Emodepside and SL0-1 potassium channels: a review

Nematode parasites infect humans and domestic animals; treatment and prophylaxis require anthelmintic drugs because vaccination and sanitation is limited. Emodepside is a more recently introduced cyclooctadepsipeptide drug that has actions against GI nematodes, lungworm, and microfilaria. It has a novel mode of action which breaks resistance to the classical anthelmintics (benzimidazoles, macrocyclic lactones and cholinergic agonists). Here we review studies on its mode of action which suggest that it acts to inhibit neuronal and muscle activity of nematodes by increasing the opening of calcium-activated potassium (SLO-1) channels.     

The pharmacology of anthelmintics in livestock

The recent development of ivermectin, netobimin, closantel, triclabendazole and clorsulon as well as new ways of using established anthelmintics has significantly improved the potential for parasite control in livestock. It is of interest to note that potential control of F. hepatica has been particularly improved with four of these new anthelmintic drugs being fasciolicidal. In addition to an appreciation of the spectrum of action of the new anthelmintics, a considerable amount of information on the pharmaco-kinetic behaviour, metabolism and mode of action of these drugs has been published. New information on pharmacology of established anthelmintics has provided new insights into the biology of parasites, drug resistance and the options for improved chemotherapy by manipulation of pharmacokinetics and metabolism. Much recent pharmacological research has been concentrated on the BZs and this has been considered here in an attempt to provide a better understanding of this significant group of anthelmintics. The development of new delivery systems, such as intraruminal slow release devices, is potentially useful with most anthelmintics. So far, this new approach to parasite control has been applied most successfully with morantel tartrate.     

Controlled release devices for the delivery of anthelmintics in cattle

Control of cattle helminths is not so much a problem of drug choice, but of drug delivery and livestock management. Several effective anthelmintics are available for domestic livestock, but their efficiency in limiting infection and disease attributed to important parasites such as Ostertagia and Haemonchus has been mainly due to good management practice and strategically timed treatment based on detailed epizootiological studies of parasite transmission. In most situations, treated animals remain fully susceptible to reinfection if continually grazed on contaminated pastures, and this is the rationale behind techniques for continuous or multiple treatment with anthelmintics. In response to these treatment requirements, the animal health industry has developed controlled release devices, or boluses, that can be implanted orally into the rumen (Fig. 1) to release anthelmintics over an extended period - either in a delayed or pulsed fashion. In this article, Gary Zimmerman and Eric Hoberg discuss same of the most promising of such devices.     

Anthelmintics for horses

Modern equine anthelmintics can be divided into at least seven principal groups based on mode of action, i.e., benzimidazoles, pro-benzimidazoles, imidothiazoles, tetrahydropyrimidines, organophosphates, piperazines, and avermectins. The spectrum of activity of these drugs varies and resistance of cyathostomes to benzimidazole and pro-benzimidazole drugs has been observed in many areas. Cross resistance with other groups has not been reported in equines, however. Control is dependent upon understanding the capabilities of anthelmintics and the epizootiology of the important parasites, e.g., large strongyles, cyathostomes, ascarids and bots. Development of effective programs in a given region should be based on routine fecal analysis to ensure that treatment schedule and products selected are adequate for local climatic conditions and management methods.     

Increased expression of ABC transport proteins is associated with ivermectin resistance in the model nematode Caenorhabditis elegans

Widespread resistance to chemotherapeutic agents is one of the biggest challenges facing human health and the agricultural industry, with resistance to all current anthelmintics now recorded and few new agents or vaccines available. Understanding the development of drug resistance in parasitic nematodes is critical to prolonging the efficacy of current anthelmintics, developing markers for monitoring drug resistance and is beneficial in the design of new chemotherapeutic agents or targets. This study describes the development of ivermectin-resistant strains of the model nematode Caenorhabditis elegans through step-wise exposure to increasing doses of ivermectin commencing with a non-toxic dose of 1 ng/ml. Resistant strains were developed that displayed a multidrug resistance phenotype with cross-resistance to the related drug moxidectin and to other anthelmintics, levamisole and pyrantel, but not albendazole. Resistance was associated with increased expression of the multidrug resistance proteins (MRPs) and P-glycoproteins. Resistance to ivermectin was reversible by the co-administration of MRP, P-glycoprotein and glutathione biosynthesis inhibitors, confirming the involvement of these proteins in resistance. In our model, resistance to low levels of ivermectin (相似文献   

Levamisole receptors: a second awakening

Levamisole and pyrantel are old (1965) but useful anthelmintics that selectively activate nematode acetylcholine ion channel receptors; they are used to treat roundworm infections in humans and animals. Interest in their actions has surged, giving rise to new knowledge and technical advances, including an ability to reconstitute receptors that reveal more details of modes of action/resistance. We now know that the receptors are plastic and may form diverse species-dependent subtypes of receptor with different sensitivities to individual cholinergic anthelmintics. Understanding the biology of the levamisole receptors is expected to inform other studies on anthelmintics (ivermectin and emodepside) that act on ion channels.     

Anthelmintic resistance in nematode parasites of cattle: a global issue?

Acceptable performance of grazing cattle frequently depends on the availability of effective broad-spectrum anthelmintics to remove, or prevent infection with, gastrointestinal nematodes. This control is increasingly threatened by populations of nematodes resistant to the most commonly used anthelmintics. Although this appears to have developed more slowly than in nematodes infecting small ruminants, the number of reports in the literature over the past five years suggests a rapidly escalating problem. This review discusses this literature, several issues unique to cattle parasitism and anthelmintics, and how previous research in small ruminants can improve the management of anthelmintic resistance in cattle.     

Genetic resistance to haemonchosis in sheep

Parasitic nematodes cause huge economic losses to the domestic livestock industry. Control requires skilled farm management and heavy use of anthelmintics, but breeding for resistant animals has long been held a possibility. For about 20 years after World War ll, the genetics of association between sheep breeds and Haemonchus contortus - the main parasite of importance in sheep - was studied in the USA and Europe. Since the early 1970s, however, such studies have received greater emphasis in Australia, where anthelmintic resistance has become widespread. In this article, Douglas Gray discusses the background to breeding programmes for resistance to hoemonchosis - paying homage to the 'Golden Ram', a Merino believed to have carried a single major gene for immunological resistance to Haemonchus and Trichostrongylus.     

Unresolved issues in anthelmintic pharmacology for helminthiases of humans

Helminth infections are an important constraint on the health and development of poor children and adults. Anthelmintic treatment programmes provide a safe and effective response, and increasing numbers of people are benefitting from these public health initiatives. Despite decades of clinical experience with anthelmintics for the treatment of human infections, relatively little is known about their clinical pharmacology. All of the drugs were developed initially in response to the considerable market for veterinary anthelmintics in high- and middle-income countries. In contrast, the greatest burden caused by these infections in humans is in resource-poor settings and as a result there has been insufficient commercial incentive to support studies on how these drugs work in humans, and how they should best be used in control programmes. The advent of mass drug administration programmes for the control of schistosomiasis, lymphatic filariasis, onchocerciasis and soil-transmitted helminthiases in humans increases the urgency to better understand and better monitor drug resistance, and to broaden the currently very narrow range of available anthelmintics. This provides fresh impetus for developing a comprehensive research platform designed to improve our understanding of these important drugs, in order to bring the scientific knowledge base supporting their use to a standard equivalent to that of drugs commonly used in developed countries. Furthermore, a better understanding of their clinical pharmacology will enable improved therapy and could contribute to the discovery of new products.     

Efficacy evaluation of anthelmintics: which methods to use in the field?

Efficacy evaluation of anthelmintics is very important even in veterinary practice due to the existence of acquired resistance, poor quality of anthelmintics in several region of the world, and nedds for quarantine strategy prior introduction of new animals. Clearly, flock evaluation can be opposed to individually based evaluations. I propose that individually based evaluations should be preferred in field condition controls: although they are more sophisticated on a statistical basis, the availability of a software for calculations renders the proposal realistic.     

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

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

Pro and cons of targeted selective treatment against digestive-tract strongyles of ruminants

The increasing prevalence of resistance to anthelmintics among gastrointestinal nematodes and the desire for lower input agriculture have promoted the idea that targeted selective treatment (treating the animals in need of such a treatment and only them) could be a sustainable solution for controlling internal parasites of ruminants. The pros are the slowing of resistance prevalence, lower residues of anthelmintics in meat and milk, and lower cost; the cons are the difficulty and time spent on selecting animals in need of treatment and the possibility of lower production. Using actual experiments and modelling we show that targeted selective treatment can be used to sustainably control gastrointestinal nematode infections in flock.     

Recent findings on the genetics of gastro-intestinal nematode resistance in ruminants

The control of helminthiases in ruminants raised in open pasture has been mainly undertaken by using prophylactic measures in the environment, but these are often inadequate due to incorrect application. With the appearance of anthelmintics, the strategy for controlling these parasitoses, passed to pharmacological treatments which became effective in reducing their impact. However, the frequent and incorrect utilisation of these molecules resulted in resistance to anthelmintics and the presence of chemical residues in animal products for human consumption. Anthelmintic resistance is widespread throughout the world, heterogeneous and probably underestimated. This has encouraged the introduction of homeopathic agents and products derived from plants whose effectiveness has not been scientifically assessed. It is well known that it is possible to detect differences in resistance to the most important parasites between breeds. In Europe, it has been reported that some ovine autochthonous breeds, Scottish Blackface and Lacaune, showed higher resistance. The implementation of breeding strategies aimed at obtaining animals with naturally low susceptibility to nematode infestations could therefore play an increasingly important role. Standard animal breeding techniques have been largely successful in improving the performance of domestic animals in the last century. Standard quantitative selection requires field data on: i) individual phenotype performance; ii) expected covariance among animals due to blood relationship between them. The whole process of predicting the breeding value of animals in order to select subsequently the genetically superior parents of the next generation is entirely based on sophisticated computations (BLUP-animal model). In sheep, the main objective is always selecting for milk yield and sometimes, in addition, milk composition. However, due to the evolution of the EU agricultural policy and consumer demand in terms of healthy and organic food, more attention is now being given to traits related to health (resistance to EST, mastitis or parasitic diseases). Some studies conducted in New Zealand and Australia showed that nematode resistance is genetically controlled with high heritabilities and quite low genetic correlations with production traits. In this sense, some studies showed that it is possible to decrease the number of parasites in the framework of a traditional breeding programme. However, in most situations, this trait is not extensively recorded due to the high cost of individual recording. Therefore, it would be useful to implement breeding strategies based on the knowledge of the genes involved in this trait expression. Traditionally, two approaches are available to locate a gene: i) genome scan; ii) candidate gene approach. The candidate gene approach attempts to link general resistance to some particular genes. To date, genetic resistance against parasites is considered to be linked with the MHC and IgE genes. Furthermore, several gene detection studies based on the genome scan approach for this trait are currently being carried out on both crossed experimental populations (fat x lean Blackface lines and Sarda x Lacaune) and pure breeds (Churra). The preliminary results seem promising as to the use of marker assisted or genotype assisted selection for this trait, which is difficult and expensive to measure on a population scale.     

Field evaluation for anthelmintic-resistant ovine gastrointestinal nematodes by in vitro and in vivo assays

A coprological survey to analyze the presence of flock resistance to benzimidazoles (BZ) and macrocyclic lactones (ML) was performed in sheep under field conditions. Fecal samples were collected from 2,625 sheep in 72 commercial farms from Galicia (NW Spain). The in vitro (FECRT, fecal egg count reduction test) and in vivo (EHA, egg hatch assay, and LFIA, larval feeding inhibition assay) tests were used to assess the efficacy of these anthelmintics. Coprocultures were also developed to obtain knowledge on the main genera of trichostrongylid nematoda prior to, and after, the administration of the anthelmintics. By using the FECRT, BZ resistance was observed in 13 (18%) flocks, whereas ML resistance was only detected in 2 (3%) farms. The number of resistant flocks to BZ was 21 (29%) by using the EHA and 7 (10%) by means of the LFIA. None of the flocks used in this study showed simultaneous resistance to both employed anthelmintics. The results from the in vitro and in vivo tests revealed that 92% of the flocks FECRT resistant to BZ were also resistant with the EHA. The LFIA confirmed all the farms resistant to ML by using the in vivo test. After the administration of BZ, nematode larvae belonging to Teladorsagia circumcincta (32.2%), Trichostrongylus spp. (29%), Nematodirus spp. (6.5%), and Chabertia ovis (3.2%) were identified. In the flocks receiving ML, only T. circumcincta was identified (57%). We recommend the use of in vitro tests because they are more efficient. As the use of macrocyclic lactones is increasing in this region, further investigation is needed for detecting resistance to the anthelmintic family compounds by the LFIA.     

A possible biochemical mode of action for benzimidazole anthelmintics

Albendazole (ABZ), cambendazole (CBZ), oxibendazole (OBZ), and thiabendazole (TBZ) are potent, orally active, broad spectrum anthelmintics widely used in human and veterinary medicine. As members of the benzimidazole series, they are closely related chemically, and it is likely that they exert their anthelmintic effects in an identical fashion. We have examined the effects of these anthelmintics on the electrical resistance of planar bimolecular lipid membranes and compared the results with those obtained with a known uncoupler, 2,4-dinitrophenol (2,4-DNP). All drugs tested markedly reduced membrane resistance at concentrations lower than 0.1 microM and were better proton conductors than 2,4-DNP by at least an order of magnitude. The sequence of proton conducting efficiency was ABZ greater than OBZ greater than TBZ greater than CBZ greater than 2,4-DNP. From 1 to 40 microM, ABZ and CBZ substantially decreased P/O (phosphorous/oxygen) ratios in coupled rat liver mitochondria in a concentration-dependent fashion using beta-hydroxybutyrate as the substrate. 2,4-DNP was also shown to decrease P/O ratios, but less effectively than the benzimidazole anthelmintics. These experiments indicate that the benzimidazole anthelmintics are lipid-soluble proton conductors that are effective in artificial (phospholipid bilayer) and natural (rat liver mitochondria) membrane systems. Dissipation of the transmembrane proton gradient should result in diminished levels of cellular ATP. In vivo treatment with a therapeutically effective dose of ABZ caused a severe disturbance in the energy balance of Hymenolepis diminuta; this was evident from a distinct drop in ATP levels, and from a decline in the ATP/ADP ratios, adenylate energy charge (AEC) and available adenylate energy (AAE) values.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reducing intestinal nematode infection: efficacy of albendazole and mebendazole

The widespread use of mebendazole and albendazole for treating intestinal nematode infections in human populations is raising concerns that careful monitoring pro- cedures should be in place to identify any emergence of drug resistance. In this article, Andy Bennett and Helen Guyatt discuss whether benchmark parasitological drug efficacy rates can be defined for these anthelmintics, by analysing published data on cure rates and egg reduction rates in the treatment of Ascaris lumbricoides, Trichuris trichiura and hookworm.     

Anthelmintic metabolism in parasitic helminths: proteomic insights

SUMMARY Anthelmintics are the cornerstone of parasitic helminth control. Surprisingly, understanding of the biochemical pathways used by parasitic helminths to detoxify anthelmintics is fragmented, despite the increasing global threat of anthelmintic resistance within the ruminant and equine industries. Reductionist biochemistry has likely over-estimated the enzymatic role of glutathione transferases in anthelmintic metabolism and neglected the potential role of the cytochrome P-450 superfamily (CYPs). Proteomic technologies offers the opportunity to support genomics, reverse genetics and pharmacokinetics, and provide an integrated insight into both the cellular mechanisms underpinning response to anthelmintics and also the identification of biomarker panels for monitoring the development of anthelmintic resistance. To date, there have been limited attempts to include proteomics in anthelmintic metabolism studies. Optimisations of membrane, post-translational modification and interaction proteomic technologies in helminths are needed to especially study Phase I CYPs and Phase III ABC transporter pumps for anthelmintics and their metabolites.     

Genetic variability following selection of Haemonchus contortus with anthelmintics

Genetic diversity in nematodes leads to variation in response to anthelmintics. Haemonchus contortus shows enormous genetic diversity, allowing anthelmintic resistance alleles to be rapidly selected. Anthelmintic resistance is now a widespread problem, especially in H. contortus. Here, I compare the genes involved in anthelmintic resistance in H. contortus with those that confer susceptibility or resistance on the free living nematode Caenorhabditis elegans. I also discuss the latest knowledge of genes associated with resistance to benzimidazoles, levamisole and the macrocyclic lactones and the need for DNA markers for anthelmintic resistance.     

The effect of selection with levamisole on benzimidazole resistance in Ostertagia spp. of sheep

A field population of Ostertagia spp. (predominantly O. circumcincta) in sheep, found to be resistant to oxfendazole, was exposed to selection with1 levamisole in the laboratory and the field. Progeny of the survivors of a single dose of levamisole in penned sheep showed a significantly lower level of resistance to oxfendazole in an anthelmintic assay, and a similarly lower level of resistance to the ovicidal activity of thiabendazole in vitro compared with the progeny of the survivors of a single dose of oxfendazole, although they remained more resistant than a known susceptible strain of 0. circumcincta. The progeny of worms present in grazing sheep after three doses of levamisole at 4-weekly intervals, and subjected to the same tests, behaved in a similar fashion. They also showed a substantial fall in benzimidazole resistance compared with the progeny of worms not exposed to any anthelmintic for 6 months. The results suggested that levamisole selected positively against benzimidazole resistance. In these circumstances some form of alternation in the use of different anthelmintics could delay the development of a high level of resistance and maintain the practical usefulness of existing anthelmintics.     

Investigation of Acetylcholine Receptor Diversity in a Nematode Parasite Leads to Characterization of Tribendimidine- and Derquantel-Sensitive nAChRs

Nicotinic acetylcholine receptors (nAChRs) of parasitic nematodes are required for body movement and are targets of important “classical” anthelmintics like levamisole and pyrantel, as well as “novel” anthelmintics like tribendimidine and derquantel. Four biophysical subtypes of nAChR have been observed electrophysiologically in body muscle of the nematode parasite Oesophagostomum dentatum, but their molecular basis was not understood. Additionally, loss of one of these subtypes (G 35 pS) was found to be associated with levamisole resistance. In the present study, we identified and expressed in Xenopus oocytes, four O. dentatum nAChR subunit genes, Ode-unc-38, Ode-unc-63, Ode-unc-29 and Ode-acr-8, to explore the origin of the receptor diversity. When different combinations of subunits were injected in Xenopus oocytes, we reconstituted and characterized four pharmacologically different types of nAChRs with different sensitivities to the cholinergic anthelmintics. Moreover, we demonstrate that the receptor diversity may be affected by the stoichiometric arrangement of the subunits. We show, for the first time, different combinations of subunits from a parasitic nematode that make up receptors sensitive to tribendimidine and derquantel. In addition, we report that the recombinant levamisole-sensitive receptor made up of Ode-UNC-29, Ode-UNC-63, Ode-UNC-38 and Ode-ACR-8 subunits has the same single-channel conductance, 35 pS and 2.4 ms mean open-time properties, as the levamisole-AChR (G35) subtype previously identified in vivo. These data highlight the flexible arrangements of the receptor subunits and their effects on sensitivity and resistance to the cholinergic anthelmintics; pyrantel, tribendimidine and/or derquantel may still be effective on levamisole-resistant worms.