Haemonchus contortus acetylcholine receptors of the DEG-3 subfamily and their role in sensitivity to monepantel

Efficacy of fenbendazole against gastrointestinal nematodes in naturally infected goats in Maputo Province, Mozambique using in vivo, in vitro and molecular assessment

Anthelmintic resistance occurs worldwide in strongyles of ruminants but data from low-income countries are sparse and rarely apply most up to date methods, while effects of management practices in these countries are poorly documented. In Mozambique, benzimidazole resistance has been previously reported; the present study followed this up in detail, applying in vivo faecal egg count (FEC) reduction test (FECRT), in vitro egg hatch test (EHT) and molecular deep amplicon sequencing approaches targeting the internal transcribed spacer 2 (ITS-2, nemabiome) and the isotype 1 β-tubulin gene to determine the resistance status on farms and the strongyle species involved. Adult Landim goats (433) from six semi-intensive and ten extensive farms (22-30 animals/farm) from Maputo Province were visited April 2021 to February 2022. Fenbendazole (5 mg/kg bw, Panacur®) was administered orally and FEC determined using Mini-FLOTAC. The eggCounts package was used to calculate FECRs with 90% confidence intervals from paired day 0 and 14 data. In vivo and in vitro tests detected AR on 5/16 (31%) farms. This included 1/10 extensive and 4/6 semi-intensive farms. The odds of finding resistant strongyles on a semi-intensive commercial farm was 40-fold higher than on an extensive farm (p = 0.016, logistic regression). A strong, negative correlation was observed between FECRT and EHT EC50 values (Pearson's R = -0.83, P = 0.001; Cohen's κ coefficient 1.0). Nemabiome data showed that Haemonchus contortus, Trichostrongylus colubriformis and unclassified Oesophagostomum closely related to Oesophagostomum columbianum were most abundant before treatment and in particular H. contortus frequencies increased after treatment. Benzimidazole resistance associated polymorphisms were detected in H. contortus and T. colubriformis. Moreover, there were hints that resistance alleles were present in Trichostrongylus axei and Teladorsagia circumcincta. Farmers should regularly test the efficacy of anthelmintics used and consider more sustainable worm control approaches to reduce selection for resistance.

A systematic review of the molecular mechanisms related to anthelmintic resistance in Haemonchus contortus: A contemporary narrative

Haemonchus contortus is a gastrointestinal parasite that affects ruminants (cattle, sheep, etc.), having a significant welfare impact worldwide. The rise of anthelmintic resistance poses a growing challenge to adequate control, compromising the success of treatments. This study presents a systematic review of the molecular mechanisms involved in the resistance of H. contortus to anthelmintic drugs. Following an extensive literature search (9075 total articles/excluding duplications), 61 articles were examined. From these, benzimidazoles (BZD) and macrocyclic lactone (ML) were the most reported drug classes (17 and 29, respectively). The mutations in the β-tubulin gene were the primary mechanism of BZD resistance. Important comparisons from early reports of resistance mechanisms to ML (published before 2020) mainly based on parasite-population gene expression (e.g., ligand-gated chloride channels, LGCC, and P-glycoproteins, PGP) with more recent genomic and transcriptomic data (e.g., transcription factor, cky-1 gene) are highlighted. Additionally, resistance mechanisms to levamisole (LEV) and monepantel are discussed, showing evidence of polymorphisms in genes related to the nicotinic acetylcholine receptors (nAChR). Considering the available data, it is possible to divide the reports into two technological periods, evidencing that the availability of a chromosome-level genome for H. contortus in association with experiments based on controlled genetic crosses and transcriptome-wide data allowed for the visualization of genes and polymorphisms that were previously indistinguishable from unrelated genetic variation (i.e., genetic noise). Therefore, the study of anthelmintic resistance mechanisms is facing new possibilities, reflecting the large data banks and the speed at which this information is being processed. We suggest that new publications on drug resistance should adopt the approaches and refer to this new era of scientific discoveries. Consistent data interpretation, including artificial intelligence (AI) support, will help us to suggest novel biological mechanisms involved in drug resistance and predict its evolution, allowing a more comprehensible approach toward sustainable parasite control strategies.

Structural insights into the molecular effects of the anthelmintics monepantel and betaine on the Caenorhabditis elegans acetylcholine receptor ACR-23

Anthelmintics are drugs used for controlling pathogenic helminths in animals and plants. The natural compound betaine and the recently developed synthetic compound monepantel are both anthelmintics that target the acetylcholine receptor ACR-23 and its homologs in nematodes. Here, we present cryo-electron microscopy structures of ACR-23 in apo, betaine-bound, and betaine- and monepantel-bound states. We show that ACR-23 forms a homo-pentameric channel, similar to some other pentameric ligand-gated ion channels (pLGICs). While betaine molecules are bound to the classical neurotransmitter sites in the inter-subunit interfaces in the extracellular domain, monepantel molecules are bound to allosteric sites formed in the inter-subunit interfaces in the transmembrane domain of the receptor. Although the pore remains closed in betaine-bound state, monepantel binding results in an open channel by wedging into the cleft between the transmembrane domains of two neighboring subunits, which causes dilation of the ion conduction pore. By combining structural analyses with site-directed mutagenesis, electrophysiology and in vivo locomotion assays, we provide insights into the mechanism of action of the anthelmintics monepantel and betaine.

Next-generation sequencing technologies for helminth diagnostics and surveillance in ruminants: shifting diagnostic barriers

Helminth infections in grazing ruminants are a major issue for livestock farming globally, but are unavoidable in outdoor grazing systems and must be effectively managed to avoid deleterious effects to animal health, and productivity. Next-generation sequencing (NGS) technologies are transforming our understanding of the genetic basis of anthelmintic resistance (AR) and epidemiological studies of ruminant gastrointestinal parasites. They also have the potential to not only help develop and validate molecular diagnostic tests but to be directly used in routine diagnostics integrating species-specific identification and AR into a single test. Here, we review how these developments have opened the pathway for the development of multi-AR and multispecies identification in a single test, with widespread implications for sustainable livestock farming for the future.

Detection of SNPs and benzimidazole resistance in strongyle nematode eggs of horses by allele-specific PCR

This study was conducted to determine single nucleotide polymorphisms (SNPs) and the benzimidazole (BZ) resistance in strongyle nematode egg populations in horses using molecular techniques. A total of 200 fecal samples were collected from horses in 26 farms in two provinces (Kayseri and Nevşehir) of the Central Anatolia Region of Türkiye between May and August 2022. The flotation method was used to detect strongyle nematode eggs in the fecal samples of the horses. Afterward, strongyle nematode eggs were collected, and the allele-specific polymerase chain reaction (AS-PCR) technique was used to detect the BZ resistance. BZ-susceptible and BZ-resistant PCR products were sequenced to determine single nucleotide polymorphisms (SNPs) in the β-tubulin isotype 1 gene. The strongyle nematode eggs were determined in 85 (42.5%) out of 200 fecal samples. AS-PCR detected 50.58% (43/85) BZ-resistant (homozygous resistant) and 36.4% (31/85) BZ-susceptible (homozygous susceptible) genes in the strongyle eggs. Both BZ-resistant and BZ-susceptible genes (heterozygous) were determined in 11 samples. BZ-resistant and BZ-susceptible allele frequencies were determined as 57.0% (48.5/85) and 43.0% (36.5/85), respectively. SNPs were detected only in codon 200 of the β-tubulin isotype 1 gene in four sequenced isolates of the two resistant and two susceptible isolates. This study is the first molecular report on BZ resistance in strongyle nematode eggs in horses in Türkiye. The widespread prevalence of BZ-resistant alleles in equine strongyle nematodes shows the requirement for the immediate usage of other anthelmintics instead of the BZ group drugs for the effective management and control of equine strongyle nematodes.

Induction of endoplasmic reticulum stress is associated with the anti-tumor activity of monepantel across cancer types

BACKGROUND

Monepantel is an anti-helminthic drug that also has anti-cancer properties. Despite several studies over the years, the molecular target of monepantel in mammalian cells is still unknown, and its mechanism-of-action is not fully understood, though effects on cell cycle, mTOR signalling and autophagy have been implicated.

METHODS

Viability assays were performed on >20 solid cancer cell cells, and apoptosis assays were performed on a subset of these, including 3D cultures. Genetic deletion of BAX/BAK and ATG were used to establish roles of apoptosis and autophagy in killing activity. RNA-sequencing was performed on four cell lines after monepantel treatment, and differentially regulated genes were confirmed by Western blotting.

RESULTS

We showed that monepantel has anti-proliferative activity on a broad range of cancer cell lines. In some, this was associated with induction of apoptosis which was confirmed using a BAX/BAK-deficient cell line. However, proliferation is still inhibited in these cells following monepantel treatment, indicating cell-cycle disruption as the major anti-cancer effect. Previous studies have also indicated autophagic cell death occurs following monepantel treatment. We showed autophagy induction in multiple cell lines; however, deletion of a key autophagy regulator ATG7 had minimal impact on monepantel's anti-proliferative activity, suggesting autophagy is associated with, but not required for its anti-tumour effects. Transcriptomic analysis of four cell lines treated with monepantel revealed downregulation of many genes involved in the cell cycle, and upregulation of genes linked to ATF4-mediated ER stress responses, especially those involved in amino-acid metabolism and protein synthesis.

CONCLUSIONS

As these outcomes are all associated with mTOR signalling, cell cycle and autophagy, we now provide a likely triggering mechanism for the anti-cancer activity of monepantel.

Functional validation of the truncated UNC-63 acetylcholine receptor subunit in levamisole resistance

Levamisole is a broad-spectrum anthelmintic which permanently activates cholinergic receptors from nematodes, inducing a spastic paralysis of the worms. Whereas this molecule is widely used to control parasitic nematodes impacting livestock, its efficacy is compromised by the emergence of drug-resistant parasites. In that respect, there is an urgent need to identify and validate molecular markers associated with resistance. Previous transcriptomic analyses revealed truncated cholinergic receptor subunits as potential levamisole resistance markers in the trichostrongylid nematodes Haemonchus contortus, Telodorsagia circumcincta and Trichostrongylus colubriformis. In the present study we used the Xenopus oocyte, as well as the free-living model nematode Caenorhabditis elegans, as heterologous expression systems to functionally investigate truncated isoforms of the levamisole-sensitive acetylcholine receptor (L-AChR) UNC-63 subunit. In the Xenopus oocyte, we report that truncated UNC-63 from C. elegans has a strong dominant negative effect on the expression of the recombinant C. elegans L-AChRs. In addition, we show that when expressed in C. elegans muscle cells, truncated UNC-63 induces a drastic reduction in levamisole susceptibility in transgenic worms, thus providing the first known functional validation for this molecular marker in vivo.

In vitro culture of parasitic stages of Haemonchus contortus

Haemonchus contortus is a constraint to sheep production. Seeking to reduce the use of hosts and produce parasitic stages in large-scale, a 42-day in vitro culture protocol of H. contortus third-stage larvae was optimized using Dulbecco's modified Eagle's medium (DMEM). In cell-free culture, larvae were maintained at 39.6°C, in acidic media (pH 6.1) for 3 or 6 days with Δ4-dafachronic acid followed by DMEM pH 7.4 supplemented or not with Fildes' reagent. In DMEM pH 7.4 at 37°C, supplementation with Caco-2 cells was compared to Fildes. On Day 14, fourth-stage larvae (L4) development rates in acidic media supplemented (86.8-88.4%) or not (74.4-77.8%) with Fildes and in Caco-2 cell co-culture (92.6%) were similar, and superior to DMEM pH 7.4 with Fildes (0.0%). On Day 21, Caco-2 cell co-culture resulted in higher larvae differentiation (25.0%) and lower degeneration (13.9%) compared to acidic media (1.5-8.1% and 48.6-69.9%, respectively). This is the first report of prolonged in vitro culture of H. contortus larvae using commercial media in co-culture with Caco-2 cells. Although no progression to the adult stage, Caco-2 cell co-culture resulted in morphological differentiation of H. contortus L4 and larval viability for up to 28 days.

Polymorphisms in exon 11 of the mptl-1 gene and monepantel resistance in Haemonchus contortus

Chemical treatments are the main strategy to control gastrointestinal nematodes in sheep, and the emergence of anthelmintic resistance, as consequence, results in control failures and leads to economic losses. Thus, molecular tests may constitute an excellent tool for the early detection of anthelmintic resistance-related mutations. Thus, a polymerase chain reaction (PCR)-based genotyping assay followed by polyacrylamide gel electrophoresis (PAGE) was developed to detect polymorphisms in exon 11 of the acetylcholine receptor monepantel-1 gene (mptl-1) that were previously associated with monepantel resistance through a genome-wide study in Haemonchus contortus. DNA samples recovered from individual and pooled third-stage larvae from two susceptible field-derived isolates and five (three in vivo-derived and two field-derived) resistant populations were used. New polymorphisms, including a 6-bp deletion and a 3-bp insertion, were detected in resistant individuals. These indels, confirmed using sequencing of cloned PCR products, are predicted to result in amino acid changes in transmembrane domain 2 (TMD2) of the MPTL-1 protein. The two susceptible isolates showed only the presence of the wild-type allele (100%), whereas lower frequencies of the wild-type allele were detected in monepantel-resistant populations (11.1 to 66.7%). These findings report new polymorphisms in the mptl-1 gene, validate the results obtained through genomic mapping for monepantel resistance, and provide a PCR-based assay to genotype indels located in exon 11 of mptl-1 in H. contortus.

Genomic signatures of selection associated with benzimidazole drug treatments in Haemonchus contortus field populations

Genome-wide methods offer a powerful approach to detect signatures of drug selection. However, limited availability of suitable reference genomes and the difficulty of obtaining field populations with well-defined, distinct drug treatment histories mean there is little information on the signatures of selection in parasitic nematodes and on how best to detect them. This study addresses these knowledge gaps by using field populations of Haemonchus contortus with well-defined benzimidazole treatment histories, leveraging a recently completed chromosomal-scale reference genome assembly. We generated a panel of 49,393 genomic markers to genotype 20 individual adult worms from each of four H. contortus populations: two from closed sheep flocks with an approximate 20 year history of frequent benzimidazole treatment, and two populations with a history of little or no treatment. Sampling occurred in the same geographical region to limit genetic differentiation and maximise the detection sensitivity. A clear signature of selection was detected on chromosome I, centred on the isotype-1 β-tubulin gene. Two additional, but weaker, signatures of selection were detected; one near the middle of chromosome I spanning 3.75 Mbp and 259 annotated genes, and one on chromosome II spanning a region of 3.3 Mbp and 206 annotated genes, including the isotype-2 β-tubulin locus. We also assessed how sensitivity was impacted by sequencing depth, worm number, and pooled versus individual worm sequence data. This study provides the first known direct genome-wide evidence for any parasitic nematode, that the isotype-1 β-tubulin gene is quantitatively the single most important benzimidazole resistance locus. It also identified two additional genomic regions that likely contain benzimidazole resistance loci of secondary importance. This study provides an experimental framework to maximise the power of genome-wide approaches to detect signatures of selection driven by anthelmintic drug treatments in field populations of parasitic nematodes.

Population replacement of benzimidazole-resistant Haemonchus contortus with susceptible strains: evidence of changes in the resistance status

The spread of anthelmintic resistance (AR) in nematode populations threatens the viability of sheep production systems worldwide, and warrants the adoption of sensitive, practical, and standardized tests to detect AR. The aim of this study was to characterize the replacement of an Haemonchus contortus population resistant to benzimidazoles (BZDs) by a susceptible one, by means of both phenotypic and genotypic techniques. Phenotypic methods to assess BZD resistance included in vivo tests, such as the fecal egg count reduction test (FECRT), and in vitro tests, such as the egg hatch assay (EHA). Additionally, genotypification of polymorphisms associated with BZD resistance by sequencing a fragment of the isotype 1 β-tubulin gene was carried out. The initial, BZD-resistant population (initial Balcarce population) exhibited an egg count reduction (ECR) of 59.3%. Following refugium replacement, the final population (final Balcarce population) exhibited an ECR of 95.2%. For the initial Balcarce population, the median effective dose (ED₅₀) for the EHA was 0.607 μg thiabendazole (TBZ)/mL, with a rate of eclosion at a discriminating dose (E_DD) of 0.1 μg TBZ/mL of 76.73%. For the final Balcarce population, ED₅₀ was 0.02 μg TBZ/mL, and E_DD was 1.97%. In the initial population, 93% of the analyzed individuals exhibited genotypic combinations associated with BZD resistance (53% Phe/Phe₁₆₇-Tyr/Tyr₂₀₀, 37% Phe/Tyr₁₆₇-Phe/Tyr₂₀₀, and 3% Phe/Tyr₁₆₇-Glu/Leu₁₉₈). Conversely, no combination associated with resistance was found in individuals from the final population. All of the tests were useful for detecting AR to BZDs. The results from the genetic and phenotypical studies were consistent, and the resulting information greatly aided in interpreting the outcomes of the population replacement and the potential impact of this strategy on management of AR.

Advances in our understanding of nematode ion channels as potential anthelmintic targets

Ion channels are specialized multimeric proteins that underlie cell excitability. These channels integrate with a variety of neuromuscular and biological functions. In nematodes, the physiological behaviors including locomotion, navigation, feeding and reproduction, are regulated by these protein entities. Majority of the antinematodal chemotherapeutics target the ion channels to disrupt essential biological functions. Here, we have summarized current advances in our understanding of nematode ion channel pharmacology. We review cys-loop ligand gated ion channels (LGICs), including nicotinic acetylcholine receptors (nAChRs), acetylcholine-chloride gated ion channels (ACCs), glutamate-gated chloride channels (GluCls), and GABA (γ-aminobutyric acid) receptors, and other ionotropic receptors (transient receptor potential (TRP) channels and potassium ion channels). We have provided an update on the pharmacological properties of these channels from various nematodes. This article catalogs the differences in ion channel composition and resulting pharmacology in the phylum Nematoda. This diversity in ion channel subunit repertoire and pharmacology emphasizes the importance of pursuing species-specific drug target research. In this review, we have provided an overview of recent advances in techniques and functional assays available for screening ion channel properties and their application.

Genome-wide analysis of the response to ivermectin treatment by a Swedish field population of Haemonchus contortus

Haemonchus contortus is a pathogenic gastrointestinal nematode of small ruminants and, in part due to its capacity to develop resistance to drugs, contributes to significant losses in the animal production sector worldwide. Despite decades of research, comparatively little is known about the specific mechanism(s) driving resistance to drugs such as ivermectin in this species. Here we describe a genome-wide approach to detect evidence of selection by ivermectin treatment in a field population of H. contortus from Sweden, using parasites sampled from the same animals before and seven days after ivermectin exposure followed by whole-genome sequencing. Despite an 89% reduction in parasites recovered after treatment measured by the fecal egg count reduction test, the surviving population was highly genetically similar to the population before treatment, suggesting that resistance has likely evolved over time and that resistance alleles are present on diverse haplotypes. Pairwise gene and SNP frequency comparisons indicated the highest degree of differentiation was found at the terminal end of chromosome 4, whereas the most striking difference in nucleotide diversity was observed in a region on chromosome 5 previously reported to harbor a major quantitative trait locus involved in ivermectin resistance. These data provide novel insight into the genome-wide effect of ivermectin selection in a field population as well as confirm the importance of the previously established quantitative trait locus in the development of resistance to ivermectin.

A High-Throughput Phenotypic Screen of the 'Pandemic Response Box' Identifies a Quinoline Derivative with Significant Anthelmintic Activity

Parasitic nematodes cause diseases in livestock animals and major economic losses to the agricultural industry worldwide. Nematodes of the order Strongylida, including Haemonchus contortus, are particularly important. The excessive use of anthelmintic compounds to treat infections and disease has led to widespread resistance to these compounds in nematodes, such that there is a need for new anthelmintics with distinctive mechanisms of action. With a focus on discovering new anthelmintic entities, we screened 400 chemically diverse compounds within the 'Pandemic Response Box' (from Medicines for Malaria Venture, MMV) for activity against H. contortus and its free-living relative, Caenorhabditis elegans-a model organism. Using established phenotypic assays, test compounds were evaluated in vitro for their ability to inhibit the motility and/or development of H. contortus and C. elegans. Dose-response evaluations identified a compound, MMV1581032, that significantly the motility of H. contortus larvae (IC₅₀ = 3.4 ± 1.1 μM) and young adults of C. elegans (IC₅₀ = 7.1 ± 4.6 μM), and the development of H. contortus larvae (IC₅₀ = 2.2 ± 0.7 μM). The favourable characteristics of MMV1581032, such as suitable physicochemical properties and an efficient, cost-effective pathway to analogue synthesis, indicates a promising candidate for further evaluation as a nematocide. Future work will focus on a structure-activity relationship investigation of this chemical scaffold, a toxicity assessment of potent analogues and a mechanism/mode of action investigation.

Suppressive treatment with monepantel and the fast selection for phenotypically resistant trichostrongylids of sheep

Gastrointestinal parasite control has been a major challenge to livestock due to the failure of anthelmintic treatments. Monepantel (MNT) was introduced in 2009 as an alternative treatment option showing a new mechanism of action against nematode parasites. To study the response of MNT in a suppressive regime, 45-Suffolk and White Dorper naturally infected sheep were divided into one of three groups, G1: control – with no treatment, G2: MNT at 2.5 mg kg−1 live weight (LW) PO every 30 days, and G3: MNT at 4.0 mg kgLW−1 PO every 30 days for 6 months. Every 15 days, the animals were individually weighed (body weight, BW) and checked for Famacha (FMC) and body condition score (BCS). The efficacy of MNT was evaluated weekly by fecal egg count (FEC) every month. FEC showed >97% efficacy at the start of the experiment, revealing a significant reduction for G2 (28%) and G3 (39%) in the following months. There was no treatment, BW or BCS effect between treatments; however, there was a period (P < 0.0001) and a treatment vs period interaction (P < 0.0001) for BW. The data revealed that MNT at a therapeutic and suppressive dose had a non-linear polynomial efficacy regression (R2) of 0.988 and 0.992, respectively. This original experiment demonstrates how short-interval and suppressive MNT treatments would rapidly select Haemonchus contortus, showing a fast susceptible-resistance phenotypic population replacement. Therefore, it is suggested that MNT might be carefully used in parasite control programmes alongside other management strategies (i.e. FMC, BCS) to reduce treatment frequency and the selection process for resistance.

Emodepside targets SLO-1 channels of Onchocerca ochengi and induces broad anthelmintic effects in a bovine model of onchocerciasis

Onchocerciasis (river blindness), caused by the filarial worm Onchocerca volvulus, is a neglected tropical disease mostly affecting sub-Saharan Africa and is responsible for >1.3 million years lived with disability. Current control relies almost entirely on ivermectin, which suppresses symptoms caused by the first-stage larvae (microfilariae) but does not kill the long-lived adults. Here, we evaluated emodepside, a semi-synthetic cyclooctadepsipeptide registered for deworming applications in companion animals, for activity against adult filariae (i.e., as a macrofilaricide). We demonstrate the equivalence of emodepside activity on SLO-1 potassium channels in Onchocerca volvulus and Onchocerca ochengi, its sister species from cattle. Evaluation of emodepside in cattle as single or 7-day treatments at two doses (0.15 and 0.75 mg/kg) revealed rapid activity against microfilariae, prolonged suppression of female worm fecundity, and macrofilaricidal effects by 18 months post treatment. The drug was well tolerated, causing only transiently increased blood glucose. Female adult worms were mostly paralyzed; however, some retained metabolic activity even in the multiple high-dose group. These data support ongoing clinical development of emodepside to treat river blindness.

Monepantel pharmaco-therapeutic evaluation in cattle: Pattern of efficacy against multidrug resistant nematodes

The goal of the current work was to perform an integrated evaluation of monepantel (MNP) pharmacokinetics (PK) and pharmacodynamics, measured as anthelmintic efficacy, after its oral administration to calves naturally infected with GI nematodes resistant to ivermectin (IVM) and ricobendazole (RBZ) on three commercial farms. On each farm, forty-five calves were randomly allocated into three groups (n = 15): MNP oral administration (2.5 mg/kg); IVM subcutaneous (SC) administration (0.2 mg/kg); and RBZ SC administration (3.75 mg/kg). Eight animals from the MNP treated group (Farm 1) were selected to perform the PK study. Drug concentrations were measured by HPLC. The efficacy was determined by the faecal egg count reduction test (FECRT). MNP and MNP-sulphone (MNPSO₂) were the main analytes recovered in plasma. MNPSO₂ systemic exposure was markedly higher compared to that obtained for MNP. Higher Cmax and AUC values were obtained for the active MNPSO₂ metabolite (96.8 ± 29.7 ng/mL and 9220 ± 1720 ng h/mL) compared to MNP (21.5 ± 4.62 ng/mL and 1709 ± 651 ng h/mL). The MNPSO₂ AUC value was 6-fold higher compared to the parent drug. Efficacies of 99% (Farm 1), 96% (Farm 2) and 98% (Farm 3) demonstrated the high activity of MNP (P < 0.05) against GI nematodes resistant to IVM (reductions between 27 and 68%) and RBZ (overall efficacy of 75% on Farm 3). While IVM failed to control Haemonchus spp. and Cooperia spp., and RBZ failed to control Coooperia spp. and Ostertagia spp., MNP achieved 100% efficacy against Haemonchus spp., Cooperia spp. and Ostertagia spp. However, a low efficacy of MNP against Oesophagostomum spp. (efficacies ranging from 22 to 74%) was observed. In conclusion, oral treatment with MNP should be considered for dealing with IVM and benzimidazole resistant nematode parasites in cattle. The work described here reports for the first time an integrated assessment of MNP pharmaco-therapeutic features and highlights the need to be considered as a highly valuable tool to manage nematode resistant to other chemical families.

Challenges and opportunities for the adoption of molecular diagnostics for anthelmintic resistance

Anthelmintic resistance is a significant threat to livestock production systems worldwide and is emerging as an important issue in companion animal parasite management. It is also an emerging concern for the control of human soil-transmitted helminths and filaria. An important aspect of managing anthelmintic resistance is the ability to utilise diagnostic tests to detect its emergence at an early stage. In host-parasite systems where resistance is already widespread, diagnostics have a potentially important role in determining those drugs that remain the most effective. The development of molecular diagnostics for anthelmintic resistance is one focus of the Consortium for Anthelmintic Resistance and Susceptibility (CARS) group. The present paper reflects discussions of this issue that occurred at the most recent meeting of the group in Wisconsin, USA, in July 2019. We compare molecular resistance diagnostics with in vivo and in vitro phenotypic methods, and highlight the advantages and disadvantages of each. We assess whether our knowledge on the identity of molecular markers for resistance towards the different drug classes is sufficient to provide some expectation that molecular tests for field use may be available in the short-to-medium term. We describe some practical aspects of such tests and how our current capabilities compare to the requirements of an 'ideal' test. Finally, we describe examples of drug class/parasite species interactions that provide the best opportunity for commercial use of molecular tests in the near future. We argue that while such prototype tests may not satisfy the requirements of an 'ideal' test, their potential to provide significant advances over currently-used phenotypic methods warrants their development as field diagnostics.

Genomic and transcriptomic variation defines the chromosome-scale assembly of Haemonchus contortus, a model gastrointestinal worm

Haemonchus contortus is a globally distributed and economically important gastrointestinal pathogen of small ruminants and has become a key nematode model for studying anthelmintic resistance and other parasite-specific traits among a wider group of parasites including major human pathogens. Here, we report using PacBio long-read and OpGen and 10X Genomics long-molecule methods to generate a highly contiguous 283.4 Mbp chromosome-scale genome assembly including a resolved sex chromosome for the MHco3(ISE).N1 isolate. We show a remarkable pattern of conservation of chromosome content with Caenorhabditis elegans, but almost no conservation of gene order. Short and long-read transcriptome sequencing allowed us to define coordinated transcriptional regulation throughout the parasite's life cycle and refine our understanding of cis- and trans-splicing. Finally, we provide a comprehensive picture of chromosome-wide genetic diversity both within a single isolate and globally. These data provide a high-quality comparison for understanding the evolution and genomics of Caenorhabditis and other nematodes and extend the experimental tractability of this model parasitic nematode in understanding helminth biology, drug discovery and vaccine development, as well as important adaptive traits such as drug resistance.

Multiple anthelmintic resistance in gastrointestinal nematodes of small ruminants in Bangladesh

Anthelmintic resistance (AR) against gastrointestinal nematodes (GINs) of sheep and goats is a global concern. To address the problem, this study assessed the status of AR in different government and private sheep and goat farms in Bangladesh. We conducted fecal egg count reduction test (FECRT) and Egg hatch assay (EHA) experiments. For the detection of resistant larvae, pooled fecal samples from treated and non-treated groups were subjected to coproculture. Furthermore, 195 adult Haemonchus parasites were genotyped to ascertain benzimidazole (BZ) resistance allele from seven topographic zones of Bangladesh using allele specific PCR (AS-PCR). In FECRT, the percentage reduction along with 95% confidence intervals indicated that GINs were resistant to albendazole (ABZ), levamisole (LEV) and ivermectin (IVM). Coproculture revealed that Haemonchus spp., Oesophagostomum spp. and Trichostrongylus spp. were resistant to anthelmintics. ABZ resistance was also confirmed by in vitro EHA in all the farms except the private goat farm in Mymensingh. The genotype frequencies were 6% for homozygous resistant (rr), 59% for heterozygous (rS) and 35% for homozygous susceptible (SS) among different topographic zones. The allelic frequency of the mutation conferring resistance (r) ranged from 25% to 47% signifying resistance to BZ in nematodes of sheep/goats. The genotype frequencies (rr, rS and SS) and allelic frequencies (r and S) varied significantly (p˂0.05) in different zones in Bangladesh. Overall, the data suggest an alarming condition created by multiple AR in Bangladesh.

A perspective on the discovery of selected compounds with anthelmintic activity against the barber's pole worm-Where to from here?

Parasitic roundworms (nematodes) cause substantial morbidity and mortality in animals worldwide. Anthelmintic treatment is central to controlling these worms, but widespread resistance to most of the commercially available anthelmintics for veterinary and agricultural use is compromising control, such that there is an urgency to discover new and effective drugs. The purpose of this article is to review information on parasitic nematodes, the treatment and control of parasitic nematode infections and aspects of discovering new anthelmintics in the context of anthelmintic resistance problems, and then to discuss some progress that our group has made in identifying selected compounds with activity against nematodes. The focus of our recent work has been on discovering new chemical entities and known drugs with anthelmintic activities against Haemonchus contortus as well as other socioeconomically important parasitic nematodes for subsequent development. Using whole worm-based phenotypic assays, we have been screening compound collections obtained via product-development-partnerships and/or collaborators, and active compounds have been assessed for their potential as anthelmintic candidates. Following the screening of 15,333 chemicals from five distinct compound collections against H. contortus, we have discovered one new chemical entity (designated SN00797439), two human kinase inhibitors (SNS-032 and AG-1295), 14 tetrahydroquinoxaline analogues, one insecticide (tolfenpyrad) and two tolfenpyrad (pyrazole-5-carboxamide) derivatives (a-15 and a-17) with anthelmintic activity in vitro. Some of these 20 'hit' compounds have selectivity against H. contortus in vitro when compared to particular human cell lines. In our opinion, some of these compounds could represent starting points for 'lead' development. Accordingly, the next research steps to be pursued include: (i) chemical optimisation of representative chemicals via structure-activity relationship (SAR) evaluations; (ii) assessment of the breadth of spectrum of anthelmintic activity on a range of other parasitic nematodes, such as strongyloids, ascaridoids, enoplids and filarioids; (iii) detailed investigations of the absorption, distribution, metabolism, excretion and toxicity (ADMET) of optimised chemicals with broad nematocidal or nematostatic activity; and (iv) establishment of the modes of action of lead candidates.

Genotypic characterisation of monepantel resistance in historical and newly derived field strains of Teladorsagia circumcincta

Recent reports of monepantel (MPTL) resistance in UK field isolates of Teladorsagia circumcincta has highlighted the need for a better understanding of the mechanism of MPTL-resistance in order to preserve its anthelmintic efficacy in this economically important species. Nine discrete populations of T. circumcincta were genotypically characterised; three MPTL-susceptible isolates, three experimentally selected MPTL-resistant strains and three field derived populations. Full-length Tci-mptl-1 gene sequences were generated and comparisons between the MPTL-susceptible isolates, MPTL-resistant strains and one field isolate, showed that different putative MPTL-resistance conferring mutations were present in different resistant isolates. Truncated forms of the Tci-mptl-1 gene were also observed. The genetic variability of individual larvae, within and between populations, was examined using microsatellite analyses at 10 'neutral' loci (presumed to be unaffected by MPTL). Results confirmed that there was little background genetic variation between the populations, global FST <0.038. Polymorphisms present in exons 7 and 8 of Tci-mptl-1 enabled genotyping of individual larvae. A reduction in the number of genotypes was observed in all MPTL-resistant strains compared to the MPTL-susceptible strains that they were derived from, suggesting there was purifying selection at Tci-mptl-1 as a result of MPTL-treatment. The potential link between benzimidazole (BZ)-resistance and MPTL-resistance was examined by screening individual larvae for the presence of three SNPs associated with BZ-resistance in the β-tubulin isotype-1 gene. The majority of larvae were BZ-susceptible homozygotes at positions 167 and 198. Increased heterozygosity at position 200 was observed in the MPTL-resistant strains compared to their respective MPTL-susceptible population. There was no decrease in the occurrence of BZ-resistant genotypes in larvae from each population. These differences, in light of the purifying selection at this locus in all MPTL-resistant isolates, suggests that Tci-mptl-1 confers MPTL-resistance in T. circumcincta, as in Haemonchus contortus, but that different mutations in Tci-mptl-1 can confer resistance in different populations.

Extreme-QTL mapping of monepantel resistance in Haemonchus contortus

Background

Haemonchus contortus, a gastrointestinal nematode parasite of sheep, is mainly controlled by anthelmintics; the occurrence of anthelmintic resistance leads to treatment failures and increases economic burden. Because molecular mechanisms involved in drug resistance can be elucidated by genomic studies, an extreme quantitative trait locus (X-QTL) mapping approach was used to identify co-segregation of the resistance phenotype with genetic markers to detect the genome-wide variants associated with monepantel resistance in H. contortus.

Methods

A cross between H. contortus isolates using parental susceptible (Par-S) males and monepantel resistant (Par-R) females resulted in SR progeny, while reciprocal cross resulted in RS progeny. Pools (n = 30,000) of infective larvae (L3) recovered from Par-R, and from SR and RS populations in the F3 generation, collected both before (unselected group) and 7 days after (selected group) selection with monepantel treatment in sheep hosts, were subjected to genome sequencing (Pool-Seq). Pairwise comparisons of allele frequencies between unselected and selected groups were performed for each population by Fisher’s exact test (FET) and for both populations combined by a Cochran-Mantel-Haenszel (CMH) test.

Results

Mapping rates varied from 80.29 to 81.77% at a 90.4X mean coverage of aligned reads. After correction for multiple testing, significant (P < 0.05) changes in allele frequencies were detected by FET for 6 and 57 single nucleotide polymorphisms (SNPs) in the SR and RS populations, respectively, and by the CMH test for 124 SNPs in both populations. The significant variants located on chromosome 2 generated a selection signal in a genomic region harboring the mptl-1, deg-3 and des-2 genes, previously reported as candidates for monepantel resistance. In addition, three new variants were identified in the mptl-1 gene.

Conclusions

This study expands knowledge on genome-wide molecular events underlying H. contortus resistance to monepantel. The identification of a genome region harboring major genes previously associated with monepantel resistance supports the results of the employed X-QTL approach. In addition, a deletion in exon 11 of the mptl-1 gene should be further investigated as the putative causal mutation leading to monepantel resistance.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3663-9) contains supplementary material, which is available to authorized users.

Helminthiases; prevalence, transmission, host-parasite interactions, resistance to common synthetic drugs and treatment

The morbidity caused by parasite worms on susceptible hosts is of great concern and studies were carried out to explain the mechanism of infection, prevalence, host-parasite interaction and resistance of the parasite to treatment. This review elucidates the prevalence of parasitic worm infections; which is on the increases with the increase in the world population, global warming, poor standard of living particularly in troubled regions and developing nations. The neglect of the disease coupled with the resistance of these parasites to the few available drugs becomes a huge challenge that influences global disease burden. Helminths infections pose a life threat and increase the disability-adjusted life year (DALYs) of the poor and vulnerable people. On the other hand, exploration of medicinal plants as an alternative source of treatment against drugs resistance helminths, attract insufficient attention. This review focused on providing a general overview of the prevalence of helminths, host-parasite interactions, the resistance of helminths and the medicinal plants used to treat helminthic infections.

Small Molecule Inhibitors of Metabolic Enzymes Repurposed as a New Class of Anthelmintics

The enormous prevalence of infections caused by parasitic nematodes worldwide, coupled to the rapid emergence of their resistance to commonly used anthelmintic drugs, presents an urgent need for the discovery of new drugs. Herein, we have identified several classes of small molecules with broad spectrum activity against these pathogens. Previously, we reported the identification of carnitine palmitoyltransferases (CPTs) as a representative class of enzymes as potential targets for metabolic chokepoint intervention that was elucidated from a combination of chemogenomic screening and experimental testing in nematodes. Expanding on these previous findings, we have discovered that several chemical classes of known small molecule inhibitors of mammalian CPTs have potent activity as anthelmintics. Cross-clade efficacy against a broad spectrum of adult parasitic nematodes was demonstrated for multiple compounds from different series. Several analogs of these initial hit compounds were designed and synthesized. The compounds we report represent a good starting point for further lead identification and optimization for development of new anthelmintic drugs with broad spectrum activity and a novel mechanism of action.

The interactions of anthelmintic drugs with nicotinic receptors in parasitic nematodes

Parasitic nematodes express a large number of distinct nicotinic acetylcholine receptors and these in turn are the targets of many classes of anthelmintic drug. This complexity poses many challenges to the field, including sorting the exact subunit composition of each of the receptor subtypes and how much they vary between species. It is clear that the model organism Caenorhabditis elegans does not recapitulate the complexity of nicotinic pharmacology of many parasite species and data using this system may be misleading when applied to them. The number of different receptors may allow nematodes some plasticity which they can exploit to evolve resistance to a specific cholinergic drug; however, this may mean that combinations of cholinergic agents may be effective at sustainably controlling them. Resistance may involve the expression of truncated receptor subunits that affect the expression levels of the receptors via mechanisms that remain to be deciphered.

Monepantel is a non-competitive antagonist of nicotinic acetylcholine receptors from Ascaris suum and Oesophagostomum dentatum

Zolvix^® is a recently introduced anthelmintic drench containing monepantel as the active ingredient. Monepantel is a positive allosteric modulator of DEG-3/DES-2 type nicotinic acetylcholine receptors (nAChRs) in several nematode species. The drug has been reported to produce hypercontraction of Caenorhabditis elegans and Haemonchus contortus somatic muscle. We investigated the effects of monepantel on nAChRs from Ascaris suum and Oesophagostomum dentatum heterologously expressed in Xenopus laevis oocytes. Using two-electrode voltage-clamp electrophysiology, we studied the effects of monepantel on a nicotine preferring homomeric nAChR subtype from A. suum comprising of ACR-16; a pyrantel/tribendimidine preferring heteromeric subtype from O. dentatum comprising UNC-29, UNC-38 and UNC-63 subunits; and a levamisole preferring subtype (O. dentatum) comprising UNC-29, UNC-38, UNC-63 and ACR-8 subunits. For each subtype tested, monepantel applied in isolation produced no measurable currents thereby ruling out an agonist action. When monepantel was continuously applied, it reduced the amplitude of acetylcholine induced currents in a concentration-dependent manner. In all three subtypes, monepantel acted as a non-competitive antagonist on the expressed receptors. ACR-16 from A. suum was particularly sensitive to monepantel inhibition (IC₅₀ values: 1.6 ± 3.1 nM and 0.2 ± 2.3 μM). We also investigated the effects of monepantel on muscle flaps isolated from adult A. suum. The drug did not significantly increase baseline tension when applied on its own. As with acetylcholine induced currents in the heterologously expressed receptors, contractions induced by acetylcholine were antagonized by monepantel. Further investigation revealed that the inhibition was a mixture of competitive and non-competitive antagonism. Our findings suggest that monepantel is active on multiple nAChR subtypes.

The novel isoxazoline ectoparasiticide lotilaner (Credelio™): a non-competitive antagonist specific to invertebrates γ-aminobutyric acid-gated chloride channels (GABACls)

BACKGROUND

The isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and, to a lesser extent, of inhibitory glutamate-gated chloride channels (GluCls). Lotilaner (Credelio™), a novel representative of this chemical class, is currently evaluated for its excellent ectoparasiticide properties.

METHODS

In this study, we investigated the molecular mode of action and pharmacology of lotilaner. We report the successful gene identification, cDNA cloning and functional expression in Xenopus oocytes of Drosohpila melanogaster (wild type and dieldrin/fipronil-resistant forms), Lepeophtheirus salmonis (an ectoparasite copepod crustacean of salmon), Rhipicephalus microplus and Canis lupus familiaris GABACls. Automated Xenopus oocyte two-electrode voltage clamp electrophysiology was used to assess GABACls functionality and to compare ion channel inhibition by lotilaner with that of established insecticides addressing GABACls as targets.

RESULTS

In these assays, we demonstrated that lotilaner is a potent non-competitive antagonist of insects (fly) GABACls. No cross-resistance with dieldrin or fipronil resistance mutations was detected, suggesting that lotilaner might bind to a site at least partly different from the one bound by known GABACl blockers. Using co-application experiments, we observed that lotilaner antagonism differs significantly from the classical open channel blocker fipronil. We finally confirmed for the first time that isoxazoline compounds are not only powerful antagonists of GABACls of acari (ticks) but also of crustaceans (sea lice), while no activity on a dog GABA_A receptor was observed up to a concentration of 10 μM.

CONCLUSIONS

Together, these results demonstrate that lotilaner is a non-competitive antagonist specific to invertebrate's γ-aminobutyric acid-gated chloride channels (GABACls). They contribute to our understanding of the mode of action of this new ectoparasiticide compound.

A BRIEF REVIEW ON THE MODE OF ACTION OF ANTINEMATODAL DRUGS

Anthelmintics are some of the most widely used drugs in veterinary medicine. Here we review the mechanism of action of these compounds on nematode parasites. Included are the older classes of compounds; the benzimidazoles, cholinergic agonists and macrocyclic lactones. We also consider newer anthelmintics, including emodepside, derquantel and tribendimidine. In the absence of vaccines for most parasite species, control of nematode parasites will continue to rely on anthelmintic drugs. As a consequence, vigilance in detecting drug resistance in parasite populations is required. Since resistance development appears almost inevitable, there is a continued and pressing need to fully understand the mode of action of these compounds. It is also necessary to identify new drug targets and drugs for the continued effective control of nematode parasites.

Functional genomics in Brugia malayi reveal diverse muscle nAChRs and differences between cholinergic anthelmintics

Many techniques for studying functional genomics of important target sites of anthelmintics have been restricted to Caenorhabditis elegans because they have failed when applied to animal parasites. To overcome these limitations, we have focused our research on the human nematode parasite Brugia malayi, which causes elephantiasis. Here, we combine single-cell PCR, whole muscle cell patch clamp, motility phenotyping (Worminator), and dsRNA for RNAi for functional genomic studies that have revealed, in vivo, four different muscle nAChRs (M-, L-, P-, and N-). The cholinergic anthelmintics had different selectivities for these receptors. We show that motility and patch-clamp responses to levamisole and pyrantel, but not morantel or nicotine, require the unc-38 and/or unc-29 genes. Derquantel behaved as a competitive antagonist and distinguished M-nAChRs activated by morantel (Kb 13.9 nM), P-nAChRs activated by pyrantel (Kb 126 nM), and L-nAChRs activated by levamisole (Kb 0.96 µM) and bephenium. Derquantel was a noncompetitive antagonist of nicotine, revealing N-type nAChRs. The presence of four diverse nAChRs on muscle is perhaps surprising and not predicted from the C. elegans model. The diverse nAChRs represent distinguishable drug targets with different functions: Knockdown of unc-38+unc-29 (L- and/or P-receptors) inhibited motility but knockdown of acr-16+acr-26 (M- and/or N-receptors) did not.

Mutations in the Hco-mptl-1 gene in a field-derived monepantel-resistant isolate of Haemonchus contortus

Resistance to the anthelmintic drug monepantel (Zolvix^®) has emerged in parasitic worms infecting sheep and goats. The mechanism of resistance in these cases is unknown. The drug targets nicotinic acetylcholine receptors belonging to the nematode-specific DEG-3 subfamily. We examined the receptor gene, Hco-mptl-1, in a highly Zolvix^®-resistant and a -susceptible isolate of the parasitic nematode Haemonchus contortus. cDNA coding for the full length receptor protein (Hco-MPTL-1) was present in all clones prepared from a pool of susceptible larvae (21/21 clones) and approximately 50% of those from the resistant isolate (17/33). On the other hand, the remaining clones from the resistant isolate showed various mutations that resulted in truncated predicted proteins, missing at least one transmembrane domain. The most common mutation (11/33 clones) resulted in the retention of intron 15, a premature stop codon, and a truncated protein. Sequencing of intron 15 genomic DNA showed very few SNPs in susceptible larvae and in 12/18 clones from resistant larvae, alongside the presence of at least 17 SNPs in the remaining resistant clones. The present study shows that the highly resistant isolate has a number of mutations in the drug target gene that would most-likely result in a non-functional receptor, thus rendering the larvae insensitive to the drug. The presence of many wild-type sequences in this highly-resistant population suggests that there was a significant presence of heterozygotes in the survivors of the field drench treatment from which the isolate was derived, and hence that at least some of the mutations may be dominant. Alternatively, their presence may be due to the additional influence of mutations at another locus contributing to the resistance phenotype. The presence of multiple separate mutations in the Hco-mptl-1 gene in this viable field-derived worm isolate may at least partly explain why resistance to Zolvix^® has arisen rapidly in the field.

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Hco-mptl-1 gene compared in susceptible and monepantel-resistant Haemonchus contortus.

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Six separate mutations detected in the resistant isolate.

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Most common mutation resulted in retention of intron and truncated predicted protein.

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Many separate mutations in the drug target site in the field-derived resistant isolate.

Increased expression of ATP binding cassette transporter genes following exposure of Haemonchus contortus larvae to a high concentration of monepantel in vitro

Background

There is some evidence that ATP binding cassette (ABC) transporters play a role in resistance to anthelmintics, particularly against macrocyclic lactones. Some anthelmintics, including ivermectin (IVM), have been shown to induce transcription of multiple ABC transporters in nematodes; however, the effects of monepantel (MPL) on transcription of these transporter genes has not been studied.

Methods

Larvae of two MPL-susceptible isolates of Haemonchus contortus were exposed to MPL at two concentrations (2.5 and 250 μg/ml) for periods of 3, 6 and 24 h. Transcription levels of sixteen ABC transporter genes were measured at the end of the incubation periods. The consequences of MPL exposure were examined by measuring rhodamine-123 efflux from the larvae, and their sensitivity to subsequent treatment with IVM or levamisole.

Results

Multiple ABC transporter genes showed significantly higher transcription in both worm isolates following exposure to MPL at 250 μg/ml for 3, 6 or 24 h, particularly the P-glycoprotein (P-gp) genes pgp-11, pgp-12 and pgp-14. Of these, only pgp-11 maintained the elevated levels 24 h after the end of the drug exposure period. In contrast, there was only a single instance of low-level upregulation as a result of exposure to MPL at 2.5 μg/ml. Larvae exposed to MPL at 250 μg/ml showed an increased efflux of rhodamine-123 and a proportion of the larval population showed an ability to subsequently tolerate higher concentrations of IVM in migration assays. There was no increased tolerance to IVM following pre-exposure to MPL at 2.5 μg/ml.

Conclusions

Exposure of H. contortus larvae to 250 μg/ml MPL results in increased transcription of multiple transporter genes and increased R-123 efflux. The subsequent ability of a proportion of the larvae to tolerate IVM suggests a protective role of ABC transporters across different chemical entities. However, these observations were only made at a concentration of MPL well above that experienced by parasitic life stages in vivo, and hence their significance remains unclear.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1806-9) contains supplementary material, which is available to authorized users.

Evaluation of an FDA approved library against laboratory models of human intestinal nematode infections

Background

Treatment options for infections with soil-transmitted helminths (STH) - Ascaris lumbricoides, Trichuris trichiura and the two hookworm species, Ancylostoma duodenale and Necator americanus - are limited despite their considerable global health burden. The aim of the present study was to test the activity of an openly available FDA library against laboratory models of human intestinal nematode infections.

Methods

All 1,600 drugs were first screened against Ancylostoma ceylanicum third-stage larvae (L3). Active compounds were scrutinized and toxic compounds, drugs indicated solely for topical use, and already well-studied anthelmintics were excluded. The remaining hit compounds were tested in parallel against Trichuris muris first-stage larvae (L1), Heligmosomoides polygyrus third-stage larvae (L3), and adult stages of the three species in vitro. In vivo studies were performed in the H. polygyrus and T. muris mice models.

Results

Fifty-four of the 1,600 compounds tested revealed an activity of > 60 % against A. ceylanicum L3 (hit rate of 3.4 %), following incubation at 200 μM for 72 h. Twelve compounds progressed into further screens. Adult A. ceylanicum were the least affected (1/12 compounds active at 50 μM), while eight of the 12 test compounds revealed activity against T. muris L1 (100 μM) and adults (50 μM), and H. polygyrus L3 (200 μM). Trichlorfon was the only compound active against all stages of A. ceylanicum, H. polygyrus and T. muris. In addition, trichlorfon achieved high worm burden reductions of 80.1 and 98.9 %, following a single oral dose of 200 mg/kg in the T. muris and H. polygyrus mouse model, respectively.

Conclusion

Drug screening on the larval stages of intestinal parasitic nematodes is feasible using small libraries and important given the empty drug discovery and development pipeline for STH infections. Differences and commonalities in drug activities across the different STH species and stages were confirmed. Hits identified might serve as a starting point for drug discovery for STH.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1616-0) contains supplementary material, which is available to authorized users.

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.

Genetic Diversity and Population Structure of Haemonchus contortus

Haemonchus contortus is one of the most successful and problematic livestock parasites worldwide. From its apparent evolutionary origins in sub-Saharan Africa, it is now found in small ruminants in almost all regions of the globe, and can infect a range of different domestic and wildlife artiodactyl hosts. It has a remarkably high propensity to develop resistance to anthelmintic drugs, making control increasingly difficult. The success of this parasite is, at least in part, due to its extremely high levels of genetic diversity that, in turn, provide a high adaptive capacity. Understanding this genetic diversity is important for many areas of research including anthelmintic resistance, epidemiology, control, drug/vaccine development and molecular diagnostics. In this article, we review the current knowledge of H. contortus genetic diversity and population structure for both field isolates and laboratory strains. We highlight the practical relevance of this knowledge with a particular emphasis on anthelmintic resistance research.

Independent origins of loss-of-function mutations conferring oxamniquine resistance in a Brazilian schistosome population

Molecular surveillance provides a powerful approach to monitoring the resistance status of parasite populations in the field and for understanding resistance evolution. Oxamniquine was used to treat Brazilian schistosomiasis patients (mid-1970s to mid-2000s) and several cases of parasite infections resistant to treatment were recorded. The gene underlying resistance (SmSULT-OR) encodes a sulfotransferase required for intracellular drug activation. Resistance has a recessive basis and occurs when both SmSULT-OR alleles encode for defective proteins. Here we examine SmSULT-OR sequence variation in a natural schistosome population in Brazil ∼40years after the first use of this drug. We sequenced SmSULT-OR from 189 individual miracidia (1-11 per patient) recovered from 49 patients, and tested proteins expressed from putative resistance alleles for their ability to activate oxamniquine. We found nine mutations (four non-synonymous single nucleotide polymorphisms, three non-coding single nucleotide polymorphisms and two indels). Both mutations (p.E142del and p.C35R) identified previously were recovered in this field population. We also found two additional mutations (a splice site variant and 1bp coding insertion) predicted to encode non-functional truncated proteins. Two additional substitutions (p.G206V, p.N215Y) tested had no impact on oxamniquine activation. Three results are of particular interest: (i) we recovered the p.E142del mutation from the field: this same deletion is responsible for resistance in an oxamniquine selected laboratory parasite population; (ii) frequencies of resistance alleles are extremely low (0.27-0.8%), perhaps due to fitness costs associated with carriage of these alleles; (iii) that four independent resistant alleles were found is consistent with the idea that multiple mutations can generate loss-of-function alleles.

Assessment of the nematocidal activity of metallocenyl analogues of monepantel

Ferrocenyl and ruthenocenyl analogues of the nematocidal drug monepantel show organometallic-dependent activity against Haemonchus contortus and Trichostrongylus colubriformis.

The atypical cation-conduction and gating properties of ELIC underscore the marked functional versatility of the pentameric ligand-gated ion-channel fold

The superfamily of pentameric ligand-gated ion channels (pLGICs) is unique among ionotropic receptors in that the same overall structure has evolved to generate multiple members with different combinations of agonist specificities and permeant-ion charge selectivities. However, aside from these differences, pLGICs have been typically regarded as having several invariant functional properties. These include pore blockade by extracellular quaternary-ammonium cations in the micromolar-to-millimolar concentration range (in the case of the cation-selective members), and a gain-of-function phenotype, which manifests as a slower deactivation time course, as a result of mutations that reduce the hydrophobicity of the transmembrane pore lining. Here, we tested this notion on three distantly related cation-selective members of the pLGIC superfamily: the mouse muscle nicotinic acetylcholine receptor (nAChR), and the bacterial GLIC and ELIC channels. Remarkably, we found that, whereas low millimolar concentrations of TMA(+) and TEA(+) block the nAChR and GLIC, neither of these two quaternary-ammonium cations blocks ELIC at such concentrations; instead, both carry measurable inward currents when present as the only cations on the extracellular side. Also, we found that, whereas lidocaine binding speeds up the current-decay time courses of the nAChR and GLIC in the presence of saturating concentrations of agonists, the binding of lidocaine to ELIC slows this time course down. Furthermore, whereas mutations that reduce the hydrophobicity of the side chains at position 9' of the M2 α-helices greatly slowed the deactivation time course of the nAChR and GLIC, these mutations had little effect--or even sped up deactivation--when engineered in ELIC. Our data indicate that caution should be exercised when generalizing results obtained with ELIC to the rest of the pLGICs, but more intriguingly, they hint at the possibility that ELIC is a representative of a novel branch of the superfamily with markedly divergent pore properties despite a well-conserved three-dimensional architecture.

Comparison of biotransformation and efficacy of aminoacetonitrile anthelmintics in vitro

The present in vitro study was designed to test and compare anthelmintic activity, hepatotoxicity, and biotransformation of four selected aminoacetonitrile derivatives (AADs): monepantel (MOP, anthelmintic approved for the treatment), AAD-970, AAD-1154, and AAD-1336. Micro-agar larval development test, MTT test of cytotoxicity, and biotransformation study coupled with Ultra high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) technique were used for this purpose. Larvae of two Haemonchus contortus strains (drug susceptible and multi-drug resistant) and primary cultures of rat and ovine hepatocytes served as model systems. All AADs (including MOP) exhibited significant larvicidal effect in H. contortus susceptible as well as multi-resistant strains, much higher than those of reference anthelmintics thiabendazole and flubendazole. AAD-1154 provides the best results for most tested parameters among all AADs in this study. The cytotoxicity test showed that all AADs can be considered as nontoxic for hepatocytes. In the biotransformation study, Phase I and Phase II metabolites of AADs were identified and schemes of possible metabolic pathways in ovine hepatocytes were proposed. Biotransformation of MOP was much more extensive than biotransformation of other AADs. Based on obtained results, AAD-1154 and AAD-1336 can be considered as promising candidates for further in vivo testing.

Identification of novel splice variants of the voltage- and Ca²⁺-dependent K⁺-channel SLO-1 of Trichuris muris

The anthelmintic cyclooctadepsipeptide emodepside is effective against nematodes showing resistance against established drug classes. Emodepside exerts its nematicidal effects mainly through its validated target, the tetrameric voltage- and calcium-activated potassium channel SLO-1. Two slo-1 genes were described in Trichuris muris. Alternative splicing is known to alter SLO-1 properties. Here, 16 T. muris splice variants for slo-1.1 and three variants for slo-1.2 were identified in addition to previously described variants. Splice variants caused by intron retentions and/or exon exclusions encode varyingly truncated subunits. Depending on the subunit composition, channels might have altered physiological and pharmacological properties including different modulation by calcium and/or voltage or reduced emodepside susceptibility which might lead to emodepside resistance as observed in Caenorhabditis elegans expressing only similarly truncated Slo-1. The comprehensive characterisation of splice variants is a prerequisite for functional analysis and confirmed conservation of remarkable differences found between both slo-1 paralogs in Trichuris suis.

Monepantel irreversibly binds to and opens Haemonchus contortus MPTL-1 and Caenorhabditis elegans ACR-20 receptors

Monepantel is a recently developed anthelmintic with a novel mode of action. Parasitic nematodes with reduced sensitivity to monepantel have led to the identification of MPTL-1, a ligand-gated ion-channel subunit of the parasitic nematode Haemonchus contortus, as a potential drug target. Homomeric MPTL-1 channels reconstituted in Xenopus oocytes are gated by µM concentrations of betaine and mM concentrations of choline. Measurement of reversal potentials indicated that the channel has a similar conductance for Na(+) and K(+) ions and does not permeate Ca(2+). Concentrations of monepantel (amino-acetonitrile derivative [AAD]-2225) >0.1 μM, but not its inactive enantiomer AAD-2224, induced channel opening in an irreversible manner. Currents elicited by monepantel alone were larger than the maximal current amplitudes achieved with betaine or choline, making monepantel a superagonist. Currents elicited by betaine or choline were allosterically potentiated by nM concentrations of monepantel and to a much smaller degree by AAD-2224. We have also reconstituted the Caenorhabditis elegans homomeric ACR-20 receptor in Xenopus oocytes. The acr-20 sequence has higher similarity to mptl-1 than acr-23, the primary target for monepantel mode of action in C. elegans. The ACR-20 channel is gated similarly as MPTL-1. Monepantel, but not AAD-2224, was able to induce channel opening in an irreversible manner at similar concentrations as for MPTL-1. Interestingly, the allosteric potentiation measured in the presence of betaine was much smaller than in MPTL-1 receptors. Together, these results establish the mode of action of monepantel in H. contortus and contribute to our understanding of the mode of action of this anthelmintic.

First report of monepantel Haemonchus contortus resistance on sheep farms in Uruguay

Background

On two farms it was noted that after routine treatment with monepantel, fecal egg counts failed to drop. This was accompanied by lambs mortality due to Haemonchus contortus infection. The aim of this work was to evaluate the efficacy of monepantel to control gastrointestinal nematodes (GIN) in two sheep farms, in Uruguay.

Findings

A Fecal Egg Count Reduction Test (FECRT) was subsequently performed at the Experimental Stations Glencoe of INIA Tacuarembó (Farm 1) and Sheep Unit of INIA La Estanzuela (Farm 2) using the World Association for the Advancement of Veterinary Parasitology guidelines. On Farm 1 the FECRT was performed using 6–8 month old Corriedale or Merino Dohne x Corriedale male lambs naturally infected with GIN. On day 0 pre-treatment, three groups of 15 lambs each were selected, blocked by fecal egg count level (FEC) and randomly assigned to one of the following: Group 0 = untreated control, Group 1 = treated with monepantel (Zolvix®, Novartis Animal Health Inc.) from stock previously purchased; Group 2 = treated with monepantel from stock provided by the supplier, at the recommended dose of 2.5 mg/kg of body weight. Fecal samples were collected directly from the rectum from each lamb on day 0 and on day 9 post-treatment. On Farm 2, the FECRT was conducted on a group of 8 month old male lambs Milchschaff x Finn. At this farm, 10 lambs were randomly allocated to be treated with monepantel (Group 1) and 10 lambs were randomly allocated to remain as untreated control (Group 0) using the same protocols as Farm 1.

On farm 1 the FECR was 0.0% (95% CI = 0.0 – 49.0) and 42.0% (95% CI = 0.0 – 75.0) for Group 1 and Group 2 respectively. For Farm 2, the FECR was 82.1% (95% CI = 36.0 – 99.0). Haemonchus spp was the resistant genus.

Conclusions

Poor effcicacy of monepantel in treating GIN parasites was demonstrated on both farms.