Amphidial structure of ivermectin-resistant and susceptible laboratory and field strains of Haemonchus contortus

The relationship between intraflagellar transport and upstream protein trafficking pathways and macrocyclic lactone resistance in Caenorhabditis elegans

Parasitic nematodes are globally important and place a heavy disease burden on infected humans, crops, and livestock, while commonly administered anthelmintics used for treatment are being rendered ineffective by increasing levels of resistance. It has recently been shown in the model nematode Caenorhabditis elegans that the sensory cilia of the amphid neurons play an important role in resistance toward macrocyclic lactones such as ivermectin (an avermectin) and moxidectin (a milbemycin) either through reduced uptake or intertissue signaling pathways. This study interrogated the extent to which ciliary defects relate to macrocyclic lactone resistance and dye-filling defects using a combination of forward genetics and targeted resistance screening approaches and confirmed the importance of intraflagellar transport in this process. This approach also identified the protein trafficking pathways used by the downstream effectors and the components of the ciliary basal body that are required for effector entry into these nonmotile structures. In total, 24 novel C. elegans anthelmintic survival-associated genes were identified in this study. When combined with previously known resistance genes, there are now 46 resistance-associated genes that are directly involved in amphid, cilia, and intraflagellar transport function.

Mapping resistance-associated anthelmintic interactions in the model nematode Caenorhabditis elegans

Parasitic nematodes infect billions of people and are mainly controlled by anthelmintic mass drug administration (MDA). While there are growing efforts to better understand mechanisms of anthelmintic resistance in human and animal populations, it is unclear how resistance mechanisms that alter susceptibility to one drug affect the interactions and efficacy of drugs used in combination. Mutations that alter drug permeability across primary nematode barriers have been identified as potential resistance mechanisms using the model nematode Caenorhabditis elegans. We leveraged high-throughput assays in this model system to measure altered anthelmintic susceptibility in response to genetic perturbations of potential cuticular, amphidial, and alimentary routes of drug entry. Mutations in genes associated with these tissue barriers differentially altered susceptibility to the major anthelmintic classes (macrocyclic lactones, benzimidazoles, and nicotinic acetylcholine receptor agonists) as measured by animal development. We investigated two-way anthelmintic interactions across C. elegans genetic backgrounds that confer resistance or hypersensitivity to one or more drugs. We observe that genetic perturbations that alter susceptibility to a single drug can shift the drug interaction landscape and lead to the appearance of novel synergistic and antagonistic interactions. This work establishes a framework for investigating combinatorial therapies in model nematodes that can potentially be translated to amenable parasite species.

Mapping resistance-associated anthelmintic interactions in the model nematode Caenorhabditis elegans

Parasitic nematodes infect billions of people and are mainly controlled by anthelmintic mass drug administration (MDA). While there are growing efforts to better understand mechanisms of anthelmintic resistance in human and animal populations, it is unclear how resistance mechanisms that alter susceptibility to one drug affect the interactions and efficacy of drugs used in combination. Mutations that alter drug permeability across primary nematode barriers have been identified as potential resistance mechanisms using the model nematode Caenorhabditis elegans. We leveraged high-throughput assays in this model system to measure altered anthelmintic susceptibility in response to genetic perturbations of potential cuticular, amphidial, and alimentary routes of drug entry. Mutations in genes associated with these tissue barriers differentially altered susceptibility to the major anthelmintic classes (macrocyclic lactones, benzimidazoles, and nicotinic acetylcholine receptor agonists) as measured by animal development. We investigated two-way anthelmintic interactions across C. elegans genetic backgrounds that confer resistance or hypersensitivity to one or more drugs. We observe that genetic perturbations that alter susceptibility to a single drug can shift the drug interaction landscape and lead to the appearance of novel synergistic and antagonistic interactions. This work establishes a framework for investigating combinatorial therapies in model nematodes that can potentially be translated to amenable parasite species.

The avermectin/milbemycin receptors of parasitic nematodes

Glutamate-gated chloride channels are the most important target of ivermectin and related compounds in parasitic nematodes. A small family of genes encode subunits of these channels, allowing the assembly of multiple channel subtypes; the subunit composition of most of the native receptors is unknown. The members of the gene family vary between species, making extrapolation from C. elegans to parasites difficult. Expression of recombinant receptors in Xenopus oocytes can identify subunits that have the ability to co-assemble into novel channels, but localisation data, ideally at the single-cell level, is required to confirm that these subunits are expressed in the same cells and tissues. Fortunately, recent advances in this area are starting to make this information available; this information is adding to our understanding of how the drugs act and of the possible subunit combinations that create their targets in vivo.

Ivermectin: An Anthelmintic, an Insecticide, and Much More

Here we tell the story of ivermectin, describing its anthelmintic and insecticidal actions and recent studies that have sought to reposition ivermectin for the treatment of other diseases that are not caused by helminth and insect parasites. The standard theory of its anthelmintic and insecticidal mode of action is that it is a selective positive allosteric modulator of glutamate-gated chloride channels found in nematodes and insects. At higher concentrations, ivermectin also acts as an allosteric modulator of ion channels found in host central nervous systems. In addition, in tissue culture, at concentrations higher than anthelmintic concentrations, ivermectin shows antiviral, antimalarial, antimetabolic, and anticancer effects. Caution is required before extrapolating from these preliminary repositioning experiments to clinical use, particularly for Covid-19 treatment, because of the high concentrations of ivermectin used in tissue-culture experiments.

The transcription factor NHR-8: A new target to increase ivermectin efficacy in nematodes

Resistance to the anthelmintic macrocyclic lactone ivermectin (IVM) has a great impact on the control of parasitic nematodes. The mechanisms by which nematodes adapt to IVM remain to be deciphered. We have identified NHR-8, a nuclear hormone receptor involved in the xenobiotic response in Caenorhabditis elegans, as a new regulator of tolerance to IVM. Loss-of-function nhr-8(ok186) C. elegans mutants subjected to larval development assays and electropharyngeogram measurements, displayed hypersensitivity to IVM, and silencing of nhr-8 in IVM-resistant worms increased IVM efficacy. In addition, compared to wild-type worms, nhr-8 mutants under IVM selection pressure failed to acquire tolerance to the drug. In addition, IVM-hypersensitive nhr-8(ok186) worms displayed low transcript levels of several genes from the xenobiotic detoxification network and a concomitant low Pgp-mediated drug efflux activity. Interestingly, some pgp and cyp genes known to impact IVM tolerance in many nematode species, were down regulated in nhr-8 mutants and inversely upregulated in IVM-resistant worms. Moreover, pgp-6 overexpression in nhr-8(ok186) C. elegans increased tolerance to IVM. Importantly, NHR-8 function was rescued in nhr-8(ok186) C. elegans with the homolog of the parasitic nematode Haemonchus contortus, and silencing of Hco-nhr-8 by RNAi on L2 H. contortus larvae increased IVM susceptibility in both susceptible and resistant H. contortus isolates. Thus, our data show that NHR-8 controls the tolerance and development of resistance to IVM in C. elegans and the molecular basis for this relates to the NHR-8-mediated upregulation of IVM detoxification genes. Since our results show that Hco-nhr-8 functions similarly to Cel-nhr-8, this study helps to better understand mechanisms underlying failure in drug efficacy and open perspectives in finding new compounds with NHR-8 antagonist activity to potentiate IVM efficacy.

IVM is the most important broad-spectrum deworming drug used today but resistance to this drug has appeared in parasites of both animals and humans. This seriously jeopardizes the success of current parasite control. Preserving IVM efficacy is a public health issue, whose outcome depends on the understanding of the molecular basis of selection for resistance to these drugs. We unambiguously show that the nuclear hormone receptor NHR-8, is crucial for protection of the nematode model Caenorhabditis elegans against IVM toxicity. Worms deficient in NHR-8 are hypersensitive to IVM and fail to become resistant to IVM under drug pressure. NHR-8 functions in the parasitic nematode of ruminants Haemonchus contortus and similar mechanisms could occur in other target pathogens. By controlling the xenobiotic detoxification network, NHR-8 may contribute to the biotransformation and elimination of IVM and help to desensitize the worm to the drug. This provides novel molecular targets involved in IVM drug tolerance in parasitic nematodes. Such findings could be exploited for targeted therapeutic intervention to treat parasitic nematode infections and delay the process of resistance development to anthelmintic drugs.

The sensory amphidial structures of Caenorhabditis elegans are involved in macrocyclic lactone uptake and anthelmintic resistance

Parasitic nematodes represent formidable pathogens of humans, livestock and crop plants. Control of these parasites is almost exclusively dependent on a small group of anthelmintic drugs, the most important of which belong to the macrocyclic lactone class. The extensive use of these drugs to control the ubiquitous trichostrongylid parasites of grazing livestock has resulted in the emergence of both single and multi-drug resistance. The expectation is that this resistance will eventually occur in the human parasites such as the common and debilitating soil transmitted nematodes and vector-borne filarial nematodes. While the modes of action of anthelmintics such as ivermectin, have been elucidated, notably in the model nematode Caenorhabditis elegans, the molecular nature of this resistance remains to be fully determined. Here we show that the anterior amphids play a key role in ivermectin uptake and mutations in these sensory structures result in ivermectin resistance in C. elegans. Random genetic mutant screens, detailed analysis of existing amphid mutants and lipophilic dye uptake indicate that the non-motile ciliated amphid neurons are a major route of ivermectin ingress; the majority of the mutants characterised in this study are predicted to be involved in intraflagellar transport. In addition to a role in ivermectin resistance, a subset of the amphid mutants are resistant to the non-related benzimidazole class of anthelmintics, raising the potential link to a multi-drug resistance mechanism. The amphid structures are present in all nematodes and are clearly defined in a drug-sensitive strain of Haemonchus contortus. It is predicted that amphidial drug uptake and intraflagellar transport may prove to be significant in the development of single and multi-drug resistance in the nematode pathogens of veterinary and human importance.

Genetic variants in dyf-7 validated by droplet digital PCR are not drivers for ivermectin resistance in Haemonchus contortus

Resistance to ivermectin (IVM) in the nematode Haemonchus contortus in small ruminants is an increasing problem throughout the world. Access to molecular diagnostics will allow early detection of IVM resistance, which in turn can limit the spread of resistant isolates. One candidate gene which has recently been suggested as a marker for IVM resistance is that for dye-filling protein (dyf-7). In this study, we critically investigated the suitability of A141G and G153T single nucleotide polymorphisms (SNP) of dyf-7 as a marker in larval cultures collected from sheep farms in Sweden, involving several isolates for which resistance status had been characterised by the faecal egg count reduction test (FECRT). Initially, we designed dyf-7 primers from a worldwide collection of adult Haemonchus contortus DNA. With the sequence data, we created a haplotype network. We then optimised and used the same sets of primers and probes in a droplet digital PCR (ddPCR) assay for precise quantification of dyf-7 allele frequencies in pre- and post-anthelmintic treatment faecal larval cultures. The fractional abundance (FA) of the mutant SNP was within the range 7.8 and 31%. However, the FA was generally stable in samples collected from the same farms, even though they were obtained on different occasions up to 25 months apart. There was also no indication that the level of IVM resistance as measured by the faecal egg count reduction test was higher on farms with high FA. Furthermore, by comparing FA in samples from the same farms pre- and post-IVM treatment, we found no evidence of a correlation between dyf-7 and level of IVM resistance. Based on these results, dyf-7 is not a suitable marker for field testing of IVM resistance in H. contortus.

Structural model, functional modulation by ivermectin and tissue localization of Haemonchus contortus P-glycoprotein-13

Haemonchus contortus, one of the most economically important parasites of small ruminants, has become resistant to the anthelmintic ivermectin. Deciphering the role of P-glycoproteins in ivermectin resistance is desirable for understanding and overcoming this resistance. In the model nematode, Caenorhabditis elegans, P-glycoprotein-13 is expressed in the amphids, important neuronal structures for ivermectin activity. We have focused on its ortholog in the parasite, Hco-Pgp-13. A 3D model of Hco-Pgp-13, presenting an open inward-facing conformation, has been constructed by homology with the Cel-Pgp-1 crystal structure. In silico docking calculations predicted high affinity binding of ivermectin and actinomycin D to the inner chamber of the protein. Following in vitro expression, we showed that ivermectin and actinomycin D modulated Hco-Pgp-13 ATPase activity with high affinity. Finally, we found in vivo Hco-Pgp-13 localization in epithelial, pharyngeal and neuronal tissues. Taken together, these data suggest a role for Hco-Pgp-13 in ivermectin transport, which could contribute to anthelmintic resistance.

Quantification of resistant alleles in the β-tubulin gene of field strains of gastrointestinal nematodes and their relation with the faecal egg count reduction test

BACKGROUND

Benzimidazole (BZ) resistance in gastrointestinal nematodes is associated with a single nucleotide polymorphism (SNP) at codons 167, 198 and 200 in the isotype 1 of beta-tubulin gene although in some species these SNPs have also been associated with resistance to macrocyclic lactones. In the present study we compared the levels of resistance in Teladorsagia circumcincta and Trichostrongylus colubriformis by means of the faecal egg reduction test (FECRT) and the percentage of resistant alleles obtained after pyrosequencing. The study was conducted in 10 naturally infected sheep flocks. Each flock was divided into three groups: i) group treated with albendazole (ABZ); ii) group treated with ivermectin (IVM); iii) untreated group. The number of eggs excreted per gram of faeces was estimated at day 0 and 14 post-treatment.

RESULTS

Resistance to ABZ was observed in 12.5% (1/8) of the flocks and to IVM in 44.4% (4/9) of them. One flock was resistant to both drugs according to FECRT. Coprocultures were performed at the same dates to collect L3 for DNA extraction from pooled larvae and to determine the resistant allele frequencies by pyrosequencing analysis. In T. circumcincta, SNPs were not found at any of the three codons before treatment; after the administration of ABZ, SNPs were present only in two different flocks, one of them with a frequency of 23.8% at SNP 167, and the other 13.2% % at SNP 198. In relation to T. colubriformis, we found the SNP200 before treatment in 33.3% (3/9) of the flocks with values between 48.5 and 87.8%. After treatment with ABZ and IVM, the prevalence of this SNP increased to 75 and 100% of the flocks, with a mean frequency of 95.1% and 82.6%, respectively.

CONCLUSION

The frequencies observed for SNP200 in T. colubriformis indicate that the presence of resistance is more common than revealed by the FECRT.

Piper aduncum against Haemonchus contortus isolates: cross resistance and the research of natural bioactive compounds

The anthelminthic activity of the essential oil (EO) of Piper aduncum L. was tested in vitro on eggs and larvae of resistant (Embrapa2010) and susceptible (McMaster) isolates of Haemonchus contortus. The EO was obtained by steam distillation and its components identified by chromatography. EO concentrations of 12.5 to 0.02 mg/mL were used in the egg hatch test (EHT) and concentrations of 3.12 to 0.01 mg/mL in the larval development test (LDT). Inhibition concentrations (IC) were determined by the SAS Probit procedure, and significant differences assessed by ANOVA followed by Tukey's test. In the EHT, the IC50 for the susceptible isolate was 5.72 mg/mL. In the LDT, the IC50 and IC90 were, respectively, 0.10 mg/mL and 0.34 mg/mL for the susceptible isolate, and 0.22 mg/mL and 0.51 mg/mL for the resistant isolate. The EO (dillapiole 76.2%) was highly efficacious on phase L1. Due to the higher ICs obtained for the resistant isolate, it was raised the hypothesis that dillapiole may have a mechanism of action that resembles those of other anthelmintic compounds. We further review and discuss studies, especially those conducted in Brazil, that quantified the major constituents of P. aduncum-derived EO.

Acquired Tolerance to Ivermectin and Moxidectin after Drug Selection Pressure in the Nematode Caenorhabditis elegans

Ivermectin and moxidectin are the most widely administered anthelmintic macrocyclic lactones (MLs) to treat human and animal nematode infections. Their widespread and frequent use has led to a high level of resistance to these drugs. Although they have the same mode of action, differences in terms of selection for drug resistance have been reported. Our objective was to study and compare changes occurring upon ivermectin or moxidectin selection in the model nematode Caenorhabditis elegans C. elegans worms were submitted to stepwise exposure to increasing doses of moxidectin. The sensitivity of moxidectin-selected worms to MLs was determined in a larval development assay and compared with those of wild-type and ivermectin-selected strains. Selection with either ivermectin or moxidectin led to acquired tolerance to ivermectin, moxidectin, and eprinomectin. Importantly, moxidectin was the most potent ML in both ivermectin- and moxidectin-selected strains. Interestingly, this order of potency was also observed in a resistant Haemonchus contortus isolate. In addition, ivermectin- and moxidectin-selected strains displayed constitutive overexpression of several genes involved in xenobiotic metabolism and transport. Moreover, verapamil potentiated sensitivity to ivermectin and moxidectin, demonstrating that ABC transporters play a role in ML sensitivity in ML-selected C. elegans strains. Finally, both ivermectin- and moxidectin-selected strains displayed a dye-filling-defective phenotype. Overall, this work demonstrated that selection with ivermectin or moxidectin led to cross-resistance to several MLs in nematodes and that the induction of detoxification systems and defects in the integrity of amphidial neurons are two mechanisms that appear to affect the responsiveness of worms to both ivermectin and moxidectin.

Molecular mechanisms for anthelmintic resistance in strongyle nematode parasites of veterinary importance

Veterinarians rely on a relatively limited spectrum of anthelmintic agents to control nematode parasites in domestic animals. Unfortunately, anthelmintic resistance has been an emerging problem in veterinary medicine. In particular, resistance has emerged among the strongyles, a group of gastrointestinal nematodes that infect a variety of hosts that range from large herbivores to small companion animals. Over the last several decades, a great deal of research effort has been directed toward developing an understanding of the mechanisms conferring resistance against the three major groups of anthelmintics: macrocyclic lactones, benzimidazoles, and nicotinic agonists. Our understanding of anthelmintic resistance has been largely formed by determining the mechanism of action for each drug class and then evaluating drug-resistant nematode isolates for mutations or differences in expression of target genes. More recently, drug efflux pumps have been recognized for their potential contribution to anthelmintic resistance. In this mini-review, we summarize the evidence for mechanisms of resistance in strongyle nematodes.

The Validation of Nematode-Specific Acetylcholine-Gated Chloride Channels as Potential Anthelmintic Drug Targets

New compounds are needed to treat parasitic nematode infections in humans, livestock and plants. Small molecule anthelmintics are the primary means of nematode parasite control in animals; however, widespread resistance to the currently available drug classes means control will be impossible without the introduction of new compounds. Adverse environmental effects associated with nematocides used to control plant parasitic species are also motivating the search for safer, more effective compounds. Discovery of new anthelmintic drugs in particular has been a serious challenge due to the difficulty of obtaining and culturing target parasites for high-throughput screens and the lack of functional genomic techniques to validate potential drug targets in these pathogens. We present here a novel strategy for target validation that employs the free-living nematode Caenorhabditis elegans to demonstrate the value of new ligand-gated ion channels as targets for anthelmintic discovery. Many successful anthelmintics, including ivermectin, levamisole and monepantel, are agonists of pentameric ligand-gated ion channels, suggesting that the unexploited pentameric ion channels encoded in parasite genomes may be suitable drug targets. We validated five members of the nematode-specific family of acetylcholine-gated chloride channels as targets of agonists with anthelmintic properties by ectopically expressing an ivermectin-gated chloride channel, AVR-15, in tissues that endogenously express the acetylcholine-gated chloride channels and using the effects of ivermectin to predict the effects of an acetylcholine-gated chloride channel agonist. In principle, our strategy can be applied to validate any ion channel as a putative anti-parasitic drug target.

Macrocyclic lactones and their relationship to the SNPs related to benzimidazole resistance

Haemonchus contortus is an abomasal nematode of ruminants that is widely present across the world. Its ability to cause death of infected animals and rapidly develop anthelmintic resistance makes it a dangerous pathogen. Ivermectin (IVM) and moxidectin (MOX) are macrocyclic lactones (MLs). They have been successfully used to treat parasitic nematodes over the last three decades. A genetic association between IVM selection and single nucleotide polymorphisms (SNPs) on the β-tubulin isotype 1 gene was reported in H. contortus. These SNPs result in replacing phenylalanine (F, TTC) with tyrosine (Y, TAC) at position 167 or 200 on the β-tubulin protein. Recently we reported a direct interaction of IVM with α- and β-tubulin. It had been hypothesized that the SNPs (F167Y and F200Y) may change tubulin dynamics and directly affect IVM binding. The goal of the current study was to observe the effects of SNPs (F167Y and F200Y) on tubulin polymerization and IVM binding. It was also of interest to evaluate the differences between IVM and MOX on tubulin polymerization. We conclude that the SNPs cause no difference in the polymerization of wild and mutant tubulins. Furthermore, neither of the SNPs reduced IVM binding. Varying results were obtained in the degree of polymerization of parasitic and mammalian tubulin for IVM and MOX, i.e., the extent of polymerization was greater for IVM compared with MOX, for H. contortus tubulin, and vice versa for mammalian tubulin. Molecular modeling showed that IVM and MOX docked into the taxane binding pocket of both mammalian and parasitic wild type and mutant tubulins. However the binding was stronger for mammalian tubulin as compared to parasitic tubulin.

The emergence of macrocyclic lactone resistance in the canine heartworm, Dirofilaria immitis

Prevention of heartworm disease caused by Dirofilaria immitis in domestic dogs and cats relies on a single drug class, the macrocyclic lactones (MLs). Recently, it has been demonstrated that ML-resistant D. immitis are circulating in the Mississippi Delta region of the USA, but the prevalence and impact of these resistant parasites remains unknown. We review published studies that demonstrated resistance in D.immitis, along with our current understanding of its mechanisms. Efforts to develop in vitro tests for resistance have not yet yielded a suitable assay, so testing infected animals for microfilariae that persist in the face of ML treatment may be the best current option. Since the vast majority of D. immitis populations continue to be drug-sensitive, protected dogs are likely to be infected with only a few parasites and experience relatively mild disease. In cats, infection with small numbers of worms can cause severe disease and so the clinical consequences of drug resistance may be more severe. Since melarsomine dihydrochloride, the drug used to remove adult worms, is not an ML, the ML-resistance should have no impact on our ability to treat diseased animals. A large refugium of heartworms that are not exposed to drugs exists in unprotected dogs and in wild canids, which may limit the development and spread of resistance alleles.

Profiling of differentially expressed genes in sheep T lymphocytes response to an artificial primary Haemonchus contortus infection

BACKGROUND

Haemonchus contortus is a common bloodsucking nematode causing widespread economic loss in agriculture. Upon H. contortus infection, a series of host responses is elicited, especially those related to T lymphocyte immunity. Existing studies mainly focus on the general immune responses of sheep T lymphocyte to H. contortus, lacking investigations at the molecular level. The objective of this study was to obtain a systematic transcriptional profiling of the T lymphocytes in H. contortus primary-infected sheep.

METHODS

Nematode-free sheep were orally infected once with H. contortus L3s. T lymphocyte samples were collected from the peripheral blood of 0, 3, 30 and 60 days post infection (dpi) infected sheep. Microarrays were used to compare gene transcription levels between samples. Quantitative RT-PCR was employed to validate the microarray data. Gene Ontology and KEGG pathway analysis were utilized for the annotation of differentially expressed genes.

RESULTS

Our microarray data was consistent with qPCR results. From microarrays, 853, 242 and 42 differentially expressed genes were obtained in the 3d vs. 0d, 30d vs. 0d and 60d vs. 0d comparison groups, respectively. Gene Ontology and KEGG pathway analysis indicated that these genes were involved in metabolism, signaling, cell growth and immune system processes. Functional analysis of significant differentially expressed genes, such as SLC9A3R2, ABCB9, COMMD4, SUGT1, FCER1G, GSK3A, PAK4 and FCER2, revealed a crucial association with cellular homeostasis maintenance and immune response. Our data suggested that maintaining both effective immunological response and natural cellular activity are important for T lymphocytes in fighting against H. contortus infection.

CONCLUSIONS

Our results provide a substantial list of candidate genes in sheep T lymphocytes response to H. contortus infection, and contribute novel insights into a general immune response upon infection.

A dyf-7 haplotype causes sensory neuron defects and is associated with macrocyclic lactone resistance worldwide in the nematode parasite Haemonchus contortus

Heavy reliance on macrocyclic lactones to treat parasitic nematodes has resulted in the evolution of widespread drug resistance that threatens human and animal health. Management strategies have been proposed that would slow the rise of resistance, however testing these strategies has been hampered by the lack of identified strong-effect resistance markers in parasites. We show that the Caenorhabditis elegans gene Cel_dyf-7, necessary for amphid sensory neuron development, also confers macrocyclic lactone sensitivity. In the sheep parasite Haemonchus contortus: (i) strains selected for macrocyclic lactone resistance were enriched in a Hco_dyf-7 haplotype that was rare in the drug-naïve population, (ii) the resistant haplotype correlated with the sensory neuron defects, and (iii) the resistant haplotype was associated with decreased Hco_dyf-7 expression. Resistant field isolates of H. contortus from five continents were enriched for the resistant haplotype, demonstrating the relevance of the Hco_dyf-7 haplotype to practise and indicating that it is a locus of strong effect. Hemizygosity resulting from sex linkage of dyf-7 likely contributes to the rise of resistance in treated populations.

Analysis of putative inhibitors of anthelmintic resistance mechanisms in cattle gastrointestinal nematodes

Effects of the cytochrome P450 inhibitor piperonyl butoxide and the P-glycoprotein inhibitor verapamil on the efficacy of ivermectin and thiabendazole were studied in vitro in susceptible and resistant isolates of the cattle parasitic nematodes Cooperia oncophora and Ostertagia ostertagi. The effects of combined use of drug and piperonyl butoxide/verapamil, respectively, were investigated in the Egg Hatch Assay, the Larval Development Assay and the Larval Migration Inhibition Assay. The effects of piperonyl butoxide and verapamil as inhibitors of thiabendazole and ivermectin responses were particularly marked for larval development, where both inhibitors were able to completely eliminate all differences between susceptible and resistant isolates. Even the lowest concentrations of anthelmintics used in combination with inhibitors caused complete inhibition of development. Differences and/or similarities among responses in different isolates were only obtained in the two other assays: in the Egg Hatch Assay piperonyl butoxide caused a shift in concentration-response curves obtained with thiabendazole to the left for all isolates tested, changing relative differences between isolates. In contrast, an effect of verapamil in the Egg Hatch Assay was only apparent for benzimidazole-resistant isolates. In the Larval Migration Inhibition Assay only ivermectin was tested and piperonyl butoxide shifted the concentration-response curves for all isolates to the left, again eliminating differences in EC50 values between susceptible and resistant isolates. This was not the case using verapamil as an inhibitor, where curves for both susceptible and benzimidazole-resistant isolates shifted to the left in Ostertagia isolates. In Cooperia the picture was more complex with ivermectin-resistant isolates showing a larger shift than the susceptible isolate. Single nucleotide polymorphisms in the β-tubulin isotype 1 gene were investigated. Significantly increased frequencies of resistance-associated alleles were observed for the codons 167 and 200 in one benzimidazole-resistant isolate but not in an isolate selected for benzimidazole resistance at an early stage of selection.

Ivermectin resistance and overview of the Consortium for Anthelmintic Resistance SNPs

Ivermectin (IVM) has transformed nematode parasite control in veterinary medicine and the control of some nematode infections in humans, such as onchocerciasais, lymphatic filariasis in Africa and strongyloidiasis. Unfortunately, IVM resistance is now a serious problem for parasite control in livestock and there is a concern about resistance development and spread in nematode parasites of humans. IVM is believed to act by opening glutamate-gated chloride channels and GABA-gated channels in invertebrate neurons or muscle cells, leading to hyperpolarisation of the cells and to an inhibitory paralysis. However, in the filarial nematodes, it is not altogether clear that the effect of IVM is confined to these actions or even whether these are the most important. Alterations in some ligand-gated ion channel (LGIC) receptor subunits may play a role in the mechanisms of IVM resistance in some nematodes, but the evidence that changes in LGICs are the most important cause of IVM resistance in nematodes is far from clear. What is evident is that IVM is an excellent substrate for some ATP-binding cassette transporters, IVM selects for changes in expression levels of ABC transporters, such as P-glycoproteins, and that altered levels of some ABC transporters contribute to IVM resistance. In addition, there is growing evidence that IVM selects on β-tubulin, at least in some nematodes. Based on these various mechanisms, which contribute to IVM resistance, it may become possible to develop panels of molecular markers for IVM resistance in different nematode parasites. In order to stimulate the development of such markers, an international Consortium for Anthelmintic Resistance SNPs (CARS) has been developed to help coordinate marker development, advance our knowledge of helminth biology and possibly assist with the development of new anthelmintic molecules.

The soluble proteome phenotypes of ivermectin resistant and ivermectin susceptible Haemonchus contortus females compared

Anthelmintics in the absence of vaccines have underpinned a parasite control strategy for over 50 years. However, the continued development of anthelmintic resistance (AR) threatens this control. Measuring early AR is difficult as there many routes that resistance can arise from within multi-nematode populations operating complex metabolism capabilities coupled to different drug management pressures. There is an urgent need to identify and measure early resistance in the field situation. Proteomic profiling of expressed soluble proteins offers a new approach to reveal a drug resistant phenotype within a complex protein pattern. The hypothesis under test was that established differences in drug response phenotypes between nematode isolates can also be measured in their comparative proteomes. As a case study, proteomic differences were measured between an ivermectin resistant and susceptible adult female Haemonchus contortus. Adult H. contortus females were extracted from the abomasa of six lambs. The nematodes had been maintained in the lambs as monospecific isolates of either ivermectin susceptible or ivermectin resistant worms. Comparative analysis of the soluble proteome was completed along with immuno-proteomic analysis using pooled infection sera from the lambs. Following image analysis, spots of interest were excised and analysed by peptide mass fingerprinting and the proteins putatively identified using BLAST. Overall, a relative increase in the expression of proteins involved in the detoxification metabolic area was observed in the resistant isolate. In addition, Western blotting analysis also revealed differences in immuno-reactivity profiles between resistant and susceptible isolates. It can be concluded from this study that proteomic differences can be detectable between ivermectin susceptible and a resistant isolates of H. contortus, which could be further explored using other isolates to confirm if proteomic based fingerprinting offers molecular phenotyping or a new panel of resistance biomarkers.

Genotypic analysis of β-tubulin in Onchocerca volvulus from communities and individuals showing poor parasitological response to ivermectin treatment

Ivermectin (IVM) has been in operational use for the control of onchocerciasis for two decades and remains the only drug of choice. To investigate the parasitological responses and genetic profile of Onchocerca volvulus, we carried out a 21 month epidemiological study to determine the response of the parasite to IVM in 10 Ghanaian endemic communities. Onchocerca nodules were surgically removed from patients in three IVM response categories (good, intermediate and poor) and one IVM naïve community. DNA from adult worms was analyzed to determine any association between genotype and IVM response phenotypic. Embryogramme analysis showed significantly higher reproductive activity in worms from poor response communities, which had up to 41% of females with live stretched microfilaria (mf) in utero, despite IVM treatment, compared with good response communities, which had no intra-uterine stretched mf. β-tubulin isotype 1 gene has been shown to be linked to IVM selection in O. volvulus and also known to be associated with IVM resistance in veterinary nematodes. We have genotyped the full length genomic DNA sequence of the β-tubulin gene from 127 adult worms obtained from the four community categories. We found SNPs at 24 sites over the entire 3696 bp. Eight of the SNPs occurred at significantly higher (p < 0.05) frequencies in the poor response communities compared with the good response communities and the IVM naïve community. Phenotypic and genotypic analyses show that IVM resistance has been selected and the genotype (1183GG/1188CC/1308TT/1545GG) was strongly associated with the resistance phenotype. Since the region in the β-tubulin gene where these four SNPs occur is within 362 bp, it is feasible to develop a genetic marker for the early detection of IVM resistance.

Ion channels and receptor as targets for the control of parasitic nematodes

Many of the anthelmintic drugs in use today act on the nematode nervous system. Ion channel targets have some obvious advantages. They tend to act quickly, which means that they will clear many infections rapidly. They produce very obvious effects on the worms, typically paralyzing them, and these effects are suitable for use in rapid and high-throughput assays. Many of the ion channels and enzymes targeted can also be incorporated into such assays. The macrocyclic lactones bind to an allosteric site on glutamate-gated chloride channels, either directly activating the channel or enhancing the effect of the normal agonist, glutamate. Many old and new anthelmintics, including tribendimidine and the amino-acetonitrile derivatives, act as agonists at nicotinic acetylcholine receptors; derquantel is an antagonist at these receptors. Nematodes express many different types of nicotinic receptor and this diversity means that they are likely to remain important targets for the foreseeable future. Emodepside may have multiple effects, affecting both a potassium channel and a pre-synaptic G protein-coupled receptor; although few other current drugs act at such targets, this example indicates that they may be more important in the future. The nematode nervous system contains many other ion channels and receptors that have not so far been exploited in worm control but which should be explored in the development of effective new compounds.

P-glycoprotein interfering agents potentiate ivermectin susceptibility in ivermectin sensitive and resistant isolates of Teladorsagia circumcincta and Haemonchus contortus

P-glycoprotein (P-gp) homologues, belonging to the ATP Binding Cassette (ABC) transporter family, are thought to play an important role in the resistance of gastro-intestinal nematode parasites against macrocyclic lactones. The aim of this study was to investigate the influence of various P-gp interfering compounds on the efficacy of ivermectin (IVM) in sensitive and resistant nematode isolates. The feeding of IVM resistant and sensitive Teladorsagia circumcincta and Haemonchus contortus first-stage larvae (L1) was assessed using a range of IVM concentrations (0.08-40 nm) with or without P-gp inhibitors: valspodar, verapamil, quercetin, ketoconazole and pluronic P85. The P-gp inhibitors were selected on the basis of their ability to interfere with P-gp transport activity in an epithelial cell line over-expressing murine P-gp. In the presence of P-gp interfering agents, the in vitro susceptibility to IVM of both sensitive and resistant isolates of T. circumcincta and H. contortus was increased. These results show that compounds interfering with P-gp transport activity could enhance IVM efficacy in sensitive isolates, and also restore IVM sensitivity in resistant nematodes. These results support the view that ABC transporters can play an important role in resistance to IVM, at least in the free-living stages of these economically important gastro-intestinal nematodes.

ABC transporters and beta-tubulin in macrocyclic lactone resistance: prospects for marker development

Macrocyclic lactones (MLs) are highly lipophilic anthelmintics which are known to bind to and open ligand-gated ion channels. However, these anthelmintics, and particularly the avermectin members of the ML class of endectocides, are potent substrates for ABC transporters and these transporters may regulate drug concentration in both the host and the parasite. There is accumulating evidence that ivermectin (IVM), and to a lesser extent moxidectin (MOX), selects for certain alleles of P-glycoprotein and other ABC transporter genes, selects for constitutive overexpression of some of these gene products, and induces overexpression of some P-glycoproteins in nematodes. However, such mechanisms of ML resistance do not easily lend themselves to the identification of SNP markers for resistance because of the diversity of ABC transporters in nematodes, the apparent diversity of effects of different MLs, and because regulatory elements for ABC transporter gene expression are not well understood in nematodes. Another non ligand-gated ion channel gene which appears to be under IVM selection, at least in Onchocerca volvulus and Haemonchus contortus, is beta-tubulin, and a simple genetic test for this selection has been described in O. volvulus. However, further work is required to elucidate a reliable marker associated with this gene in H. contortus or other parasitic nematodes of livestock. The possible involvement of ABC transporter genes and beta-tubulin in ML resistance provides a start in developing our understanding of this phenotype and markers for its detection in field populations of parasitic nematodes. However, more work is required before these leads can provide practical SNP markers for ML resistance.

Nematode ligand-gated chloride channels: an appraisal of their involvement in macrocyclic lactone resistance and prospects for developing molecular markers

Ligand-gated chloride channels, including the glutamate-(GluCl) and GABA-gated channels, are the targets of the macrocyclic lactone (ML) family of anthelmintics. Changes in the sequence and expression of these channels can cause resistance to the ML in laboratory models, such as Caenorhabditis elegans and Drosophila melanogaster. Mutations in multiple GluCl subunit genes are required for high-level ML resistance in C. elegans, and this can be influenced by additional mutations in gap junction and amphid genes. Parasitic nematodes have a different complement of channel subunit genes from C. elegans, but a few genes, including avr-14, are widely present. A polymorphism in an avr-14 orthologue, which makes the subunit less sensitive to ivermectin and glutamate, has been identified in Cooperia oncophora, and polymorphisms in several subunits have been reported from resistant isolates of Haemonchus contortus. This has led to suggestions that ML resistance may be polygenic. Possible reasons for this, and its consequences for the development of molecular tests for resistance, are explored.

Drug transfer into target helminth parasites

The pharmacokinetics of an anthelmintic drug includes the time course of drug absorption, distribution, metabolism and elimination from the host and determines the concentration of the active drug that reaches the location of the parasite. However, the action of the anthelmintic also depends on the ability of the active drug to reach its specific receptor within the target parasite. Thus, drug entry and accumulation in target helminths are important issues when considering how best to achieve optimal efficacy. Passive drug transfer through the external helminth surface is the predominant entry mechanism for most widely used anthelmintics and is discussed in this article. Despite the structural differences between the external surface of nematodes (the cuticle) and the external surface of cestodes and trematodes (the tegument), the mechanism of drug entrance into both types of helminth depends on the lipophilicity of the anthelmintic and this is the major physicochemical determinant for the drug to reach a therapeutic concentration in the target parasite. Understanding the processes that regulate drug transfer into helminth parasites is an important aspect in improving the control of parasites in human and veterinary medicine.

Ivermectin selection on beta-tubulin: evidence in Onchocerca volvulus and Haemonchus contortus

Ivermectin resistance is common in trichostrongylid nematodes of livestock, such as Haemonchus contortus. This anthelmintic is the only drug approved for mass administration to control onchocerciasis caused by the nematode parasite, Onchocerca volvulus. In parts of West Africa up to 18 rounds of ivermectin treatment have been administered to communities and there are reports of poor parasitological responses to treatment. Understanding ivermectin resistance and ivermectin selection is an important step to reduce selection pressure for resistance, and to develop molecular markers which can be used to monitor the development of resistance and its spread. Here we report evidence that ivermectin selection changes the frequency of beta-tubulin alleles in both the sheep parasite, H. contortus, and the human parasite, O. volvulus. In O. volvulus we have been able to look at the frequency of beta-tubulin alleles in O. volvulus obtained before any ivermectin was used in humans in Africa, and following its widespread use. In H. contortus, we have been able to look at the frequency of beta-tubulin alleles in a strain which has not seen any anthelmintic selection and in an ivermectin selected strain derived from the unselected strain. We have found ivermectin selects on beta-tubulin in both of these nematode species. In the case of O. volvulus, we had previously reported that ivermectin selects for specific single nucleotide polymorphisms in the O. volvulus beta-tubulin gene. This polymorphism results in three amino acid changes in the H3 helix of beta-tubulin, as well as deletions in an associated intron. We report a simple PCR assay to detect the amplicon length polymorphism, resulting from these intronic deletions, which can be used to monitor the frequency of the beta-tubulin allele selected for by ivermectin in O. volvulus.

Glutamate-gated chloride channels and the mode of action of the avermectin/milbemycin anthelmintics

The macrocyclic lactones are the biggest selling and arguably most effective anthelmintics currently available. They are good substrates for the P-glycoproteins, which might explain their selective toxicity for parasites over their vertebrate hosts. Changes in the expression of these pumps have been implicated in resistance to the macrocyclic lactones, but it is clear that they exert their anthelmintic effects by binding to glutamate-gated chloride channels expressed on nematode neurones and pharyngeal muscle cells. This effect is quite distinct from the channel opening induced by glutamate, the endogenous transmitter acting at these receptors, which produces rapidly opening and desensitising channels. Ivermectin-activated channels open very slowly but essentially irreversibly, leading to a very long-lasting hyperpolarisation or depolarisation of the neurone or muscle cell and therefore blocking further function. Molecular and genetic studies have shown that there are multiple GluCl isoforms in both free-living and parasitic nematodes: the exact genetic make-up and functions of the GluCl may vary between species. The known expression patterns of the GluCl explain most of the observed biological effects of treatment with the macrocyclic lactones, though the reason for the long-lasting inhibition of larval production in filarial species is still poorly understood.

Resistance as a tool for discovering and understanding targets in parasite neuromusculature

The problem of anthelmintic resistance prevents efficient control of parasites of livestock and may soon compromise human parasite control. Research into the mechanisms of resistance and the quest for diagnostic tools to aid control has required research that focuses on field resistance. On the other hand, resistant worms, including those kept in the laboratory, provide useful tools for studying drug action, especially at neuromuscular targets in worms. While the needs and directions of these research aims overlap, this review concentrates on research on drug targets. In this context, resistance is a useful tool for site of action confirmation. For example, correlations between molecular expression studies and resistance assays conducted on whole worms can strengthen claims for sites of anthelmintic action. Model systems such as Caenorhabditis elegans have been very useful in understanding targets but give a limited picture as it is now clear that resistance mechanisms in this worm are different from those in parasites. Accordingly, research on parasites themselves must also be performed. Resistant isolates of the sheep nematode parasite Haemonchus contortus are the most widely used for this purpose as in vivo, in vitro, physiological and molecular studies can be performed with this species. Neuromuscular target sites for the anthelmintics levamisole and ivermectin are the best studied and have benefited most from the use of resistant worm isolates. Resistance to praziquantel and the newer chemical groups should provide new tools to explore targets in the future.

Will technology provide solutions for drug resistance in veterinary helminths?

Drug resistance in veterinary helminths affects a growing number of livestock producers on a global basis. The parasites infecting the major species of livestock are presently showing resistance in varying degrees to the commonly used classes of anthelmintics. The degree and extent of this problem especially with respect to multidrug resistance (MDR) in nematode populations is likely to increase. Finding solutions to the spread of resistance requires knowledge of the drugs' modes of action and mechanisms of resistance. This knowledge can then be applied to detect and monitor the state of resistance. Here we present a brief overview of resistance mechanisms and some of the technologies being used to study them. We also discuss some of the strategies for slowing the spread of resistance. The issue of reversal of drug resistance is analysed under consideration of recent progress in the field of MDR reversal in non-infectious diseases. Finally, we propose an application of currently available technologies that could assist in the detection and monitoring of anthelmintic resistance. Taking into account the significant complexity of the genetic mechanism of anthelmintic resistance in and between the various species, we suggest to undertake a co-ordinated effort to systematically identify anthelmintic-related single nucleotide polymorphisms (SNPs) in the most important helminth parasites. Monitoring the state of resistance in field populations could be achieved with a SNP-based protocol for genotyping the many genes known or suspected to contribute to the modes of action or mechanisms of resistance to the various classes of anthelmintics. If significant associations between genotypes and phenotypes exist within a species, then a single test with sufficient SNPs could potentially have universal applicability. These could then be explored for the development of new molecular diagnostic procedures. New classes of anthelmintics are needed, but until they are developed and available to the producers, technology can assist to achieve the goal of better sustainability in anthelmintic usage.

Genetic polymorphism of the β-tubulin gene of Onchocerca volvulus in ivermectin naïve patients from Cameroon, and its relationship with fertility of the worms

Observations of low response of patients infected with Onchocerca volvulus to ivermectin suggest that the parasite may be under a selection process toward potential resistance. To limit the extension of this phenomenon, it is crucial to characterize the genes of O. volvulus that are involved. For this, O. volvulus adult worms collected before the introduction of ivermectin in an onchocerciasis endemic area of central Cameroon were genotyped for beta-tubulin. To derive a baseline to investigate the selective pressure of ivermectin, we analysed (1) the frequency distribution of the beta-tubulin alleles, and (2) the relationship between the different beta-tubulin related genotypes and the fertility status of the female worms. The frequency of allele b of the beta-tubulin gene was very low, as it was observed in West Africa. We observed a deficit of heterozygous female worms leading to Hardy Weinberg disequilibrium, which might be explained by a shorter life-span of these worms compared to the homozygous worms. Unexpectedly, our results also show that the heterozygous female worms were much less fertile than the homozygotes: more than two thirds of the homozygotes were fertile, whereas only 37% of the heterozygotes were fertile. These results will be further considered when analysing post-treatment data.

A comparison of genetic polymorphism in populations of Onchocerca volvulus from untreated- and ivermectin-treated patients

An analysis of the polymorphism of 16 genes from Onchocerca volvulus was undertaken, in two populations of worms from either ivermectin-naïve patients or patients who had been repeatedly treated with ivermectin, in Ghana. Six genes were selected for analysis because studies in other nematodes had suggested a possible association with ivermectin resistance. The other 10 genes were included as control genes and have not been associated with ivermectin resistance. Twelve of the 16 genes were polymorphic, including five of the candidate genes and seven of the control genes. In all of the control genes and four of the candidate genes, there were no differences in genetic polymorphism between the untreated and ivermectin treatment worms. However, there were statically significant differences (chi2=0.05) in allelic frequencies between the untreated and treatment derived worms for P-glycoprotein and beta-tubulin genes; both genes which have been previously associated with ivermectin resistance in other nematodes. These genes were in Hardy-Weinberg equilibrium in the untreated population. However, the P-glycoprotein alleles, in the worms from the patients under treatment were not in Hardy-Weinberg equilibrium, and analysis of the allele frequencies of beta-tubulin suggested that this gene may have also been under selection in the worms from the ivermectin-treated patients. This data provides evidence of genetic selection by ivermectin on O. volvulus and indicates that investigations should be made to determine whether ivermectin resistance is developing. The beta-tubulin and P-glycoprotein genes may prove useful for monitoring for possible development of ivermectin resistance.

Is anthelmintic resistance a concern for heartworm control? What can we learn from the human filariasis control programs?

Heartworm prophylaxis is currently largely dependent on the ability of avermectins and milbemycins to arrest the development of third and fourth stages of Dirofilaria immitis for prolonged periods, without producing adulticidal effects. Major control programs, dependent on the activity of ivermectin, are being implemented for human onchocerciasis and lymphatic filariasis. The avermectins and milbemycins act on glutamate-gated and gamma-aminobutyrate-gated chloride channel subunit proteins in nematodes. Ivermectin resistance has been widely described in trichostrongylid nematodes of ruminants. There is evidence that when ivermectin resistance occurs in nematodes, there may be selection on some, but not all of the genes that code for ligand-gated chloride channel subunit proteins as well as on some ABC-transporter genes, whose products may be involved in regulating macrocyclic lactone drug concentrations at receptors, and on some structural protein genes of amphidial neurones. Although ivermectin resistance has not been reported in filarial nematodes, there have recently been reports of suboptimal responses to ivermectin in Onchocerca volvulus. Evidence has been found of ivermectin selection on at least ABC-transporter genes and some neuronal structural protein genes in O. volvulus. To date, there is no evidence of avermectin/milbemycin resistance in D. immitis, also a filarial nematode. Chemotherapy against trichostrongylids of animals, human filariae, and D. immitis, relies on avermectins or milbemycins. However, control involves targeting different stages or processes in the nematode life cycles, different control strategies, different proportions of the nematode population in refugia, and different drug dosage rates. Consideration of the proportion of the D. immitis population normally in refugia, the life cycle stage targeted, and the anthelmintic dosages used suggest that it is unlikely that significant avermectin/milbemycin resistance will be selected in D. immitis with current treatment strategies.

Drug resistance in veterinary helminths

At present, there is no effective alternative to chemical control of parasitic helminths where livestock are grazed intensively. Resistance to anthelmintics has become a major problem in veterinary medicine, and threatens both agricultural income and animal welfare. The molecular and biochemical basis of this resistance is not well understood. The lack of reliable biological and molecular tests means that we are not able to follow the emergence and spread of resistance alleles and clinical resistance as well as we need. This review summarizes some of the recent findings on resistance mechanisms, puts forward some recommendations for limiting its impact and suggests some priorities for research in this area.

Distribution of glutamate-gated chloride channel subunits in the parasitic nematode Haemonchus contortus

Glutamate-gated chloride channels (GluCl) are related to gamma-aminobutyric acid-A (GABA(A)) receptors and are the target sites for the avermectin/milbemycin (A/M) anthelmintics, drugs that cause paralysis of the somatic and pharyngeal muscles in nematodes. We have previously identified four GluCl subunits, HcGluClalpha, HcGluClbeta, HcGluClalpha3A, and HcGluClalpha3B from the sheep parasite Haemonchus contortus. We raised specific antisera against all of these subunits and used them in immunofluorescence experiments on adult parasites. All of the subunits were expressed in the motor nervous system, especially motor neuron commissures. Double-immunostaining experiments suggested that HcGluClalpha and HcGluClbeta were expressed on the same commissures; these were also stained with an anti-GABA antibody, suggesting that they may be inhibitory motor neurons. The HcGluClbeta subunit was also detected in lateral and sublateral nerve cords. The HcGluClalpha3A and -B subunits, products of an alternatively spliced gene, were expressed in different neurons. We found HcGluClalphaA in a pair of sensory, possibly amphid, neurons in the head, in addition to the motor neuron commissures. HcGluClalpha3B was detected in three cell bodies, probably of pharyngeal neurons, and to ventral and lateral cords. These results indicate that the GluCl are widely distributed in the H. contortus nervous system and suggest that they have critical roles controlling locomotion, pharyngeal function, and possibly sensory processing in parasitic nematodes. They also provide an explanation for the observed effects of the A/M anthelmintics.