SNP Analysis Infers that Recombination Is Involved in the Evolution of Amitraz Resistance in Rhipicephalus microplus

Novel polymorphisms in the octopamine receptor gene of amitraz resistant population of Rhipicephalus sanguineus sensu lato, in south India

The brown dog tick or Rhipicephalus sanguineus sensu lato is an ixodid tick, responsible for the dissemination of pathogens that cause canine infectious diseases besides inflicting the direct effects of tick bite. The hot humid climate of Kerala, a south Indian state, is favorable for propagation of tick vectors and acaricides are the main stay of tick control. Though the resistance against synthetic pyrethroids is reported among these species, the status of amitraz resistance in R. sanguineus s. l. in the country is uncertain due to the lack of molecular characterisation data and scarce literature reports. Hence the present study was focused on the phenotypic detection and preliminary genotypic characterisation of amitraz resistance in the R. sanguineus s. l. A modified larval packet test (LPT) on a susceptible isolate was performed to determine the discriminating dose (DD). Further LPT-DD on 35 tick isolates was carried out to detect amitraz resistance robustly, along with that full dose response bioassays on the resistant isolates were performed. The results indicated that amitraz resistance is prevalent with 49 per cent of the samples being resistant. Amplification of exon 3 of octopamine receptor gene from both the susceptible and resistant larval isolates was carried out. Amplicons of ten pooled amitraz susceptible and ten pooled amitraz resistant representative samples were sequenced and analysed, unveiling a total of three novel non-synonymous mutations in the partial coding region at positions V32A, N41D and V58I in phenotypically resistant larval DNA samples. In silico analysis by homology modelling and molecular docking of the mutated and unmutated receptors showed that these mutations had reduced the binding affinity to amitraz. However, lack of mutations in the octopamine receptor gene in three of the pooled low order resistant R. sanguineus s. l. larval samples could be suggestive of other mechanisms associated with amitraz resistance in the region. Hence, further association studies should be carried out to confirm the association of these mutations with target insensitivity in R. sanguineus s. l. ticks, along with exploring the status of metabolic resistance and other mechanisms of resistance.

Risk factors contributing to tick-acaricide control failure in communal areas of the Oliver Tambo district eastern cape province, South Africa

Application of chemical acaricides in the control of ticks has led to the problem of tick-acaricide control failure. To obtain an understanding of the possible risk factors involved in this tick-acaricide control failure, this study investigated tick control practices on communal farms in the north-eastern part of the Eastern Cape Province (ECP) of South Africa. A semi-structured questionnaire designed to document specific farm attributes and acaricide usage practices was administered at 94 communal farms from the Oliver Tambo District municipality of the ECP. Data collected indicated that the main acaricide chemicals used at plunge dips of inland and coastal areas were synthetic pyrethroid formulations. Most (75%) farmers claimed not to have noticed a significant reduction in numbers of actively feeding and growing ticks on cattle after several acaricide treatments. Based on the farmers' perceptions, leading factors that could have led to tick-acaricide control failure included: weak strength of the dip solution (76%); poor structural state of dip tanks (42%); and irregular tick control (21%). The rearing of crossbreeds of local and exotic cattle breeds, perceived weak strength of the dip solution and high frequency of acaricide treatment, were statistically associated with proportions of farms reporting tick-acaricide control failure. Furthermore, approximately 50% of farms reported at least four tick control malpractices, which could have resulted in the emergence and spread of tick-acaricide control failure. Other sub-optimal tick control practices encountered included incorrect acaricide rotation, and failure to treat all cattle in a herd. This data will inform and guide the development of management strategies for tick-acaricide control failure and resistance in communal farming areas.

Transcriptome wide identification of neuropeptides and G protein-coupled receptors (GPCRs) in Sunn pest, Eurygaster integriceps Puton

Sunn pest (Eurygaster integriceps Puton) is major wheat pest causing economic damage. Neuropeptides and their receptors, G protein-coupled receptors (GPCRs), are involved in the regulation of insect physiology and behavior. Herein, a transcriptome-wide analysis was conducted in order to identify genes encoding neuropeptides, and putative GPCRs to gain insight into neuropeptide-modulated processes. De novo transcriptome assembly was undertaken using paired-end sequence reads derived from RNA samples collected from whole adults and yielded 582,398 contigs. In total, 46 neuropeptides have been identified, encompassing various known insect neuropeptide families. In addition, we discovered four previously uncharacterized neuroparsin peptides, which contributes to our understanding of the neuropeptide landscape. Furthermore, 85 putative neuropeptide GPCRs were identified, comprising three classes of GPCRs, A, B, C, and LGR, of which class C is not widely reported in insects. In addition, the identified GPCRs exhibited a remarkable 80% homology with the GPCRs found in the brown marmorated stink bug. It is noteworthy that these GPCRs displayed only a 20% homology to GPCRs from many other insect species. This information may be used to understand the neuropeptide-modulated physiology and behavior of Eurygaster integriceps, and to develop specific neuropeptide-based pest management strategies.

Contrasting roles of cytochrome P450s in amitraz and chlorfenapyr resistance in the crop pest Tetranychus urticae

The molecular mechanisms of amitraz and chlorfenapyr resistance remain only poorly understood for major agricultural pests and vectors of human diseases. This study focusses on a multi-resistant field strain of the crop pest Tetranychus urticae, which could be readily selected in the laboratory to high levels of amitraz and chlorfenapyr resistance. Toxicity experiments using tralopyril, the active toxophore of chlorfenapyr, suggested decreased activation as a likely mechanism underlying resistance. Starting from the same parental strain, transcriptome profiling revealed that a cluster of detoxifying genes was upregulated after amitraz selection, but unexpectedly downregulated after chlorfenapyr selection. Further functional validation associated the upregulation of CYP392A16 with amitraz metabolism and the downregulation of CYP392D8 with reduced activation of chlorfenapyr to tralopyril. Genetic mapping (QTL analysis by BSA) was conducted in an attempt to unravel the genetic mechanisms of expression variation and resistance. This revealed that chlorfenapyr resistance was associated with a single QTL, while 3 QTLs were uncovered for amitraz resistance. Together with the observed contrasting gene expression patterns, we argue that transcriptional regulators most likely underly the distinct expression profiles associated with resistance, but these await further functional validation.

A review of the molecular mechanisms of acaricide resistance in mites and ticks

The Arachnida subclass of Acari comprises many harmful pests that threaten agriculture as well as animal health, including herbivorous spider mites, the bee parasite Varroa, the poultry mite Dermanyssus and several species of ticks. Especially in agriculture, acaricides are often used intensively to minimize the damage they inflict, promoting the development of resistance. Beneficial predatory mites used in biological control are also subjected to acaricide selection in the field. The development and use of new genetic and genomic tools such as genome and transcriptome sequencing, bulked segregant analysis (QTL mapping), and reverse genetics via RNAi or CRISPR/Cas9, have greatly increased our understanding of the molecular genetic mechanisms of resistance in Acari, especially in the spider mite Tetranychus urticae which emerged as a model species. These new techniques allowed to uncover and validate new resistance mutations in a larger range of species. In addition, they provided an impetus to start elucidating more challenging questions on mechanisms of gene regulation of detoxification associated with resistance.

Control of ticks and tick-borne diseases in Africa through improved diagnosis and utilisation of data on acaricide resistance

A meeting, sponsored by the Bill and Melinda Gates Foundation (BMGF) and organised by Clinglobal, was held at The International Livestock Research Institute (ILRI) in Nairobi, Kenya, from 19th - to 21st October 2022. The meeting assembled a unique group of experts on tick control in Africa. Academia, international agencies (FAO and ILRI), the private Animal Health sector and government veterinary services were represented. The significant outcomes included: (i) a shared commitment to standardisation and improvement of acaricide resistance bioassay protocols, particularly the widely used larval packet test (LPT); (ii) development of novel molecular assays for detecting acaricide resistance; (3) creation of platforms for disseminating acaricide resistance data to farmers, veterinary service providers and veterinary authorities to enable more rational evidence-based control of livestock ticks. Implementation of enhanced control will be facilitated by several recently established networks focused on control of parasites in Africa and globally, whose activities were presented at the meeting. These include a newly launched community of practice on management of livestock ticks, coordinated by FAO, an African module of the World Association for the Advancement of Veterinary Parasitology (WAAVP-AN) and the MAHABA (Managing Animal Health and Acaricides for a Better Africa) initiative of Elanco Animal Health.

Assessment of cypermethrin and amitraz resistance and molecular profiling of voltage-gated sodium channel and octopamine tyramine genes of Rhipicephalus microplus

Control of ticks and tick-borne pathogens is a priority for human and animal health. Livestock-holders extensively rely on acaricide applications for tick control. Different groups of acaricides including cypermethrin and amitraz have been consistently used in Pakistan. There has been a gap in understanding the susceptibility or resistance of Rhipicephalus microplus, the most prevalent tick in Pakistan, to acaricides. The present study aimed to molecularly characterize cypermethrin and amitraz targeted genes such as voltage-gated sodium channel (VGSC) and octopamine tyramine (OCT/Tyr) of R. microplus ticks in Khyber Pakhtunkhwa (KP), Pakistan to monitor the acaricides resistance. Tick specimens were collected from cattle and buffaloes in northern (Chitral, Shangla, Swat, Dir, and Buner), central (Peshawar, Mardan, Charsadda, Swabi, and Nowshera), and southern districts (Kohat, Karak, Lakki Marwat, Tank, and Dera Ismail Khan) of KP, Pakistan. Different concentrations of commercially available cypermethrin (10%) and amitraz (12.5%) were prepared for in vitro larval immersion tests (LIT). In LIT, the average mortality rate of immersed larvae was recorded that was increased gradually with an increase in the concentration of specific acaricide. The larvae's highest mortality rates (94.5% and 79.5%) were observed at 100-ppm of cypermethrin and amitraz, respectively. A subset of 82 R. microplus ticks was subjected to extract genomic DNA, followed by PCR to amplify partial fragments of VGSC (domain-II) and OCT/Tyr genes. The BLAST results of the consensus sequence of VGSC gene (domain-II) showed 100% identity with the acaricides susceptible tick sequence from the United States (reference sequence). Obtained identical sequences of OCT/Tyr genes showed maximum identity (94-100%) with the identical sequences reported from Australia (reference sequence), India, Brazil, Philippines, USA, South Africa, and China. Thirteen single nucleotide polymorphisms (10 synonymous and three non-synonymous) were observed at various positions of partial OCT/Tyr gene fragments. The SNP at position A-22-C (T-8-P) in OCT/Tyr gene has been linked to amitraz resistance in R. microplus ticks. Molecular analysis and LIT bioassay's findings indicate the availability of resistant R. microplus ticks in the KP region. To our understanding, this is the first preliminary study to monitor cypermethrin and amitraz resistance via molecular profiling of cypermethrin and amitraz targeted genes (VGSC and OCT/Tyr) in combination with in vitro bioassays (LIT) in R. microplus ticks from Pakistan.

Molecular Characterization of Octopamine/Tyramine Receptor Gene of Amitraz-Resistant Rhipicephalus (Boophilus) decoloratus Ticks from Uganda

We previously reported the emergence of amitraz-resistant Rhipicephalus (Boophilus) decoloratus ticks in the western region of Uganda. This study characterized the octopamine/tyramine receptor gene (OCT/Tyr) of amitraz-resistant and -susceptible R. (B.) decoloratus ticks from four regions of Uganda. The OCT/Tyr gene was amplified from genomic DNA of 17 R. (B.) decoloratus larval populations of known susceptibility to amitraz. The amplicons were purified, cloned and sequenced to determine mutations in the partial coding region of the OCT/Tyr gene. The amplified R. (B.) decoloratus OCT/Tyr gene was 91-100% identical to the R. (B.) microplus OCT/Tyr gene. Up to 24 single nucleotide polymorphisms (SNPs) were found in the OCT/Tyr gene from ticks obtained from high acaricide pressure areas, compared to 8 from the low acaricide pressure areas. A total of eight amino acid mutations were recorded in the partial OCT/Tyr gene from ticks from the western region, and four of them were associated with amitraz-resistant tick populations. The amino acid mutations M1G, L16F, D41G and V72A were associated with phenotypic resistance to amitraz with no specific pattern. Phylogenetic analysis revealed that the OCT/Tyr gene sequence from this study clustered into two distinct groups that separated the genotype from high acaricide pressure areas from the susceptible populations. In conclusion, this study is the first to characterize the R. (B.) decoloratus OCT/Tyr receptor gene and reports four novel amino acid mutations associated with phenotypic amitraz resistance in Uganda. However, lack of mutations in the ORF of the OCT/Tyr gene fragment for some of the amitraz-resistant R. (B.) decoloratus ticks could suggest that other mechanisms of resistance may be responsible for amitraz resistance, hence the need for further investigation.

Acaricides Resistance in Ticks: Selection, Diagnosis, Mechanisms, and Mitigation

Ticks are blood-feeding ecto-parasites that have a cosmopolitan distribution in tropical and subtropical regions of the world. Ticks cause economic losses in the form of reduced blood, meat and dairy products, as well as pathogen transmission. Different acaricides such as organochlorines, organophosphates, formamidines (e.g. amitraz), synthetic pyrethroids, macrocyclic lactones, fipronil, and fluazuron are currently used sequentially or simultaneously to control tick infestations. Most acaricide treatments now face increasingly high chances of failure, due to the resistance selection in different tick populations against these drugs. Acaricide resistance in ticks can be developed in different ways, including amino acid substitutions that result in morphological changes in the acaricide target, metabolic detoxification, and reduced acaricide entry through the outer layer of the tick body. The current literature brings a plethora of information regarding the use of different acaricides for tick control, resistance selection, analysis of mutations in target sites, and resistance mitigation. Alternatives such as synergistic use of different acaricides, plant-derived phytochemicals, fungi as biological control agents, and anti-tick vaccines have been recommended to avoid and mitigate acaricide resistance. The purpose of this review was to summarize and discuss different acaricides applied for tick control, their mechanisms of action and resistance selection, genetic polymorphisms in their target molecules, as well as the approaches used for diagnosis and mitigation of acaricide resistance, specifically in Rhipicephalus microplus ticks.

Molecular targets for the development of new acaricides against Rhipicephalus microplus: a review

The cattle tick Rhipicephalus microplus is an ectoparasite with high economic importance to bovine culture, mainly in tropical and subtropical regions. The resistance of the tick from the commercial acaricides has hindered its control, thus motivating the search for new strategies. The purpose of this study was to perform a critical review about the main molecular targets of R. microplus that are useful for the discovery of new acaricides. Bibliographic search was conducted in the databases PubMed, ScienceDirect and CAB Direct, using the following descriptors: ‘Rhipicephalus microplus’, ‘Boophilus microplus’, ‘molecular targets’ and ‘action’, published between 2010 and 2021. Out of the 212 publications identified, 17 articles were selected for study inclusion. This review described 14 molecular targets and among these 4 are targets from commercial acaricides. Most of them are enzymes to catalyse important reactions to tick survival, related to energetic metabolism, mechanisms of biotransformation and neurotransmission. The data will be helpful in the development of new more effective and selective acaricides.

Whole genome sequencing and bulked segregant analysis suggest a new mechanism of amitraz resistance in the citrus red mite, Panonychus citri (Acari: Tetranychidae)

BACKGROUND

Amitraz is a broad-spectrum insecticide/acaricide for the control of aphids, psyllids, ticks and mites. Current evidence suggests that ticks and phytophagous mites have developed strong resistance to amitraz. Previous studies have shown that multiple mechanisms are associated with amitraz resistance in ticks, but very few reports have involved Panonychus citri. We therefore used whole genome sequencing and bulked segregant analysis (BSA) to identify the mechanism underlying P. citri's resistance to amitraz.

RESULTS

High-quality assembly of the whole P. citri genome was completed, resulting in a genome of approximately 83.97 Mb and a contig N50 of approximately 1.81 Mb. Gene structure predictions revealed 11 577 genes, of which 10 940 genes were annotated. Trait-associated regions in the genome were mapped with bulked segregant analysis and 38 candidate SNPs were obtained, of which T752C had the strongest correlation with the resistant trait, located at the 5' untranslated region (UTR) of the β-2R adrenergic-like octopamine receptor gene. The mutation resulted in the formation of a short hairpin loop structure in mRNA and gene expression was down-regulated by more than 50% in the amitraz-resistant strain. Validation of the T752C mutation in field populations of P. citri found that the correlation between the resistance ratio and the base mutation was 94.40%.

CONCLUSION

Our results suggest that this 5' UTR mutation of the β-2R octopamine receptor gene, confers amitraz resistance in P. citri. This discovery provides a new explanation for the mechanism of pest resistance: base mutations in the 5' untranslated region of target gene may regulate the susceptibility of pests to pesticides.

An octopamine receptor confers selective toxicity of amitraz on honeybees and Varroa mites

The Varroa destructor mite is a devastating parasite of Apis mellifera honeybees. They can cause colonies to collapse by spreading viruses and feeding on the fat reserves of adults and larvae. Amitraz is used to control mites due to its low toxicity to bees; however, the mechanism of bee resistance to amitraz remains unknown. In this study, we found that amitraz and its major metabolite potently activated all four mite octopamine receptors. Behavioral assays using Drosophila null mutants of octopamine receptors identified one receptor subtype Octβ2R as the sole target of amitraz in vivo. We found that thermogenetic activation of octβ2R-expressing neurons mimics amitraz poisoning symptoms in target pests. We next confirmed that the mite Octβ2R was more sensitive to amitraz and its metabolite than the bee Octβ2R in pharmacological assays and transgenic flies. Furthermore, replacement of three bee-specific residues with the counterparts in the mite receptor increased amitraz sensitivity of the bee Octβ2R, indicating that the relative insensitivity of their receptor is the major mechanism for honeybees to resist amitraz. The present findings have important implications for resistance management and the design of safer insecticides that selectively target pests while maintaining low toxicity to non-target pollinators.

Mechanisms of amitraz resistance in a Rhipicephalus microplus strain from southern Brazil

Amitraz is one of the most used acaricides for the control of ticks of domestic animals, however, extensive use of this active ingredient has favored the development of resistant populations of Rhipicephalus microplus worldwide. The possible mechanisms of metabolic and/or target-site alterations mechanisms of amitraz resistance were investigated in a Brazilian field population of R. microplus (São Gabriel strain). Bioassays with the synergists piperonylbutoxide, triphenylphosphate and diethyl-maleate were used to evaluate the metabolic mechanisms involved. Target-site insensitivity was investigated by amplification and sequencing of a fragment of the octopamine/tyramine (OCT/TYR) receptor gene. Piperonylbutoxide synergism (synergism ratio = 2.8) indicated the participation of the P450 pathway in the detoxification of amitraz. Previously reported single nucleotide polymorphisms that confer amino acid changes in the OCT/TYR receptor, threonine to proline (T8P) and leucine to serine (L22S), were found in the amitraz-resistant strain but not in the susceptible reference strain. The results suggest that amitraz resistance in the studied strain is multi-factorial and may result from cytochrome P450 detoxification and mutations in octopamine receptors.

G-Protein Coupled Receptors (GPCRs) in Insects-A Potential Target for New Insecticide Development

G-protein coupled receptors (GPCRs) play important roles in cell biology and insects' physiological processes, toxicological response and the development of insecticide resistance. New information on genome sequences, proteomic and transcriptome analysis and expression patterns of GPCRs in organs such as the central nervous system in different organisms has shown the importance of these signaling regulatory GPCRs and their impact on vital cell functions. Our growing understanding of the role played by GPCRs at the cellular, genome, transcriptome and tissue levels is now being utilized to develop new targets that will sidestep many of the problems currently hindering human disease control and insect pest management. This article reviews recent work on the expression and function of GPCRs in insects, focusing on the molecular complexes governing the insect physiology and development of insecticide resistance and examining the genome information for GPCRs in two medically important insects, mosquitoes and house flies, and their orthologs in the model insect species Drosophila melanogaster. The tissue specific distribution and expression of the insect GPCRs is discussed, along with fresh insights into practical aspects of insect physiology and toxicology that could be fundamental for efforts to develop new, more effective, strategies for pest control and resistance management.

G-Protein Coupled Receptors (GPCRs): Signaling Pathways, Characterization, and Functions in Insect Physiology and Toxicology

G-protein-coupled receptors (GPCRs) are known to play central roles in the physiology of many organisms. Members of this seven α-helical transmembrane protein family transduce the extracellular signals and regulate intracellular second messengers through coupling to heterotrimeric G-proteins, adenylate cyclase, cAMPs, and protein kinases. As a result of the critical function of GPCRs in cell physiology and biochemistry, they not only play important roles in cell biology and the medicines used to treat a wide range of human diseases but also in insects’ physiological functions. Recent studies have revealed the expression and function of GPCRs in insecticide resistance, improving our understanding of the molecular complexes governing the development of insecticide resistance. This article focuses on the review of G-protein coupled receptor (GPCR) signaling pathways in insect physiology, including insects’ reproduction, growth and development, stress responses, feeding, behaviors, and other physiological processes. Hormones and polypeptides that are involved in insect GPCR regulatory pathways are reviewed. The review also gives a brief introduction of GPCR pathways in organisms in general. At the end of the review, it provides the recent studies on the function of GPCRs in the development of insecticide resistance, focusing in particular on our current knowledge of the expression and function of GPCRs and their downstream regulation pathways and their roles in insecticide resistance and the regulation of resistance P450 gene expression. The latest insights into the exciting technological advances and new techniques for gene expression and functional characterization of the GPCRs in insects are provided.

The Insect Type 1 Tyramine Receptors: From Structure to Behavior

Simple Summary

This review aims to describe the type 1 tyramine receptors (TAR1s) in insects with a multidisciplinary approach and might be an important tool for a wide scientific audience, including biochemists, molecular physiologists, ethologists, and neurobiologists with a biological entomology background. In fact, in the last years, TAR1 has received much attention due to its broad general interest. The review is composed of a general introduction about the tyraminergic and octopaminergic systems and the corresponding tyramine (TA) and octopamine (OA) receptors, including the recent classification as well as their brief structural and functional information. The four chapters then describe TAR1s: (1) Molecular and structural characterization, with the purpose to provide a clear biochemical overview of the receptor that ensures a well-defined TAR1 identity; (2) pharmacology, in which a clear TAR1-mediated intracellular signaling pathway is detailed; (3) physiology and behavior, focusing on the TAR1-controlled traits in insects; (4) insecticide target, in which the knowledge on TAR1 roles in insects is associated with the growing evidence about the pest management strategies based on this receptor. The conclusions summarize TAR1 features as well as future directions on which the receptor research should move.

Abstract

Tyramine is a neuroactive compound that acts as neurotransmitter, neuromodulator, and neurohormone in insects. Three G protein-coupled receptors, TAR1-3, are responsible for mediating the intracellular pathway in the complex tyraminergic network. TAR1, the prominent player in this system, was initially classified as an octopamine receptor which can also be activated by tyramine, while it later appeared to be a true tyramine receptor. Even though TAR1 is currently considered as a well-defined tyramine receptor and several insect TAR1s have been characterized, a defined nomenclature is still inconsistent. In the last years, our knowledge on the structural, biochemical, and functional properties of TAR1 has substantially increased. This review summarizes the available information on TAR1 from different insect species in terms of basic structure, its regulation and signal transduction mechanisms, and its distribution and functions in the brain and the periphery. A special focus is given to the TAR1-mediated intracellular signaling pathways as well as to their physiological role in regulating behavioral traits. Therefore, this work aims to correlate, for the first time, the physiological relevance of TAR1 functions with the tyraminergic system in insects. In addition, pharmacological studies have shed light on compounds with insecticidal properties having TAR1 as a target and on the emerging trend in the development of novel strategies for pest control.

Determination of resistance status to amitraz in the cattle tick Rhipicephalus (Boophilus) microplus from Luzon, Philippines, through bioassay and molecular analysis

Tick infestations and tick-borne diseases are among the leading causes of economic losses in the cattle industry worldwide. Amitraz is one of the most commonly used chemical acaricides against the cattle tick Rhipicephalus (Boophilus) microplus. Resistance to amitraz has been reported in many countries but not in the Philippines. This study aimed to determine whether cattle ticks from provinces in northern and southern Luzon, Philippines, are resistant to amitraz. Engorged or nearly engorged female ticks were collected from 21 farms and allowed to lay eggs to produce larvae. Larval packet test (LPT) was performed using three concentrations of amitraz, as well as a negative control (diluent). Reverse-transcription polymerase chain reaction (RT-PCR) was also performed to amplify a fragment of the octopamine/tyramine receptor gene and was subjected to sequence analysis by multiple nucleotide and amino acid sequence alignments together with reference strain, amitraz-susceptible strain, and amitraz-resistant strain reported in other countries. LPT results suggest the absence of resistance in all of the tested populations. Interestingly, analysis of the octopamine/tyramine receptor amino acid sequence revealed four out of 14 larval pools having substitutions similar to that of the reported amitraz-resistant strains. Although no apparent resistance was observed in this study, prudent use of amitraz should be practiced as the development of resistance is still likely to occur in the future.

An update on amitraz efficacy against Rhipicephalus microplus after 15 years of disuse

Amitraz is an acaricide that is widely used in veterinary medicine to control the cattle tick Rhipicephalus microplus. However, controversy exists in the literature regarding the resistance of R. microplus to this product. The present work provides an update on the acaricidal efficacy of amitraz (Triatox®, 12.5 % amitraz) after 15 years without its use on a property. Two in vivo (bovines treated with amitraz and submitted to tick counts, n = 20 animals) and one in vitro (adult immersion test, n = 40 ticks) assays were performed to determine product efficacy. The efficacy of the commercial formulation tested in the first in vivo trial ranged from 14.1 to 47.0%, and in the second from 3.6 to 35.1%, for the 28 days of the experiments. Efficacy for the in vitro trial was 47.38%. The dose recommended by the manufacturer of the product did not cause mortality to most of the ticks of this strain, and efficacy/resistance was not reverted or modified after 15 years (estimated 60 tick generations).

Genotyping amitraz resistance profiles in Rhipicephalus microplus Canestrini (Acari: Ixodidae) ticks from Punjab, India

Acaricide resistance is one of the greatest threats to sustainable and effective control of vector ticks worldwide. The amitraz resistance status in cattle tick, Rhipicephalus microplus populations collected from 18 districts of Punjab in north-western India were characterized using bioassay and molecular assays. The modified larval packet test was used and the resistance factors (RF) against amitraz for the field populations were in the range of 0.36-4.85, indicating level I resistance status in ten populations. Characterization of a partial segment of the octopamine/tyramine (OCT/Tyr) receptor gene of R. microplus field populations from Punjab revealed a total of 18 nucleotide substitutions in the coding region out of which 5 were non-synonymous substitutions. Three of these non-synonymous substitutions (T8P, V15I and A20 T) were earlier reported in American and South African populations of R. microplus. Among the two single nucleotide polymorphisms (A22C-T8P; T65C-L22S) potentially linked to amitraz resistance in American, South African and Zimbabwean resistant populations, only the T8P substitution was recorded from the Barnala population. The PCR-RFLP assay using EciI restriction enzyme was used for genotyping of the larvae as homozygous resistant (RR), homozygous susceptible (SS) and heterozygous (SR). Genotyping of 514 larval DNA samples from 18 field populations revealed 92.8 % larval population as SR and the remaining 7.2 % as RR genotypes. The percentage of resistant alleles in the tick populations was 53.6 (range 50.0-57.2) indicating its moderate distribution in the region. The present study is the pioneer report establishing the hypothesis that amitraz-resistance is recessively inherited and heterozygous individuals show phenotypic susceptibility to the drug in the Indian tick populations.

Inventory of Varroa destructor susceptibility to amitraz and tau-fluvalinate in France

Varroa destructor is one of the greatest threats for the European honeybee, Apis mellifera. Acaricides are required to control mite infestation. Three conventional chemical acaricide substances are used in France: tau-fluvalinate, flumethrin and amitraz. Tau-fluvalinate was used for over 10 years before experiencing a loss of effectiveness. In 1995, bioassay trials showed the first mite resistance to tau-fluvalinate. In some countries, amitraz was widely used, also leading to resistance of V. destructor to amitraz. In France, some efficiency field tests showed a loss of treatment effectiveness with amitraz. We adapted the bioassay from Maggi and collaborators to determine mite susceptibility to tau-fluvalinate and amitraz in France in 2018 and 2019. The lethal concentration (LC) which kills 90% of susceptible mite strains (LC₉₀) is 0.4 and 12 µg/mL for amitraz and tau-fluvalinate, respectively. These concentrations were chosen as the determining factors to evaluate mite susceptibility. Some mites, collected from different apiaries, present resistance to amitraz and tau-fluvalinate (71% of the mite samples show resistance to amitraz and 57% to tau-fluvalinate). As there are few active substances available in France, and if mite resistance to acaricides continues to increase, the effectiveness of the treatments will decrease and therefore more treatments per year will be necessary. To prevent this situation, a new strategy needs to be put in place to include mite resistance management. We suggest that a bioassay would be a good tool with which to advise the policymakers.

Molecular screening of cattle ticks, tick-borne pathogens and amitraz resistance in ticks of Santo Domingo de los Tsáchilas province in Ecuador

The province of Santo Domingo de los Tsáchilas in Ecuador is a strategic place for cattle production and trade. The lack of knowledge about tick species, tick control and tick-borne diseases in Ecuador prompted this study with the goal of identifying the cattle-tick species and tick-borne agents present in the area and molecularly determining the potential acaricide resistance to amitraz of the major cattle tick species. Eighty-four cattle farms were visited and in 88 % of them, cattle were infested with ticks. Additionally, 24 historical samples from other surrounding Ecuadorian provinces, were screened as well. Besides morphological keys, PCR-RFLP MspI was used to confirm the presence of the Rhipicephalus ticks. The tick samples were also screened for tick-borne agents using PCR-RFLP BseDI and Hhal tests to identify circulating Babesia sp. and Anaplasma spp. Furthermore, the PCR-RFLP EciI technique was used to identify the amitraz resistance gene in populations of Rhipicephalus microplus in the province. Pooled testing was used to determine prevalence at individual-tick level. The presence of R. microplus and Amblyomma cajennense sensu lato (s.l.) ticks was found in 83 % and 21 % of the cattle farms respectively, showing R. microplus is widespread in the province of Santo Domingo de los Tsáchilas. Regarding tick-borne agents, only Anaplasma marginale was observed in 50 % of the visited farms of the province, while about 27 % of the ticks tested positive according to estimations from the data of the tick pools. The presence of Babesia bigemina was only confirmed in samples collected outside the province. The amitraz resistance allele in R. microplus was found in 62 % of the farms, but the percentage of farms with cattle ticks completely resistant to this acaricide was low (2%). The findings of this study should prompt cattle producers and animal health authorities to monitor control strategies, which address the management of resistant tick populations and the epidemiologically-unstable areas of tick-borne diseases.

Voltage-gated sodium channel gene mutations and pyrethroid resistance in Rhipicephalus microplus

Pyrethroid pesticides are extensively used to manage animal and human disease vectors including the southern cattle tick Rhipicephalus microplus (Canestrini). The indiscriminate and incorrect use of pyrethroids has led to the almost ubiquitous development of resistance to this pesticide class for this tick species. Voltage-gated sodium channels (Na-channels) are the primary target-site of pyrethroids and several studies on the involvement of mutations in the coding gene among pyrethroid-resistant R. microplus populations from different parts of the world have shown that there are various single nucleotide polymorphisms (SNPs) that are associated with resistance to pyrethroids. Identification of the exact location of the mutations in the protein coding regions of the targeted gene facilitates the design of various molecular tools for genotyping the resistant populations and thus promotes the rapid detection of resistance. This review aims to provide an update on the identification of pyrethroid resistance-associated Na-channel mutations from R. microplus.The database of diverse mutations from different regions of the world helped us to develop the molecular markers for resistance monitoring in a rapid and efficient manner. Their role and the development of different forms of molecular tools for genotyping ticks for mutations in the Na-channel gene are also discussed. In this review, the word mutation is used interchangeably with SNP.

Menace of acaricide resistance in cattle tick, Rhipicephalus microplus in India: Status and possible mitigation strategies

Livestock in general and dairy sector in particular plays a vital role in the Indian economy and in the socio-economic development of millions of people. Rhipicephalus microplus, a one-host and the most prevalent cattle tick in various agro climatic zones of India. This tick species is responsible for severe economic losses to livestock owners both through the direct effects of blood sucking, injection of toxins and indirectly by transmitting pathogens. Due to inadvertent and indiscriminate use of chemicals to kill ticks, field tick population has developed resistance to almost all chemicals that are used to manage them. The intention of this review is to provide the current status of resistance in ticks, the possible mechanisms of resistance operating in the tick population, factors contributing to the development of resistance and the managemental strategies.

Molecular markers and their application in the monitoring of acaricide resistance in Rhipicephalus microplus

Monitoring acaricide resistance and understanding the underlying mechanisms are critically important in developing strategies for resistance management and tick control. Identification of single nucleotide polymorphisms in the acaricide-resistant associated gene of Rhipicephalus microplus has enabled the development of molecular markers for detection and monitoring of resistance against different types of acaricide. There are many molecular markers developed for resistance monitoring, including mutations on target genes such as sodium channel, acetylcholinesterase, carboxylesterase, β-adrenergic octopamine receptor, octopamine-tyramine etc. Molecular genotyping through molecular markers can detect the presence of resistance-associated genes in a tick population before it reaches high frequency. This review aims to provide an update on the various molecular markers discovered to date from different regions of the world.

The genetic relationship between R. microplus and R. decoloratus ticks in South Africa and their population structure

Rhipicephalus microplus and R. decoloratus are one-host ticks that preferentially feed on cattle. They are capable of transmitting various tick-borne pathogens which may be detrimental to the agricultural and livestock industry in South Africa. Previous studies have shown that R. microplus forms five lineages in the R. microplus complex, segregating into different geographical areas based on mitochondrial markers. This study examined the phylogenetic relationship within and between R. microplus and R. decoloratus using the nuclear internal transcribed spacer 2 (ITS2) and mitochondrial cytochrome oxidase subunit I (COI) genes. The results showed that the nuclear ITS2 marker is informative for interspecific variation but lacks the resolution for intraspecific variation. Analysis of the mitochondrial COI gene revealed that R. microplus ticks from South Africa grouped into a clade comprised of ticks from Asia and South America. The population structure of these two tick species was also investigated using novel microsatellite markers. Population structure analyses revealed that both the R. microplus and R. decoloratus populations presented with two genetic clusters. Rhipicephalus microplus ticks from the Kwa-Zulu Natal (KZN) province belonged to cluster 1, and those from the Eastern Cape (EC) province predominantly grouped into cluster 2. No observable population structure was noted for R. decoloratus. The overlap of genetic clusters in both species could be attributed to inbreeding between the regions by unrestricted movement of cattle across provinces. Such movement promotes tick mobility, gene flow and the homogenisation of tick populations.

Differentially expressed genes in response to amitraz treatment suggests a proposed model of resistance to amitraz in R. decoloratus ticks

The widespread geographical distribution of Rhipicephalus decoloratus in southern Africa and its ability to transmit the pathogens causing redwater, gallsickness and spirochaetosis in cattle makes this hematophagous ectoparasite of economic importance. In South Africa, the most commonly used chemical acaricides to control tick populations are pyrethroids and amitraz. The current amitraz resistance mechanism described in R. microplus, from South Africa and Australia, involves mutations in the octopamine receptor, but it is unlikely that this will be the only contributing factor to mediate resistance. Therefore, in this study we aimed to gain insight into the more complex mechanism(s) underlying amitraz resistance in R. decoloratus using RNA-sequencing. Differentially expressed genes (DEGs) were identified when comparing amitraz susceptible and resistant ticks in the presence of amitraz while fed on bovine hosts. The most significant DEGs were further analysed using several annotation tools. The predicted annotations from these genes, as well as KEGG pathways potentially point towards a relationship between the α-adrenergic-like octopamine receptor and ionotropic glutamate receptors in establishing amitraz resistance. All genes with KEGG pathway annotations were further validated using RT-qPCR across all life stages of the tick. In susceptible ticks, the proposed model is that in the presence of amitraz, there is inhibition of Ca²⁺ entry into cells and subsequent membrane hyperpolarization which prevents the release of neurotransmitters. In resistant ticks, we hypothesize that this is overcome by ionotropic glutamate receptors (NMDA and AMPA) to enhance synaptic transmission and plasticity in the presence of neurosteroids. Activation of NMDA receptors initiates long term potentiation (LTP) which may allow the ticks to respond more rapidly and with less stimulus when exposed to amitraz in future. Overactivation of the NMDA receptor and excitotoxicity is attenuated by the estrone, NAD⁺ and ATP hydrolysing enzymes. This proposed pathway paves the way to future studies on understanding amitraz resistance and should be validated using in vivo activity assays (through the use of inhibitors or antagonists) in combination with metabolome analyses.

Strategies for the control of Rhipicephalus microplus ticks in a world of conventional acaricide and macrocyclic lactone resistance

Infestations with the cattle tick, Rhipicephalus microplus, constitute the most important ectoparasite problem for cattle production in tropical and subtropical regions worldwide, resulting in major economic losses. The control of R. microplus is mostly based on the use of conventional acaricides and macrocyclic lactones. However, the intensive use of such compounds has resulted in tick populations that exhibit resistance to all major acaricide chemical classes. Consequently, there is a need for the development of alternative approaches, possibly including the use of animal husbandry practices, synergized pesticides, rotation of acaricides, pesticide mixture formulations, manual removal of ticks, selection for host resistance, nutritional management, release of sterile male hybrids, environmental management, plant species that are unfavourable to ticks, pasture management, plant extracts, essential oils and vaccination. Integrated tick management consists of the systematic combination of at least two control technologies aiming to reduce selection pressure in favour of acaricide-resistant individuals, while maintaining adequate levels of animal production. The purpose of this paper is to present a current review on conventional acaricide and macrocyclic lactone resistance for better understanding and control of resistant ticks with particular emphasis on R. microplus on cattle.

Genetic mutations in sodium channel domain II and carboxylesterase genes associated with phenotypic resistance against synthetic pyrethroids by Rhipicephalus (Boophilus) decoloratus ticks in Uganda

We previously reported emergence of super synthetic pyrethroid (SP) resistant Rhipicephalus (Boophilus) decoloratus ticks in Uganda. This study investigated the genetic basis of phenotypic resistance against SP in R. (B.) decoloratus and sought to identify novel diagnostic mutations for rapid detection of SP resistance in the above tick species. Genomic DNA was extracted from pooled larvae of 20 tick populations (19 of known SP susceptibility and 1 unknown susceptibility). The voltage sensitive sodium channel (VSSC) domain II S4-5 linker (SP target) and partial carboxylesterase (SP metabolizing enzyme) genes were amplified by PCR, cloned and sequenced. The resultant sequences were analyzed to determine single nucleotide polymorphisms (SNPs) associated with phenotypic resistance in the various tick populations investigated. Novel SNPs that introduced Eco RI and Eco RII restriction sites in carboxylesterase gene were identified in silco and validated with restriction fragment length polymorphism (RFLP) against 18 tick populations of known SP susceptibility. The study identified a super knock down resistance (kdr) mutation T58C in R. (B.) decoloratus VSSC associated with stable SP resistance. We further identified multiple nonsynonymous mutations in carboxylesterase of SP resistant ticks; one of which conferred novel EcoRII (G195C) restriction site for PCR-RFLP detection of SP resistance. In conclusion, this study is the first to report super kdr mutation in sodium channel domain II and multiple mutations in carboxylesterase genes that may concurrently mediate stable resistance against synthetic pyrethroids in R. (B.) decoloratus ticks from Uganda. The Eco RII based PCR-RFLP is a useful tool for rapid detection of stable SP resistant R. (B.) decoloratus ticks.

An integrative approach to understanding pyrethroid resistance in Rhipicephalus microplus and R. decoloratus ticks

Rhipicephalus microplus and Rhipicephalus decoloratus species occur in regions with savannah and temperate climates, typically in grassland and wooded areas used as cattle pasture. Both species are associated with the transmission of Anaplasma and Babesia spp., impacting livestock health and quality of livestock-associated products. In Africa, tick control is predominantly mediated with the use of acaricides, such as synthetic pyrethroids. After several years on the market, reports of resistance to synthetic pyrethroids escalated but limited field data and validation studies have been conducted to determine the extent of acaricide resistance in Africa. Without this data, knowledge-based tick control will remain problematic and selection pressure will remain high increasing the rate of resistance acquisition. To date, several pyrethroid resistance associated single nucleotide polymorphisms (SNPs) have been reported for arthropods within the voltage-gated sodium channel. Three SNPs have been identified within this channel in pyrethroid resistant R. microplus ticks, but none has been reported for R. decoloratus. This study is the first to report the presence of a shared SNP within the voltage-gated sodium channel in both R. microplus and R. decoloratus, which is directly linked to pyrethroid resistance in R. microplus. As the mode of action by which these SNPs mediate pyrethroid resistance remains unknown, this study aims to set hypotheses by means of predictive structural modelling. This not only paves the way forward to elucidating the underlying biological mechanisms involved in pyrethroid resistance, but also improvement of existing acaricides and ultimately sustainable tick control management.