Investigation of the mechanism of levamisole resistance trichostrongylid nematodes of sheep

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.

Natural Products Are a Promising Source for Anthelmintic Drug Discovery

Parasitic nematodes infect almost all forms of life. In the human context, parasites are one of the major causative factors for physical and intellectual growth retardation in the developing world. In the agricultural setting, parasites have a great economic impact through a reduction in livestock performance or control cost. The main method of controlling these devastating conditions is the use of anthelmintic drugs. Unfortunately, there are only a few anthelmintic drug classes available in the market and significant resistance has developed in most of the parasitic species of livestock. Therefore, development of new anthelmintics with different modes of action is critical for sustainable parasitic control in the future. The drug development pipeline is broadly limited to two types of molecules, namely synthetic compounds and natural plant products. Compared to synthetic compounds, natural products are highly diverse, and many have historically proven valuable in folk medicine to treat various gastrointestinal ailments. This review focus on the use of traditional knowledge-based plant extracts in the development of new therapeutic leads, the approaches used as screening techniques, and common bottlenecks and opportunities in plant-based anthelmintic drug discovery.

Using droplet digital PCR for the detection of hco-acr-8b levamisole resistance marker in H. contortus

The nematode Haemonchus contortus is one of the most prevalent and pathogenic parasites in small ruminants. Although usually controlled using anthelmintics, the development of drug resistance by the parasite has become a major issue in livestock production. While the molecular detection of benzimidazole resistance in H. contortus is well developed, the molecular tools and protocols are far less advanced for the detection of levamisole resistance. The hco-acr-8 gene encodes a critical acetylcholine susceptible subunit that confers levamisole-sensitivity to the receptor. Here, we report the development of a droplet digital PCR assay as a molecular tool to detect a 63 bp deletion in the hco-acr-8 that has been previously associated with levamisole resistance. Sanger sequencing of single adult H. contortus yielded 56 high-quality consensus sequences surrounding the region containing the deletion. Based on the sequencing data, new primers and probes were designed and validated with a novel droplet digital PCR assay for the quantification of the deletion containing “resistant” allele in genomic DNA samples. Single adult worms from six phenotypically described isolates (n = 60) and from two Swedish sheep farms (n = 30) where levamisole was effective were tested. Even though a significant difference in genotype frequencies between the resistant and susceptible reference isolates was found (p = 0.01), the homozygous “resistant” genotype was observed to be abundantly present in both the susceptible isolates as well as in some Swedish H. contortus samples. Furthermore, field larval culture samples, collected pre- (n = 7) and post- (n = 6) levamisole treatment on seven Swedish sheep farms where levamisole was fully efficacious according to Fecal Egg Count Reduction Test results, were tested to evaluate the frequency of the “resistant” allele in each. Frequencies of the deletion ranged from 35 to 80% in the pre-treatment samples, whereas no amplifiable H. contortus genomic DNA was detected in the post-treatment samples. Together, these data reveal relatively high frequencies of the 63 bp deletion in the hco-acr-8 both on individual H. contortus and field larval culture scales, and cast doubt on the utility of the deletion in the hco-acr-8 as a molecular marker for levamisole resistance detection on sheep farms.

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Acr8b – levamisole resistance marker investigated in single worms and larval cultures.

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Individuals homozygous for acr8b found more commonly, even in susceptible isolates.

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Levamisole treatment efficacy was unaffected by increased acr8b frequencies in larvae.

Nicotine-sensitive acetylcholine receptors are relevant pharmacological targets for the control of multidrug resistant parasitic nematodes

The control of parasitic nematodes impacting animal health relies on the use of broad spectrum anthelmintics. However, intensive use of these drugs has led to the selection of resistant parasites in livestock industry. In that respect, there is currently an urgent need for novel compounds able to control resistant parasites. Nicotine has also historically been used as a de-wormer but was removed from the market when modern anthelmintics became available. The pharmacological target of nicotine has been identified in nematodes as acetylcholine-gated ion channels. Nicotinic-sensitive acetylcholine receptors (N-AChRs) therefore represent validated pharmacological targets that remain largely under-exploited. In the present study, using an automated larval migration assay (ALMA), we report that nicotinic derivatives efficiently paralyzed a multiple (benzimidazoles/levamisole/pyrantel/ivermectin) resistant field isolate of H. contortus. Using C. elegans as a model we confirmed that N-AChRs are preferential targets for nornicotine and anabasine. Functional expression of the homomeric N-AChR from C. elegans and the distantly related horse parasite Parascaris equorum in Xenopus oocytes highlighted some striking differences in their respective pharmacological properties towards nicotine derivative sensitivity. This work validates the exploitation of the nicotine receptors of parasitic nematodes as targets for the development of resistance-breaking compounds.

Gaining Insights Into the Pharmacology of Anthelmintics Using Haemonchus contortus as a Model Nematode

Progress made in understanding pharmacokinetic behaviour and pharmacodynamic mechanisms of drug action/resistance has allowed deep insights into the pharmacology of the main chemical classes, including some of the few recently discovered anthelmintics. The integration of pharmaco-parasitological research approaches has contributed considerably to the optimization of drug activity, which is relevant to preserve existing and novel active compounds for parasite control in livestock. A remarkable amount of pharmacology-based knowledge has been generated using the sheep abomasal nematode Haemonchus contortus as a model. Relevant fundamental information on the relationship among drug influx/efflux balance (accumulation), biotransformation/detoxification and pharmacological effects in parasitic nematodes for the most traditional anthelmintic chemical families has been obtained by exploiting the advantages of working with H. contortus under in vitro, ex vivo and in vivo experimental conditions. The scientific contributions to the pharmacology of anthelmintic drugs based on the use of H. contortus as a model nematode are summarized in the present chapter.

Anthelmintic Resistance in Haemonchus contortus: History, Mechanisms and Diagnosis

Haemonchus contortus has shown a great ability to develop resistance to anthelmintic drugs. In many instances, resistance has appeared less than 10years after the introduction of a new drug class. Field populations of this species now show resistance to all major anthelmintic drug classes, including benzimidazoles (BZs), imidazothiazoles and macrocyclic lactones. In addition, resistance to the recently introduced amino-acetonitrile derivative class (monepantel) has already been reported. The existence of field populations showing resistance to all three major drug classes, and the early appearance of resistance to monepantel, threatens the sustainability of sheep and goat production systems worldwide. This chapter reviews the history of the development of resistance to the various anthelmintics in H. contortus and examines the mechanisms utilized by this species to resist the effects of these drugs. Some of these mechanisms are well understood, particularly for BZ drugs, while our knowledge and understanding of others are increasing. Finally, we summarize methods available for the diagnosis of resistance. While such diagnosis currently relies largely on the faecal egg count reduction test, which suffers from issues of expense and sensitivity, we describe past and current efforts to utilize cheaper and less laborious phenotypic assays with free-living life stages, and then describe progress on the development of molecular assays to provide sensitive resistance-detection tests.

Functional Characterization of a Novel Class of Morantel-Sensitive Acetylcholine Receptors in Nematodes

Acetylcholine receptors are pentameric ligand-gated channels involved in excitatory neuro-transmission in both vertebrates and invertebrates. In nematodes, they represent major targets for cholinergic agonist or antagonist anthelmintic drugs. Despite the large diversity of acetylcholine-receptor subunit genes present in nematodes, only a few receptor subtypes have been characterized so far. Interestingly, parasitic nematodes affecting human or animal health possess two closely related members of this gene family, acr-26 and acr-27 that are essentially absent in free-living or plant parasitic species. Using the pathogenic parasitic nematode of ruminants, Haemonchus contortus, as a model, we found that Hco-ACR-26 and Hco-ACR-27 are co-expressed in body muscle cells. We demonstrated that co-expression of Hco-ACR-26 and Hco-ACR-27 in Xenopus laevis oocytes led to the functional expression of an acetylcholine-receptor highly sensitive to the anthelmintics morantel and pyrantel. Importantly we also reported that ACR-26 and ACR-27, from the distantly related parasitic nematode of horses, Parascaris equorum, also formed a functional acetylcholine-receptor highly sensitive to these two drugs. In Caenorhabditis elegans, a free-living model nematode, we demonstrated that heterologous expression of the H. contortus and P. equorum receptors drastically increased its sensitivity to morantel and pyrantel, mirroring the pharmacological properties observed in Xenopus oocytes. Our results are the first to describe significant molecular determinants of a novel class of nematode body wall muscle AChR.

Development of the larval migration inhibition test for comparative analysis of ivermectin sensitivity in cyathostomin populations

Cyathostomins are the most prevalent parasitic pathogens of equids worldwide. These nematodes have been controlled using broad-spectrum anthelmintics; however, cyathostomin resistance to each anthelmintic class has been reported and populations insensitive to more than one class are relatively commonplace. The faecal egg count reduction test (FECRT) is considered the most suitable method for screening anthelmintic sensitivity in horses, but is subject to variation and is relatively time-consuming to perform. Here, we describe a larval migration inhibition test (LMIT) to assess ivermectin (IVM) sensitivity in cyathostomin populations. This test measures the paralysing effect of IVM on the ability of third stage larvae (L3) to migrate through a pore mesh. When L3 from a single faecal sample were examined on multiple occasions, variation in migration was observed: this was associated with the length of time that the L3 had been stored before testing but the association was not significant. Half maximal effective concentration (EC50) values were then obtained for cyathostomin L3 from six populations of horses or donkeys that showed varying sensitivity to IVM in previous FECRTs. Larvae from populations indicated as IVM resistant by FECRT displayed significantly higher EC50 values in the LMIT than L3 from populations classified as IVM sensitive or L3 from populations that had not been previously exposed to IVM or had limited prior exposure. The analysis also showed that EC50 values obtained using L3 from animals in which IVM faecal egg count reduction (FECR) levels had been recorded as <95% were significantly higher than EC50 values obtained using L3 from animals for which FECR was measured as >95%. For one of the populations, time that had elapsed since IVM administration had an effect on the EC50 value obtained, with a longer time since treatment associated with lower EC50 values. These results indicate that the LMIT has value in discriminating IVM sensitivity amongst cyathostomin populations, but several factors were identified that need to be taken into account when executing the test and interpreting the derived data.

The in vitro assay profile of macrocyclic lactone resistance in three species of sheep trichostrongyloids

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Ivermectin and its two components contribute to action and resistance.

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Moxidectin tended to have lower resistance ratios than ivermectin in the LDA.

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Moxidectin was the most potent inhibitor of migration in susceptible H. contortus.

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LMIA performs better in detecting resistance to MOX than LDA.

Anthelmintic resistance has emerged as an important problem in animal industries. Understanding resistance mechanisms, especially against macrocyclic lactones (MLs), is the first step in developing better diagnostic tools. Effects of several MLs including ivermectins and milbemycins were tested using two well established in vitro assays: the larval development assay (LDA) and the larval migration inhibition assay (LMIA). These were performed on free-living stages of susceptible and ML-resistant isolates of three trichostrongyloid nematode species of sheep. In general, dose response curves shifted to the right in the resistant isolates. Data showed that resistance was present to ivermectin and its two components suggesting that both components contribute to action and resistance. There were no consistent patterns of potency and resistance of the tested substances for the different isolates in the LDA except that moxidectin (MOX) tended to have lower resistance ratios than ivermectin (IVM). MOX was the most potent inhibitor in the LMIA in susceptible Haemonchus contortus while being less potent in Trichostrongylus colubriformis and particularly in Ostertagia circumcincta. MOX showed high resistance ratios in the LMIA in all three species. Based on these results, resistance to MOX has unique characteristics and the LMIA may perform better in detecting resistance to MOX in these parasite species.

Functional reconstitution of Haemonchus contortus acetylcholine receptors in Xenopus oocytes provides mechanistic insights into levamisole resistance

BACKGROUND AND PURPOSE

The cholinergic agonist levamisole is widely used to treat parasitic nematode infestations. This anthelmintic drug paralyses worms by activating a class of levamisole-sensitive acetylcholine receptors (L-AChRs) expressed in nematode muscle cells. However, levamisole efficacy has been compromised by the emergence of drug-resistant parasites, especially in gastrointestinal nematodes such as Haemonchus contortus. We report here the first functional reconstitution and pharmacological characterization of H. contortus L-AChRs in a heterologous expression system.

EXPERIMENTAL APPROACH

In the free-living nematode Caenorhabditis elegans, five AChR subunit and three ancillary protein genes are necessary in vivo and in vitro to synthesize L-AChRs. We have cloned the H. contortus orthologues of these genes and expressed them in Xenopus oocytes. We reconstituted two types of H. contortus L-AChRs with distinct pharmacologies by combining different receptor subunits.

KEY RESULTS

The Hco-ACR-8 subunit plays a pivotal role in selective sensitivity to levamisole. As observed with C. elegans L-AChRs, expression of H. contortus receptors requires the ancillary proteins Hco-RIC-3, Hco-UNC-50 and Hco-UNC-74. Using this experimental system, we demonstrated that a truncated Hco-UNC-63 L-AChR subunit, which was specifically detected in a levamisole-resistant H. contortus isolate, but not in levamisole-sensitive strains, hampers the normal function of L-AChRs, when co-expressed with its full-length counterpart.

CONCLUSIONS AND IMPLICATIONS

We provide the first functional evidence for a putative molecular mechanism involved in levamisole resistance in any parasitic nematode. This expression system will provide a means to analyse molecular polymorphisms associated with drug resistance at the electrophysiological level.

Adaptation and evaluation of three different in vitro tests for the detection of resistance to anthelmintics in gastro intestinal nematodes of cattle

Three different in vitro methods, the Larval Development Test (LDT), the Larval Migration Inhibition Test (LMIT) and the Micromotility Meter Test (MMT) have been adapted to detect anthelmintic resistance in cattle nematodes. Nematode eggs and third stage larvae of different Ostertagia ostertagi and Cooperia oncophora isolates were obtained from faecal cultures of experimentally infected calves. Additionally, adult C. oncophora were evaluated in the MMT for the detection of resistance to ivermectin (IVM). For all three in vitro tests standard operating procedures (SOPs) were established and successfully used for the detection of responses of non-parasitic and parasitic stages to different anthelmintic substances and the description of dose-response curves. In the LDT ivermectin (IVM) and thiabendazole (TBZ) were tested, in the LMIT IVM and levamisole (LEV) and in the MMT only IVM was evaluated. Susceptible isolates of C. oncophora and O. ostertagi, an IVM-resistant isolate of C. oncophora and a TBZ-selected isolate of O. ostertagi were used in all (C. oncophora) or only some of these tests (O. ostertagi). For all isolates sigmoidal dose-response curves and EC(50) values for the tested substances were obtained using a four-parameter logistic model. For the LDT, the previously reported problem in development of larvae was successfully overcome with mean development rates between 80% and 87% in negative controls. Following optimization of incubation times, temperatures, mesh sizes (LMIT only), nutritive medium (LDT only) and group size (MMT only) all three test systems reliably detected significant differences in the response to IVM between the susceptible and IVM-resistant isolate of C. oncophora (p<0.0001), resulting in an resistance ratio (RR) value of approximately 5 for IVM and 2.8 for LEV in C. oncophora. The LDT also detected differences in the response to TBZ between the susceptible and BZ-selected O. ostertagi isolates (p<0.001) with an RR of 2 for TBZ. With the standardization of the described tests we report reproducible and reliable in vitro methods for the detection of resistance to IVM (LDT, LMIT and MMT) and TBZ (LDT) for cattle parasitic nematodes.

Identification of levamisole resistance markers in the parasitic nematode Haemonchus contortus using a cDNA-AFLP approach

cDNA-AFLP (cDNA-Amplified Fragment Length Polymorphism)-based strategy has been used to identify levamisole (LEV) resistance markers in the nematode Haemonchus contortus. Transcript profiles of adult nematodes from two LEV-resistant and two susceptible isolates were compared. Among the 17 280 transcript-derived fragments (TDFs) amplified, 26 presented a polymorphic pattern between resistant and susceptible nematodes: 11 TDFs were present in both resistant isolates and absent from both susceptible isolates whereas 15 TDFs were present in both susceptible isolates and absent from both resistant isolates. 8 TDFs specifically present in resistant isolates were cloned and sequenced. Some of these TDFs could represent novel genes, as their sequences presented no homologies in databases. Interestingly, specific expression of one candidate (HA17) in resistant nematodes from different isolates was confirmed by RT-PCR experiments. The finding that HA17 expression correlates with LEV resistance in three H. contortus isolates vs five susceptible isolates strongly suggest that we identified a new potential marker of LEV resistance. This differential approach at the transcriptome level could be of great interest for the identification of the molecular mechanism involved in this phenotype.

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

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

Anthelmintic resistance detection methods

The development of species and populations of parasitic helminths with resistance to one or more anthelmintics is an increasing problem world-wide. The majority of currently available anthelmintics used to control parasitic nematodes of cattle and sheep belong to only three main groups, the benzimidazoles, imidazothiazoles and the avermectins/milbemycins. The successful implementation of helminth control programmes designed to limit the development of resistance in nematode populations depends to some degree on the availability of effective and sensitive methods for its detection and monitoring. A variety of in vivo and in vitro tests have been developed for the detection of nematode populations resistant to the main anthelmintic groups, but each suffers to some degree from reliability, reproducibility, sensitivity and ease of interpretation. This review covers those tests that have been reported and described and highlights some of their strengths and weaknesses.

Cloning and structural analysis of partial acetylcholine receptor subunit genes from the parasitic nematode Teladorsagia circumcincta

Nematode nicotinic acetylcholine receptors (nAChRs) are the sites of action for the anthelmintic drug levamisole. Recent findings indicate that the molecular mechanism of levamisole resistance may involve changes in the number and/or functions of target nAChRs. Accordingly, we have used an RT-PCR approach to isolate and characterise partial cDNA clones (tca-1 and tca-2) encoding putative nAChR subunits from the economically important trichostrongyloid, Teladorsagia circumcincta. The predicted tca-1 gene product is a 248 aa fragment (TCA-1) which contains structural motifs typical of ligand-binding (alpha-) subunits, and which shows very high sequence similarities (98.8 and 97.2% amino acid identities) to the alpha-subunits encoded by tar-1 and hca-1 from Trichostrongylus colubriformis and Haemonchus contortus, respectively. Sequence analyses of partial tca-1 cDNAs from one levamisole-resistant and two susceptible populations of T. circumcincta revealed polymorphism at the predicted amino acid level, but there was no apparent association of any particular tca-1 allele with resistance. tca-2 encodes a 67 aa fragment (TCA-2) containing the TM4 transmembrane domain and carboxyl terminus of a putative nAChR structural (non-alpha) subunit. The deduced amino acid sequence of TCA-2 shows highest similarity (75% amino acid identity) to ACR-2, a structural subunit involved in forming levamisole-gated ion channels in Caenorhabditis elegans, but low similarity (43% identity) to the corresponding regions of TAR-1 and HCA-1. tca-2 is the first nAChR subunit gene of this type to be isolated from parasitic nematodes, and it provides a basis for further characterisation of structural subunits in trichostrongyloids.

Genomic and genetic research on bursate nematodes: significance, implications and prospects

Molecular genetic research on parasitic nematodes (order Strongylida) is of major significance for many fundamental and applied areas of medical and veterinary parasitology. The advent of gene technology has led to some progress for this group of nematodes, particularly in studying parasite systematics, drug resistance and population genetics, and in the development of diagnostic assays and the characterisation of potential vaccine and drug targets. This paper gives an account of the molecular biology and genetics of strongylid nematodes, mainly of veterinary socio-economic importance, indicates the implications of such research and gives a perspective on genome research for this important parasite group, in light of recent technological advances and knowledge of the genomes of other metazoan organisms.

Investigation of intestinal nematode responses to naphthalophos and pyrantel using a larval development assay

Responses of several nematode species to naphthalophos and pyrantel/levamisole were examined using a larval development assay in order to determine the potential of this assay for detection of resistance. Haemonchus contortus and Ostertagia circumcincta showed concentration-dependent responses to naphthalophos, however, the assay was unsuitable for Trichostrongylus colubriformis due to the low toxicity of the drug to the larval stages of this nematode. Measurement of concentration-dependent response to pyrantel in susceptible T. colubriformis was limited by a reduced toxicity against larvae at high drug concentrations, resulting in a parabolic response with a development-inhibition maxima of less than 100%. This limits the usefulness of the assay to detect pyrantel resistance in this species as the presence of a small resistant fraction in a field isolate may be indistinguishable from the parabolic susceptible response. On the other hand, responses of susceptible T. colubriformis to levamisole, and susceptible H. contortus to pyrantel and levamisole showed 100% development inhibition over a range of drug concentrations, indicating that the appearance of a resistant fraction in a field population would be readily discernible from the susceptible response, allowing resistance detection for these drug/parasite combinations. This study has highlighted the varied suitability of the larval development assay technique for resistance detection with different combinations of drugs and parasite species.

Haemonchus contortus: sequence heterogeneity of internucleotide binding domains from P-glycoproteins

P-Glycoproteins are transmembrane proteins associated with acquired multidrug resistance in mammalian cells and some protozoan parasites by a process of active drug export. P-glycoproteins contain two nucleotide binding domains which couple ATP to the drug transport process. The region between the nucleotide binding domains of P-glycoproteins, termed the internucleotide binding domain (IBD), was PCR-amplified from adult and larval cDNA libraries using degenerate primers. The 11 clones isolated by this method fall into several distinct groups, with one group of alleles displaying between 82 and 99% identity at the nucleotide level. This sets a baseline for sequence variation of transcribed alleles from a parasitic nematode. Northern blotting showed that P-glycoprotein genes are transcribed in a developmentally regulated fashion in Haemonchus contortus. Southern blots of H. contortus drug-resistant isolates with an IBD probe revealed a pattern consistent with the involvement of P-glycoprotein in resistance to avermectin/milbemycin anthelmintics.

Comparison of six in vitro tests in determining benzimidazole and levamisole resistance in Haemonchus contortus and Ostertagia circumcincta of sheep

Six in vitro methods for the detection of anthelmintic resistance were compared using benzimidazole/levamisole-resistant Haemonchus contortus and benzimidazole/levamisole/ivermectin-resistant Ostertagia circumcincta as well as susceptible strains of both parasite species. The degree of resistance to thiabendazole and levamisole was compared by (1) an egg hatch assay, (2) an egg hatch paralysis assay, (3) a larval development assay, (4) a larval paralysis assay (5) a larval paralysis assay with physostigmine and (6) larval micromotility assay. The degree of resistance for each assay was expressed as resistance factor--RF. For the detection of thiabendazole and levamisole resistance, the larval development test was observed as the most sensitive to measure quantitatively a degree of resistance between susceptible and resistant strains. For this test the RF for thiabendazole and levamisole was 14.3 and >32.5, respectively in H. contortus strains and 21.1 and 3.5 in strains of O. circumcincta. Egg hatch assay was also found to be sensitive and accurate in determining of resistance to benzimidazole. For measurement of levamisole resistance the egg hatch paralysis assay and larval paralysis assay were found to be able to distinguish between strains, but some disadvantages of these techniques make it unsuitable for field detection of levamisole resistance. Other in vitro assays as larval paralysis assay with physostigmine and larval micromotility assay were also investigated. Significant differences in paralysis of the larvae were observed using larval paralysis assay.

Polymorphic DNA markers in the genome of parasitic nematodes

Polymorphic molecular markers are being identified to characterize the genomes of parasitic nematodes. The aim is to construct a map with markers evenly spread over the six chromosomes. With such a map, regions can be identified that are under selection pressure when attempts are being made to eradicate worms, be it by drugs, vaccines or genetic resistance in the sheep. Several types of markers have been developed, microsatellites, transposon-associated markers, amplified fragment length polymorphism (AFLP) and expressed sequence tag (EST) markers. Linkage groups can be constructed using several genetic crosses between inbred and drug resistant strains. EST markers will be especially important for comparative mapping with the genome of Caenorhabditis elegans, and therefore localization of the linkage group on a chromosome. It will then be possible to identify functional genes close to markers that have changed allele frequencies under selection pressure and identify the mechanisms of resistance to parasite control.

Binding of [3H]m-aminolevamisole to receptors in levamisole-susceptible and -resistant Haemonchus contortus

M-aminolevamisole, a potent analogue of the commercial anthelmintic levamisole, was used to investigate ligand-binding properties of homogenates of larval and parasitic stages of the nematode parasite of sheep, Haemonchus contortus. Kinetics of the binding of [3H]m-aminolevamisole to homogenates was measured in a drug-susceptible isolate and compared with a levamisole-resistant isolate. Equilibrium binding studies and kinetic studies revealed a high affinity binding component with a KD of 3 nM. A low affinity component (KD = 2.4 microM) was also apparent in equilibrium studies. High affinity [3H]m-aminolevamisole binding was displaced in a concentration-dependent manner by levamisole analogues and cholinergic agonists. Compared with the susceptible isolate, binding in a levamisole-resistant isolate of the parasite, was quantitatively similar over a range of developmental stages and binding conditions. However, under the conditions of binding there was a reduced affinity (larger KD) and more binding sites (larger Bmax) at the low affinity site in the resistant compared with the susceptible isolate. It was concluded that the ligand was binding to acetylcholine receptor populations of the nematode and that resistance may be associated with alterations in the low affinity site of this receptor.

Levamisole binding sites in Haemonchus contortus

Larval and adult extracts from isolates of Haemonchus contortus were assayed for specific [3H]levamisole binding activity. All of the tissue preparations displayed [3H]levamisole binding sites. The sensitive isolate SE and resistant isolate RJ showed no differences in larval and adult binding data. Larval SE extracts had higher receptor density (Bmax = 648 fmol mg-1) and dissociation constant (Kd = 1.28 microM) for [3H]levamisole than larval extracts of the American isolate RUSA (Bmax = 87 fmol mg-1 and Kd = 0.15 microM). Extracts of adult SE and RUSA isolates contain as much as 327 fmol mg-1 of protein and 205 fmol mg-1 of protein, respectively, and similar dissociation constants (Kd = 0.77 microM and Kd = 0.81 microM, respectively). There was a good correlation between specific binding activity of larval and adult extracts in both SE and RUSA isolates. The nicotinic cholinergic antagonist alpha-bungarotoxin had no effects in either isolate on [3H]levamisole binding activity. The results confirm that levamisole acts at a cholinergic receptor in H. contortus, and suggest that target site modification could be involved in the development of levamisole resistance.

Inheritance of levamisole and benzimidazole resistance in an isolate of Haemonchus contortus

Reciprocal crosses between an isolate of Haemonchus contortus resistant to both benzimidazole and levamisole anthelmintics and a susceptible isolate were performed in order to determine the mode of inheritance of these resistances. F1 and F2 generations and parent isolates were assayed for susceptibility to thiabendazole and levamisole in vitro. For each drug all of the filial generations were intermediate in susceptibility between the parent isolates, and analysis indicated that resistance was inherited as an incompletely recessive character determined by more than one gene in each case. There was no evidence of maternal inheritance. Results of both the in vitro assays and in vitro selection, followed by determination of sex ratio in the survivors, as well as studies on adult worms, provided no evidence for sex-linkage. This work illustrates that in vitro assays coupled with minimal studies in sheep are useful for determining inheritance of resistance, yet use fewer experimental animals than traditional studies.

The nicotinic acetylcholine alpha-subunit gene tar-1 is located on the X chromosome but its coding sequence is not involved in levamisole resistance in an isolate of Trichostrongylus colubriformis

The polymerase chain reaction was used to amplify fragments comprising the known reading frame of the nematode nicotinic acetylcholine alpha-subunit gene tar-1. Sequences were derived from DNA prepared from bulk collections of worms and from individual male and female Trichostrongylus colubriformis. In each case a levamisole-resistant (BCk) and a drug susceptible population were examined. Although several nucleotide transitions were detected no amino acid sequence variations were found between the isolates and between individual worms, indicating that the coding sequence of this gene is not responsible for levamisole-resistance in the isolate tested. However, an intronic allelic T/C variation at position 4955 was observed in both populations. It has been reported that levamisole-resistance in the BCk isolate of T. colubriformis is due to a sex-linked recessive gene or gene complex. A restriction fragment length polymorphism formed by the allelic variation was found and was detectable by digestion with the restriction endonuclease NlaIII. Statistical comparison of allele frequencies from individual male and female worms was consistent with sex-linkage of tar-1 (P < 0.05) but showed no correlation with levamisole resistance status. The polymorphism described will provide a useful X-chromosome marker and represents the first mapped genetic locus in this species.

Selection for high levamisole resistance in Haemonchus contortus monitored with an egg-hatch assay

To investigate the characteristics of selection for levamisole resistance in Haemonchus contortus, the consecutive nematode generations of an in vivo selection were monitored with a newly developed egg-hatch assay. The in vivo selection was started with a population not previously exposed to any anthelmintics (SHS). At first, the levamisole resistance progressed gradually in successive nematode generations by treating sheep with increasing doses of levamisole, the initial dose being 1 mg kg-1. Treatment with 5 mg kg-1 levamisole resulted, however, in a steep increase of resistance. The selection was ended after six generations, since a level of 30 mg kg-1 levamisole, which is not far from the toxic level for sheep, was reached. The final population, RHS6, was studied in a controlled test. Treatment of RHS6-infected sheep with 30 mg kg-1 levamisole caused an 80% decrease of faecal egg output, and a reduction of 34% in worm numbers. It was remarkable that only the number of female adults was reduced. RHS6 showed a reduced viability, but a fertility similar to the starting population SHS.

Characterization of an acetylcholine receptor gene of Haemonchus contortus in relation to levamisole resistance

The anthelminitic drug levamisole is thought to bind to nicotinic acetylcholine receptors of nematodes. It is possible that resistance to this drug is associated with either a change in binding characteristics or a reduction in the number of nicotinic acetylcholine receptors. Therefore, the molecular mechanism of levamisole resistance in the parasitic nematode Haemonchus contortus was studied by isolating and characterising cDNA clones encoding a putative ligand binding nicotinic acetylcholine receptor subunit, HCAl, of two susceptible and one levamisole resistant population. Hcal is related to unc-38, a nicotinic acetylcholine receptor subunit gene associated with levamisole resistance in Caenorhabditis elegans. Although extensive sequence analyses of hcal sequences revealed polymorphism at amino acid level, no association with levamisole resistance could be detected. Restriction fragment length polymorphism analyses confirmed that, although polymorphism was detected, no selection of a specific allele of hcal has taken place during selection for levamisole resistance in various levamisole resistant populations.

In vitro characterization of lines of Oesophagostomum dentatum selected or not selected for resistance to pyrantel, levamisole and ivermectin

Lines of Oesophagostomum dentatum artificially selected or not selected for resistance to pyrantel, levamisole and ivermectin were used in this study. From the 10th generation of selection eggs were collected from each line and subjected to an in vitro larval development assay (LDA) and an egg hatch assay (EHPA). Significant differences were observed between an unselected line of O. dentatum and the lines selected for resistance to levamisole or pyrantel in both assays. The LDA was more sensitive than EHPA in detecting anthelmintic resistance in O. dentatum. The results obtained from the LDA confirmed side-resistance between levamisole and morantel/pyrantel. The in vitro tests failed to show significant differences between ivermectin-sensitive and resistant lines.

Pharmacology of anthelmintic resistance

Anthelmintic resistance has compromised the control of nematode parasites in several animal-based industries. Studies of resistance have not only improved our understanding of this phenomenon but also shed light on physiological systems of parasitic helminths. In addition, research on molecular aspects of anthelmintic resistance may provide selectable markers for use in future transfection studies with helminths. Several anthelmintics act on helminth neuromuscular systems. Drugs such as levamisole are cholinergic agonists and, based on pharmacological studies, levamisole-resistant nematodes appear to have altered acetylcholine receptors. It is likely that anticholinesterase anthelmintics share cross resistance with levamisole. Ivermectin appears to be a glutamate agonist. In vitro studies of ivermectin-resistant nematodes suggest that IVM receptors are located on pharyngeal and somatic muscle. The free-living nematode Caenorhabditis elegans may provide a model for anthelmintic resistance. It has been useful in cloning drug receptors from parasites but differences between its life history and habitat compared with parasitic nematodes may limit its usefulness for studying resistance in these parasites.

Levamisole resistance in Haemonchus contortus selected at different stages of infection

Resistance to the anthelmintic levamisole is common amongst most genera of trichostrongylid nematodes of sheep, but is relatively rare in the species Haemonchus contortus. The level of resistance in at least one strain of this species varies during the life cycle. Investigation of this phenomenon, especially in parasitic stages of the parasite may reveal information on resistance mechanisms and factors affecting the selection of resistance. Compared with adult worms of the same resistant strain, immature worms are more susceptible to levamisole both in in vitro contraction assays and an in vivo efficacy experiment. To ascertain if immature parasitic worms of a susceptible strain could be selected for resistance, 3 lines of worms were selected with levamisole for 9 generations. Lines were selected with the same dose rate (2 mg kg-1) at the 4th and 24th day of infection. In addition, because levamisole is less active against immature worms a further line was selected at the 4th day with a higher dose (8 mg kg-1) but similar efficacy to the selection at day 24. In in vitro assays the line selected at day 4 with the lower dose of levamisole showed no resistance compared with an unselected line. The other 2 lines developed high levels of resistance indicating that resistance can develop after selection at either stage. In addition these data directly show that selection pressure is important in determining the rate of development of resistance.

Resistance to benzimidazole anthelmintics by Haemonchus contortus in goats in peninsular Malaysia

A previous study had suggested that local strains of goat trichostrongyles, comprising largely Haemonchus contortus, might have developed resistance to benzimidazole anthelmintics. A trial involving 18 goats was conducted to confirm this. There was a significant (P < 0.01) reduction in worm burdens in goats given levamisole, but this was not so for those animals given albendazole, fenbendazole, oxfendazole and mebendazole (P > 0.05).

Anthelmintic resistance

Anthelmintic resistance is widespread in nematode parasites of sheep, goats and horses. Resistance is also developing in nematode parasites of cattle and has been detected in pig parasites. Benzimidazole, levamisole/morantel and ivermectin resistances occur in nematodes of sheep and goats and closantel resistance has been found in Haemonchus contortus. Anthelmintic resistance is likely to develop wherever anthelmintics are frequently used and be detected if it is investigated. Worm count or egg count reduction after treatment are useful for the detection of all types of anthelmintic resistances. More economical, faster and more sensitive in vitro assays for the detection of anthelmintic resistance have been developed. Some, such as the egg hatch assay are specific for a particular class of anthelmintic, whilst others such as larval development assays can be used with most anthelmintics. Improvements in our understanding of the biochemistry and molecular genetics of anthelmintic actions should lead to the development of more sensitive assays for the detection of anthelmintic resistance in individual nematodes. Levamisole/morantel resistance appears to be associated with alterations in cholinergic receptors in resistant nematodes. Ivermectin appears to act by binding to a glutamate receptor of a membrane chloride channel. This receptor has been expressed in vitro so that further studies of the interaction of ivermectin with this receptor and its possible alteration in ivermectin resistance will be feasible. Benzimidazole resistance in nematodes and fungi appears to be associated with an alteration in beta-tubulin genes which reduces or abolishes the high affinity binding of benzimidazoles for tubulin in these organisms. This knowledge can be exploited for DNA probes for benzimidazole resistance/susceptibility in individual organisms.

The effects of anthelmintics on ovine larval nematode parasite migration in vitro

The anthelmintics ivermectin, levamisole, morantel tartrate and thiabendazole all inhibited, in vitro, the motility of third stage larvae (L3) of Trichostrongylus colubriformis. The bioassay, based on the inhibition of L3 migration from agar gels, yielded sigmoid dose-response curves for ivermectin, levamisole and morantel tartrate, but not thiabendazole. The concentration of levamisole giving 50% inhibition of migration (EC50) was determined for Cooperia curticei, Haemonchus contortus, Nematodirus spathiger, Ostertagia circumcincta, Trichostrongylus axei, T. colubriformis and T. vitrinus. EC50s differed between species but within species the EC50s for ensheathed and exsheathed L3 were similar except for N. spathiger which showed significantly higher EC50 for the ensheathed L3. No difference between EC50s for levamisole-resistant and susceptible strains of T. colubriformis were found. Similarly, morantel-resistant and susceptible strains of T. colubriformis could not be differentiated in this bioassay. The inhibition of L3 motility by known anthelmintic compounds in this bioassay suggests that the bioassay could be used as a screen for potential new anthelmintics.

Relationship between pharmacological properties and clinical efficacy of ruminant anthelmintics

The purpose of this review article is to establish a relationship between pharmacological properties and clinical efficacy of the most widely used broad-spectrum veterinary anthelmintics. The impact of drug pharmacokinetics/pharmacodynamics on anthelmintic efficacy and resistance is discussed. We review the clinical pharmacokinetics of the currently available anthelmintics used in livestock-imidazothiazoles, tetrahydropyrimidines, benzimidazoles/pro-benzimidazoles, salicylanilides (closantel) and avermectin-type compounds. Understanding the pharmacokinetic and metabolic behaviour of broad-spectrum anthelmintics in the host, and factors modulating that behaviour, is highly important for maximizing anthelmintic utility and efficacy.

An in vitro assay utilising parasitic larval Haemonchus contortus to detect resistance to closantel and other anthelmintics

A new assay for detecting resistance to anthelmintics in vitro is described. The assay uses a simple culture system in which the ability of anthelmintics to kill or inhibit the migration of parasitic third and fourth stage Haemonchus contortus larvae through a 50 microns aperture mesh is assessed. The assay detects 2-10-fold resistance to closantel. Resistance to benzimidazoles, levamisole and ivermectin can also be detected.

Resistance to benzimidazole anthelmintics by trichostrongyles in sheep and goats in north-west Cameroon

Following the routine use of tiabendazole at monthly intervals for several years, the trichostrongyle parasites of sheep and goats on a government research station in North-West Cameroon had become resistant to benzimidazole anthelmintics.

Effects of cholinergic drugs on longitudinal contraction in levamisole-susceptible and -resistant Haemonchus contortus

A novel force transducer was used to measure the effects of cholinergic agonists on longitudinal contraction in Haemonchus contortus. Drugs were applied to whole worms or injected via a cannula in the pseudocoelomic cavity. A number of agonists, including nicotine and the anthelmintics m-aminolevamisole, levamisole and morantel, caused contractions in whole worms. Four- to 25-fold increases in concentration of the active compounds were required to cause contractions in each of two levamisole-resistant strains of H. contortus. Of the other compounds tested, bephenium had equivalent activity against susceptible and resistant strains. Anticholinesterase compounds caused contractions after a slight delay in susceptible, but not resistant worms. Numerous cholinergic agonists and other compounds did not cause contraction when applied to whole worms. One of these, acetylcholine, caused contractions in cannulated worms. Compared with the susceptible strain, five- to six-fold higher concentrations of acetylcholine were required to cause equivalent contractions in the resistant strains. Levamisole resistance in adult H. contortus is likely to be due to a change in the characteristics of the cholinergic receptor(s).

Levamisole resistance in Trichostrongylus colubriformis: a sex-linked recessive character

Reciprocal crosses between susceptible and levamisole resistant strains of Trichostrongylus colubriformis produced F1 offspring consistent with resistance being inherited as a sex-linked recessive character. The resistance status of the offspring of the backcrosses of the F1 to both parental strains supported this hypothesis. The results are consistent with resistance being controlled by a single gene, or a tightly linked group of genes, but indicate that other autosomal loci have minor effects. The results contrast with the reported observations that resistance to the benzimidazole anthelmintics is polygenic and autosomal. The results are discussed relative to a general evolutionary model for anthelmintic resistance which predicts that selection from the upper extreme of an anthelmintic tolerance distribution results in polygenicity.

Resistance to levamisole and cross-resistance between pyrantel and levamisole in Oesophagostomum quadrispinulatum and Oesophagostomum dentatum of pigs

Two strains of Oesophagostomum spp., consisting of both O. quadrispinulatum and O. dentatum, were subjected to a controlled in vivo assay for resistance to levamisole and pyrantel by comparison with susceptible isolates. One strain (LEM) was recently isolated from a commercial herd, where sows showed high numbers of strongyle eggs in faeces within 2 weeks of farrowing and following treatment with levamisole at the manufacturer's recommended dose rate 1 week before farrowing. Levamisole had been used as the sole anthelmintic for treatment for at least 7 years on this farm. Treatment with pyrantel in this herd also indicated cross-resistance to this drug. A mixed population of O. quadrispinulatum and O. dentatum of this strain was subjected to controlled in vivo assays. Faecal egg count reduction (FECR) was found to be -573.3% (P greater than 0.05) and worm count reductions (WCR) of O. quadrispinulatum and O. dentatum were estimated as 44.5% (P greater than 0.05) and 96.4% (P less than 0.001), respectively. Treatment with pyrantel showed that cross-resistance existed to this drug, with FECR of 10.4% (P greater than 0.05) and WCR of 64.5% (P greater than 0.05) and 90.7% (P less than 0.05) for O. quadrispinulatum and O. dentatum, respectively. Another strain (VJ) was isolated from another commercial pig herd, which was dosed with pyrantel citrate four times a year for at least 8 years. This strain showed resistance to pyrantel, with FECR of 43.8% (P greater than 0.05) and WCR of 65.9% (P greater than 0.05) and 49.4% (P greater than 0.05) for O. quadrispinulatum and O. dentatum, respectively. However, both species were susceptible to levamisole. Our results suggested that selection with levamisole gave rise to levamisole resistance and automatically conferred resistance to pyrantel, whereas selection with pyrantel only resulted in resistance to this drug alone. These findings are discussed in relation to the location of the two species of Oesophagostomum in the large intestine of pigs and the mode of action of this class of anthelmintics.

Anthelmintic resistance in nematodes: extent, recent understanding and future directions for control and research

Resistance has now been reported to all of the broad spectrum anthelmintic types currently available, namely to the benzimidazoles, levamisole/morantel and to ivermectin. The problem causes most concern for parasite control in sheep, but anthelmintic resistance has also been reported in nematodes of horses, goats, pigs and more recently cattle. Our understanding of the factors which select rapidly for resistance has increased and programmes of worm control which minimize selection for anthelmintic resistance are being developed and tested. One of the greatest problems encountered in attempting to reduce the selection for overt drug resistance is the need for more sensitive tests for developing resistance. In the long term, new approaches to chemotherapy and to overcoming anthelmintic resistance problems will arise from improving our understanding of the modes of action of, and mechanisms of resistance to, anthelmintics at the level of the receptor proteins and their genes.

Xenobiotic metabolism in helminths

During its life cycle a parasite, like mammals and other organisms, comes into contact with a variety of toxic molecules. In mammals the main line of defence against such compounds is a group of oxidative enzymes that occur predominantly in the liver. Many of these oxidations are dependent on the haemoprotein cytochrome P-450 which serves as a terminal oxidase accepting electrons from NADPH and cytochrome [Formula: see text] . This review by Wendy Precious and John Barrett illustrates that in contrast to mammals, xenobiotic metabolism in both parasitic and free-living platyhelminths and nematodes is predominantly reductive and hydrolytic as the cytochrome P-450 system is absent. This is surprising since it is present in many groups including bacteria, fungi and protozoa, which suggests an ancient origin. The absence of the cytochrome P-450 system not only severely limits the ability of helminths to detoxify compounds but also limits their ability to activate prodrugs.