Rapid selection for ivermectin resistance in Haemonchus contortus

Anthelmintic Treatment of Sheep and the Role of Parasites Refugia in a Local Context

Gastrointestinal nematodes in grazing livestock are ubiquitous and can cause severe damage, leading to substantial losses in agricultural yields. It is undeniable that the integrated use of anthelmintics is often an essential component of successful intensive livestock management. However, anthelmintic resistance has been a major challenge for several decades, especially in pasture-based lamb production. Measures are therefore needed to reduce the risk and prevent further spread. In many countries with more extensive lamb production and pronounced resistance problems than in Sweden, the importance of keeping parasites in refugia is emphasised. To ensure that treatment is necessary, the Swedish model is based on deworming certain groups of ewes based on the parasitological results of a faecal examination and then releasing them with their lambs to safe pastures. This is intended to reduce the risk of infection, which ultimately reduces the number of subsequent treatments. Whether this preventive strategy in turn means an increased risk of resistance is debatable. In this review, we explain the importance of parasites in refugia and how they can help delay the development of resistance to anthelmintics. We also discuss how likely it is that our model contributes to an increase in resistance risk and whether there is reason to question whether it is a sustainable strategy in the long term.

Genomic Analysis of Gastrointestinal Parasite Resistance in Akkaraman Sheep

Genome-wide association studies (GWAS) have been used as an effective tool to understand the genetics of complex traits such as gastrointestinal parasite (GIP) resistance. The aim of this study was to understand the genetics of gastrointestinal parasite (nematodes, Moniezia spp., Eimeria spp.) resistance in Akkaraman sheep by performing genomic heritability estimations and conducting GWAS to uncover responsible genomic regions. This is one of the first studies to examine the genetic resistance of Akkaraman sheep to the tapeworm parasite. The samples from 475 animals were genotyped using the Axiom 50K Ovine Genotyping Array. Genomic heritability estimates ranged from 0.00 to 0.34 for parasite resistance traits. This indicates that measured phenotypes have low to moderate heritability estimates. A total of two genome-wide significant SNP associated with TNEM3 and ATRNL1 genes and 10 chromosome-wide significant SNPs related with 10 genes namely NELL1, ST6GALNAC3, HIPK1, SYT1, ALK, ZNF596, TMCO5A, PTH2R, LARGE1, and SCG2 were suggested as candidates for parasite resistance traits. The majority of these candidate genes were involved in several basic biological processes that are essential and important for immune system functions and cellular growth; specifically, inflammatory responses, cellular transport, cell apoptosis, cell differentiation, histone de-acetylation, and endocytosis. These results have implications for animal breeding program studies due to the effect that the genetic background has on parasite resistance, which underlies many productive, health, and wellness-related traits.

Estimation of the impact of three different bioinformatic pipelines on sheep nemabiome analysis

Background

Next-generation sequencing (NGS) has provided an alternative strategy to study the composition of nematode communities with increased resolution and sensitivity. However, the handling and processing of gigabytes worth of amplicon sequence data produced by an NGS platform is still a major hurdle, limiting the use and adoption of faster and more convenient analysis software.

Methods

In total 32 paired, fecal samples from Swedish sheep flocks were cultured and the larvae subsequently harvested subjected to internal transcribed spacer 2 (ITS2) amplicon sequencing using the PacBio platform. Samples were analyzed with three different bioinformatic pipelines, i.e. the DADA2, Mothur and SCATA pipelines, to determine species composition and richness.

Results

For the the major species tested in this study (Haemonchus contortus, Teladorsagia circumcinta and Trichostrongylus colubriformis) neither relative abundances nor species diversity differed significantly between the three pipelines, effectively showing that all three analysis pipelines, although different in their approaches, yield nearly identical outcomes. In addition, the samples analyzed here had especially high frequencies of H. contortus (90–95% across the three pipelines) both before and after sample treatment, followed by T. circumcinta (3.5–4%). This shows that H. contortus is the parasite of primary importance in contemporary Swedish sheep farms struggling with anthelmintic resistance. Finally, although on average a significant reduction in egg counts was achieved post-treatment, no significant shifts in major species relative frequencies occurred, indicating highly rigid community structures at sheep farms where anthelmintic resistance has been reported.

Conclusions

The findings presented here further contribute to the development and application of NGS technology to study nemabiome compositions in sheep, in addition to expanding our understanding about the most recent changes in parasite species abundances from Swedish sheep farms struggling with anthelmintic resistance.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05399-0.

Chemogenomic approach to identifying nematode chemoreceptor drug targets in the entomopathogenic nematode Heterorhabditis bacteriophora

Parasitic nematodes constitute one of the major threats to human health, causing diseases of major socioeconomic importance worldwide. Recent estimates indicate that more than 1 billion people are infected with parasitic nematodes around the world. Current measures to combat parasitic nematode infections include anthelmintic drugs. However, heavy exposure to anthelmintics has selected populations of livestock parasitic nematodes that are no longer susceptible to the drugs, rendering several anthelmintics useless for parasitic nematode control in many areas of the world. The rapidity with which anthelmintic resistance developed in response to these drugs suggests that increasing the selective pressure on human parasitic nematodes will also rapidly generate resistant worm populations. Therefore, development of new anthelmintics is of major importance before resistance becomes widespread in human parasitic nematode populations. G-Protein Coupled Receptors (GPCRs) represent an important target for many pharmacological interventions due to their ubiquitous expression in various cell types. GPCRs contribute to numerous physiological processes, and their ligand binding sites located on cell surfaces make them accessible targets and attractive substrates in terms of druggability. In fact, ∼35 % of Food and Drug Administration (FDA) and European Medicines Agency (EMA) approved drugs target GPCRs and their associated proteins, with over 300 additional drugs targeting GPCRs at the clinical trial stage. Nematode Chemosensory GPCRs (NemChRs) are unique to nematodes, and therefore represent ideal substrates for target-based drug discovery. Here we set out to identify NemChRs that are transcriptionally active inside the host, and to use these NemChRs in a reverse pharmacological screen to impede parasitic development. Our data identified several NemChRs, and we focused on one that was expressed in neuronal cells and exhibited the highest fold change in transcription after host activation. Next, we performed homology modelling and molecular dynamics simulations of this NemChR in order to conduct a virtual screening campaign to identify candidate drug targets which were ranked and selected for experimental testing in bioassays. Taken together, our results identify and characterize a candidate NemChR drug target, and provide a chemogenomic pipeline for identifying nematicide substrates.

Suppression of hyaluronidase reduces invasion and establishment of Haemonchus contortus larvae in sheep

Haemonchus contortus is a hematophagous endoparasite of small ruminants, which is responsible for huge economic losses in livestock sector. Hyaluronidase produced by infective larvae of H. contortus can degrade hyaluronic acid present in the host’s abomasal tissue. Thus, it facilitates larval tissue invasion and early establishment. We herein explored this ability of hyaluronidase in H. contortus, and tested whether hyaluronidase is utilized as a virulence factor by H. contortus while establishing the infection. We first successfully blocked the hyaluronidase gene in L3 larvae by RNA interference (RNAi), which was subsequently confirmed by qPCR, enzymatic activity, and immunohistochemistry assays. Using these larvae we then conducted in vivo and in vitro assays on sheep to assess the effects of hyaluronidase suppression on larval invasion and establishment of infection. The in vivo assay showed a significant drop in worm burden in siRNA treated group in comparison to control group. During in vitro assay we applied an ovine ex vivo model where siRNA treated group of larvae showed significantly reduced invasion of the abomasal tissue explants as compared to control group. These findings indicate that hyaluronidase plays a key role in host’s tissue invasion and larval establishment, and it is used as a virulence factor by H. contortus while establishing the infection. As an invasive virulence molecule, its functional research is thus conducive to the prevention of haemonchosis.

Assessment of the F200Y mutation frequency in the β tubulin gene of Haemonchus contortus following the exposure to a discriminating concentration of thiabendazole in the egg hatch test

The ruminant livestock production sector is under threat due to the infections with gastrointestinal nematode parasites and the subsequent development of anthelmintic resistance. One of most common and pathogenic species in small ruminants is Haemonchus contortus. The ability to control the infections with this and other gastrointestinal nematodes relies heavily on the use of anthelmintic drugs. Although resistance to all major classes of anthelmintics has been shown in H. contortus, the precise mechanism of resistance acquisition is only known for benzimidazoles. F200Y (TAC) is a common point mutation in the isotype 1 β tubulin gene which is associated with an effective increase in the resistance towards benzimidazole drugs. Here, we show the utility of using this mutation as a marker in a droplet digital PCR assay to track how two H. contortus laboratory strains, characterized by different resistance levels, change with respect to this mutation, when subjected to increasing concentrations of thiabendazole. Additionally, we wanted to investigate whether exposure to a discriminating dose of thiabendazole in the egg hatch test resulted in the death of all H. contortus eggs with a susceptible genotype. We found the MHco5 strain to maintain an overall higher frequency of the F200Y mutation (80-100%) over all drug concentrations, whilst a steady, gradual increase from around 30%-60% was observed in the case of the MHco4 strain. This is further supported by the dose-response curves, displaying a much higher tolerance of the MHco5 strain (LD₅₀ = 0.38 μg/ml) in comparison to the MHco4 strain (LD₅₀ = 0.07 μg/ml) to the effects of thiabendazole. All things considered, we show that the F200Y mutation is still a viable and reliable marker for the detection and surveillance of benzimidazole drug resistance in H. contortus in Europe.

Use of bootstrapped, regularised regression to identify factors associated with lamb-derived revenue on commercial sheep farms

The profitability of UK sheep farms is variable with many farms making a net loss. For economic sustainability, farms have to be profitable, therefore it is important to maximise income whilst controlling costs. The most important source of income in sheep flocks is from lamb production but there is little information on factors that explain variability between farms in revenue from lamb sales. The aim of this research was to identify farm, farmer and management factors likely to have the largest, most reliable associations with lamb-derived revenue. From a population of 830 sheep farms, 408 farmers completed an online questionnaire comprising over 300 variables. Total lamb-derived revenue was calculated for each farm using abattoir information including carcass classification. The median flock size was 560 ewes, median land size 265 acres, median revenue per acre from lambs sold was £197 (IQR = 120-296) and median revenue per ewe £95 (IQR = 72-123). A robust analytic approach using regularised (elastic net) regression with bootstrapping was implemented to account for multicollinearity in the data and to reduce the likelihood of model over-fitting. To provide model inference and allow ranking of variables in terms of relevance, covariate stability and coefficient distributions were evaluated. Factors with high stability and relatively large positive associations with revenue per acre were (median effect size (£); 95 % bootstrap probability interval); an increased stocking rate of 0.2 ewe/acre (13; 6-17), fertilizer being used on most of the grazing land (18; 0.1-37), the use of rotational grazing (13; 0.3-34), decreased proportion of ewes with prolapses (4; 0.3-9), separation of lame sheep from the rest of the flock (16; 0.9-37), selecting ewes for culling based on prolapses (20; 0.2-55) and infertility (20; 2-46), conducting body condition scoring of ewes at lambing (28; 3-58), early lactation (21; 1-54) or weaning (25; 2-70), increased farmer education (20; 2-54) and farmers with a positive business attitude (15; 0.2-38). Additional factors with a high stability and relatively large associations with increased revenue per ewe were; never trimming diseased feet of lame ewes (9; 0.8-22) and making use of farm records (5; 0.3-12). This is the first study in animal health epidemiology to use bootstrapped regularised regression to evaluate a wide dataset to provide a ranking of the importance of explanatory covariates. We conclude that the relatively small set of variables identified, with a potentially large influence on lamb-derived revenue, should be considered prime candidates for future intervention studies.

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

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

In vivo selection for Haemonchus contortus resistance to monepantel

Gastrointestinal nematodes significantly affect the ovine industry, and Haemonchus contortus is considered the most pathogenic parasite in tropical regions. This situation is aggravated when the main strategy to control worms fails because of the genetic resistance that parasites acquire against anthelmintics. Aiming to anticipate the events involved in anthelmintic resistance, we induced monepantel resistance in H. contortus by in vivo subdosing of sheep hosts. Four successive passages of a monepantel-susceptible H. contortus isolate in Santa Ines or Ile de France sheep hosts resulted in three monepantel-resistant (efficacy varying from 0 to 58.5%) H. contortus isolates. Sheep hosts were treated from 0.075 mg/kg to the therapeutic dose of 2.5 mg/kg of monepantel in 19-26 rounds of selection for 112-133 weeks. Success in inducing H. contortus resistance to monepantel may have been affected by worm burden and by host-parasite interactions, including a possible effect of the breed of sheep hosts. We conclude that subdosing of sheep, although time-consuming, is an efficient in vivo strategy for the induction of monepantel resistance in H. contortus. The resistant parasites can be used in further studies to elucidate the genetic and biochemical events involved in the acquisition of anthelmintic resistance.

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.

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.

Comparison of two in vitro methods for the detection of ivermectin resistance in Haemonchus contortus in sheep

Gastrointestinal parasitic nematodes in sheep cause severe economic losses. Anthelmintics are the most commonly used drugs for prophylaxis and therapy against parasitic helminths. The problem of drug resistance has developed for all commercially available anthelmintics in several genera and classes of helminths. In vitro and in vivo tests are used to detect anthelmintic resistance. Two in vitro methods (larval migration inhibition test and micromotility test) for the detection of ivermectin (IVM) resistance were compared using IVM-resistant and IVM-susceptible isolates of Haemonchus contortus. The degree of resistance for each test was expressed as a resistance factor (RF). The micromotility test was more sensitive for quantitatively measuring the degree of resistance between susceptible and resistant isolates. The RFs for this test for IVM and eprinomectin ranged from 1.00 to 108.05 and from 3.87 to 32.32, respectively.

Evolution of drug-tolerant nematode populations in response to density reduction

Resistance to xenobiotics remains a pressing issue in parasite treatment and global agriculture. Multiple factors may affect the evolution of resistance, including interactions between life‐history traits and the strength of selection imposed by different drug doses. We experimentally created replicate selection lines of free‐living Caenorhabditis remanei exposed to Ivermectin at high and low doses to assess whether survivorship of lines selected in drug‐treated environments increased, and if this varied with dose. Additionally, we maintained lines where mortality was imposed randomly to control for differences in density between drug treatments and to distinguish between the evolutionary consequences of drug‐treatment versus ecological processes due to changes in density‐dependent feedback. After 10 generations, we exposed all of the selected lines to high‐dose, low‐dose and drug‐free environments to evaluate evolutionary changes in survivorship as well as any costs to adaptation. Both adult and juvenile survival were measured to explore relationships between life‐history stage, selection regime and survival. Intriguingly, both drug‐selected and random‐mortality lines showed an increase in survivorship when challenged with Ivermectin; the magnitude of this increase varied with the intensity of selection and life‐history stage. Our results suggest that interactions between density‐dependent processes and life history may mediate evolved changes in susceptibility to control measures.

Failure of ivermectin treatment in Haemonchus contortus infected-Swedish sheep flocks

Control of gastrointestinal nematodes of veterinary importance in Swedish sheep flocks is primarily based on recurring strategic anthelmintic treatments after detection of strongyle eggs in faeces samples. This study reports reduced efficacy of ivermectin (IVM) against Haemonchus contortus in naturally infected Swedish sheep flocks. Faecal egg count reduction tests (FECRT) and examinations of H. contortus-specific DNA with qPCR on larval cultures were applied to samples from 11 sheep flocks (A-K) in south-eastern Sweden between 2013 and 2014. Four of these flocks (D, E, J and K) had been in direct contact with flock H, where IVM treatment failure was first observed in October 2013, some years after the introduction of imported dairy sheep. In flock H, the resistance status to IVM was also confirmed by a larval developmental test. IVM concentrations 15-20 times higher than for susceptible strains of H. contortus were required to kill the larvae. In addition, faeces samples were obtained from 37 other Swedish sheep farms where the treatment response to IVM was screened initially in six animals using FEC and qPCR 7-10days after administration of IVM. Six farms where the majority was identified with this pre-screening test (B, C, F, G, I and K), were also investigated in more detail with FECRT as described above after the animals had been allocated to groups and treated orally or injected with a minimum of 0.2mg IVM, 0.2mg doramectin (farm F) or 0.2mg moxidectin per kg body weight (farm A and B). Four flocks (farm A, D, G and I) were also treated with 4.8mg albendazole and/or 7.5mg levamisole per kg body weight. Pre-treatment faeces samples were collected from 15 animals on the same day as deworming. Post-treatment samples were collected 7-10days later, whenever possible from 10 animals per group with the highest pre-treatment egg counts. Based on FECRT results, IVM efficacy to H. contortus was reduced on six farms (C, D, E, G, H and I) out of 11 farms studied with FECRT. This is the first report of IVM treatment failure in H. contortus-infected sheep in Sweden.

Protein profiling of Haemonchus contortus found in sheep of Kashmir valley

Economic losses due to helminth parasites in sheep throughout the world are considerable. Haemonchus contortus is a blood sucking intestinal helminth that lives in the abomasum of small ruminants worldwide. This parasite can be devastating to producers as it causes decreased production levels due to clinical signs such as anaemia, edema and death. For isolation of the proteins of the parasite, a well defined methodology was adopted. The abomasae of sheep in which this parasite resides were collected from abattoirs of various districts and were then carried to laboratory for screening. In case of collection sites falling in far areas, the organs were screened on spot. The parasites were collected in normal saline, washed and stored in 0.05 M PBS with pH of 7.4 at 0 °C. After refrigeration, frozen nematodes were thawed, homogenized and centrifuged at 1,000-15,000 rpm for 15 min. The supernatant was thus collected as a protein mixture and stored at -20 °C. Protein concentration of the samples was estimated by Lowry method. The samples were then analyzed through PAGE and then through SDS-PAGE. Protein estimation of the samples was estimated to be 4.2 mg/ml. The processed parasite samples were then subjected to PAGE and SDS-PAGE to determine the presence of the proteins. It showed high concentration of proteins in its whole protein profile. The proteins were seen as continuous bands intermixing with each other in PAGE analysis. The present study revealed two bands of molecular weights-55 and 33 kDa in PAGE analysis. The proteins when analyzed through SDS-PAGE were mostly found in the range of 25-70 kDa. The SDS-PAGE analysis showed four prominent bands. These bands were of the molecular weights of 66, 40, 33 and 26 kDa. The present work was a challenging one since only a single study was conducted in this region on this aspect and thus obviously was a big task to peep into the field where scanty input was available.

Selection and characterisation of monepantel resistance in Teladorsagia circumcincta isolates

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Monepantel resistance was selected in three isolates of Teladorsagia circumcincta.

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Selection for resistance was generated through rounds of sub-optimal dosing.

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Selection for monepantel resistance occurred within 9–13 generations.

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Selection for resistance impacted on the life history traits of the isolates.

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Increased egg output and shortened time to patency was seen in a resistant isolate.

Monepantel (MPTL) is one of two new anthelmintic compounds introduced onto the sheep market to control gastro-intestinal nematodes. Resistance to this compound is rare but has been reported. In order to preserve the efficacy of this and other anthelmintics, it is essential to understand both (a) the mechanisms involved in the selection of resistance and (b) how the parasites evolve to deal with these compounds.

To address these questions three MPTL-resistant Teladorsagia circumcincta isolates (MTci2-11, MTci5-13 and MTci7-12) have been artificially selected in vivo from phenotypically characterised parent isolates (MTci2, MTci5, MTci7 respectively). The selection process involved collecting and culturing eggs from surviving worms from sheep administered sub-optimal dosages of MPTL (Zolvix®) to provide infective larvae to infect further sheep until resistant isolates were generated (between 9 and 13 rounds of selection). A controlled efficacy test was conducted using the original parental isolates and the newly generated MPTL resistant isolates (n = 5 per group). Selected isolates were assessed both under anthelmintic stress (Zolvix®, 2.5 mg/kg bodyweight; MTci-MPTL) and at rest (untreated, MTci-CON). A number of life-history traits were assessed, namely, worm establishment rates, time to patency, faecal egg output, body length of adults and eggs in utero. The estimated resistance status of the selected isolates was confirmed with 48%, 28% and 9% reductions in worm burden at 7-days post Zolvix® administration for MTci2-11-MPTL, MTci5-13-MPTL and MTci7-12-MPTL, respectively, compared with untreated controls. One of the selected isolates MTci7-12-CON showed significantly greater total worm burden (p = 0.025), greater establishment rate (p = 0.033), decreased time to patency (p = 0.048), higher cumulative egg outputs (p = 0.002) compared with its parental derivative MTci7. The trial results suggest that anthelmintic selection in T. circumcincta, albeit under experimental conditions, can select for more prolific/fecund and quicker maturing populations. These data provide an insight into how parasites evolve in response to anthelmintic pressure.

Wild deer as potential vectors of anthelmintic-resistant abomasal nematodes between cattle and sheep farms

Gastrointestinal (GI) nematodes are among the most important causes of production loss in farmed ruminants, and anthelmintic resistance is emerging globally. We hypothesized that wild deer could potentially act as reservoirs of anthelmintic-resistant GI nematodes between livestock farms. Adult abomasal nematodes and faecal samples were collected from fallow (n = 24), red (n = 14) and roe deer (n = 10) from venison farms and areas of extensive or intensive livestock farming. Principal components analysis of abomasal nematode species composition revealed differences between wild roe deer grazing in the areas of intensive livestock farming, and fallow and red deer in all environments. Alleles for benzimidazole (BZ) resistance were identified in β-tubulin of Haemonchus contortus of roe deer and phenotypic resistance confirmed in vitro by an egg hatch test (EC₅₀ = 0.149 µg ml⁻¹ ± 0.13 µg ml⁻¹) on H. contortus eggs from experimentally infected sheep. This BZ-resistant H. contortus isolate also infected a calf experimentally. We present the first account of in vitro BZ resistance in wild roe deer, but further experiments should firmly establish the presence of phenotypic BZ resistance in vivo. Comprehensive in-field studies should assess whether nematode cross-transmission between deer and livestock occurs and contributes, in any way, to the development of resistance on livestock farms.

Detection of ivermectin resistance by a larval development test--back to the past or a step forward?

The aim of this study was to evaluate the potential of the larval development test for the detection of ivermectin (IVM) resistance in Haemonchus contortus of sheep. Single infections with 5000 third-stage larvae of five resistant and two susceptible isolates of H. contortus were given to sheep. Fecal samples were collected four times during patency, and the micro-agar version of the larval development test (MALDT) was performed. Three macrocyclic lactone drugs (IVM, eprinomectin and IVM aglycone) were tested. The results of the tests are presented as LC50 and LC99 values. The MALDT was well able to distinguish between susceptible and resistant isolates. Resistance factors (RF) for the LC99 values were generally higher than those obtained by comparing LC50 values. The highly resistant isolates were readily distinguishable from the susceptible isolates, particularly when using IVM aglycone and eprinomectin, with RFs above 20.

Caenorhabditis elegans: modest increase of susceptibility to ivermectin in individual P-glycoprotein loss-of-function strains

P-glycoproteins (Pgps) are members of the ABC transporter superfamily and are involved in detoxification mechanisms of single- and multicellular organisms. Their importance for survival of organisms in the presence of harmful drug concentrations has been widely studied in cancer cells but Pgp-dependent drug resistance of parasites has also been demonstrated. Ivermectin (IVM), a widely used anthelmintic in human and veterinary medicine, is a known substrate at least of mammalian Pgps and resistance against IVM is proposed to be associated with Pgps. The consequences of loss of Pgp function for the development of the model nematode Caenorhabditis elegans were analysed in the presence of IVM. Either strains missing only a single Pgp were used or Pgp activity generally was inhibited using verapamil (VPL). Loss-of-function of individual Pgp resulted in a statistically significant increase in IVM susceptibility in terms of impaired development with decreases in EC₅₀ values between 1.5- and 4.3-fold. Absence of seven Pgps resulted in a higher impact on IVM susceptibility of C. elegans since it resulted in EC₅₀ values decreased by 2.4- to 4.3-fold. This increase in IVM susceptibility was even more pronounced than that observed when Pgp function was blocked in general by VPL (approximately 2.5-fold). This study demonstrates clearly that Pgps are of importance for IVM detoxification in the model organism C. elegans and that some Pgps obviously have a higher impact than others.

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.

Is the micro-agar larval development test reliable enough to detect ivermectin resistance?

To aid the diagnosis of anthelmintic resistance, a range of in vivo and in vitro techniques have been developed. Amongst in vitro techniques, the larval development test is the most widely employed. Six lambs were infected with susceptible (three) and ivermectin-resistant (three) isolates of Haemonchus contortus. The micro-agar larval development test (MALDT) was able to easily distinguish between susceptible and resistant isolates. Different proportions of resistant and susceptible eggs were subsequently incubated, i.e. development to the third larval stage occurred only in the resistant isolate. The percentage of resistant eggs ranged from 2 to 20.0 % of all eggs in the wells. In all cases, the MALDT was able to detect the presence of a minimum of 10 % of resistant worms amongst a susceptible background population. The probability was approximately 87 % of positively diagnosing a proportion of resistant worms of only 2-4 % within the population.

Introgression of ivermectin resistance genes into a susceptible Haemonchus contortus strain by multiple backcrossing

Anthelmintic drug resistance in livestock parasites is already widespread and in recent years there has been an increasing level of anthelmintic drug selection pressure applied to parasitic nematode populations in humans leading to concerns regarding the emergence of resistance. However, most parasitic nematodes, particularly those of humans, are difficult experimental subjects making mechanistic studies of drug resistance extremely difficult. The small ruminant parasitic nematode Haemonchus contortus is a more amenable model system to study many aspects of parasite biology and investigate the basic mechanisms and genetics of anthelmintic drug resistance. Here we report the successful introgression of ivermectin resistance genes from two independent ivermectin resistant strains, MHco4(WRS) and MHco10(CAVR), into the susceptible genome reference strain MHco3(ISE) using a backcrossing approach. A panel of microsatellite markers were used to monitor the procedure. We demonstrated that after four rounds of backcrossing, worms that were phenotypically resistant to ivermectin had a similar genetic background to the susceptible reference strain based on the bulk genotyping with 18 microsatellite loci and individual genotyping with a sub-panel of 9 microsatellite loci. In addition, a single marker, Hcms8a20, showed evidence of genetic linkage to an ivermectin resistance-conferring locus providing a starting point for more detailed studies of this genomic region to identify the causal mutation(s). This work presents a novel genetic approach to study anthelmintic resistance and provides a “proof-of-concept” of the use of forward genetics in an important model strongylid parasite of relevance to human hookworms. The resulting strains provide valuable resources for candidate gene studies, whole genome approaches and for further genetic analysis to identify ivermectin resistance loci.

The use of drugs (anthelmintics) to control nematode parasites (roundworms) is common in both humans and animals. This has led to the widespread development of drug resistance in livestock parasites and serious concerns regarding its emergence in human parasites. Haemonchus contortus is a parasitic nematode of sheep that has a high propensity to develop resistance and is the most widely used model system in which to study anthelmintic drug resistance. Ivermectin is an extremely important drug for parasite control in both humans and animals. Here, we report a novel approach using genetic crossing to transfer a region of the H. contortus genome containing ivermectin resistance genes from resistant strains into a susceptible strain. During our backcrossing approach, we have identified a genetic marker showing evidence of genetic linkage to ivermectin resistance. The susceptible strain we have used is currently having its complete genome sequenced making the information and strains generated here extremely valuable for the identification of ivermectin resistance genes. This work represents an important proof of concept for using genetic approaches to identify genomic regions containing drug resistant genes in parasitic nematodes.

ABC transporter efflux pumps: a defense mechanism against ivermectin in Rhipicephalus (Boophilus) microplus

ATP-binding cassette (ABC) transporters are efflux transporters found in all organisms. These proteins are responsible for pumping xenobiotic and endogenous metabolites through extra- and intracellular membranes, thereby reducing cellular concentrations of toxic compounds. ABC transporters have been associated with drug resistance in several nematodes and parasitic arthropods. Here, the ability of ABC transporter inhibitors to enhance ivermectin (IVM) sensitivity was tested in larvae and adult females of Rhipicephalus (Boophilus) microplus. Larvae of susceptible and IVM-resistant tick populations were pre-exposed to sub-lethal doses of the ABC transporter inhibitors Cyclosporin A (CsA) and MK571, and subsequently treated with IVM in a Larval Packet Test (LPT). ABC transporter inhibition by both drugs significantly reduced the concentration for 50% lethality (LC(50)) values of four IVM-resistant populations but IVM sensitivity of a susceptible population remained unchanged. IVM sensitivity in adults was assessed through an artificial feeding assay. The addition of CsA to a blood meal substantially affected IVM toxicity in adult female ticks from a resistant population by reducing oviposition and egg viability, although it did not alter IVM toxicity in susceptible females. Three partial nucleotide sequences with similarity to ABC transporters were retrieved from the DFCI Boophilus microplus Gene Index (http://compbio.dfci.harvard.edu/index.html). Their transcriptional levels in the midgut of resistant and susceptible females were determined by quantitative PCR, showing that one of these sequences was significantly up-regulated in IVM-resistant females and suggesting its participation in IVM detoxification. We believe this work reports the first known evidence for the participation of ABC transporters in IVM resistance in R. microplus.

P-glycoproteins of Haemonchus contortus: development of real-time PCR assays for gene expression studies

P-glycoproteins (P-gps) are proteins that function as efflux pumps, removing lipophilic xenobiotic compounds from cells. There is evidence that P-gps play a role in the resistance of parasitic nematodes to anthelmintic drugs such as benzimidazoles and macrocyclic lactones. As anthelmintic resistance becomes more common, it is important to identify candidate resistance genes with the aim of understanding the molecular basis of resistance, and of developing assays to detect these resistance-associated changes. We identified several sequences from the genome of the parasite Haemonchus contortus with convincing homology to the known P-gp coding genes of the model nematode Caenorhabditis elegans. Nine of these sequences were successfully amplified by polymerase chain reaction (PCR) and shown to be most similar to the C. elegans sequences for pgp-1, pgp-2, pgp-3, pgp-4, pgp-9, pgp-10, pgp-11, pgp-12 and pgp-14. These partial P-gp sequences from H. contortus were used to design and optimize a quantitative real-time PCR assay to investigate potential changes in the expression levels of P-gp transcripts associated with drug resistance. No significant changes in P-gp mRNA expression levels were found in a rapidly selected ivermectin-resistant parasite isolate compared to its drug-sensitive parent, but the assay has the potential to be used on other isolates in the future to further investigate resistance-associated changes in P-gp gene expression.

Importance of ivermectin to human onchocerciasis: past, present, and the future

Ivermectin (registered for human use as Mectizan^®) was donated by Merck & Co Inc in 1987 for the treatment and control of human onchocerciasis ("river blindness"). This philanthropic gesture has had a remarkable effect in reducing the incidence and prevalence of this serious ocular and dermatological disease, while changing health system support for millions of people worldwide. Over 800 million doses have been given to more than 80 million people for onchocerciasis during the past 23 years. As a result, onchocerciasis has been significantly reduced in more than 25 countries, transmission has been interrupted in foci in at least 10 countries, and the disease is no longer seen in children in many formerly endemic foci. Recent communications have suggested that the drug's efficacy as the major therapeutic agent for these control and elimination programs may be threatened, but alternative interpretations for suboptimal response/resistance suggest otherwise. Current research needs and control methods by which the public health community in endemic countries may respond to resistance, should it occur in their area, are discussed, along with the continuing importance of this anthelmintic as the mainstay in onchocerciasis control programs.

Control of important helminthic infections vaccine development as part of the solution

Among the tools available for the control of helminth infections, chemotherapy has come to totally dominate the field. In the veterinary field, development of drug resistance has appeared but this is not (yet) a problem in the control of human diseases. Although there is no vaccine commercially available for any human parasitic infection yet, recent progress in vaccine development is making this a future possibility for several diseases. The goal of chemotherapy is to alleviate infection and morbidity in the definitive host, or reduce transmission, while the effect of available vaccine candidates would mainly be to influence transmission through targeting the intermediate or reservoir host, when the infection is zoonotic. Apart from this general scheme, there are also vaccine candidates targeting the parasites in the definitive host, in particular the early developmental stages, which should reduce the risk of drug failure. Since the biological targets in most cases are different, vaccination would be synergistic with drug therapy. This review covers diseases caused by helminthes in both humans and animals and includes examples of diseases caused by cestodes, nematodes and trematodes. The focus is on infections for which vaccine development has been undertaken for a long time, resulting in products that could realistically become integrated into control strategies in the near future.

Phylogenomics of ligand-gated ion channels predicts monepantel effect

The recently launched veterinary anthelmintic drench for sheep (Novartis Animal Health Inc., Switzerland) containing the nematocide monepantel represents a new class of anthelmintics: the amino-acetonitrile derivatives (AADs), much needed in view of widespread resistance to the classical drugs. Recently, it was shown that the ACR-23 protein in Caenorhabditis elegans and a homologous protein, MPTL-1 in Haemonchus contortus, are potential targets for AAD action. Both proteins belong to the DEG-3 subfamily of acetylcholine receptors, which are thought to be nematode-specific, and different from those targeted by the imidazothiazoles (e.g. levamisole). Here we provide further evidence that Cel-ACR-23 and Hco-MPTL-1-like subunits are involved in the monepantel-sensitive phenotype. We performed comparative genomics of ligand-gated ion channel genes from several nematodes and subsequently assessed their sensitivity to anthelmintics. The nematode species in the Caenorhabditis genus, equipped with ACR-23/MPTL-1-like receptor subunits, are sensitive to monepantel (EC₅₀<1.25 µM), whereas the related nematodes Pristionchus pacificus and Strongyloides ratti, which lack an ACR-23/MPTL-1 homolog, are insensitive (EC₅₀>43 µM). Genome sequence information has long been used to identify putative targets for therapeutic intervention. We show how comparative genomics can be applied to predict drug sensitivity when molecular targets of a compound are known or suspected.

Increased use of anthelmintics has contributed to the emergence of drug-resistant nematodes, causing serious problems for more than one billion sheep worldwide. The last class of compounds indicated for livestock was introduced 28 years ago. Recently, however, Novartis AH developed a new anthelmintic active against drug-resistant nematodes of sheep, the amino-acetonitrile derivative (AAD) monepantel. We have previously indirectly shown that the AADs have a novel mode of action involving acetylcholine receptor subunits: the ACR-23 protein in Caenorhabditis elegans and a homologous protein, MPTL-1 in Haemonchus contortus. To better understand the mode of action of the AADs, we performed comparative genomics of all ligand-gated ion channel genes from a range of organisms, including members from all nematode clades. We confirmed that MPTL-1 belongs to a unique, nematode-specific sub-family of receptor subunits. We also found that some nematode species lack ACR-23/MPTL-1 and predicted them to be monepantel insensitive. We challenged this hypothesis in a panel of drug tests: several species of Caenorhabditis nematodes equipped with ACR-23/MPTL-1-like receptor subunits were found susceptible to monepantel, whereas Pristionchus pacificus, closely related to these worms but lacking an ACR-23/MPTL-1 homolog, was tolerant. The parasitic nematode Strongyloides ratti, which has only a remote homolog of DES-2 and ACR-23/MPTL-1, was also tolerant to monepantel. This confirms our prediction and highlights how comparative genomic data can be used to predict a drug effect.

Anthelminthic resistance of Haemonchus contortus in small ruminants in Switzerland and Southern Germany

Two goat and two sheep flocks have been found to be suspicious of a clinically evident reduced anthelmintic efficacy, i.e. lacking improvement of gastrointestinal disorders, insufficient weight gain and continuing inappetence after anthelmintic treatments. In order to conduct an appropriate evaluation of the efficacy the following trials were performed: the faecal egg count reduction test on the studied goats of the two herds revealed a reduction of the egg-excretion after the eprinomectin-treatment (1 mg/kg BW, pour-on) of 17.4% and 27.5%, respectively, which clearly confirms the occurrence of anthelmintic resistance against eprinomectin in these two herds. The alternatively administered moxidectin-treatment (1 mg/kg BW, pour-on) of one flock resulted in a 99.1% faecal egg count reduction. In both sheep flocks, 30 randomly selected sheep were divided in three groups and each group was treated with a different anthelmintic, according to the instructions for use. The faecal egg count reductions for the various groups treated orally with benzimidazoles were 70.8% and 55.3% (albendazole), 52.4% (fenbendazole) and 47.3% (oxfendazole). The two moxidectin-treated groups (0.2 mg/kg BW, oral) showed an EpG-reduction of 100% and 44.3%, respectively, thus also demonstrating resistance against macrocyclic lactones. Pre- and post-treatment faecal larval cultures revealed Haemonchus contortus as the predominant resistant species.

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.

Population genetics of anthelmintic resistance in parasitic nematodes

A key aim of anthelmintic resistance research is to identify molecular markers that could form the basis of sensitive and accurate diagnostic tests. These would provide powerful tools to study the origin and spread of anthelmintic resistance in the field and to monitor strategies aimed at preventing and managing resistance. Molecular markers could also form the basis of routine diagnostic tests for use in surveillance and clinical veterinary practice. Much of the research conducted to date has focused on the investigation of possible associations of particular candidate genes with the resistance phenotype. In the future, as full parasite genome sequences become available, there will be an opportunity to apply genome-wide approaches to identify the genetic loci that underlie anthelmintic resistance. Both the interpretation of candidate gene studies and the application of genome-wide approaches require a good understanding of the genetics and population biology of the relevant parasites as well as knowledge of how resistance mutations arise and are selected in populations. Unfortunately, much of this information is lacking for parasitic nematodes. This review deals with a number of aspects of genetics and population biology that are pertinent to these issues. We discuss the possible origins of resistance mutations and the likely effects of subsequent selection on the genetic variation at the resistance-conferring locus. We also review some of the experimental approaches that have been used to test associations between candidate genes and anthelmintic resistance phenotypes and highlight implications for future genome-wide studies.

A purified Bacillus thuringiensis crystal protein with therapeutic activity against the hookworm parasite Ancylostoma ceylanicum

Crystal (Cry) proteins produced by the soil bacterium Bacillus thuringiensis (Bt) are harmless to vertebrates, but they are highly toxic to insects and nematodes. Their value in controlling insects that destroy crops and transmit human diseases is well established. Although it has recently been demonstrated that a few individual Bt Cry proteins, such as Cry5B, are toxic to a wide range of free-living nematodes, the potential activity of purified Cry proteins against parasitic nematodes remains largely unknown. We report here studies aimed at characterizing in vitro and in vivo anthelminthic activities of purified recombinant Cry5B against the hookworm parasite Ancylostoma ceylanicum, a bloodfeeding gastrointestinal nematode for which humans are permissive hosts. By using in vitro larval development assays, Cry5B was found to be highly toxic to early stage hookworm larvae. Exposure of adult A. ceylanicum to Cry5B was also associated with significant toxicity, including a substantial reduction in egg excretion by adult female worms. To demonstrate therapeutic efficacy in vivo, hamsters infected with A. ceylanicum were treated with three daily oral doses of purified Cry5B, the benzimidazole anthelminthic mebendazole, or buffer. Compared with control (buffer-treated) animals, infected hamsters that received Cry5B showed statistically significant improvements in growth and blood hemoglobin levels as well as reduced worm burdens that were comparable to the mebendazole-treated animals. These data demonstrate that Cry5B is highly active in vitro and in vivo against a globally significant nematode parasite and that Cry5B warrants further clinical development for human and veterinary use.