Detection of knockdown resistance (kdr) mutations in Anopheles gambiae: a comparison of two new high-throughput assays with existing methods

Insecticide resistance and the future of malaria control in Zambia

BACKGROUND

In line with the Global trend to improve malaria control efforts a major campaign of insecticide treated net distribution was initiated in 1999 and indoor residual spraying with DDT or pyrethroids was reintroduced in 2000 in Zambia. In 2006, these efforts were strengthened by the President's Malaria Initiative. This manuscript reports on the monitoring and evaluation of these activities and the potential impact of emerging insecticide resistance on disease transmission.

METHODS

Mosquitoes were captured daily through a series of 108 window exit traps located at 18 sentinel sites. Specimens were identified to species and analyzed for sporozoites. Adult Anopheles mosquitoes were collected resting indoors and larva collected in breeding sites were reared to F1 and F0 generations in the lab and tested for insecticide resistance following the standard WHO susceptibility assay protocol. Annual cross sectional household parasite surveys were carried out to monitor the impact of the control programme on prevalence of Plasmodium falciparum in children aged 1 to 14 years.

RESULTS

A total of 619 Anopheles gambiae s.l. and 228 Anopheles funestus s.l. were captured from window exit traps throughout the period, of which 203 were An. gambiae malaria vectors and 14 An. funestus s.s.. In 2010 resistance to DDT and the pyrethroids deltamethrin, lambda-cyhalothrin and permethrin was detected in both An. gambiae s.s. and An. funestus s.s.. No sporozoites were detected in either species. Prevalence of P. falciparum in the sentinel sites remained below 10% throughout the study period.

CONCLUSION

Both An. gambiae s.s. and An. funestus s.s. were controlled effectively with the ITN and IRS programme in Zambia, maintaining a reduced disease transmission and burden. However, the discovery of DDT and pyrethroid resistance in the country threatens the sustainability of the vector control programme.

Extensive permethrin and DDT resistance in Anopheles arabiensis from eastern and central Sudan

Background

The distribution of insecticide treated nets (ITN) has been dramatically scaled up in eastern and central Sudan. Resistance to insecticides has already been reported in this region and there is an urgent need to develop appropriate resistance management strategies, which requires detailed information on the extent and causes of resistance. This study assessed resistance to permethrin and DDT in seven populations of Anopheles arabiensis from Sudan.

Results

Three out of the seven populations were defined as resistant to permethrin and five of six populations resistant to DDT according to WHO criteria. The 1014F kdr allele was present in all six populations tested and the presence of this allele was significantly correlated with resistance to permethrin (P = 0.0460). While homozygous 1014F individuals were statistically not more likely to survive (53.7%) permethrin than to be killed (38.6%) by the diagnostic dose, there was no difference in the likelihood of permethrin survival in heterozygotes (P = 0.7973). The susceptible genotypes were more likely to be killed by permethrin exposure than to survive (P = 0.0460). The 1014F allele failed to confer a survival advantage to the WHO diagnostic dose of DDT in either the homozygous or heterozygous state. The 1014S allele was not detected in any of the populations tested.

Conclusion

The kdr allele is certainly contributing to the extensive resistance to permethrin and DDT in Sudan but the high number of DDT (43%) and permethrin (16.7%) survivors that did not contain either kdr alleles suggests that other resistance mechanisms are also present in these populations. The high frequency of permethrin resistance throughout central and eastern Sudan is a cause of great concern for malaria control activities.

Pyrethroid resistance in the tomato red spider mite, Tetranychus evansi, is associated with mutation of the para-type sodium channel

BACKGROUND

The tomato red spider mite, Tetranychus evansi (Baker and Pritchard), is a serious pest of solanaceous crops in many African countries. In this study an investigation has been conducted to establish whether mutation of the para-type sodium channel underlies pyrethroid resistance in T. evansi strains collected in Southern Malawi.

RESULTS

Two T. evansi strains from Malawi showed tolerance to the organophosphate chlorpyrifos and resistance (20-40-fold) to the pyrethroid bifenthrin, but were susceptible to two contemporary acaricides (abamectin and fenpyroximate) in insecticide bioassays. Cloning of a 3.1 kb fragment (domains IIS5 to IVS5) of the T. evansi para gene from pyrethroid-resistant and pyrethroid-susceptible strains revealed a single non-synonymous mutation in the resistant strains that results in an amino acid substitution (M918T) within the domain II region of the channel. Although novel to mites, this mutation confers high levels of resistance to pyrethroids in several insect species where it has always been associated with another mutation (L1014F). This is the first report of the M918T mutation in the absence of L1014F in any arthropod species. Diagnostic tools were developed that allow sensitive detection of this mutation in individual mites.

CONCLUSION

This is the first study of pyrethroid resistance in T. evansi and provides contemporary information for resistance management of this pest in Southern Malawi.

Pyrethroid resistance in Sitophilus zeamais is associated with a mutation (T929I) in the voltage-gated sodium channel

The maize weevil, Sitophilus zeamais, is the most important pest affecting stored grain in Brazil and its control relies heavily on the use of insecticides. The intensive use of compounds such as the pyrethroids has led to the emergence of resistance, and previous studies have suggested that resistance to both pyrethroids and 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) may result from reduced sensitivity of the insecticide target, the voltage-gated sodium channel. To identify the molecular mechanisms underlying pyrethroid resistance in S. zeamais, the domain II region of the voltage-gated sodium channel (para-orthologue) gene was amplified by PCR and sequenced from susceptible and resistant laboratory S. zeamais strains that were selected with a discriminating dose of DDT. A single point mutation, T929I, was found in the para gene of the resistant S. zeamais populations and its presence in individual weevils was strongly associated with survival after DDT exposure. This is the first identification of a target-site resistance mutation in S. zeamais and unusually it is a super-kdr type mutation occurring in the absence of the more common kdr (L1014F) substitution. A high-throughput assay based on TaqMan single nucleotide polymorphism genotyping was developed for sensitive detection of the mutation and used to screen field-collected strains of S. zeamais. This showed that the mutation is present at low frequency in field populations and is a useful tool for informing control strategies.

Insecticide resistance in malaria vector mosquitoes at four localities in Ghana, West Africa

Background

Malaria vector control programmes that rely on insecticide-based interventions such as indoor house spraying with residual insecticides or insecticide treated bed nets, need to base their decision-making process on sound baseline data. More and more commercial entities in Africa, such as mining companies, are realising the value to staff productivity of controlling malaria transmission in their areas of operation.

This paper presents baseline entomological data obtained during surveys conducted for four mining operations in Ghana, West Africa.

Results

The vast majority of the samples were identified as Anopheles gambiae S form with only a few M form specimens being identified from Tarkwa. Plasmodium falciparum infection rates ranged from 4.5 to 8.6% in An. gambiae and 1.81 to 8.06% in An. funestus. High survival rates on standard WHO bioassay tests were recorded for all insecticide classes except the organophosphates that showed reasonable mortality at all locations (i.e. > 90%). The West African kdr mutation was detected and showed high frequencies in all populations.

Conclusions

The data highlight the complexity of the situation prevailing in southern Ghana and the challenges facing the malaria vector control programmes in this region. Vector control programmes in Ghana need to carefully consider the resistance profiles of the local mosquito populations in order to base their resistance management strategies on sound scientific data.

Anopheles gambiae distribution and insecticide resistance in the cities of Douala and Yaoundé (Cameroon): influence of urban agriculture and pollution

BACKGROUND

Urban malaria is becoming a major health priority across Africa. A study was undertaken to assess the importance of urban pollution and agriculture practice on the distribution and susceptibility to insecticide of malaria vectors in the two main cities in Cameroon.

METHODS

Anopheline larval breeding sites were surveyed and water samples analysed monthly from October 2009 to December 2010. Parameters analysed included turbidity, pH, temperature, conductivity, sulfates, phosphates, nitrates, nitrites, ammonia, aluminium, alkalinity, iron, potassium, manganese, magnesium, magnesium hardness and total hardness. Characteristics of water bodies in urban areas were compared to rural areas and between urban sites. The level of susceptibility of Anopheles gambiae to 4% DDT, 0.75% permethrin, 0.05% deltamethrin, 0.1% bendiocarb and 5% malathion were compared between mosquitoes collected from polluted, non polluted and cultivated areas.

RESULTS

A total of 1,546 breeding sites, 690 in Yaoundé and 856 in Douala, were sampled in the course of the study. Almost all measured parameters had a concentration of 2- to 100-fold higher in urban compare to rural breeding sites. No resistance to malathion was detected, but bendiocarb resistance was present in Yaounde. Very low mortality rates were observed following DDT or permethrin exposure, associated with high kdr frequencies. Mosquitoes collected in cultivated areas, exhibited the highest resistant levels. There was little difference in insecticide resistance or kdr allele frequency in mosquitoes collected from polluted versus non-polluted sites.

CONCLUSION

The data confirm high selection pressure on mosquitoes originating from urban areas and suggest urban agriculture rather than pollution as the major factor driving resistance to insecticide.

High-throughput assays for detection of the F1534C mutation in the voltage-gated sodium channel gene in permethrin-resistant Aedes aegypti and the distribution of this mutation throughout Thailand

OBJECTIVES

To develop rapid monitoring tools to detect the F1534C permethrin-resistance mutation in domain IIIS6 of the Aedes aegypti voltage-gated sodium channel gene and determine the frequency and distribution of this mutation in Thailand.

METHODS

A TaqMan SNP genotyping and an allele specific PCR (AS-PCR) assay were developed and validated by comparison with DNA sequencing of homozygous susceptible and homozygous resistant laboratory strains, their reciprocal-cross progenies, and field-caught mosquitoes. To determine the resistance phenotype of wild-caught A. aegypti, mosquitoes were exposed to 0.75% permethrin paper. The AS-PCR assay was used to screen 619 individuals from 20 localities throughout Thailand.

RESULTS

Overall, both assays gave results consistent with DNA sequencing for laboratory strains of known genotype and for wild-caught A. aegypti. The only slight discrepancy was for the AS-PCR method, which overestimated the mutant allele frequency by 1.8% in wild-caught samples. AS-PCR assays of permethrin-exposed samples show that the mutant C1534 allele is very closely associated with the resistant phenotype. However, 19 permethrin-resistant individuals were homozygous for the wild-type F1534 allele. DNA sequencing revealed all these individuals were homozygous for two other mutations in domain II, V1016G and S989P, which are known to confer resistance (Srisawat et al. 2010). The F1534C mutation is widespread in Thailand with mutant allele frequencies varying among populations from 0.20 to 1.00.

CONCLUSIONS

These assays can be used for the rapid detection of the F1534C resistance mutation in A. aegypti populations. The F1534C, and other, mutations underlie an extremely high prevalence of pyrethroid resistance in Thailand.

High-throughput approach to detection of knockdown resistance (kdr) mutation in mosquitoes, Culex quinquefasciatus, based on real-time PCR using single-labelled hybridisation probe/melting curve analysis

BACKGROUND

Knockdown resistance (kdr) mutation (L1014F) is a well-defined mechanism of resistance to pyrethroids and DDT in many insect species. Sensitive detection of the mutations associated with resistance is a prerequisite for resistance management strategies. The authors have developed a new real-time molecular diagnostic assay based on SimpleProbe(®)/melting curve analysis for large-scale kdr genotyping in the wild population of Culex quinquefasciatus Say, the principal vector of bancroftian filariasis. Melting curve analysis is based on the thermal stability difference between matched and mismatched DNA duplexes. The application of SimpleProbe(®) chemistry in insects described here is novel in entomology research.

RESULTS

The mosquitoes homozygous for knockdown-resistant and knockdown-susceptible allele showed melting peaks at 60.45 °C (±0.25) and 64.09 °C (±0.24) respectively. The heterozygous mosquitoes yielded both peaks at approximately 60.5 °C (±0.2) and 64.20 °C (±0.23). Among the 92 samples genotyped, 16 were found to be homozygous resistant, 44 homozygous susceptible and 32 heterozygous. Comparative assessments were made of all the reported methods for kdr genotyping.

CONCLUSION

The present method is cheaper, faster, more reliable and versatile than other alternatives proposed in detecting correct kdr genotypes in mosquitoes. This is the first report using a single-labelled hybridisation probe to detect point mutations in insect populations.

Spatial and temporal variation in the kdr allele L1014S in Anopheles gambiae s.s. and phenotypic variability in susceptibility to insecticides in Western Kenya

BACKGROUND

Malaria vector control in Africa depends upon effective insecticides in bed nets and indoor residual sprays. This study investigated the extent of insecticide resistance in Anopheles gambiae s.l., Anopheles gambiae s.s. and Anopheles arabiensis in western Kenya where ownership of insecticide-treated bed nets has risen steadily from the late 1990s to 2010. Temporal and spatial variation in the frequency of a knock down resistance (kdr) allele in A. gambiae s.s. was quantified, as was variation in phenotypic resistance among geographic populations of A. gambiae s.l.

METHODS

To investigate temporal variation in kdr frequency, individual specimens of A. gambiae s.s. from two sentinel sites were genotyped using RT-PCR from 1996-2010. Spatial variation in kdr frequency, species composition, and resistance status were investigated in additional populations of A. gambiae s.l. sampled in western Kenya in 2009 and 2010. Specimens were genotyped for kdr as above and identified to species via conventional PCR. Field-collected larvae were reared to adulthood and tested for insecticide resistance using WHO bioassays.

RESULTS

Anopheles gambiae s.s. showed a dramatic increase in kdr frequency from 1996 - 2010, coincident with the scale up of insecticide-treated nets. By 2009-2010, the kdr L1014S allele was nearly fixed in the A. gambiae s.s. population, but was absent in A. arabiensis. Near Lake Victoria, A. arabiensis was dominant in samples, while at sites north of the lake A. gambiae s.s was more common but declined relative to A. arabiensis from 2009 to 2010. Bioassays demonstrated that A. gambiae s.s. had moderate phenotypic levels of resistance to DDT, permethrin and deltamethrin while A. arabiensis was susceptible to all insecticides tested.

CONCLUSIONS

The kdr L1014S allele has approached fixation in A. gambiae s.s. populations of western Kenya, and these same populations exhibit varying degrees of phenotypic resistance to DDT and pyrethroid insecticides. The near absence of A. gambiae s.s. from populations along the lakeshore and the apparent decline in other populations suggest that insecticide-treated nets remain effective against this mosquito despite the increase in kdr allele frequency. The persistence of A. arabiensis, despite little or no detectable insecticide resistance, is likely due to behavioural traits such as outdoor feeding and/or feeding on non-human hosts by which this species avoids interaction with insecticide-treated nets.

Multiple insecticide resistance: an impediment to insecticide-based malaria vector control program

BACKGROUND

Indoor Residual Spraying (IRS), insecticide-treated nets (ITNs) and long-lasting insecticidal nets (LLINs) are key components in malaria prevention and control strategy. However, the development of resistance by mosquitoes to insecticides recommended for IRS and/or ITNs/LLINs would affect insecticide-based malaria vector control. We assessed the susceptibility levels of Anopheles arabiensis to insecticides used in malaria control, characterized basic mechanisms underlying resistance, and evaluated the role of public health use of insecticides in resistance selection.

METHODOLOGY/PRINCIPAL FINDINGS

Susceptibility status of An. arabiensis was assessed using WHO bioassay tests to DDT, permethrin, deltamethrin, malathion and propoxur in Ethiopia from August to September 2009. Mosquito specimens were screened for knockdown resistance (kdr) and insensitive acetylcholinesterase (ace-1(R)) mutations using AS-PCR and PCR-RFLP, respectively. DDT residues level in soil from human dwellings and the surrounding environment were determined by Gas Chromatography with Electron Capture Detector. An. arabiensis was resistant to DDT, permethrin, deltamethrin and malathion, but susceptible to propoxur. The West African kdr allele was found in 280 specimens out of 284 with a frequency ranged from 95% to 100%. Ace-1(R) mutation was not detected in all specimens scored for the allele. Moreover, DDT residues were found in soil samples from human dwellings but not in the surrounding environment.

CONCLUSION

The observed multiple-resistance coupled with the occurrence of high kdr frequency in populations of An. arabiensis could profoundly affect the malaria vector control programme in Ethiopia. This needs an urgent call for implementing rational resistance management strategies and integrated vector control intervention.

The Vector Population Monitoring Tool (VPMT): High-Throughput DNA-Based Diagnostics for the Monitoring of Mosquito Vector Populations

Regular monitoring of mosquito vector populations is an integral component of most vector control programmes. Contemporary data on mosquito species composition, infection status, and resistance to insecticides are a prerequisite for effective intervention. For this purpose we, with funding from the Innovative Vector Control Consortium (IVCC), have developed a suite of high-throughput assays based on a single "closed-tube" platform that collectively comprise the "Vector Population Monitoring Tool" (VPMT). The VPMT can be used to screen mosquito disease vector populations for a number of traits including Anopheles gambiae s.l. and Anopheles funestus species identification, detection of infection with Plasmodium parasites, and identification of insecticide resistance mechanisms. In this paper we focus on the Anopheles-specific assays that comprise the VPMT and include details of a new assay for resistance todieldrin Rdl detection. The application of these tools, general and specific guidelines on their use based on field testing in Africa, and plans for further development are discussed.

Staggered larval time-to-hatch and insecticide resistance in the major malaria vector Anopheles gambiae S form

Background

Anopheles gambiae is a major vector of malaria in the West African region. Resistance to multiple insecticides has been recorded in An. gambiae S form in the Ahafo region of Ghana. A laboratory population (GAH) established using wild material from this locality has enabled a mechanistic characterization of each resistance phenotype as well as an analysis of another adaptive characteristic - staggered larval time-to-hatch.

Methods

Individual egg batches obtained from wild caught females collected from Ghana and the Republic of the Congo were monitored for staggered larval time-to-hatch. In addition, early and late larval time-to-hatch sub-colonies were selected from GAH. These selected sub-colonies were cross-mated and their hybrid progeny were subsequently intercrossed and back-crossed to the parental strains. The insecticide susceptibilities of the GAH base colony and the time-to-hatch selected sub-colonies were quantified for four insecticide classes using insecticide bioassays. Resistance phenotypes were mechanistically characterized using insecticide-synergist bioassays and diagnostic molecular assays for known reduced target-site sensitivity mutations.

Results

Anopheles gambiae GAH showed varying levels of resistance to all insecticide classes. Metabolic detoxification and reduced target-site sensitivity mechanisms were implicated. Most wild-caught families showed staggered larval time-to-hatch. However, some families were either exclusively early hatching or late hatching. Most GAH larvae hatched early but many egg batches contained a proportion of late hatching larvae. Crosses between the time-to-hatch selected sub-colonies yielded ambiguous results that did not fit any hypothetical models based on single-locus Mendelian inheritance. There was significant variation in the expression of insecticide resistance between the time-to-hatch phenotypes.

Conclusions

An adaptive response to the presence of multiple insecticide classes necessarily involves the development of multiple resistance mechanisms whose effectiveness may be enhanced by intra-population variation in the expression of resistance phenotypes. The variation in the expression of insecticide resistance in association with selection for larval time-to-hatch may induce this kind of enhanced adaptive plasticity as a consequence of pleiotropy, whereby mosquitoes are able to complete their aquatic life stages in a variable breeding environment using staggered larval time-to-hatch, giving rise to an adult population with enhanced variation in the expression of insecticide resistance.

A comparison of DNA sequencing and the hydrolysis probe analysis (TaqMan assay) for knockdown resistance (kdr) mutations in Anopheles gambiae from the Republic of the Congo

BACKGROUND

Knockdown resistance (kdr) caused by a single base pair mutation in the sodium channel gene is strongly associated with pyrethroid insecticide resistance in Anopheles gambiae in West-Central Africa. Recently, various molecular techniques have been developed to screen for the presence of the kdr mutations in vector populations with varying levels of accuracy. In this study, the results of the hydrolysis probe analysis for detecting the kdr mutations in An. gambiae s.s. from the Republic of the Congo were compared with DNA sequence analysis.

METHODS

A total of 52 pyrethroid and DDT resistant An. gambiae from Pointe-Noire (Congo-Brazzaville) were tested for detection of the two kdr mutations (kdr-e and kdr-w) that are known to occur in this species. Results from the hydrolysis probe analysis were compared to DNA sequencing to verify the accuracy of the probe analysis for this vector population.

RESULTS

Fifty-one specimens were found to be An. gambiae S-form and one was a M/S hybrid. DNA sequencing revealed that more than half of the specimens (55.8%) carried both the kdr-e and kdr-w resistance mutations, seven specimens (13.5%) were homozygous for the kdr-e mutation, and 14 specimens (26.9%) were homozygous for the kdr-w mutation. A single individual was genotyped as heterozygous kdr-e mutation (1.9%) only and another as heterozygous kdr-w mutation (1.9%) only. Analysis using hydrolysis probe analysis, without adjustment of the allelic discrimination axes on the scatter plots, revealed six specimens (11.5%) carrying both mutations, 30 specimens (57.8%) as homozygous kdr-w, six specimens (11.5%) homozygous for the kdr-e mutation, one specimen (1.9%) heterozygous for the kdr-w mutation and one specimen (1.9%) present in wild type form. Eight of the specimens (15.4%) could not be identified using unadjusted hydrolysis probe analysis values. No heterozygous kdr-e mutations were scored when adjustment for the allelic discrimination axes was omitted. However, when the axes on the scatter plots were adjusted the results were consistent with those of the DNA sequence analysis, barring two individuals that were mis-scored in the hydrolysis probe analysis.

CONCLUSION

Both the kdr-e and kdr-w mutations were abundant in An. gambiae S-form from Pointe-Noire. The hydrolysis probe analysis can lead to misleading results if adjustment to allelic discrimination axes is not investigated. This is mainly relevant when both kdr-e and kdr-w are present in a population in a high frequency. This report highlights the importance of concurrent screening for both mutations. Therefore, performing routine assay protocols blindly can result in the misinterpretation of results. Although hydrolysis probe analysis of kdr is still held as the gold standard assay, this paper highlights the importance of kdr mutation confirmation via sequencing especially in regions where kdr frequency has never been reported before or where both the kdr-e and kdr-w mutations are present simultaneously.

Molecular ecology of pyrethroid knockdown resistance in Culex pipiens pallens mosquitoes

Pyrethroid insecticides have been extensively used in China and worldwide for public health pest control. Accurate resistance monitoring is essential to guide the rational use of insecticides and resistance management. Here we examined the nucleotide diversity of the para-sodium channel gene, which confers knockdown resistance (kdr) in Culex pipiens pallens mosquitoes in China. The sequence analysis of the para-sodium channel gene identified L1014F and L1014S mutations. We developed and validated allele-specific PCR and the real-time TaqMan methods for resistance diagnosis. The real-time TaqMan method is more superior to the allele-specific PCR method as evidenced by higher amplification rate and better sensitivity and specificity. Significant positive correlation between kdr allele frequency and bioassay-based resistance phenotype demonstrates that the frequency of L1014F and L1014S mutations in the kdr gene can be used as a molecular marker for deltamethrin resistance monitoring in natural Cx. pipiens pallens populations in the East China region. The laboratory selection experiment found that L1014F mutation frequency, but not L1014S mutation, responded to deltamethrin selection, suggesting that the L1014F mutation is the key mutation conferring resistance to deltamethrin. High L1014F mutation frequency detected in six populations of Cx. pipens pallens suggests high prevalence of pyrethroid resistance in Eastern China, calling for further surveys to map the resistance in China and for investigating alternative mosquito control strategies.

High-resolution DNA melting analysis in clinical research and diagnostics

Among nucleic acid analytical methods, high-resolution melting analysis is gaining more and more attention. High-resolution melting provides simple, homogeneous solutions for variant scanning and genotyping, addressing the needs of today's overburdened laboratories with rapid turnaround times and minimal cost. The flexibility of the technique has allowed it to be adopted by a wide range of disciplines for a variety of applications. In this review we examine the broad use of high-resolution melting analysis, including gene scanning, genotyping (including small amplicon, unlabeled probe and snapback primers), sequence matching and methylation analysis. Four major application arenas are examined to demonstrate the methods and approaches commonly used in particular fields. The appropriate usage of high-resolution melting analysis is discussed in the context of known constraints, such as sample quality and quantity, with a particular focus placed on proper experimental design in order to produce successful results.

Uncertainty in mapping malaria epidemiology: implications for control

Malaria is a location-specific, dynamic infectious disease transmitted by mosquitoes to humans and is influenced by environmental, vector, parasite, and host factors. The principal purposes of malarial epidemiology are 1) to describe the malarial distribution in space and time along with the physical, biologic, and social etiologic factors and 2) to guide control objectives for either modeling impact or measuring progress of control tactics. Mapping malaria and many of its causative factors has been achieved on many different levels from global distribution to biologic quantitative trait localization in humans, parasites, and mosquitoes. Despite these important achievements, a large degree of uncertainty still exists on the annual burden of malarial cases. Accurate, sensitive detection and treatment of asymptomatic reservoirs important to infectious transmission are additional components necessary for future control measures. Presently spurred by the leadership and funding of Bill and Melinda Gates, the malarial community is developing and implementing plans for elimination of malaria. The challenge for malariologists is to digitally integrate and map epidemiologic factors and intervention measures in space and time to target effective, sustainable control alongside research efforts.

Field, genetic, and modeling approaches show strong positive selection acting upon an insecticide resistance mutation in Anopheles gambiae s.s

Alleles subject to strong, recent positive selection will be swept toward fixation together with contiguous sections of the genome. Whether the genomic signatures of such selection will be readily detectable in outbred wild populations is unclear. In this study, we employ haplotype diversity analysis to examine evidence for selective sweeps around knockdown resistance (kdr) mutations associated with resistance to dichlorodiphenyltrichloroethane and pyrethroid insecticides in the mosquito Anopheles gambiae. Both kdr mutations have significantly lower haplotype diversity than the wild-type (nonresistant) allele, with kdr L1014F showing the most pronounced footprint of selection. We complement these data with a time series of collections showing that the L1014F allele has increased in frequency from 0.05 to 0.54 in 5 years, consistent with a maximum likelihood-fitted selection coefficient of 0.16 and a dominance coefficient of 0.25. Our data show that strong, recent positive selective events, such as those caused by insecticide resistance, can be identified in wild insect populations.

Development of multiplex real-time PCR assays for identification of members of the Anopheles funestus species group

Background

The malaria vector and non-vector species of the Anopheles funestus group are morphologically very similar and accurate identification is required as part of effective control strategies. In the past, this has relied on morphological and cytogenetic methods but these have been largely superseded by a robust allele-specific PCR (AS-PCR). One disadvantage of AS-PCR is the requirement for post-PCR processing by gel electrophoresis of PCR products. In this study, three new high-throughput 'closed-tube' assays were developed and compared with the previously described AS-PCR technique.

Methods

Protocols for three fluorescence-based assays based on Melt Curve Analysis (MCA), High Resolution Melt (HRM) and TaqMan SNP genotyping were developed to detect and discriminate Anopheles parensis, Anopheles leesoni, Anopheles vaneedeni, Anopheles rivulorum and An. funestus s.s. The sensitivity and specificity of these assays were compared with the widely used AS-PCR in a blind trial using DNA extracted from wild-caught mosquitoes.

Results

The TaqMan assay proved to be the most sensitive and specific of the three new assays. The MCA and HRM assays initially gave promising results, but were more sensitive to both DNA quality and quantity and consequently showed a higher rate of incorrect identifications.

Conclusion

The TaqMan assay proved to be the most robust of the three protocols tested in this study. This assay very effectively identified all five members of the An. funestus group using fluorescently-labeled probes with distinct emission and excitation spectra allowing their independent detection in a single reaction. This method is at least as sensitive and specific as the gold standard AS-PCR approach and because it has no requirement for post-PCR processing is simpler and more rapid to run. The one disadvantage of the TaqMan assay is the cost of this assay, both in terms of initial capital outlay and running cost per sample, which is higher than AS-PCR. However, the cost of both the real-time PCR machine and fluorescently labelled probes required is falling and in the future the cost of this assay is likely to become closer to that of standard PCR.

Malaria vector composition and insecticide susceptibility status in Guinea Conakry, West Africa

This study provides data on malaria vector species composition and insecticide susceptibility status from three localities in Guinea Conakry. A total of 497 mosquitoes were collected resting indoors and morphologically identified as belonging to the Anopheles gambiae complex. The majority of these were An. gambiae s.s. (99.6%), but a small percentage (0.4%) were identified as Anopheles arabiensis. Thirty-four Anopheles funestus s.s. were also collected. The molecular S form of An. gambiae s.s. was predominant over the M form in Siguiri (95%) and Boffa (97.4%), whereas at Mt Nimba the M form was more abundant (61.4%) than the S form (38.1%). One hybrid M/S specimen was recorded from Mt Nimba. Siguiri populations showed high levels of resistance to DDT, dieldrin and bendiocarb. Anopheles gambiae from Boffa were largely susceptible to the insecticides tested. At Mt Nimba, resistance to DDT and bendicocarb was detected. Biochemical enzyme analysis showed that an altered acetylcholinesterase is operating in the field at low levels. The frequency of the 1014F kdr allele in the An. gambiae S form was 0.24 at Siguiri and 0.14 at Mt Nimba. A single RR specimen was found in the M form. The heterogeneity in species composition and resistance profiles between sites requires vector control interventions to be tailored to each site based on the data collected from ongoing monitoring and surveillance.

Trends in DDT and pyrethroid resistance in Anopheles gambiae s.s. populations from urban and agro-industrial settings in southern Cameroon

BACKGROUND

Pyrethroid insecticides are widely used for insect pest control in Cameroon. In certain insect species, particularly the malaria vector Anopheles gambiae, resistance to this class of insecticides is a source of great concern and needs to be monitored in order to sustain the efficacy of vector control operations in the fields. This study highlights trends in DDT and pyrethroid resistance in wild An. gambiae populations from South Cameroon.

METHODS

Mosquitoes were collected between 2001 and 2007 in four sites in South Cameroon, where insecticides are used for agricultural or personal protection purposes. Insecticide use was documented in each site by interviewing residents. Batches of 2-4 days old adult female mosquitoes reared from larval collections were tested for susceptibility to DDT, permethrin and deltamethrin using standard WHO procedures. Control, dead and survivors mosquitoes from bioassays were identified by PCR-RFLP and characterized for the kdr mutations using either the AS-PCR or the HOLA method.

RESULTS

Four chemical insecticide groups were cited in the study sites: organochlorines, organophosphates, carbamates and pyrethroids. These chemicals were used for personal, crop or wood protection. In the four An. gambiae populations tested, significant variation in resistance levels, molecular forms composition and kdr frequencies were recorded in the time span of the study. Increases in DDT and pyrethroid resistance, as observed in most areas, were generally associated with an increase in the relative frequency of the S molecular form carrying the kdr mutations at higher frequencies. In Mangoum, however, where only the S form was present, a significant increase in the frequency of kdr alleles between 2003 to 2007 diverged with a decrease of the level of resistance to DDT and pyrethroids. Analyses of the kdr frequencies in dead and surviving mosquitoes showed partial correlation between the kdr genotypes and resistance phenotypes, suggesting that the kdr mechanism may act with certain co-factors to be identified.

CONCLUSION

These results demonstrate the ongoing spread of kdr alleles in An. gambiae in Central Africa. The rapid evolution of insecticide resistance in this highly dynamic and genetically polymorphic species remains a challenge for its control.

Does kdr genotype predict insecticide-resistance phenotype in mosquitoes?

Several groups are developing and applying DNA-based technologies to monitor insecticide-based disease control programmes. However, several recent papers have concluded that the knockdown resistance (kdr) genotype-phenotype correlation that is observed in a wide variety of taxa might not hold in all mosquitoes. In this article, we review the evidence to support this putative breakdown and argue that the conclusion follows from unreliable data or the unparsimonious interpretation of data. We assert that the link between kdr genotype and DDT- and pyrethroid-susceptibility phenotype is clear. However, we emphasize that kdr genotype might explain only a portion of heritable variation in resistance and that diagnostic assays to test the importance of other resistance mechanisms in field populations are required.

Frequency of Val1016Ile mutation in the voltage-gated sodium channel gene of Aedes aegypti Brazilian populations

One of the major insect pyrethroid resistance mechanisms affects its target site, the voltage-gated sodium channel (Na(v)). In Aedes aegypti, the Val1016Ile substitution of the AaNa(v) gene is associated to resistance in several Latin American countries. Genotyping of susceptible and resistant mosquitoes from seven Brazilian localities detected the 1016Ile mutation in five populations with a higher frequency of this substitution in resistant specimens in all cases. Furthermore, analysis of nine additional field populations revealed that five also presented the 1016Ile mutation. Our data suggest a recent dissemination and involvement of this substitution with pyrethroid resistance in Brazil.

Assessing high-resolution melt curve analysis for accurate detection of gene variants in complex DNA fragments

Mutation detection has, until recently, relied heavily on the use of gel-based methods that can be both time consuming and difficult to design. Nongel-based systems are therefore important to increase simplicity and improve turn around time without compromising assay sensitivity and accuracy, especially in the diagnostic/clinical setting. In this study, we assessed the latest of the nongel-based methods, namely high-resolution melt (HRM) curve analysis. HRM is a closed-tube method that incorporates a saturating dye during DNA amplification followed by a monitoring of the change in fluorescence as the DNA duplex is denatured by an increasing temperature. We assessed 10 amplicons derived from eight genes, namely SERPINA1, CXCR7, MBL, VDR, NKX3A, NPY, TP53, and HRAS using two platforms, the LightScanner System using LC Green PLUS DNA binding dye (Idaho Technology, Salt Lake City, UT, USA) and the LightCycler 480 using the HRM Master dye (Roche Diagnostics, Indianapolis, IN, USA). DNA variants (mutations or polymorphims) were previously identified using denaturing gradient gel electrophoresis (DGGE) a method, similarly to HRM, based upon the different melting properties of double-stranded DNA. Fragments were selected based on variant and fragment complexity. This included the presence of multiple sequence variants, variants in alternate orientations, and single or multiple variants (constitutional or somatic) in GC-rich fragments. We demonstrate current limitations of the HRM method for the analysis of complex DNA regions and call for caution when using HRM as the sole method to make a clinical diagnosis based on genetic analysis.

Quantifying the drivers of larval density patterns in two tropical mosquito species to maximize control efficiency

Understanding the contributions of environmental variation and density feedbacks to changes in vector populations is essential for designing effective vector control. We analyzed monitoring datasets describing larval densities over 7 yr of the two dominant mosquito species, Aedes vigilax (Skuse) and Culex annulirostris (Skuse), of the greater Darwin area (Northern Territory, Australia). Using generalized linear and linear mixed-effects models, we tested hypotheses regarding the environmental determinants of spatio-temporal patterns in relative larval abundance in both species. The most important spatial drivers of Ae. vigilax and Cx. annulirostris larval densities were elevation and water presence. Ae. vigilax density correlates negatively with elevation, whereas there was a positive relationship between Cx. annulirostris density and elevation. These results show how larval habitats used by the saltwater-influenced breeder Ae. vigilax and the obligate freshwater breeder Cx. annulirostris are separated in a tidally influenced swamp. The models examining temporal drivers of larval density also identified this discrimination between freshwater and saltwater habitats. Ae. vigilax larval densities were positively related to maximum tide height and high tide frequency, whereas Cx. annulirostris larval densities were positively related to elevation and rainfall. Adult abundance in the previous month was the most important temporal driver of larval densities in both species, providing a clear dynamical link between the two main life phases in mosquito development. This study shows the importance of considering both spatial and temporal drivers, and intrinsic population dynamics, when planning vector control strategies to reduce larval density, adult population density, and disease transmission effectively.

PCR-based methods for the detection of L1014 kdr mutation in Anopheles culicifacies sensu lato

BACKGROUND

Anopheles culicifacies s.l., a major malaria vector in India, has developed widespread resistance to DDT and is becoming resistant to pyrethroids-the only insecticide class recommended for the impregnation of bed nets. Knock-down resistance due to a point mutation in the voltage gated sodium channel at L1014 residue (kdr) is a common mechanism of resistance to DDT and pyrethroids. The selection of this resistance may pose a serious threat to the success of the pyrethroid-impregnated bed net programme. This study reports the presence of kdr mutation (L1014F) in a field population of An. culicifacies s.l. and three new PCR-based methods for kdr genotyping.

METHODS

The IIS4-IIS5 linker to IIS6 segments of the para type voltage gated sodium channel gene of DDT and pyrethroid resistant An. culicifacies s.l. population from the Surat district of India was sequenced. This revealed the presence of an A-to-T substitution at position 1014 leading to a leucine-phenylalanine mutation (L1014F) in a few individuals. Three molecular methods viz. Allele Specific PCR (AS-PCR), an Amplification Refractory Mutation System (ARMS) and Primer Introduced Restriction Analysis-PCR (PIRA-PCR) were developed and tested for kdr genotyping. The specificity of the three assays was validated following DNA sequencing of the samples genotyped.

RESULTS

The genotyping of this An. culicifacies s.l. population by the three PCR based assays provided consistent result and were in agreement with DNA sequencing result. A low frequency of the kdr allele mostly in heterozygous condition was observed in the resistant population. Frequencies of the different genotypes were in Hardy-Weinberg equilibrium.

CONCLUSION

The Leu-Phe mutation, which generates the kdr phenotype in many insects, was detected in a pyrethroid and DDT resistant An. culicifacies s.l. population. Three PCR-based methods were developed for kdr genotyping. All the three assays were specific. The ARMS method was refractory to non-specific amplification in non-stringent amplification conditions. The PIRA-PCR assay is able to detect both the codons for the phenylalanine mutation at kdr locus, i.e., TTT and TTC, in a single assay, although the latter codon was not found in the population genotyped.

A novel and simple method for genotyping the mdx mouse using high-resolution melt polymerase chain reaction

The mdx mouse mutation arises from a C-to-T point mutation, which terminates the translation of dystrophin and results in the loss of a functional dystrophin protein. mdx mice are used widely in studies of the role of dystrophin and of potential treatments for Duchenne muscular dystrophy, thus accurate genotyping is essential. Current methods require labor-intensive efforts and can often lead to misconstrued results. This study describes a simple and highly reliable, sensitive, and user-friendly, high-resolution melt (HRM) assay that is able to utilize DNA obtained from a variety of sources in order to genotype the known sequence variant of the mdx mouse. Muscle Nerve 39: 603-608, 2009.

Rapid detection of point mutations in Plasmodium falciparum genes associated with antimalarial drugs resistance by using High-Resolution Melting analysis

We have developed a High-Resolution DNA Melting method to detect mutations related to Plasmodium falciparum resistance. This method is based on real-time PCR followed by High Resolution Melting ramping from 67 degrees C to 80 degrees C with fluorescence data acquisition set at 0.1 degrees C increments. The accuracy of the technique was assessed using 177 P. falciparum clinical isolates and two reference strains. Results perfectly matched those obtained by DNA sequencing for some important genetic markers of P. falciparum resistance. This technique could be of great value for epidemiological studies, especially in developing countries.

Insecticide resistance and its association with target-site mutations in natural populations of Anopheles gambiae from eastern Uganda

Insecticide resistance in Anopheles gambiae threatens the success of malaria vector control programmes in sub-Saharan Africa. In order to manage insecticide resistance successfully, it is essential to assess continuously the target mosquito population. Here, we collected baseline information on the distribution and prevalence of insecticide resistance and its association with target-site mutations in eastern Uganda. Anopheles gambiae s.l. adults were raised from wild-caught larvae sampled from two ecologically distinct breeding sites and exposed to WHO discriminating concentrations of DDT, permethrin, deltamethrin, bendiocarb and malathion. Survival rates to DDT were as high as 85.4%, alongside significant resistance levels to permethrin (38.5%), reduced susceptibility to deltamethrin, but full susceptibility to bendiocarb and malathion. Using molecular diagnostics, susceptible and resistant specimens were further tested for the presence of knockdown resistance (kdr) and acetylcholinesterase 1 resistance (ace-1(R)) alleles. While ace-1(R) and kdrL1014F ('kdr west') alleles were absent, the kdr L1014S ('kdr east') allele was present in both populations. In A. gambiae s.s., L1014S was closely associated with DDT and, to a lesser degree, with permethrin resistance. Intriguingly, the association between DDT resistance and the presence of L1014S is consistent with a co-dominant effect, with heterozygous individuals showing an intermediate phenotype.

Development and application of a simple colorimetric assay reveals widespread distribution of sodium channel mutations in Thai populations of Aedes aegypti

Dengue fever and its more serious complications dengue haemorrhagic fever and dengue shock syndrome are growing public health problems in tropical and subtropical countries. In the absence of a vaccine, most dengue control programmes rely heavily on the use of insecticides to target the Aedes mosquito vectors. As a limited number of insecticides are routinely used in control, monitoring for the presence of resistance is an essential component of dengue prevention programmes. The pyrethroid insecticides target the voltage-gated sodium channel on the insects' neurons. Substitutions at residue 1016 of this protein have been associated with pyrethroid and DDT resistance in Aedes aegypti populations from Latin America and Asia. Here we report on the development of a simple colorimetric assay to detect these mutations in individual mosquitoes. Evaluation of this diagnostic assay on 180 Ae. aegypti individuals from Thailand revealed the presence of high frequencies of the Val1016Gly mutation throughout the country. The assay requires no specialised equipment and will enable monitoring for insecticide resistance associated alleles to be routinely incorporated into dengue surveillance operations.

A pre-intervention study of malaria vector abundance in Rio Muni, Equatorial Guinea: their role in malaria transmission and the incidence of insecticide resistance alleles

BACKGROUND

Following the success of the malaria control intervention on the island of Bioko, malaria control by the use of indoor residual spraying (IRS) and long-lasting insecticide-treated nets (LLITN) was extended to Rio Muni, on the mainland part of Equatorial Guinea. This manuscript reports on the malaria vectors present and the incidence of insecticide resistant alleles prior to the onset of the programme.

METHODS

Anopheles mosquitoes were captured daily using window traps at 30 sentinel sites in Rio Muni, from December 2006 to July 2007. The mosquitoes were identified to species and their sporozoite rates, knockdown resistance (kdr) and acetylcholinesterase (AChE) sensitivity measured, to define the role of vector species in malaria transmission and their potential susceptibility to insecticides.

RESULTS

A total of 6,162 Anopheles mosquitoes were collected of which 4,808 were morphologically identified as Anopheles gambiae s.l., 120 Anopheles funestus, 1,069 Anopheles moucheti, and 165 Anopheles nili s.l.. Both M and S molecular forms of Anopheles gambiae s.s. and Anopheles melas were identified. Anopheles ovengensis and Anopheles carnevalei were the only two members of the An. nili group to be identified. Using the species-specific sporozoite rates and the average number of mosquitoes per night, the number of infective mosquitoes per trap per 100 nights for each species complex was calculated as a measure of transmission risk. Both kdr-w and kdr-e alleles were present in the S-form of An. gambiae s.s. (59% and 19% respectively) and at much lower frequencies in the M-form (9.7% and 1.8% respectively). The kdr-w and kdr-e alleles co-occurred in 103 S-form and 1 M-form specimens. No insensitive AChE was detected.

CONCLUSION

Anopheles gambiae s.s, a member of the Anopheles gambiae complex was shown to be the major vector in Rio Muni with the other three groups playing a relatively minor role in transmission. The demonstration of a high frequency of kdr alleles in mosquito populations before the onset of a malaria control programme shows that continuous entomological surveillance including resistance monitoring will be of critical importance to ensure the chosen insecticide remains effective.

PCR-based detection of Plasmodium in Anopheles mosquitoes: a comparison of a new high-throughput assay with existing methods

Background

Detection of the four malaria-causing Plasmodium species (Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale and Plasmodium malariae) within their mosquito hosts is an essential component of vector control programmes. Several PCR protocols have been developed for this purpose. Many of these methods, while sensitive, require multiple PCR reactions to detect and discriminate all four Plasmodium species. In this study a new high-throughput assay was developed and compared with three previously described PCR techniques.

Methods

A new assay based on TaqMan SNP genotyping was developed to detect all four Plasmodium species and discriminate P. falciparum from P. vivax, P. ovale and P. malariae. The sensitivity and the specificity of the new assay was compared to three alternative PCR approaches and to microscopic dissection of salivary glands in a blind trial of 96 single insect samples that included artificially infected Anopheles stephensi mosquitoes. The performance of the assays was then compared using more than 450 field-collected specimens that had been stored on silica gel, in ethanol or in isopropanol.

Results

The TaqMan assay was found to be highly specific when using Plasmodium genomic DNA as template. Tests of analytical sensitivity and the results of the blind trial showed the TaqMan assay to be the most sensitive of the four methods followed by the 'gold standard' nested PCR approach and the results generated using these two methods were in good concordance. The sensitivity of the other two methods and their agreement with the nested PCR and TaqMan approaches varied considerably. In trials using field collected specimens two of the methods (including the nested protocol) showed a high degree of non-specific amplification when using DNA derived from mosquitoes stored in ethanol or isopropanol. The TaqMan method appeared unaffected when using the same samples.

Conclusion

This study describes a new high-throughput TaqMan assay that very effectively detects the four Plasmodium species that cause malaria in humans and discriminates the most deadly species, P. falciparum, from the others. This method is at least as sensitive and specific as the gold standard nested PCR approach and because it has no requirement for post-PCR processing is cheaper, simpler and more rapid to run. In addition this method is not inhibited by the storage of mosquito specimens by drying or in ethanol or isopropanol.

Reduced susceptibility to DDT in field populations of Anopheles quadriannulatus and Anopheles arabiensis in Malawi: evidence for larval selection

Bioassays for insecticide resistance in adult mosquitoes were conducted on samples of Anopheles gambiae Giles s.l. (Diptera: Culicidae) species collected as larvae from breeding sites in the lower Shire Valley, Malawi. The results indicate full susceptibility to permethrin, deltamethrin and malathion, but reduced susceptibility to DDT in one sample from Thom (LT(50) of 8.39 min for females and 25.09 min for males). Polymerase chain reaction-based species identification of the mosquitoes assayed revealed a mixture of Anopheles arabiensis Patton and Anopheles quadriannulatus (Theobold). The LT(50) did not differ significantly between species. Genotyping of the L1014F and L1014S kdr alleles showed all mosquito specimens to be homozygous wild type; thus the reduced susceptibility detected is not attributable to target site insensitivity and instead is likely to be metabolic in nature. Anopheles quadriannulatus is characteristically zoophagic and exophilic. Indeed, of 82 Anopheles collected through knockdown collections within dwellings, only one was An. quadriannulatus and the rest were An. arabiensis. They are unlikely, therefore, to have been exposed to selection pressure arising from insecticide-treated net usage or to DDT indoor residual spraying. Therefore, it is suggested that this example of reduced susceptibility to DDT in An. quadriannulatus reflects selection in the larval stages.

Lessons from the past: managing insecticide resistance in malaria control and eradication programmes

The distribution of insecticide-treated bednets to help combat the burden of malaria in sub-Saharan Africa has accelerated in the past 5 years. Additionally, many countries are also considering, or have already begun, indoor residual spraying campaigns. These are positive developments, since vector control has repeatedly proven to be an effective means of reducing malaria transmission. However, the sustainability of these insecticide-based interventions relies on the continuing susceptibility of the anopheles vectors to the limited number of available insecticides. Continual monitoring for early signs of insecticide resistance and the adoption of carefully considered resistance management strategies are therefore required. Regrettably, this essential monitoring component is frequently given a low priority in the push to meet ambitious coverage targets. We outline the key requirements for establishing an insecticide resistance surveillance system and urge all those involved in malaria vector control, either directly or as facilitators, to ensure that these measures are incorporated into control programmes. Failure to act now will inevitably lead to a future breakdown in disease control and jeopardise hopes of eradicating this major public-health problem.

A primer-introduced restriction analysis-polymerase chain reaction method to detect knockdown resistance mutations in Anopheles gambiae

In the major malaria vector Anopheles gambiae Giles, two point mutations at the voltage-gated sodium channel have been associated with knockdown resistance (kdr) to DDT and pyrethroid insecticides. Simple allele-specific polymerase chain reaction (PCR) assays to detect these single-nucleotide polymorphisms are prone to lack of specificity and therefore alternative techniques have been proposed. However, these may not be easily implemented in many laboratories from malaria endemic regions. Here, we describe a primer-introduced restriction analysis (PIRA)-PCR method to detect kdr mutations in An. gambiae. This method unambiguously identified all six genotypes for the kdr locus in a sample of 113 field-collected mosquitoes for which kdr genotypes had been confirmed by DNA sequencing. Co-occurrence of both kdr alleles was found in sites from Equatorial Guinea and Gabon and the L1014F mutation was detected in M-form individuals from Angola. The PIRA-PCR proved to be a reliable, robust, and simpler alternative for the detection of kdr mutations in this malaria vector.