Pharmacology of anthelmintic resistance

Occurrence, ecology risk assessment and exposure evaluation of 19 anthelmintics in dust and soil from China

In order to fill the blank of domestic research on anthelmintics in dust and soil, 159 paired dust (including indoor and outdoor dust) and soil samples were collected nationwide. All 19 kinds of the anthelmintics were detected in the samples. The total concentration of the target substances in the outdoor dust, indoor dust and soil samples ranged from 1.83 to 1.30 × 10³ ng/g, from 2.99 to 6.00 × 10³ ng/g and from 0.23 to 8.03 × 10² ng/g, respectively. The total concentration of the 19 anthelmintics in northern China were significantly higher than those in southern China in the outdoor dust and soil samples. No significant correlation was found in the total concentration of anthelmintics between the indoor and outdoor dust because of strong human activities interference, however, a significant correlation existed between the outdoor dust and soil samples and between the indoor dust and soil samples. High ecological risk was found at 35% and 28% of all the sampling sites to non-target organisms in the soil respectively for IVE and ABA, and merits further study. The daily anthelmintics intakes were evaluated via ingestion and dermal contact of soil and dust samples for both children and adults. Ingestion was the predominant way for anthelmintics exposure, and the anthelmintics in soil and dust did not pose a health threat to human health at present.

A new enabling proteomics methodology to investigate membrane associated proteins from parasitic nematodes: case study using ivermectin resistant and ivermectin susceptible isolates of Caenorhabditis elegans and Haemonchus contortus

The mechanisms involved in anthelmintic resistance (AR) are complex but a greater understanding of AR management is essential for effective and sustainable control of parasitic helminth worms in livestock. Current tests to measure AR are time consuming and can be technically problematic, gold standard diagnostics are therefore urgently required to assist in combatting the threat from drug resistant parasites. For anthelmintics such as ivermectin (IVM), target proteins may be present in the cellular membrane. As proteins usually act in complexes and not in isolation, AR may develop and be measurable in the target associated proteins present in the parasite membrane. The model nematode Caenorhabditis elegans was used to develop a sub-proteomic assay to measure protein expression differences, between IVM resistant and IVM susceptible isolates in the presence and absence of drug challenge. Evaluation of detergents including CHAPS, ASB-14, C7BzO, Triton ×100 and TBP (tributyl phosphine) determined optimal conditions for the resolution of membrane proteins in Two Dimensional Gel Electrophoresis (2DE). These sub-proteomic methodologies were then translated and evaluated using IVM-susceptible and IVM-resistant Haemonchus contortus; a pathogenic blood feeding parasitic nematode which is of global importance in livestock health, welfare and productivity. We have demonstrated the successful resolution of membrane associated proteins from both C. elegans and H. contortus isolates, using a combination of CHAPS and the zwitterionic amphiphilic surfactant ASB-14 to further support the detection of markers for AR.

Functional analysis of genetic polymorphism in Wuchereria bancrofti glutathione S-transferase antioxidant gene: impact on protein structure and enzyme catalysis

Wuchereria bancrofti glutathione S-transferase (Wb-GST) is referred as a promising chemotherapeutic target for lymphatic filariasis. GST represents the major class of detoxifying enzymes of the tissue dwelling parasitic helminths. Though many inhibition studies were carried out for Wb-GST, understanding its genetic distribution in parasite population is necessary to develop ideal inhibitor. Our genetic polymorphic studies exposed the existence of three variant Wb-GST alleles in the four endemic regions of India. Moreover, it also revealed the variability in the distribution of Wb-GST alleles in the studied population. Therefore we cloned, expressed and purified the recombinant variant Wb-GST proteins to study the mutation impact on its structure and hence on its catalysis. Among the studied mutations, the I60F/G78S substitutions in the N-terminal domain and loop region connecting the two domains of Wb-GST lowered the affinity for glutathione and its analog, S-hexyl glutathione. Moreover, molecular modeling and docking studies revealed that the I60F/G78S mutations affected the proximity of Trp38 and Arg95 in glutathione binding site resulting in weaker interaction with S-hexyl glutathione. Besides, the variants also had lower affinity (Ki) and higher IC50 values for well-known GST inhibitors. Interestingly, the Wb-GST variant proteins showed enhanced catalytic efficiency for lipid peroxidation products which are produced due to oxidative stress. Thus, our study provides evidence for the functional impact of mutations on Wb-GST protein and also spotlights the mechanisms of parasite survival against the host oxidative stress environment.

Why don't we have a schistosomiasis vaccine?

Over the past 30 years there has been a concerted effort to understand host immune responses to schistosomes, with the ultimate aim of producing a vaccine for human use. In this issue, Bergquist and Colley, in summarizing recent meetings in Cairo, provide a detailed appraisal of progress towards that goal. It seems an appropriate time to ask why, with reference to Schistosoma mansoni, the development of a vaccine has proved so difficult. This question is explored here by Alan Wilson and Pat Coulson.

First transcriptomic analysis of the economically important parasitic nematode, Trichostrongylus colubriformis, using a next-generation sequencing approach

Trichostrongylus colubriformis (Strongylida), a small intestinal nematode of small ruminants, is a major cause of production and economic losses in many countries. The aims of the present study were to define the transcriptome of the adult stage of T. colubriformis, using 454 sequencing technology and bioinformatic analyses, and to predict the main pathways that key groups of molecules are linked to in this nematode. A total of 21,259 contigs were assembled from the sequence data produced from a normalized cDNA library; 7876 of these contigs had known orthologues in the free-living nematode Caenorhabditis elegans, and encoded, amongst others, proteins with 'transthyretin-like' (8.8%), 'RNA recognition' (8.4%) and 'metridin-like ShK toxin' (7.6%) motifs. Bioinformatic analyses inferred that relatively high proportions of the C. elegans homologues are involved in biological pathways linked to 'peptidases' (4%), 'ribosome' (3.6%) and 'oxidative phosphorylation' (3%). Highly represented were peptides predicted to be associated with the nervous system, digestion of host proteins or inhibition of host proteases. Probabilistic functional gene networking of the complement of C. elegans orthologues (n=2126) assigned significance to particular subsets of molecules, such as protein kinases and serine/threonine phosphatases. The present study represents the first, comprehensive insight into the transcriptome of adult T. colubriformis, which provides a foundation for fundamental studies of the molecular biology and biochemistry of this parasitic nematode as well as prospects for identifying targets for novel nematocides. Future investigations should focus on comparing the transcriptomes of different developmental stages, both genders and various tissues of this parasitic nematode for the prediction of essential genes/gene products that are specific to nematodes.

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

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

Environmental risk and toxicology of human and veterinary waste pharmaceutical exposure to wild aquatic host-parasite relationships

Pollution of the aquatic environment by human and veterinary waste pharmaceuticals is an increasing area of concern but little is known about their ecotoxicological effects on wildlife. In particular the interactions between pharmaceuticals and natural stressors of aquatic communities remains to be elucidated. A common natural stressor of freshwater and marine organisms are protozoan and metazoan parasites, which can have significant effects on host physiology and population structure, especially under the influence of many traditional kinds of toxic pollutants. However, little is known about the effects of waste pharmaceuticals to host-parasite dynamics. In order to assess the risk waste pharmaceuticals pose to aquatic wildlife it has been suggested the use of toxicological data derived from mammals during the product development of pharmaceuticals may be useful for predicting toxic effects. An additional similar source of information is the extensive clinical studies undertaken with numerous classes of drugs against parasites of human and veterinary importance. These studies may form the basis of preliminary risk assessments to aquatic populations and their interactions with parasitic diseases in pharmaceutical-exposed habitats. The present article reviews the effects of the most common classes of pharmaceutical medicines to host-parasite relationships and assesses the risk they may pose to wild aquatic organisms. In addition the effects of pharmaceutical mixtures, the importance of sewage treatment, and the risk of developing resistant strains of parasites are also assessed.

LC-MS-MS identification of albendazole and flubendazole metabolites formed ex vivo by Haemonchus contortus

Resistance of helminth parasites to common anthelminthics is a problem of increasing importance. The full mechanism of resistance development is still not thoroughly elucidated. There is also limited information about helminth enzymes involved in metabolism of anthelminthics. Identification of the metabolites formed by parasitic helminths can serve to specify which enzymes take part in biotransformation of anthelminthics and may participate in resistance development. The aim of our work was to identify the metabolic pathways of the anthelminthic drugs albendazole (ABZ) and flubendazole (FLU) in Haemonchus contortus, a world-wide distributed helminth parasite of ruminants. ABZ and FLU are benzimidazole anthelminthics commonly used in parasitoses treatment. In our ex vivo study one hundred living adults of H. contortus, obtained from the abomasum of an experimentally infected lamb, were incubated in 5 mL RPMI-1640 medium with 10 micromol L(-1) benzimidazole drug (10% CO(2), 38 degrees C) for 24 h. The parasite bodies were then removed from the medium. After homogenization of the parasites, both parasite homogenates and medium from the incubation were separately extracted using solid-phase extraction. The extracts were analyzed by liquid chromatography-mass spectrometry (LC-MS) with electrospray ionization (ESI) in positive-ion mode. The acquired data showed that H. contortus can metabolize ABZ via sulfoxidation and FLU via reduction of a carbonyl group. Albendazole sulfoxide (ABZSO) and reduced flubendazole (FLUR) were the only phase I metabolites detected. Concerning phase II of biotransformation, the formation of glucose conjugates of ABZ, FLU, and FLUR was observed. All metabolites mentioned were found in both parasite homogenates and medium from the incubation.

P-glycoprotein selection in strains of Haemonchus contortus resistant to benzimidazoles

Anthelmintic resistance in parasitic nematodes of livestock is a chronic problem in many parts of the world. Benzimidazoles are effective, broad-spectrum anthelmintics that bind to and selectively depolymerise microtubules. Resistance to the benzimidazoles, however, developed quickly and is caused by genetic changes in genes encoding beta-tubulins, subunits of microtubules. In Haemonchus contortus, resistance to avermectins has been correlated with genetic changes at a gene encoding a P-glycoprotein, a cell membrane transport protein that has a very high affinity for ivermectin. The substrate specificity of P-glycoprotein is very broad, and resistance to benzimidazoles can be modulated by lectins specific for P-glycoprotein. We investigated the possibility that genetic changes in P-glycoprotein might be correlated with benzimidazole resistance in nematodes. An analysis of restriction fragment length polymorphisms of a P-glycoprotein gene from a sensitive and a cambendazole-selected strain of H. contortus, derived from the sensitive strain, showed a significant difference in allele frequencies between strains. The frequency of one allele in particular increased substantially. The same allele was also found at a high frequency in an independently derived thiabendazole-selected field isolate. We present genetic evidence of selection at a P-glycoprotein locus during selection for benzimidzole resistance in H. contortus.

Anthelmintic resistance: The state of play revisited

Helminthosis is one of the major constraints in the successful wool and mutton industry throughout the world. Anthelmintic Resistance (AR) is said to have been established when previously effective drug ceases to kill exposed parasitic population at the therapeutically recommended dosages. Anthelmintic resistance is almost cosmopolitan in distribution and it has been reported in almost all species of domestic animals and even in some parasites of human beings. Some of the most important species of parasites of small ruminants in which AR has been reported include: Haemonchus spp., Trichostrongylus spp. Teladorsagia spp., Cooperia spp. Nematodirus spp., and Oesophagostomum spp. All the major groups of anthelmintics have been reported for development of variable degrees of resistance in different species of gastrointestinal nematodes. This paper describes the global scenario of prevalence and methods used for detection of AR in small ruminants. Different mechanisms and contributory factors for the development of AR are discussed. Various options and alternate strategies for the control and/or delay in the onset of AR are suggested in the light of available information.

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.

Synergism of rotenone by piperonyl butoxide in Haemonchus contortus and Trichostrongylus colubriformis in vitro: potential for drug-synergism through inhibition of nematode oxidative detoxification pathways

The anthelmintic properties of rotenone and its activity in combination with the cytochrome P450 inhibitor piperonyl butoxide, were examined in in vitro assays with adults and larvae of Haemonchus contortus and larvae of Trichostrongylus colubriformis. Rotenone was toxic to larvae of both species, with LC(50) values in larval development assays of 0.54 and 0.64 microg/ml for H. contortus and T. colubriformis, respectively. The compound also caused complete cessation of movement in adult H. contortus after 72 h at a concentration of 20 microg/ml. Toxicity of rotenone towards the larvae of both species was increased in the presence of piperonyl butoxide (synergism ratios of 3-4-fold at the LC(50)) and the activity against adult H. contortus was also significantly enhanced following pre-treatment with piperonyl butoxide. This significant synergism suggests that these nematode species are able to utilize a cytochrome P450 enzyme system to detoxify rotenone and indicates that a role may exist for cytochrome P450 inhibitors to act as synergists for other anthelmintics which are susceptible to oxidative metabolism within the nematode.

The larval feeding inhibition assay for the diagnosis of nematode anthelmintic resistance

A larval feeding assay for detection of nematode anthelmintic resistance to macrocyclic lactones and imidazothiazoles is described. The estimated concentration of anthelmintic required to inhibit larval feeding in 50% of L1's (IC50) that were resistant to either macrocyclic lactones or imidazothiazoles were significantly higher (P < or = 0.05) than those of susceptible isolates. Some variations in IC50 values were observed during the patent period of infection in all strains, although the pattern of the IC50 followed the same course. IC50 values varied in larvae developing from eggs shed throughout the patent period, with low values in the earliest larvae followed by higher values as the infection progressed, before decreasing at 70-90 days post-infection, although the low values of the first part of the patent period were not reached. However, the IC50 differences between all resistant and susceptible strains were significant throughout the whole patent period for ivermectin and levamisole. These results suggest that this technique may provide an alternative in vitro to detect anthelmintic resistance.

Monitoring drug efficacy and early detection of drug resistance in human soil-transmitted nematodes: a pressing public health agenda for helminth control

Control of soil-transmitted helminth infection and elimination of lymphatic filariasis by periodic chemotherapy increase drug pressure for possible occurrence of resistance against single dose anthelminthics. In veterinary practice, frequent treatment of closed populations has led to a serious problem of anthelminthic drug resistance which is now largely irreversible. Reduced efficacy of single dose drugs against nematodes of humans should be taken as early warnings to tackle the issue in due time. Research and development of sensitive tools for monitoring and early detection of drug resistance is urgently needed to sustain the benefits of helminth control programs gained so far. A concerted action with international partners and the creation of a network of scientists to address this issue is the next pressing public health issue for helminth control.

Structure of the major cytosolic glutathione S-transferase from the parasitic nematode Onchocerca volvulus

Onchocerciasis is a debilitating parasitic disease caused by the filarial worm Onchocerca volvulus. Similar to other helminth parasites, O. volvulus is capable of evading the host's immune responses by a variety of defense mechanisms, including the detoxification activities of the glutathione S-transferases (GSTs). Additionally, in response to drug treatment, helminth GSTs are highly up-regulated, making them tempting targets both for chemotherapy and for vaccine development. We analyzed the three-dimensional x-ray structure of the major cytosolic GST from O. volvulus (Ov-GST2) in complex with its natural substrate glutathione and its competitive inhibitor S-hexylglutathione at 1.5 and 1.8 angstrom resolution, respectively. From the perspective of the biochemical classification, the Ov-GST2 seems to be related to pi-class GSTs. However, in comparison to other pi-class GSTs, in particular to the host's counterpart, the Ov-GST2 reveals significant and unusual differences in the sequence and overall structure. Major differences can be found in helix alpha-2, an important region for substrate recognition. Moreover, the binding site for the electrophilic co-substrate is spatially increased and more solvent-accessible. These structural alterations are responsible for different substrate specificities and will form the basis of parasite-specific structure-based drug design investigations.

Variation and polymorphism in helminth parasites

There are strong biological, evolutionary and immunological arguments for predicting extensive polymorphism among helminth parasites, but relatively little data and few instances from which the selective forces acting on parasite diversity can be discerned. The paucity of information on intraspecific variation stands in contrast to the fine detail with which helminth species have been delineated by morphological techniques, accentuating a trend towards considering laboratory strains as representative of a relatively invariant organism. However, in the fast-moving evolutionary race between host and parasite one would predict a monomorphic species would be driven to extinction. We review the arena of intraspecific variation for the major helminth parasites, ranging from biological properties such as host or vector preference, to biochemical and immunological characteristics, as well as molecular markers such as DNA sequence variants. These data are summarized, before focusing in more detail on polymorphisms within protein-coding genes of potential relevance to the host-parasite relationship, such as vaccine candidates. In particular, we discuss the available data on a number of major antigens from the filarial nematode Brugia malayi. Information is currently too sparse to answer the question of whether there is antigenic variation in filariasis, but the indications are that proteins from the blood-borne microfilarial stage show significant intraspecific variability. Future work will define whether polymorphisms in these antigens may be driven by exposure to the host immune response or reflect some other facet of parasite biology.

Resistance to antiparasitic drugs: the role of molecular diagnosis

Chemotherapy is central to the control of many parasite infections of both medical and veterinary importance. However, control has been compromised by the emergence of drug resistance in several important parasite species. Such parasites cover a broad phylogenetic range and include protozoa, helminths and arthropods. In order to achieve effective parasite control in the future, the recognition and diagnosis of resistance will be crucial. This demand for early, accurate diagnosis of resistance to specific drugs in different parasite species can potentially be met by modern molecular techniques. This paper summarises the resistance status of a range of important parasites and reviews the available molecular techniques for resistance diagnosis. Opportunities for applying successes in some species to other species where resistance is less well understood are explored. The practical application of molecular techniques and the impact of the technology on improving parasite control are discussed.

Caenorhabditis elegans: how good a model for veterinary parasites?

The organism about which most is known on a molecular level is a nematode, the free-living organism Caenorhabditis elegans. This organism has served as a reasonable model for the discovery of anthelmintic drugs and for research on the mechanism of action of anthelmintics. Useful information on mechanisms of anthelmintic resistance has also been obtained from studies on C. elegans. Unfortunately, there has not been a large-scale extension of genetic techniques developed in C. elegans to research on parasitic species of veterinary (or human) parasites. Much can be learned about the essentials of nematode biology by studying C. elegans, but discovering the basic biology of nematode parasitism can only be gained through comparative studies on multiple parasitic species.

Caenorhabditis elegans and the study of gene function in parasites

The free-living nematode Caenorhabditis elegans is a tractable experimental model system for the study of both vertebrate and invertebrate biology. Its most significant advantages are its simplicity, both in anatomy and in genomic organization, and the elaborate methods that have been developed to attribute function to previously uncharacterized genes. Importantly, > 40% of parasitic nematode genes exhibit high levels of homology to genes within the C. elegans genome. Studying such genes using the C. elegans model should yield new insights into key molecules and their possible implications in parasite survival, leading to the discovery of new drug targets and vaccine candidates.

Identification of potential vaccine and drug target candidates by expressed sequence tag analysis and immunoscreening of Onchocerca volvulus larval cDNA libraries

The search for appropriate vaccine candidates and drug targets against onchocerciasis has so far been confronted with several limitations due to the unavailability of biological material, appropriate molecular resources, and knowledge of the parasite biology. To identify targets for vaccine or chemotherapy development we have undertaken two approaches. First, cDNA expression libraries were constructed from life cycle stages that are critical for establishment of Onchocerca volvulus infection, the third-stage larvae (L3) and the molting L3. A gene discovery effort was then initiated by random expressed sequence tag analysis of 5,506 cDNA clones. Cluster analyses showed that many of the transcripts were up-regulated and/or stage specific in either one or both of the cDNA libraries when compared to the microfilariae, L2, and both adult stages of the parasite. Homology searches against the GenBank database facilitated the identification of several genes of interest, such as proteinases, proteinase inhibitors, antioxidant or detoxification enzymes, and neurotransmitter receptors, as well as structural and housekeeping genes. Other O. volvulus genes showed homology only to predicted genes from the free-living nematode Caenorhabditis elegans or were entirely novel. Some of the novel proteins contain potential secretory leaders. Secondly, by immunoscreening the molting L3 cDNA library with a pool of human sera from putatively immune individuals, we identified six novel immunogenic proteins that otherwise would not have been identified as potential vaccinogens using the gene discovery effort. This study lays a solid foundation for a better understanding of the biology of O. volvulus as well as for the identification of novel targets for filaricidal agents and/or vaccines against onchocerciasis based on immunological and rational hypothesis-driven research.

Drug resistance in human helminths: current situation and lessons from livestock

In this review the available reports on drug resistance in human helminths, particularly hookworms and schistosomes, are critically analyzed. The experiences with helminths of livestock are then reviewed, in particular the factors contributing to the development of anthelmintic resistance, the mechanisms and genetics of resistance to various anthelmintic classes, and the methods available for detection. These experiences appear to be worryingly similar and relevant to the potential development of drug resistance in human helminths. Recommendations to reduce its risks are suggested.

Drug resistance in human helminths: current situation and lessons from livestock

In this review the available reports on drug resistance in human helminths, particularly hookworms and schistosomes, are critically analyzed. The experiences with helminths of livestock are then reviewed, in particular the factors contributing to the development of anthelmintic resistance, the mechanisms and genetics of resistance to various anthelmintic classes, and the methods available for detection. These experiences appear to be worryingly similar and relevant to the potential development of drug resistance in human helminths. Recommendations to reduce its risks are suggested.

The genetics of ivermectin resistance in Caenorhabditis elegans

The ability of organisms to evolve resistance threatens the effectiveness of every antibiotic drug. We show that in the nematode Caenorhabditis elegans, simultaneous mutation of three genes, avr-14, avr-15, and glc-1, encoding glutamate-gated chloride channel (GluCl) alpha-type subunits confers high-level resistance to the antiparasitic drug ivermectin. In contrast, mutating any two channel genes confers modest or no resistance. We propose a model in which ivermectin sensitivity in C. elegans is mediated by genes affecting parallel genetic pathways defined by the family of GluCl genes. The sensitivity of these pathways is further modulated by unc-7, unc-9, and the Dyf (dye filling defective) genes, which alter the structure of the nervous system. Our results suggest that the evolution of drug resistance can be slowed by targeting antibiotic drugs to several members of a multigene family.

Inheritance of avermectin resistance in Haemonchus contortus

A larval development assay was used to compare the responses of the Chiswick Avermectin Resistant (CAVRS) isolate of Haemonchus contortus, an avermectin-susceptible isolate (VRSG) and their crosses to avermectins. The F(1) and F(2) generations of reciprocal crosses between CAVRS and VRSG were denoted as CAVRS malesxVRSG females=CXV, and VRSG malesxCAVRS females=VXC. The levels of avermectin resistance in the developing larvae of the F(1) of both CXV and VXC were indistinguishable from that in the avermectin-resistant parent, indicating that the resistance trait is completely dominant. Avermectin dose-response curves for the CXV F(1) did not show a 50% mortality rate at low concentrations, indicating that avermectin resistance is not sex-linked. This conclusion was confirmed when adult male worms of the F(1) of the CXV mating were found to have survived treatment of the host with 200microgkg(-1) ivermectin. This dose rate (200microgkg(-1) ivermectin) caused a 50% reduction in the number of adult males in the F(1) from both CXV and VXC crosses, but only a non-significant reduction in the number of adult females in the F(1). Dose-response curves obtained for the F(2) generations in the larval development assay indicated the presence of 25% of avermectin-susceptible individuals, suggesting that a single major gene largely controls the avermectin-resistance trait. This genetic analysis of avermectin resistance in an Australian H. contortus isolate indicates that the expression of the gene for avermectin resistance is an autosomal, complete dominant in the larvae; however, in adults its expression is sex-influenced, with males having a lower resistance to avermectin than females.

Fasciolicides: efficacy, actions, resistance and its management

The modes of action of fasciolicides are described. Closantel and other salicylanilides interfere with energy metabolism by uncoupling oxidative phosphorylation in the fluke. Other fasciolicides are believed to have a metabolic action-halogenated phenols (via uncoupling) and clorsulon (via inhibition of glycolysis)-but direct evidence is lacking. Benzimidazoles (in particular, triclabendazole) bind to fluke tubulin and disrupt microtubule-based processes. Diamphenethide inhibits protein synthesis in the fluke. Other potential drug actions may contribute to overall drug efficacy. In particular, a number of fasciolicides-salicylanilides, phenols, diamphenethide-induce a rapid paralysis of the fluke, so their action may have a neuromuscular basis, although the actions remain ill-defined. Resistance to salicylanilides and triclabendazole has been detected in the field, although drug resistance does not appear to be a major problem yet. Strategies to minimize the development of resistance include the use of synergistic drug combinations, together with the design of integrated management programmes and the search for alternatives to drugs, in particular, vaccines.

Parasite diversity and anthelmintic resistance in two herds of horses

Diversity of parasite populations was compared between two herds of horses, one a regularly treated herd the other a feral herd which has bad no anthelmintic treatment for at least 25 years. Eggs obtained from fecal samples of both herds were tested for anthelmintic resistance by use of an in-vitro larval hatch/development assay (LDA), DrenchRite. A fecal egg reduction test was also performed with the domesticated herd using fenbendazole, pyrantel pamoate and ivermectin. Cyathostomes were the predominant group of worms present in both herds. Trichostrongylus axei was seen in both herds, but Strongylus equinus, Strongylus vulgaris, Gyalocephalus capitatus, Poteriostomum spp. and Strongyloides westeri were only found in the feral horses. Larvae of Strongylus edentatus were found in a single domesticated horse. Fecal egg reduction tests with the domesticated herd showed a 32% egg count reduction for fenbendazole, a 93% reduction with pyrantel, and a 99% reduction with ivermectin. From the LDA, anthelmintic resistance was evaluated by determining the resistance ratio of the domesticated herd compared with the feral herd. For benzimidazoles in the domesticated herd, 45% of the cyathostome population was 9.4 times more tolerant than the feral herd's parasite population. The parasite population in the domesticated herd was 1.5 times more tolerant to Levamisole, and 1.7 times more tolerant to the benzimidazole/levamisole combination than the parasite population within the feral herd. 9% of the parasite population in the domesticated herd was 90 times more tolerant to avermectins than the feral herd's parasite population, even though a subpopulation of worms in the feral herd were tolerant to low concentrations of avermectins despite never being previously exposed to this class of anthelmintic.

Pharmacology of anthelmintic resistance in cyathostomes: will it occur with the avermectin/milbemycins?

Anthelmintic-resistance has emerged as a problem in several animal industries. In the horse, cyathostome resistance to all available treatments except for the avermectin/milbemycins means that these drugs provide the cornerstone of control. Ivermectin has been available for several years; the related compound moxidectin is more recent. Although we do not know for sure, aspects of moxidectin such as its persistent action and its efficacy against mucosal stages of cyathostomes, may enhance the rate of development of resistance. On the other hand, selection pressure would be reduced if the persistence of moxidectin allows it to be used less frequently in the field. Reduced anthelmintic usage and surveillance of egg reappearance period are the most useful tools in managing resistance.

Frontiers in anthelmintic pharmacology

Research in anthelmintic pharmacology faces a grim future. The parent field of veterinary parasitology has seemingly been devalued by governments, universities and the animal industry in general. Primarily due to the success of the macrocyclic lactone anthelmintics in cattle, problems caused by helminth infections are widely perceived to be unimportant. The market for anthelmintics in other host species that are plagued by resistance, such as sheep and horses, is thought to be too small to sustain a discovery program in the animal health pharmaceutical industry. These attitudes are both alarming and foolish. The recent history of resistance to antibiotics provides more than adequate warning that complacency about the continued efficacy of any class of drugs for the chemotherapy of an infectious disease is folly. Parasitology remains a dominant feature of veterinary medicine and of the animal health industry. Investment into research on the basic and clinical pharmacology of anthelmintics is essential to ensure chemotherapeutic control of these organisms into the 21st century. In this article, we propose a set of questions that should receive priority for research funding in order to bring this field into the modern era. While the specific questions are open for revision, we believe that organized support of a prioritized list of research objectives could stimulate a renaissance in research in veterinary helminthology. To accept the status quo is to surrender.

Haemonchus contortus: effects of glutamate, ivermectin, and moxidectin on inulin uptake activity in unselected and ivermectin-selected adults

Using [(3)H]inulin uptake as a measure of pharyngeal pumping activity, we have investigated and compared the effects of glutamate, ivermectin, and moxidectin on inulin uptake in susceptible and ivermectin-selected Haemonchus contortus. Inulin uptake is inhibited by glutamate, ivermectin, and moxidectin, at biologically relevant concentrations. Glutamate influences the responses to both ivermectin and moxidectin, suggesting that these three substances share a common mechanism of action. The effects of ivermectin on inulin uptake, but not moxidectin, are significantly altered as a result of selection with ivermectin. These results suggest that ivermectin and moxidectin may differ, to some extent, in their mode of action responses or mechanisms of resistance.

Haemonchus contortus: characterization of a glutamate binding site in unselected and ivermectin-selected larvae and adults

A specific ivermectin-sensitive, glutamate binding site has been identified in the parasitic nematode Haemonchus contortus. Glutamate binding in H. contortus was saturable and occurred in a single class of high-affinity binding sites which appeared to have pharmacological properties different from those of mammalian glutamate receptors. Adult and larval forms of H. contortus had dramatically different glutamate binding kinetics, the larvae showing nearly up to 200-fold higher Bmax values and up to 9-fold increases in Kd values compared to adults. Treatment of adult H. contortus with the anthelmintic, ivermectin, decreased the Bmax value for glutamate binding in the susceptible strain but not in the resistant parasites. Furthermore, selection for ivermectin resistance was associated with a significant increase in Bmax for glutamate binding in adults and a similarly significant increase in glutamate binding affinity in larvae. These results suggest that the H. contortus glutamate binding site identified in this study may be involved in the phenomenon of ivermectin resistance.

Resistance to levamisole resolved at the single-channel level

Levamisole is commonly used to treat nematode parasite infections but therapy is limited by resistance. The purpose of this study was to determine the mechanism of resistance to this selective nicotinic drug. Levamisole receptor channel currents in muscle patches from levamisole-sensitive and levamisole-resistant isolates of the parasitic nematode Oesophagostomum dentatum were compared. The number of channels present in patches of sensitive and resistant isolates was similar at 10 microM levamisole, but at 30 microM and 100 microM the resistant isolate contained fewer active patches, suggesting desensitization. Mean Po and open times were reduced in resistant isolates. The distribution of conductances of channels in the sensitive isolate revealed a heterogeneous receptor population and the presence of G25, G35, G40, and G45 subtypes. A G35 subtype was missing in the resistant isolate. Resistance to levamisole was produced by changes in the averaged properties of the levamisole receptor population, with some receptors from sensitive and resistant isolates having indistinguishable characteristics.

Pharmacology of anthelmintic resistance

Anthelmintic resistance has grown from a curiosity to an important economic problem in several animal industries and is now set to threaten the control of human parasites. The pharmacology of anthelmintics and anthelmintic resistance has been studied most extensively in the nematode parasites of sheep. Here, Nick Sangster and Jenny Gill discuss this veterinary experience, summarizing the progress made in understanding anthelmintic resistance and highlighting the tools available for research.

Moxidectin: persistence and efficacy against drug-resistant Ostertagia circumcincta

In order to determine whether the efficacy of moxidectin against Ostertagia circumcincta is enhanced by its persistency, therapeutic efficacy was compared at intervals after treatment and with that of ivermectin, a closely related but more transient endectocide. Groups of 7-month-old New Zealand Romney lambs were infected with a strain of O. circumcincta known to be resistant to moxidectin. At patency of the infections, groups of lambs were treated with either moxidectin or ivermectin at the manufacturer's recommended dosages, or left untreated. At 3, 6 and 10 days post-treatment, faecal egg count was measured and groups of lambs were slaughtered for estimation of adult worm burden. Drug-resistant worm burdens were significantly reduced in those animals treated with moxidectin but not in those treated with ivermectin. No effect of time of slaughter on worm burden was observed with either drug, demonstrating that the higher therapeutic efficacy of moxidectin against this parasite was not due to an increased period of drug exposure. Faecal egg counts in the moxidectin treated animals increased with time after treatment indicating a temporary suppression of egg output by surviving worms. The implications of these findings on selection for anthelmintic resistance are discussed.

Anthelmintic resistance: past, present and future

Anthelmintic resistance continues to increase in geographic range, in the number of species affected and the range of drugs involved. Several aspects of resistance have emerged as important issues. They include lack of genetic reversion, presence of side resistance and lack of universality. Furthermore, resistant isolates recovered from the field may have different characteristics to those selected in pen passage. Research into anthelmintic resistance has not progressed far beyond the stage of descriptive research. Some progress has been made in developing control strategies and in diagnosing resistance, especially in the development and adoption of in-vitro tests. However, these still need improvements in their ability to detect resistance to closantel and avermectin/milbemycin anthelmintics. Less progress into understanding the basis of resistance has occurred. Research priorities include improvement of diagnostic tests and the development of molecular tests, particularly for resistance to levamisole and the avermectin/milbemycins. Resistance itself, as a selectable marker for genetic transfection in parasites, is a potential tool for investigating parasite biology.

Parasitic peptides! The structure and function of neuropeptides in parasitic worms

Parasitic worms come from two very different phyla-Platyhelminthes (flatworms) and Nematoda (roundworms). Although both phyla possess nervous systems with highly developed peptidergic components, there are key differences in the structure and action of native neuropeptides in the two groups. For example, the most abundant neuropeptide known in platyhelminths is the pancreatic polypeptide-like neuropeptide F, whereas the most prevalent neuropeptides in nematodes are FMRFamide-related peptides (FaRPs), which are also present in platyhelminths. With respect to neuropeptide diversity, platyhelminth species possess only one or two distinct FaRPs, whereas nematodes have upwards of 50 unique FaRPs. FaRP bioactivity in platyhelminths appears to be restricted to myoexcitation, whereas both excitatory and inhibitory effects have been reported in nematodes. Recently interest has focused on the peptidergic signaling systems of both phyla because elucidation of these systems will do much to clarify the basic biology of the worms and because the peptidergic systems hold the promise of yielding novel targets for a new generation of antiparasitic drugs.

Genetic and biochemical evidence for a novel avermectin-sensitive chloride channel in Caenorhabditis elegans. Isolation and characterization

Avermectins are a class of macrocyclic lactones that is widely used in crop protection and to treat helminth infections in man and animals. Two complementary DNAs (GluClalpha and GluClbeta) encoding chloride channels that are gated by avermectin and glutamate, respectively, were isolated from Caenorhabditis elegans. To study the role of these subunits in conferring avermectin sensitivity we isolated a mutant C. elegans strain with a Tc1 transposable element insertion that functionally inactivated the GluClalpha gene (GluClalpha::Tc1). GluClalpha::Tc1 animals exhibit a normal phenotype including typical avermectin sensitivity. Xenopus oocytes expressing GluClalpha::Tc1 strain mRNA elicited reduced amplitude avermectin and glutamate-dependent chloride currents. Avermectin binding assays in GluClalpha::Tc1 strain membranes showed the presence of high affinity binding sites, with a reduced Bmax. These experiments suggest that GluClalpha is a target for avermectin and that additional glutamate-gated and avermectin-sensitive chloride channel subunits exist in C. elegans. We isolated a cDNA (GluClalpha2) encoding a chloride channel that shares 75% amino acid identity with GluClalpha. This subunit forms homomeric channels that are gated irreversibly by avermectin and reversibly by glutamate. GluClalpha2 coassembles with GluClbeta to form heteromeric channels that are gated by both ligands. The presence of subunits related to GluClalpha may explain the low level and rarity of target site involvement in resistance to the avermectin class of compounds.

mle-1, a mariner-like transposable element in the nematode Trichostrongylus colubriformis

A mariner-like element termed mle-1 was discovered in the parasitic nematode Trichostrongylus colubriformis. The mle-1 has features which support its assignment as a mariner-like transposable element. Cloned mle-1 was derived from an intron of the tar-1 gene. It comprises 893 bp, includes two 27 bp flanking perfect inverted repeats and is present at approximately 50 copies in the genome. The element contains a coding region which displays homology to transposases, with the greatest amino acid similarity to a Caenorhabditis elegans mariner-like transposase. The coding region contains two 12 bp repeats; these repeats flank an 11 bp segment which accounts for a frameshift in this region. As a candidate transposon, mle-1 provides potential for genetic manipulation of this and related organisms.

Haemonchus contortus: the uptake and metabolism of closantel

Closantel is an anthelmintic which associates with plasma albumin and is useful for the control of sheep parasites, such as Haemonchus contortus, that ingest blood. However, the utility of closantel for parasite control has been threatened by the emergence of resistance. The mechanisms of resistance are unknown. A closantel-resistant and a closantel-susceptible isolate of H. contortus were compared with respect to the distribution and metabolism of closantel. Neither strain appeared to metabolise closantel in vitro or in vivo. Following treatment of infected sheep with radioactively labelled closantel, isotope levels in closantel-resistant adult H. contortus were significantly lower than in susceptible worms. This reduced accumulation of drug could contribute to closantel resistance by mechanisms such as reduced feeding, failure to dissociate the drug-albumin complex in the gut or increased efflux of closantel from resistant worms.