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

Investigation of pyrethroid resistance mutations in Linognathus stenopsis lice collected from goats in western and northwestern Iran

Introduction

Linognathus stenopsis lice are an extensive parasitic concern in goat populations worldwide, posing significant economic and health risks. This study examined the identification of alleles of resistance to pyrethroid and mutations in L. stenopsis samples obtained from goats in five provinces in western and northwestern Iran.

Methods

Morphological and molecular techniques were employed to identify the louse species. Molecular identification methods and gene sequencing were used to identify resistance-associated mutations in the voltage-gated sodium channel (VGSC) gene.

Results and discussion

The results revealed that six amino acid substitutions, including threonine-to-isoleucine (T917I), leucine-to-phenylalanine (L920F), isoleucine-to-phenylalanine (I927F), phenylalanine-to-alanine (F928A), valine-to-arginine (V929R), and arginine-to-leucine (R930L) mutations, were present in the VGSC gene of L. stenopsis lice from various regions of Iran. These findings suggest the potential for pyrethroid resistance development in this louse species, highlighting the importance of integrated pest management (IPM) strategies. Such strategies, which combine selective insecticides, regular grooming, and environmental sanitation, are crucial for effectively managing L. stenopsis infestations and preserving the efficacy of pyrethroids for pest control. Moreover, the emergence of novel kdr mutations underscores the need for ongoing research into the molecular mechanisms underlying these mutations. This research is vital for developing strategies to combat pyrethroid resistance and maintaining the efficacy of insecticides in controlling lice.

Knockdown resistance mutations are common and widely distributed in Xenopsylla cheopis fleas that transmit plague in Madagascar

BACKGROUND

Plague, caused by the bacterium Yersinia pestis, remains an important disease in Madagascar, where the oriental rat flea, Xenopsylla cheopis, is a primary vector. To control fleas, synthetic pyrethroids (SPs) have been used for >20 years, resulting in resistance in many X. cheopis populations. The most common mechanisms of SP resistance are target site mutations in the voltage-gated sodium channel (VGSC) gene.

METHODOLOGY/PRINCIPAL FINDINGS

We obtained 25 collections of X. cheopis from 22 locations across Madagascar and performed phenotypic tests to determine resistance to deltamethrin, permethrin, and/or dichlorodiphenyltrichloroethane (DDT). Most populations were resistant to all these insecticides. We sequenced a 535 bp segment of the VGSC gene and identified two different mutations encoding distinct substitutions at amino acid position 1014, which is associated with knockdown resistance (kdr) to SPs in insects. Kdr mutation L1014F occurred in all 25 collections; a rarer mutation, L1014H, was found in 12 collections. There was a significant positive relationship between the frequency of kdr alleles and the proportion of individuals surviving exposure to deltamethrin. Phylogenetic comparisons of 12 VGSC alleles in Madagascar suggested resistant alleles arose from susceptible lineages at least three times. Because genotype can reasonably predict resistance phenotype, we developed a TaqMan PCR assay for the rapid detection of kdr resistance alleles.

CONCLUSIONS/SIGNIFICANCE

Our study provides new insights into VGSC mutations in Malagasy populations of X. cheopis and is the first to report a positive correlation between VGSC genotypes and SP resistance phenotypes in fleas. Widespread occurrence of these two SP resistance mutations in X. cheopis populations in Madagascar reduces the viability of these insecticides for flea control. However, the TaqMan assay described here facilitates rapid detection of kdr mutations to inform when use of these insecticides is still warranted to reduce transmission of plague.

High levels of pyrethroid resistance and super-kdr mutations in the populations of tropical bed bug, Cimex hemipterus, in Iran

Background

The tropical bed bug, Cimex hemipterus, is an important ectoparasite causing various health problems. This species is mainly confined to tropical regions; however, insecticide resistance, global warming, and globalization have changed its distribution map. Molecular information on pyrethroid resistance, which is essential for the development of control programs, is unknown for C. hemipterus in expanded areas. The present study was designed to determine the permethrin resistance status, characterize the pyrethroid receptor sites in voltage-gated sodium channel (vgsc) gene, and identify the resistance-related mutations in the populations of tropical bed bug in Iran.

Methods

Live bed bugs were collected, and adults of C. hemipterus were selected for bioassay and molecular surveys. Bioassay was performed by tarsal contact with permethrin 0.75% in mixed-sex of samples. Conventional and quantitative TaqMan and SYBR Green real-time PCR assays were conducted to characterize the vgsc gene and genotypes of studied populations.

Results

In the bioassay tests, the mortality rates were in the range of 30.7–38.7% and 56.2–77.4% in 24 and 48 h, respectively. The knockdown rates of studied populations were in the range of 32.2–46.6% and 61.5–83.8% in the first and second days, respectively. The KT₅₀ and KT₉₀ values in the Cimex lectularius Kh1 strain were presented as 5.39 and 15.55 h, respectively. These values in the selected populations of C. hemipterus varied from 27.9 to 29.5 and from 82.8 to 104.4 h, respectively. Knockdown time ratios (KR₅₀ and KR₉₀) in these populations varied from 5.17 to 6.17-fold compared with those of the C. lectularius Kh1 strain. Fragments of vgsc gene with 355 bp and 812 bp were amplified. Analysis of sequences revealed the A468T substitution, kdr-associated D953G, and super-kdr M918I and L1014F mutations in all populations.

Conclusions

The specific/sensitive, safe, and rapid diagnostic assays developed in this study are recommended for detection of kdr/super-kdr mutations and frequency of mutant alleles. The presence of super-kdr mutations and high resistance to permethrin in all the populations necessitate the reconsideration of control approaches against C. hemipterus.

Graphical Abstract

Two classic mutations in the linker-helix IIL45 and segment IIS6 of Apolygus lucorum sodium channel confer pyrethroid resistance

BACKGROUND

Pyrethroids are classified as type I and type II for distinct symptomology. Voltage-gated sodium channel is a primary target of pyrethroids. Mutations of the insect sodium channel have been identified to result in resistance to pyrethroids. Double mutation (L¹⁰⁰² F/M⁹⁰⁶ I) was detected in field-strain of Apolygus lucorum (Meyer-Dür). Although, it was illuminated the function of the same position mutation in other pests, it is necessary to demonstrate the role in A. lucorum .

RESULTS

In this study, we examined the effects of mutations on channel gating and pyrethroid sensitivity in Xenopus oocytes. L¹⁰⁰² F, M⁹⁰⁶ I and L¹⁰⁰² F/M⁹⁰⁶ I all shifted the voltage dependence of activation in the depolarizing direction. L¹⁰⁰² F, M⁹⁰⁶ I and L¹⁰⁰² F/M⁹⁰⁶ I all reduced the amplitude of tail currents induced by type I (bifenthrin and permethrin) and type II (λ-cyhalothrin and deltamethrin). The double mutation, L¹⁰⁰² F/M⁹⁰⁶ I, reduced integral channel modification by 10-fold compared with the L¹⁰⁰² F and M⁹⁰⁶ I mutations alone, respectively. Computational analysis based on the model of dual pyrethroid receptors, the two resistance mutations, L¹⁰⁰² F and M⁹⁰⁶ I are facing two opposite sides of this newly identified pocket. Both mutations affect the optimal binding of the ligands by changing the shape of the pocket but in different ways.

CONCLUSION

Our results illustrate the distinct effect of mutations on pyrethroids. It is predicted with computer modeling that these mutations allosterically affect pyrethroid binding. © 2020 Society of Chemical Industry.

First detection of the kdr mutation (L1014F) in the plague vector Xenopsylla cheopis (Siphonaptera: Pulicidae)

BACKGROUND

The oriental rat flea, Xenopsylla cheopis, is the most efficient vector of the plague. Pyrethroid insecticides such as cypermethrin, cyhalothrin and deltamethrin have been often used to limit plague transmission via controlling the vector during outbreaks. However, this strategy is threatened by the development of insecticide resistance. Understanding the mechanisms underlying pyrethroid resistance is the prerequisite for successful flea control.

METHODS

Partial DNA sequences of X. cheopis voltage gated sodium channel (VGSC) gene were amplified from a total of 111 individuals, collected from a natural plague epidemic foci in Baise City, Guangxi Zhuang Autonomous Region of China. These DNA fragments were sequenced. The frequency and distribution of kdr mutations were assessed in four X. cheopis populations. The origin of kdr mutations was investigated by phylogenetic and network analysis.

RESULTS

The classical knockdown resistance (kdr) mutation (L1014F) was detected in four field populations at frequencies ranging between 0.021-0.241. The mutant homozygote was observed only in one of the four populations. Seven haplotypes were identified, with two of them carrying the resistance L1014F mutation. Phylogenetic tree and network analysis indicated that the L1014F allele was not singly originated. Based on polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) profiling, an easy-to-use and accurate molecular assay for screening individual fleas for the L1014F mutation was developed.

CONCLUSIONS

To our knowledge, this work represents the first report of the L1014F mutation in the plague vector X. cheopis. The incidence of the L1014F allele highlights the need of further studies on the phenotypic effect of this mutation in this plague vector. Early detection and monitoring of insecticide resistance is suggested in order to make effective control strategies in case of plague outbreaks in this region.

Field assessment of insecticide dusting and bait station treatment impact against rodent flea and house flea species in the Madagascar plague context

Bubonic is the most prevalent plague form in Madagascar. Indoor ground application of insecticide dust is the conventional method used to control potentially infected rodent fleas that transmit the plague bacterium from rodents to humans. The use of bait stations is an alternative approach for vector control during plague epidemics, as well as a preventive control method during non-epidemic seasons. Bait stations have many advantages, principally by reducing the amount of insecticide used, lowering the cost of the treatment and minimizing insecticide exposure in the environment. A previous study reported promising results on controlling simultaneously the reservoir and vectors, when slow-acting rodenticide was incorporated in bait stations called “Boîtes de Kartman”. However, little evidence of an effective control of the fleas prior to the elimination of rodents was found. In this study, we evaluated bait stations containing insecticide powder and non-toxic attractive rodent bait for their potential to control rat fleas. Its efficacy was compared to the standard method. The impact of both methods on indoor and outdoor rodent fleas, as well as the human household flea Pulex irritans were analyzed at different time points after treatments. Bait stations did not cause any significant immediate or delayed reduction of rat fleas and increasing the number of operational bait stations per household did not significantly improve their efficacy. Insecticide ground dusting appeared to be the most efficient method to control indoor rat fleas. Both methods appeared to have little impact on the density of outdoor rat fleas and human fleas. These results demonstrate limited effectiveness for bait stations and encourage the maintenance of insecticide dusting as a first-line control strategy in case of epidemic emergence of plague, when immediate effect on rodent fleas is needed. Recommendations are given to improve the efficacy of the bait station method.

Insecticide ground dusting inside houses is the recommended measure to control rat fleas responsible for bubonic plague transmission. The main inconvenience of this method is the direct contact of houseowners to the toxic insecticide dust and spillage in environment. A bait station approach, where the insecticide is confined in a box or tunnel containing rodent attractant, seems to be a valuable complementary or alternative vector control tool. However currently, little is known about its real efficacy on reducing or eliminating fleas harbored by rats. Guidelines regarding its implementation (density and duration of use) as vector control tool are lacking. Those questions were addressed during a field trial study, where bait stations were deployed at different densities per household and followed up at different time points. The efficacy of bait station was compared to the standard method. The present study allowed to demonstrate that bait station approach requires more improvements to be efficient. Meanwhile, insecticide ground dusting is still recommended for to control rat fleas during epidemics.

Transcript-Level Analysis of Detoxification Gene Mutation-Mediated Chlorpyrifos Resistance in Laodelphax striatellus (Hemiptera: Delphacidae)

Enhanced detoxification and target mutations that weaken insecticide binding ability are major mechanisms of insecticide resistance. Among these, over-expression or site mutations of carboxylesterase (CarE), cytochrome P450s (CYP450), and glutathione-S-transferase (GST) were the main form responsible for insecticide detoxification; however, transcript-level analysis of the relationship of detoxification gene mutations with chlorpyrifos (an organophosphorus insecticide) resistance is scarce thus far. In this study, multiple sites exhibiting polymorphisms within three detoxification genes were firstly examined via sequencing among different chlorpyrifos-resistant and susceptible individuals of Laodelphax striatellus. For example, the mutation frequencies of A374V in LsCarE16 were 83, 33, and 3%, S277A in LsCarE24 were 88, 28, and 3%, E36K in LsCYP426A1 were 100, 65, and 0% for chlorpyrifos-resistant, resistant decay, and susceptible individuals, respectively. Analysis also found expression levels of GSTd1, GSTt1, GSTs2, CYP4DE1U1, and CYP425B1 are coordinated with chlorpyrifos resistance levels; moreover, we found the deficiencies of 43S and 44A as well as two point mutations of E60D and Q61H at N-terminal region of the OP potential target acetylcholinesterase (AChE) in high resistant but not in low-chlorpyrifos resistant individuals. The results above all demonstrated the dynamic evolutionary process of insecticide resistance and revealed some resistance factors that only played roles at certain resistance level; high insecticide resistance in this example is the result of synergistic impact from multiple resistance factors.