledges on bacterial community in Anopheles, especially in wild and infected specimens from diverse origin and species. Bacterial culture and 16S rRNA gene-PCR associated to Temporal Temperature Gradient Electrophoresis (TTGE) were applied to explore the bacterial diversity in the abdomen of 100 wild specimens (eight Anopheles species) collected in the Binh Phuoc Province, Vietnam. Culture and PCR-TTGE were complementary. The bacterial richness of the mosquito collection encompassed 105 genera belonging to seven phyla, mostly Proteobacteria, Firmicutes, and Actinobacteria. Staphylococcus, Clostridium, and Bacillus in Firmicutes were the most prevalent genera. However, Proteobacteria represented by 57 genera was the most diversified phylum in Anopheles microbiota. The high overall of Anopheles-associated bacteria is confirmed with, to our knowledge, 51 genera described for the first time in Anopheles microbiota. However, the diversity per specimen was low with average diversity index and the average Shannon-Wiener score (H) of 4.843 and 5.569, respectively. The most represented bacterial genera were present in <30% of the specimens. Consequently, the core microbiota share by Anopheles from Binh Phuoc was very narrow, suggesting that Anopheles microbiota was greatly influenced by local environments. The repertory of bacterial genera in two specimens of An. dirus and An. pampanai naturally infected by Plasmodium vivax was also described as preliminary results. Finally, this study completed the repertory of bacteria associated to wild Anopheles. Anopheles associated-bacteria appeared specimen-dependent rather than mosquitoe species- or group-dependent. Their origin and the existence of Anopheles-specific bacterial taxa are discussed.

Past and new challenges for malaria control and elimination: the role of operational research for innovation in designing interventions

This meeting report presents the outcomes of a workshop held in Bangkok on December 1st 2014, where the following challenges were discussed: the threat of resistance to artemisinin and artemisinin-based combination therapy in the Greater Mekong Sub-region (GMS) and in Africa; access to treatment for most at risk and hard to reach population; insecticide resistance, residual and outdoors transmission. The role of operational research and the interactions between research institutions, National Malaria Control Programmes, Civil Society Organizations, and of financial and technical partners to address those challenges and to accelerate translation of research into policies and programmes were debated. The threat and the emergency of the artemisinin resistance spread and independent emergence in the GMS was intensely debated as it is now close to the border of India. The need for key messages, based on scientific evidence and information available and disseminated without delay, was highlighted as crucial for an effective and urgent response.

Knockdown resistance of Anopheles sinensis in Henan province, China

Background

Vivax malaria was historically epidemic in Henan Province of China and Anopheles sinensis was the main vectors and poor farming communities bare the greatest burden of disease. Knockdown resistance in An. sinensis is one of the mechanisms of resistance against pyrethroids. In the present study, the frequency of mutations from An. sinensis was examined in Henan province, China.

Methods

Anopheles was collected from Kaifeng, Tongbai, Tanghe, Pingqiao, Shihe, and Yongcheng counties of Henan province in 2013. Molecular identification of Anopheles species was conducted by polymerase chain reaction (PCR) amplifying the internal transcribed spacer 2 (ITS2). Part of the IIS6 domain of the para-type sodium channel protein gene was polymerase chain reaction-amplified and directly sequenced. Frequency and geographic difference of kdr gene mutant types were analysed.

Results

208 Anopheles were received molecular identification, of which 169 (81.25%) were An. sinensis, 25 (12.02%) were Anopheles yatsushiroensis, and 12 (5.77%) were Anopheles lesteri. A 325 bp fragment of the para-type sodium channel gene including position 1014 was successfully sequenced from 139 Anopheles, of which 125 (89.93%) were An. sinensis, 12 (8.63%) were An. yatsushiroensis, 2 (1.44%) were An. lesteri. The molecular analyses revealed that mutations existed at codon 1014 in An. sinensis but not in An. yatsushiroensis and An. lesteri. Frequency of kdr mutation was 73.60% (92/125) from population of An. sinensis in Henan province, of which L1014F (TTT + TTC) allele frequencies accounted for 46.40% (58/125), and was higher than that of L1014C(TGT) which accounted for 27.20% (34/125) ( χ2 = 55.423, P < 0.001). The frequency of kdr mutation in Kaifeng county was 100% (49/49), and was higher than that of 37.93% (11/29) in Tongbai, 54.55% (6/11) in Pingqiao, 50.00% (3/3) in Shihe, and 62.50% (10/16) in Yongcheng county, respectively (χ2 = 39.538, P < 0.001; χ2 = 24.298, P < 0.001; χ2 = 25.913, P < 0.001; χ2 = 20.244, P < 0.001). While 92.86% (13/14) frequency of kdr mutation was found in Tanghe county, which was higher than that in Tongbai county (χ2 = 11.550, P = 0.0018).

Conclusions

A high frequency of kdr gene mutations from population of An. sinensis in Henan province was found.

Malaria transmission, infection, and disease at three sites with varied transmission intensity in Uganda: implications for malaria control

The intensification of control interventions has led to marked reductions in malaria burden in some settings, but not others. To provide a comprehensive description of malaria epidemiology in Uganda, we conducted surveillance studies over 24 months in 100 houses randomly selected from each of three subcounties: Walukuba (peri-urban), Kihihi (rural), and Nagongera (rural). Annual entomological inoculation rate (aEIR) was estimated from monthly Centers for Disease Control and Prevention (CDC) light trap mosquito collections. Children aged 0.5-10 years were provided long-lasting insecticidal nets (LLINs) and followed for measures of parasite prevalence, anemia and malaria incidence. Estimates of aEIR were 2.8, 32.0, and 310 infectious bites per year, and estimates of parasite prevalence 7.4%, 9.3%, and 28.7% for Walukuba, Kihihi, and Nagongera, respectively. Over the 2-year study, malaria incidence per person-years decreased in Walukuba (0.51 versus 0.31, P = 0.001) and increased in Kihihi (0.97 versus 1.93, P < 0.001) and Nagongera (2.33 versus 3.30, P < 0.001). Of 2,582 episodes of malaria, only 8 (0.3%) met criteria for severe disease. The prevalence of anemia was low and not associated with transmission intensity. In our cohorts, where LLINs and prompt effective treatment were provided, the risk of complicated malaria and anemia was extremely low. However, malaria incidence was high and increased over time at the two rural sites, suggesting improved community-wide coverage of LLIN and additional malaria control interventions are needed in Uganda.

A mutation in the intracellular loop III/IV of mosquito sodium channel synergizes the effect of mutations in helix IIS6 on pyrethroid resistance

Activation and inactivation of voltage-gated sodium channels are critical for proper electrical signaling in excitable cells. Pyrethroid insecticides promote activation and inhibit inactivation of sodium channels, resulting in prolonged opening of sodium channels. They preferably bind to the open state of the sodium channel by interacting with two distinct receptor sites, pyrethroid receptor sites PyR1 and PyR2, formed by the interfaces of domains II/III and I/II, respectively. Specific mutations in PyR1 or PyR2 confer pyrethroid resistance in various arthropod pests and disease vectors. Recently, a unique mutation, N(1575)Y, in the cytoplasmic loop linking domains III and IV (LIII/IV) was found to coexist with a PyR2 mutation, L(1014)F in IIS6, in pyrethroid-resistant populations of Anopheles gambiae. To examine the role of this mutation in pyrethroid resistance, N(1575)Y alone or N(1575)Y + L(1014)F were introduced into an Aedes aegypti sodium channel, AaNav1-1, and the mutants were functionally examined in Xenopus oocytes. N(1575)Y did not alter AaNav1-1 sensitivity to pyrethroids. However, the N(1575)Y + L(1014)F double mutant was more resistant to pyrethroids than the L(1014)F mutant channel. Further mutational analysis showed that N(1575)Y could also synergize the effect of L(1014)S/W, but not L(1014)G or other pyrethroid-resistant mutations in IS6 or IIS6. Computer modeling predicts that N(1575)Y allosterically alters PyR2 via a small shift of IIS6. Our findings provide the molecular basis for the coexistence of N(1575)Y with L(1014)F in pyrethroid resistance, and suggest an allosteric interaction between IIS6 and LIII/IV in the sodium channel.

The colonization of pyrethroid resistant strain from wild Anopheles sinensis, the major Asian malaria vector

Background

Anopheles sinensis is one of the most important malaria vectors in Asian countries. The rapid spread of insecticide resistance has become a major obstacle for insecticide-based strategies for vector control. Therefore, it is necessary to prepare an insecticide-resistant strain of An. sinensis to further understand the insecticide resistance mechanisms in this species to facilitate genetic approaches to targeting the insecticide-resistant population of this important malaria vector.

Methods

An. sinensis mosquitoes were collected from regions where pyrethroid resistance had been reported. The mosquitoes were subjected to continuous pyrethroid selection after species confirmation, and the forced copulation method was used to increase the mating rate. In addition, the knockdown-resistance (kdr) mutation frequencies of each generation of An. sinensis were measured; and the metabolic enzyme activities of cytochrome P450 monoxygenases (P450s) and glutathione S-transferases (GSTs) were detected.

Results

The identification of field-captured An. sinensis was confirmed by both morphological and molecular methods. The population of An. sinensis exhibited stable resistance to pyrethroid after continuous generations of pyrethroid selection in the laboratory with high kdr mutation frequencies; and elevated levels of both P450s and GSTs were significantly found in field selected populations comparing with the laboratory susceptible strain. So far, the colonised strain has reached its eleventh generation and culturing well in the laboratory.

Conclusions

We colonised a pyrethroid-resistant population of An. sinensis in the laboratory, which provides a fundamental model for genetic studies of this important malaria vector.

Insecticide resistance and its underlying mechanisms in field populations of Aedes aegypti adults (Diptera: Culicidae) in Singapore

Background

In Singapore, dose–response bioassays of Aedes aegypti (L.) adults have been conducted, but the mechanisms underlying resistance to insecticides remain unclear. In this study, we evaluated insecticide resistance and its underlying mechanism in field populations of Ae. aegypti adults.

Methods

Seven populations of Ae. aegypti were collected from public residential areas and assays were conducted according to WHO guidelines to determine their susceptibility to several commonly used insecticides.

Results

Various levels of pyrethroid resistance (RR₅₀ = 3.76 to 142.06-fold) and low levels of pirimiphos-methyl resistance (RR₅₀ = 1.01 to 1.51-fold) were detected. The insecticide susceptibility profile of Ae. aegypti adults was homogenous among the different study sites. Addition of the synergists piperonyl butoxide, S,S,S,-tributyl phosphorotrithioate, and triphenyl phosphate generally failed to enhance the toxicity of the insecticides investigated, suggesting an insignificant role of metabolic-based insecticide resistance and possible involvement of target site resistance. Further biochemical investigation of specific metabolic enzyme activities provided further evidence that detoxifying enzymes such as mono-oxygenases, esterases, glutathione S-transferases and altered acethylcholinesterases generally did not contribute to the resistance observed.

Conclusions

This study confirmed the presence of pyrethroid resistance among Ae. aegypti adults in Singapore and documented the early onset of organophosphate resistance.

Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids - a review

Constant and extensive use of chemical insecticides has created a selection pressure and favored resistance development in many insect species worldwide. One of the most important pyrethroid resistance mechanisms is classified as target site insensitivity, due to conformational changes in the target site that impair a proper binding of the insecticide molecule. The voltage-gated sodium channel (NaV) is the target of pyrethroids and DDT insecticides, used to control insects of medical, agricultural and veterinary importance, such as anophelines. It has been reported that the presence of a few non-silent point mutations in the NaV gene are associated with pyrethroid resistance, termed as 'kdr' (knockdown resistance) for preventing the knockdown effect of these insecticides. The presence of these mutations, as well as their effects, has been thoroughly studied in Anopheles mosquitoes. So far, kdr mutations have already been detected in at least 13 species (Anopheles gambiae, Anopheles arabiensis, Anopheles sinensis, Anopheles stephensi, Anopheles subpictus, Anopheles sacharovi, Anopheles culicifacies, Anopheles sundaicus, Anopheles aconitus, Anopheles vagus, Anopheles paraliae, Anopheles peditaeniatus and Anopheles albimanus) from populations of African, Asian and, more recently, American continents. Seven mutational variants (L1014F, L1014S, L1014C, L1014W, N1013S, N1575Y and V1010L) were described, with the highest prevalence of L1014F, which occurs at the 1014 site in NaV IIS6 domain. The increase of frequency and distribution of kdr mutations clearly shows the importance of this mechanism in the process of pyrethroid resistance. In this sense, several species-specific and highly sensitive methods have been designed in order to genotype individual mosquitoes for kdr in large scale, which may serve as important tolls for monitoring the dynamics of pyrethroid resistance in natural populations. We also briefly discuss investigations concerning the course of Plasmodium infection in kdr individuals. Considering the limitation of insecticides available for employment in public health campaigns and the absence of a vaccine able to brake the life cycle of the malaria parasites, the use of pyrethroids is likely to remain as the main strategy against mosquitoes by either indoor residual spraying (IR) and insecticide treated nets (ITN). Therefore, monitoring insecticide resistance programs is a crucial need in malaria endemic countries.

Sodium channel point mutations associated with pyrethroid resistance in Chinese strains of Culex pipiens quinquefasciatus (Diptera: Culicidae)

BACKGROUND

Pesticide resistance due to sodium channel point mutations has been well documented in many mosquito species.

METHODS

We tested the resistance of six, wild, Chinese populations of the mosquito Culex pipiens quinquefasciatus to deltamethrin and cyhalothrin. The full length of the sodium channel gene was cloned and sequenced from a pooled sample of mosquitoes from each population.

RESULTS

Seven amino acid alterations were found (V250M, R436K, M943V, I973T, L1035F, L1035S and E1901D). Correlation between the frequencies of these mutations and the level of pesticide resistance (LC50) associated with them indicates that those at position L1035 (corresponding to position L1014F in the house fly, Musca domestica; GenBank Accession No.: X96668) are associated with resistance to deltamethrin and cyhalothrin. The frequency of the L1035F mutation was significantly correlated with resistance to deltamethrin (R2 = 0.536, P = 0.049) and cyhalothrin (R2 = 0.626, P = 0.030), and the combined frequency of the L1035F and L1035S mutations was significantly correlated with resistance to both deltamethrin (R2 = 0.661, P = 0.025), and cyhalothrin (R2 = 0.803, P = 0.008). None of the other mutations were correlated with either deltamethrin or cyhalothrin resistance. Interestingly, an HWE test indicated significant linkage between the M943V and I973T mutations (P < 0.01), but further research is required to determine the biological significance of this linkage.

CONCLUSIONS

Identifying these mutations may be of practical benefit to the development of pesticide resistance management programs.

Insecticide resistance of Anopheles sinensis and An. vagus in Hainan Island, a malaria-endemic area of China

Background

Malaria is one of the most important public health problems in Southeast Asia, including Hainan Island, China. Vector control is the main malaria control measure, and insecticide resistance is a major concern for the effectiveness of chemical insecticide control programs. The objective of this study is to determine the resistance status of the main malaria vector species to pyrethroids and other insecticides recommended by the World Health Organization (WHO) for indoor residual sprays.

Methods

The larvae and pupae of Anopheles mosquitoes were sampled from multiple sites in Hainan Island, and five sites yielded sufficient mosquitoes for insecticide susceptibility bioassays. Bioassays of female adult mosquitoes three days after emergence were conducted in the two most abundant species, Anopheles sinensis and An. vagus, using three insecticides (0.05% deltamethrin, 4% DDT, and 5% malathion) and following the WHO standard tube assay procedure. P450 monooxygenase, glutathione S-transferase and carboxylesterase activities were measured. Mutations at the knockdown resistance (kdr) gene and the ace-1gene were detected by DNA sequencing and PCR-RFLP analysis, respectively.

Results

An. sinensis and An. vagus were the predominant Anopheles mosquito species. An. sinensis was found to be resistant to DDT and deltamethrin. An. vagus was susceptible to deltamethrin but resistant to DDT and malathion. Low kdr mutation (L1014F) frequency (<10%) was detected in An. sinensis, but no kdr mutation was detected in An. vagus populations. Modest to high (45%-75%) ace-1 mutation frequency was found in An. sinensis populations, but no ace-1 mutation was detected in An. vagus populations. Significantly higher P450 monooxygenase and carboxylesterase activities were detected in deltamethrin-resistant An. sinensis, and significantly higher P450 monooxygenase, glutathione S-transferase and carboxylesterase activities were found in malathion-resistant An. vagus mosquitoes.

Conclusions

Multiple insecticide resistance was found in An. sinensis and An. vagus in Hainan Island, a malaria-endemic area of China. Cost-effective integrated vector control programs that go beyond synthetic insecticides are urgently needed.

Anopheles sinensis mosquito insecticide resistance: comparison of three mosquito sample collection and preparation methods and mosquito age in resistance measurements

Background

Insecticide resistance monitoring in malaria mosquitoes is essential for guiding the rational use of insecticides in vector control programs. Resistance bioassay is the first step for insecticide monitoring and it lays an important foundation for molecular examination of resistance mechanisms. In the literature, various mosquito sample collection and preparation methods have been used, but how mosquito sample collection and preparation methods affect insecticide susceptibility bioassay results is largely unknown. The objectives of this study were to determine whether mosquito sample collection and preparation methods affected bioassay results, which may cause incorrect classification of mosquito resistance status.

Methods

The study was conducted in Anopheles sinensis mosquitoes in two study sites in central China. Three mosquito sample collection and preparation methods were compared for insecticide susceptibility, kdr frequencies and metabolic enzyme activities: 1) adult mosquitoes collected from the field; 2) F1 adults from field collected, blood-fed mosquitoes; and 3) adult mosquitoes reared from field collected larvae.

Results

Mosquito sample collection and preparation methods significantly affected mortality rates in the standard WHO tube resistance bioassay. Mortality rate of field-collected female adults was 10-15% higher than in mosquitoes reared from field-collected larvae and F1 adults from field collected blood-fed females. This pattern was consistent in mosquitoes from the two study sites. High kdr mutation frequency (85-95%) with L1014F allele as the predominant mutation was found in our study populations. Field-collected female adults consistently exhibited the highest monooxygenase and GST activities. The higher mortality rate observed in the field-collected female mosquitoes may have been caused by a mixture of mosquitoes of different ages, as older mosquitoes were more susceptible to deltamethrin than younger mosquitoes.

Conclusions

Female adults reared from field-collected larvae in resistance bioassays are recommended to minimize the effect of confounding factors such as mosquito age and blood feeding status so that more reliable and reproducible mortality may be obtained.

Molecular evidence for historical presence of knock-down resistance in Anopheles albimanus, a key malaria vector in Latin America

Background

Anopheles albimanus is a key malaria vector in the northern neotropics. Current vector control measures in the region are based on mass distributions of long-lasting insecticidal nets (LLINs) and focal indoor residual spraying (IRS) with pyrethroids. Resistance to pyrethroid insecticides can be mediated by increased esterase and/or multi-function oxidase activity and/or mutations in the voltage-gated sodium channel gene. The aim of this work was to characterize the homologous kdr region of the voltage-gated sodium channel gene in An. albimanus and to conduct a preliminary retrospective analysis of field samples collected in the 1990’s, coinciding with a time of intense pyrethroid application related to agricultural and public health insect control in the region.

Methods

Degenerate primers were designed to amplify the homologous kdr region in a pyrethroid-susceptible laboratory strain (Sanarate) of An. albimanus. Subsequently, a more specific primer pair was used to amplify and sequence the region that contains the 1014 codon associated with pyrethroid resistance in other Anopheles spp. (L1014F, L1014S or L1014C).

Results

Direct sequencing of the PCR products confirmed the presence of the susceptible kdr allele in the Sanarate strain (L1014) and the presence of homozygous-resistant kdr alleles in field-collected individuals from Mexico (L1014F), Nicaragua (L1014C) and Costa Rica (L1014C).

Conclusions

For the first time, the kdr region in An. albimanus is described. Furthermore, molecular evidence suggests the presence of kdr-type resistance in field-collected An. albimanus in Mesoamerica in the 1990s. Further research is needed to conclusively determine an association between the genotypes and resistant phenotypes, and to what extent they may compromise current vector control efforts.

Diversity and Convergence of Sodium Channel Mutations Involved in Resistance to Pyrethroids

Pyrethroid insecticides target voltage-gated sodium channels, which are critical for electrical signaling in the nervous system. The intensive use of pyrethroids in controlling arthropod pests and disease vectors has led to many instances of pyrethroid resistance around the globe. In the past two decades, studies have identified a large number of sodium channel mutations that are associated with resistance to pyrethroids. The purpose of this review is to summarize both common and unique sodium channel mutations that have been identified in arthropod pests of importance to agriculture or human health. Identification of these mutations provides valuable molecular markers for resistance monitoring in the field and helped the discovery of the elusive pyrethroid receptor site(s) on the sodium channel.

Susceptibility of Anopheles sinensis to Plasmodium vivax in malarial outbreak areas of central China

BACKGROUND

Anopheles sinensis, Anopheles anthropophagus, Anopheles minimus and Anopheles dirus are the major vectors of malaria transmission in China. Anopheles sinensis is considered a secondary vector due to its relatively low malaria-transmission ability. However, in 2005, an outbreak of over 40,000 Plasmodium vivax malaria cases was reported in areas where Anopheles sinensis was the only major vector. Therefore, it is necessary to reassess the malaria transmission ability of this vector species in China.

METHODS

Laboratory colonies of An. sinensis and An. anthropophagus, and first-generation progeny (F1) of An. sinensis that had been collected in central China, were infected by direct membrane feeding assay with mono-vivax gametocyte-containing blood collected from vivax-infected patients. The mosquitoes were kept for 7 to 14 days post-blood feeding to allow parasites to develop into oocysts and sporozoites. Infectivity was measured by dissecting midguts and salivary glands. The presence of oocysts and sporozoites was determined by microscopy at 7 and 14 days post-blood feeding, and the numbers of gametocytes and asexual parasites, as well as mosquito parasite infections, were determined.

RESULTS

The positive oocyst and sporozoite feed rates of the 142 pairs of lab-colony An. sinensis and An. anthropophagus were not significantly different, and the same results were found with the 10 pairs of laboratory and F1 An. sinensis. An. sinensis had more oocysts/midgut at 7 days post-feeding than An. anthropophagus, but the gametocytemia, asexual parasitemia, and ratio of macrogametocytes to microgametocytes, did not correlate with either oocyst or sporozoite infection. However, in the oocyst-positive mosquitoes, there was a correlation between gametocytemia and the average oocyst number/midgut.

CONCLUSIONS

The susceptibility of An. sinensis (both laboratory and F1) to P. vivax-infected blood is similar to Anopheles anthropophagus, when evaluated by membrane feeding assay under laboratory conditions. In recent years, in central China, the vivax malaria transmission ability of An. sinensis has probably been underestimated. Further studies of this species in other regions are needed. An. sinensis could also be a good candidate vector for evaluating candidate malaria transmission-blocking vaccines (TBV).

Relationship between knockdown resistance, metabolic detoxification and organismal resistance to pyrethroids in Anopheles sinensis

Anopheles sinensis is the most important vector of malaria in Southeast Asia, including China. Currently, the most effective measure to prevent malaria transmission relies on vector control through the use of insecticides, primarily pyrethroids. Extensive use of insecticides poses strong selection pressure on mosquito populations for resistance. Resistance to insecticides can arise due to mutations in the insecticide target site (target site resistance), which in the case of pyrethroids is the para-type sodium channel gene, and/or the catabolism of the insecticide by detoxification enzymes before it reaches its target (metabolic detoxification resistance). In this study, we examined deltamethrin resistance in An. sinensis from China and investigated the relative importance of target site versus metabolic detoxification mechanisms in resistance. A high frequency (>85%) of nonsynonymous mutations in the para gene was found in populations from central China, but not in populations from southern China. Metabolic detoxification as measured by the activity of monooxygenases and glutathione S-transferases (GSTs) was detected in populations from both central and southern China. Monooxygenase activity levels were significantly higher in the resistant than the susceptible mosquitoes, independently of their geographic origin. Stepwise multiple regression analyses in mosquito populations from central China found that both knockdown resistance (kdr) mutations and monooxygenase activity were significantly associated with deltamethrin resistance, with monooxygenase activity playing a stronger role. These results demonstrate the importance of metabolic detoxification in pyrethroid resistance in An. sinensis, and suggest that different mechanisms of resistance could evolve in geographically different populations.

First detection of multiple knockdown resistance (kdr)-like mutations in voltage-gated sodium channel using three new genotyping methods in Anopheles sinensis from Guangxi Province, China

To investigate knockdown resistance (kdr)-like mutations associated with pyrethroid resistance in Anopheles sinensis (Wiedemann, 1828), from Guangxi province, southwest China, a segment of a sodium channel gene was sequenced and genotyped using three new genotyping assays. Direct sequencing revealed the presence of TTG-to-TCG and TG-to-TTT mutations at allele position L1014, which led to L1014S and L1014F substitutions in a few individual and two novel substitutions of N1013S and L1014W in two DNA templates. A low frequency of the kdr allele mostly in the heterozygous state of L1014S and L1014F was observed in this mosquito population. In this study, the genotyping of An. sinensis using three polymerase chain reaction-based methods generated consistent results, which agreed with the results of DNA sequencing. In total, 52 mosquitoes were genotyped using a direct sequencing assay. The number of mosquitoes and their genotypes were as follows: L/L = 24, L/S = 19, L/F = 8, and F/W = 1. The allelic frequency of L1014, 1014S, and 1014F were 72, 18, and 9%, respectively.

The polymorphism and the geographical distribution of the knockdown resistance (kdr) of Anopheles sinensis in the Republic of Korea

Background

In the Republic of Korea (ROK), six sibling species of the Anopheles sinensis complex are considered the vector species of malaria, but data on their susceptibilities to malaria and vector capacities have been controversial. The intensive use of insecticides has contributed to the rapid development and spread of insecticide resistance in the An. sinensis complex. Knockdown resistance (kdr) to pyrethroids and DDT in the An. sinensis complex is associated with a mutation in codon 1014 of the voltage-gated sodium channel (VGSC) gene. Because the degree of insecticide resistance varies among mosquito species and populations, the detection of kdr mutations among the six sibling species of the An. sinensis complex is a prerequisite for establishing effective long-term vector control strategies in the ROK

Methods

In order to investigate species-specific kdr mutations, An. sinensis complex specimens have been collected from 22 sites in the ROK. Because of the difficulties with species identifications that are based only on morphological characteristics, molecular identification methods have been conducted on every specimen. Part of the IIS6 domain of the VGSC was polymerase chain reaction-amplified and directly sequenced.

Results

The molecular analyses revealed that mutations existed at codon 1014 only in An. sinensis sensu stricto and no mutations were found in the other five Anopheles species. In An. sinensis s.s., one wild type (TTG L1014) and three mutant types (TTT L1014F, TTC L1014F, and TGT L1014C) of kdr alleles were detected. The TTC L1014F mutation was observed for the first time in this species.

Conclusions

The fact that the highly polymorphic kdr gene is only observed in An. sinensis s.s., out of the six Anopheles species and their geographical distribution suggest the need for future studies of insecticide resistance monitoring and investigations of species-specific resistance mechanisms in order to build successful malaria vector control programmes in the ROK.

Malaria in the Greater Mekong Subregion: heterogeneity and complexity

The Greater Mekong Subregion (GMS), comprised of six countries including Cambodia, China's Yunnan Province, Lao PDR, Myanmar (Burma), Thailand and Vietnam, is one of the most threatening foci of malaria. Since the initiation of the WHO's Mekong Malaria Program a decade ago, malaria situation in the GMS has greatly improved, reflected in the continuous decline in annual malaria incidence and deaths. However, as many nations are moving towards malaria elimination, the GMS nations still face great challenges. Malaria epidemiology in this region exhibits enormous geographical heterogeneity with Myanmar and Cambodia remaining high-burden countries. Within each country, malaria distribution is also patchy, exemplified by 'border malaria' and 'forest malaria' with high transmission occurring along international borders and in forests or forest fringes, respectively. 'Border malaria' is extremely difficult to monitor, and frequent malaria introductions by migratory human populations constitute a major threat to neighboring, malaria-eliminating countries. Therefore, coordination between neighboring countries is essential for malaria elimination from the entire region. In addition to these operational difficulties, malaria control in the GMS also encounters several technological challenges. Contemporary malaria control measures rely heavily on effective chemotherapy and insecticide control of vector mosquitoes. However, the spread of multidrug resistance and potential emergence of artemisinin resistance in Plasmodium falciparum make resistance management a high priority in the GMS. This situation is further worsened by the circulation of counterfeit and substandard artemisinin-related drugs. In most endemic areas of the GMS, P. falciparum and Plasmodium vivax coexist, and in recent malaria control history, P. vivax has demonstrated remarkable resilience to control measures. Deployment of the only registered drug (primaquine) for the radical cure of vivax malaria is severely undermined due to high prevalence of glucose-6-phosphate dehydrogenase deficiency in target human populations. In the GMS, the dramatically different ecologies, diverse vector systems, and insecticide resistance render traditional mosquito control less efficient. Here we attempt to review the changing malaria epidemiology in the GMS, analyze the vector systems and patterns of malaria transmission, and identify the major challenges the malaria control community faces on its way to malaria elimination.

Multiplex assay for species identification and monitoring of insecticide resistance in Anopheles punctulatus group populations of Papua New Guinea

Anopheles punctulatus sibling species (An. punctulatus s.s., Anopheles koliensis, and Anopheles farauti species complex [eight cryptic species]) are principal vectors of malaria and filariasis in the Southwest Pacific. Given significant effort to reduce malaria and filariasis transmission through insecticide-treated net distribution in the region, effective strategies to monitor evolution of insecticide resistance among An. punctulatus sibling species is essential. Mutations in the voltage-gated sodium channel (VGSC) gene have been associated with knock-down resistance (kdr) to pyrethroids and DDT in malarious regions. By examining VGSC sequence polymorphism we developed a multiplex assay to differentiate wild-type versus kdr alleles and query intron-based polymorphisms that enable simultaneous species identification. A survey including mosquitoes from seven Papua New Guinea Provinces detected no kdr alleles in any An. punctulatus species. Absence of VGSC sequence introgression between species and evidence of geographic separation within species suggests that kdr must be monitored in each An. punctulatus species independently.