Inheritance of avermectin resistance in Haemonchus contortus

Effect of Ivermectin on the Expression of P-Glycoprotein in Third-Stage Larvae of Haemonchus contortus Isolated from China

Haemonchus contortus poses a severe hazard to the healthy development of the sheep industry and threatens the welfare of sheep. Ivermectin is the primary anthelmintic used for the prevention and treatment of H. contortus parasitism. However, the widespread and uncontrolled application of ivermectin has resulted in the development and spread of resistant strains of H. contortus. P-glycoprotein (P-gp) plays important roles in the pharmacology and toxicology of ivermectin, and changes in P-gp expression levels can be used to analyze the resistance of H. contortus to ivermectin. This study aimed to analyze the effects of ivermectin on P-gp expression in H. contortus L3 larvae isolated from China and to evaluate whether changes in P-gp expression levels can be used to analyze resistant H. contortus strains. In the absence of drug treatment, the ivermectin-resistant strains isolated in China showed increased expression of P-gp11 (p < 0.01) compared with sensitive strains from elsewhere, whereas the expressions of P-gp2 and P-gp9.1 were downregulated (p < 0.01). When the same strain was compared before and after drug treatment, obvious differences in expression were observed between the different strains. Ivermectin-induced P-gp expression was found to be very complex among the L3 larvae of different strains. In addition, it was confirmed that using P-gp to determine ivermectin resistance in H. contortus strains from different geographic environments can yield different results.

[Five past twelve - the resistance situation in small ruminant gastrointestinal nematodes in Germany]

Anthelmintic resistance has reached alarming levels worldwide and already seriously threatens pasture-based small ruminant production in certain geographic regions. The situation in Europe has also dramatically deteriorated in the last decade. This review provides an overview of the occurrence of anthelmintic resistance in small ruminants with a particular focus on Germany and its neighbouring countries. It also covers mechanisms leading to the development and spread of anthelmintic resistance, and recommendations for a responsible use of anthelmintics in veterinary practice.

Genomic landscape of drug response reveals mediators of anthelmintic resistance

Like other pathogens, parasitic helminths can rapidly evolve resistance to drug treatment. Understanding the genetic basis of anthelmintic drug resistance in parasitic nematodes is key to tracking its spread and improving the efficacy and sustainability of parasite control. Here, we use an in vivo genetic cross between drug-susceptible and multi-drug-resistant strains of Haemonchus contortus in a natural host-parasite system to simultaneously map resistance loci for the three major classes of anthelmintics. This approach identifies new alleles for resistance to benzimidazoles and levamisole and implicates the transcription factor cky-1 in ivermectin resistance. This gene is within a locus under selection in ivermectin-resistant populations worldwide; expression analyses and functional validation using knockdown experiments support that cky-1 is associated with ivermectin survival. Our work demonstrates the feasibility of high-resolution forward genetics in a parasitic nematode and identifies variants for the development of molecular diagnostics to combat drug resistance in the field.

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

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

Transcriptomic analyses implicate neuronal plasticity and chloride homeostasis in ivermectin resistance and response to treatment in a parasitic nematode

The antiparasitic drug ivermectin plays an essential role in human and animal health globally. However, ivermectin resistance is widespread in veterinary helminths and there are growing concerns of sub-optimal responses to treatment in related helminths of humans. Despite decades of research, the genetic mechanisms underlying ivermectin resistance are poorly understood in parasitic helminths. This reflects significant uncertainty regarding the mode of action of ivermectin in parasitic helminths, and the genetic complexity of these organisms; parasitic helminths have large, rapidly evolving genomes and differences in evolutionary history and genetic background can confound comparisons between resistant and susceptible populations. We undertook a controlled genetic cross of a multi-drug resistant and a susceptible reference isolate of Haemonchus contortus, an economically important gastrointestinal nematode of sheep, and ivermectin-selected the F2 population for comparison with an untreated F2 control. RNA-seq analyses of male and female adults of all populations identified high transcriptomic differentiation between parental isolates, which was significantly reduced in the F2, allowing differences associated specifically with ivermectin resistance to be identified. In all resistant populations, there was constitutive upregulation of a single gene, HCON_00155390:cky-1, a putative pharyngeal-expressed transcription factor, in a narrow locus on chromosome V previously shown to be under ivermectin selection. In addition, we detected sex-specific differences in gene expression between resistant and susceptible populations, including constitutive upregulation of a P-glycoprotein, HCON_00162780:pgp-11, in resistant males only. After ivermectin selection, we identified differential expression of genes with roles in neuronal function and chloride homeostasis, which is consistent with an adaptive response to ivermectin-induced hyperpolarisation of neuromuscular cells. Overall, we show the utility of a genetic cross to identify differences in gene expression that are specific to ivermectin selection and provide a framework to better understand ivermectin resistance and response to treatment in parasitic helminths.

Parasitic helminths (worms) infect people and animals throughout the world and are largely controlled with mass administration of anthelmintic drugs. There are a very limited number of anthelmintics available and parasitic helminths can rapidly develop resistance to these drugs. Ivermectin is a widely used anthelmintic in both humans and animals, but resistance is now widespread in the veterinary field. We crossed ivermectin resistant and ivermectin susceptible parasitic helminths and treated them with ivermectin or left them as untreated controls. This provided resistant and susceptible populations with a similar genetic background with which to study differences in gene expression associated with ivermectin resistance. We identified upregulation of a gene with no previous association with drug resistance (HCON_00155390:cky-1) in male and female worms in all resistant populations. This gene is thought to be expressed in the helminth pharynx (mouthpart) and, in mammals, plays a role in controlling nerve function and protecting nerves from damage. This is consistent with the known effects of ivermectin in inhibiting helminth feeding through pharyngeal paralysis and may implicate a novel mechanism that allows resistant worms to survive treatment.

Emerging Anthelmintic Resistance in Poultry: Can Ethnopharmacological Approaches Offer a Solution?

Limited pharmacological studies have been conducted on plant species used against poultry helminths. The objective of this study was to provide a basis for plant based anthelmintics as possible alternatives against poultry anthelmintic resistance. The study justified the need for alternative anthelmintics. The study places emphasis on the increasing anthelmintic resistance, mechanism of resistance, and preparational protocols for plant anthelmintics and their associated mechanism of action. Pharmaceutical studies on plants as alternative therapies for the control of helminth parasites have not been fully explored especially in several developing countries. Plants from a broad range of species produce a wide variety of compounds that are potential anthelmintics candidates. Important phenolic acids have been found in Brassica rapa L. and Terminalia avicenniodes Guill. and Perri that affect the cell signaling pathways and gene expression. Benzo (c) phenanthridine and isoquinoline alkaloids are neurotoxic to helminths. Steroidal saponins (polyphyllin D and dioscin) interact with helminthic mitochondrial activity, alter cell membrane permeability, vacuolation and membrane damage. Benzyl isothiocyanate glucosinolates interfere with DNA replication and protein expression, while isoflavones from Acacia oxyphylla cause helminth flaccid paralysis, inhibit energy generation, and affect calcium utilization. Condensed tannins have been shown to cause the death of nematodes and paralysis leading to expulsion from the gastro-intestinal tract. Flavonoids from Chenopodium album L and Mangifera indica L act through the action of phosphodiesterase and Ca2+-ATPase, and flavonoids and tannins have been shown to act synergistically and are complementary to praziquantel. Artemisinins from Artemisia cina O. Berg are known to disrupt mitochondrial ATP production. Terpenoids from Cucurbita moschata L disrupt neurotransmission leading to paralysis as well as disruption of egg hatching. Yeast particle encapsulated terpenes are effective for the control of albendazole-resistant helminths.

Benchmarking Australian sheep parasite control: Changes in gastrointestinal nematode control practices reported from surveys between 2003 and 2019

Cross-sectional surveys of sheep parasite control practices in Australia were conducted in 2004, 2012 and 2019 to document parasite problems, control practices and measure change over time. This article reports the results pertaining to gastrointestinal nematode infection; comparisons between years are mostly descriptive and not based on statistical inference. There was a general increase in the use of grazing management to prepare clean pastures for sheep to control gastrointestinal nematode infections with the largest increases in the use of: cropping, long acting anthelmintics to prepare clean pastures, feeding strategies, selecting rams for resistance to nematodes, and leaving some sheep untreated. The proportion of respondents using faecal worm egg count monitoring (WEC) and the number of WEC monitors per year were similar in 2003 (weaners: 3.0 WEC/year, adult ewes: 2.6 WEC/year) and 2018 (lambs and weaners and adult ewes both 3.1 WEC/year) but lower in 2011(lambs and weaners: 2.0 WEC/year, adult ewes: 2.9 WEC/year), whilst there was a higher number of anthelmintic treatments given in 2011 than 2003 and 2018 which were similar. There was an increase in the proportion of respondents carrying out WEC monitors themselves from 2011 (21%) to 2018 (30%) and a corresponding reduction in the use of private laboratories, government laboratories and veterinarians with regional differences in who carried out WECs. The top three anthelmintic groups used did not differ between the three survey years with macrocyclic lactones the most used followed by benzimidazoles (BZ) and levamisole (LEV) although the percentage of MLs used in 2011 and 2018 was lower than in 2003 with higher proportions of respondents using BZ and LEV groups in the latter two survey years. Moxidectin was in the most commonly used active in all survey years. There was an increase in the use of combination of anthelmintics from different groups, especially for a combination of three anthelmintics (2003: 4.4%, 2011: 19.1%, 2018: 21.5%), with lower use of single anthelmintics in 2011 (57.0%) and 2018 (55.4%) compared with 2003 (74.5%). The use of testing for anthelmintic resistance was generally low across the survey years with a lower proportion of respondents using tests in 2011 and 2018 than in 2003 (2003:48%, 2011: 29%, 2018: 37%). Time of year, results from WEC and seasonal weather condition were the most important factors when deciding when to apply anthelmintic treatment.

A journey through 50 years of research relevant to the control of gastrointestinal nematodes in ruminant livestock and thoughts on future directions

This review article provides an historical perspective on some of the major research advances of relevance to ruminant livestock gastrointestinal nematode control over the last 50 years. Over this period, gastrointestinal nematode control has been dominated by the use of broad-spectrum anthelmintic drugs. Whilst this has provided unprecedented levels of successful control for many years, this approach has been gradually breaking down for more than two decades and is increasingly unsustainable which is due, at least in part, to the emergence of anthelmintic drug resistance and a number of other factors discussed in this article. We first cover the remarkable success story of the discovery and development of broad-spectrum anthelmintic drugs, the changing face of anthelmintic drug discovery research and the emergence of anthelmintic resistance. This is followed by a review of some of the major advances in the increasingly important area of non-pharmaceutical gastrointestinal nematode control including immunology and vaccine development, epidemiological modelling and some of the alternative control strategies such as breeding for host resistance, refugia-based methods and biological control. The last 50 years have witnessed remarkable innovation and success in research aiming to improve ruminant livestock gastrointestinal nematode control, particularly given the relatively small size of the research community and limited funding. In spite of this, the growing global demand for livestock products, together with the need to maximise production efficiencies, reduce environmental impacts and safeguard animal welfare - as well as specific challenges such as anthelmintic drug resistance and climate change- mean that gastrointestinal nematode researchers will need to be as innovative in the next 50 years as in the last.

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.

Emodepside has sex-dependent immobilizing effects on adult Brugia malayi due to a differentially spliced binding pocket in the RCK1 region of the SLO-1 K channel

Filariae are parasitic nematodes that are transmitted to their definitive host as third-stage larvae by arthropod vectors like mosquitoes. Filariae cause diseases including: lymphatic filariasis with distressing and disturbing symptoms like elephantiasis; and river blindness. Filarial diseases affect millions of people in 73 countries throughout the topics and sub-tropics. The drugs available for mass drug administration, (ivermectin, albendazole and diethylcarbamazine), are ineffective against adult filariae (macrofilariae) at the registered dosing regimen; this generates a real and urgent need to identify effective macrofilaricides. Emodepside, a veterinary anthelmintic registered for treatment of nematode infections in cats and dogs, is reported to have macrofilaricidal effects. Here, we explore the mode of action of emodepside using adult Brugia malayi, one of the species that causes lymphatic filariasis. Whole-parasite motility measurement with Worminator and patch-clamp of single muscle cells show that emodepside potently inhibits motility by activating voltage-gated potassium channels and that the male is more sensitive than the female. RNAi knock down suggests that emodepside targets SLO-1 K channels. We expressed slo-1 isoforms, with alternatively spliced exons at the RCK1 (Regulator of Conductance of Potassium) domain, heterologously in Xenopus laevis oocytes. We discovered that the slo-1f isoform, found in muscles of males, is more sensitive to emodepside than the slo-1a isoform found in muscles of females; and selective RNAi of the slo-1a isoform in female worms increased emodepside potency. In Onchocerca volvulus, that causes river blindness, we found two isoforms in adult females with homology to Bma-SLO-1A and Bma-SLO-1F at the RCK1 domain. In silico modeling identified an emodepside binding pocket in the same RCK1 region of different species of filaria that is affected by these splice variations. Our observations show that emodepside has potent macrofilaricidal effects and alternative splicing in the RCK1 binding pocket affects potency. Therefore, the evaluation of potential sex-dependent effects of an anthelmintic compound is of importance to prevent any under-dosing of one or the other gender of nematodes once given to patients.

Population genomic and evolutionary modelling analyses reveal a single major QTL for ivermectin drug resistance in the pathogenic nematode, Haemonchus contortus

BACKGROUND

Infections with helminths cause an enormous disease burden in billions of animals and plants worldwide. Large scale use of anthelmintics has driven the evolution of resistance in a number of species that infect livestock and companion animals, and there are growing concerns regarding the reduced efficacy in some human-infective helminths. Understanding the mechanisms by which resistance evolves is the focus of increasing interest; robust genetic analysis of helminths is challenging, and although many candidate genes have been proposed, the genetic basis of resistance remains poorly resolved.

RESULTS

Here, we present a genome-wide analysis of two genetic crosses between ivermectin resistant and sensitive isolates of the parasitic nematode Haemonchus contortus, an economically important gastrointestinal parasite of small ruminants and a model for anthelmintic research. Whole genome sequencing of parental populations, and key stages throughout the crosses, identified extensive genomic diversity that differentiates populations, but after backcrossing and selection, a single genomic quantitative trait locus (QTL) localised on chromosome V was revealed to be associated with ivermectin resistance. This QTL was common between the two geographically and genetically divergent resistant populations and did not include any leading candidate genes, suggestive of a previously uncharacterised mechanism and/or driver of resistance. Despite limited resolution due to low recombination in this region, population genetic analyses and novel evolutionary models supported strong selection at this QTL, driven by at least partial dominance of the resistant allele, and that large resistance-associated haplotype blocks were enriched in response to selection.

CONCLUSIONS

We have described the genetic architecture and mode of ivermectin selection, revealing a major genomic locus associated with ivermectin resistance, the most conclusive evidence to date in any parasitic nematode. This study highlights a novel genome-wide approach to the analysis of a genetic cross in non-model organisms with extreme genetic diversity, and the importance of a high-quality reference genome in interpreting the signals of selection so identified.

A Genome Resequencing-Based Genetic Map Reveals the Recombination Landscape of an Outbred Parasitic Nematode in the Presence of Polyploidy and Polyandry

The parasitic nematode Haemonchus contortus is an economically and clinically important pathogen of small ruminants, and a model system for understanding the mechanisms and evolution of traits such as anthelmintic resistance. Anthelmintic resistance is widespread and is a major threat to the sustainability of livestock agriculture globally; however, little is known about the genome architecture and parameters such as recombination that will ultimately influence the rate at which resistance may evolve and spread. Here, we performed a genetic cross between two divergent strains of H. contortus, and subsequently used whole-genome resequencing of a female worm and her brood to identify the distribution of genome-wide variation that characterizes these strains. Using a novel bioinformatic approach to identify variants that segregate as expected in a pseudotestcross, we characterized linkage groups and estimated genetic distances between markers to generate a chromosome-scale F1 genetic map. We exploited this map to reveal the recombination landscape, the first for any helminth species, demonstrating extensive variation in recombination rate within and between chromosomes. Analyses of these data also revealed the extent of polyandry, whereby at least eight males were found to have contributed to the genetic variation of the progeny analyzed. Triploid offspring were also identified, which we hypothesize are the result of nondisjunction during female meiosis or polyspermy. These results expand our knowledge of the genetics of parasitic helminths and the unusual life-history of H. contortus, and enhance ongoing efforts to understand the genetic basis of resistance to the drugs used to control these worms and for related species that infect livestock and humans throughout the world. This study also demonstrates the feasibility of using whole-genome resequencing data to directly construct a genetic map in a single generation cross from a noninbred nonmodel organism with a complex lifecycle.

Recent advances in candidate-gene and whole-genome approaches to the discovery of anthelmintic resistance markers and the description of drug/receptor interactions

Anthelmintic resistance has a great impact on livestock production systems worldwide, is an emerging concern in companion animal medicine, and represents a threat to our ongoing ability to control human soil-transmitted helminths. The Consortium for Anthelmintic Resistance and Susceptibility (CARS) provides a forum for scientists to meet and discuss the latest developments in the search for molecular markers of anthelmintic resistance. Such markers are important for detecting drug resistant worm populations, and indicating the likely impact of the resistance on drug efficacy. The molecular basis of resistance is also important for understanding how anthelmintics work, and how drug resistant populations arise. Changes to target receptors, drug efflux and other biological processes can be involved. This paper reports on the CARS group meeting held in August 2013 in Perth, Australia. The latest knowledge on the development of molecular markers for resistance to each of the principal classes of anthelmintics is reviewed. The molecular basis of resistance is best understood for the benzimidazole group of compounds, and we examine recent work to translate this knowledge into useful diagnostics for field use. We examine recent candidate-gene and whole-genome approaches to understanding anthelmintic resistance and identify markers. We also look at drug transporters in terms of providing both useful markers for resistance, as well as opportunities to overcome resistance through the targeting of the transporters themselves with inhibitors. Finally, we describe the tools available for the application of the newest high-throughput sequencing technologies to the study of anthelmintic resistance.

Trichostrongylus and Haemonchus anthelmintic resistance in naturally infected sheep from southern Brazil

The anthelmintic resistance in small ruminants is a common problem and concern worldwide. The aim of this study was to verify anthelmintic treatment efficacy in naturally infected sheep. This study was conducted on nine herds that used the same anthelmintic management for over a year. In each farm, the animals were divided into two groups: untreated control group (n = 5) and treated (n = 10) according to the number of eggs per gram of feces (EPG). The treatment effect was checked based on EPG results and larval culture performed before treatment and 10 days after treatment. Significant differences were not observed (P> 0.05) on EPG results between untreated and treated groups. The coproculture showed that the animals were infected primarily byHaemonchus spp., Trichostrongylus spp.,Teladorsagia spp., Cooperia spp. andOesophagostomum spp. In all farms, anthelmintic resistance by genera Haemonchus and Trichostrongylus was found, but this resistance varied greatly between farms.Haemonchus spp. showed resistance to closantel, levamisole, and albendazole. Trichostrongylus spp. was shown to be resistant to closantel, levamisole, and albendazole. The drugs tested showed to be efficient against the genera Teladorsagia,Cooperia, and Oesophagostomum. Based on these results, we conclude that the anthelmintic resistance to the tested drugs is a problem present in the farms evaluated.

Sea lice population and sex differences in P-glycoprotein expression and emamectin benzoate resistance on salmon farms in the Bay of Fundy, New Brunswick, Canada

BACKGROUND

Parasitic sea lice are a major challenge for salmon aquaculture. This is especially due to the recent development of resistance to emamectin benzoate (EMB) in the parasite. We investigated: (1) whether EMB treatment success in Grand Manan, Bay of Fundy, NB, Canada can be explained through EMB bioassay and P-glycoprotein (P-gp) mRNA expression studies; (2) if other populations of sea lice not under EMB selective pressure possess similar EMB sensitivity as Grand Manan sea lice populations; and (3) the heritability of EMB resistance in Lepeophtheirus salmonis.

RESULTS

EMB bioassay results indicated population, species, sex and temporal differences in EMB EC50 values. RT-qPCR analyses revealed population and sex differences in P-gp mRNA levels, correlating with the bioassay results. Laboratory-reared sea lice maintained their EMB sensitivity status up to the F3 generation. Caligus elongatus, collected from Grand Manan showed more than twofold lower EMB EC50 values compared with L. salmonis collected from the same site. Concurrent exposure to EMB and verapamil yielded no increase in C. elongatus sensitivity to the parasiticide.

CONCLUSION

Sea lice bioassay and P-gp mRNA studies can be used to track EMB resistance and sex differences in EMB sensitivity and P-gp mRNA levels exist in the parasite.

Haemonchus contortus as a paradigm and model to study anthelmintic drug resistance

Anthelmintic resistance is a major problem for the control livestock parasites and a potential threat to the sustainability of community-wide treatment programmes being used to control human parasites in the developing world. Anthelmintic resistance is essentially a complex quantitative trait in which multiple mutations contribute to the resistance phenotype in an additive manner. Consequently, a combination of forward genetic and genomic approaches are needed to identify the causal mutations and quantify their contribution to the resistance phenotype. Therefore, there is a need to develop genetic and genomic approaches for key parasite species identified as relevant models. Haemonchus contortus, a gastro-intestinal parasite of sheep, has shown a remarkable propensity to develop resistance to all the drugs used in its control. Partly because of this, and partly because of its experimental amenability, research on this parasite has contributed more than any other to our understanding of anthelmintic resistance. H. contortus offers a variety of advantages as an experimental system including the ability to undertake genetic crosses; a prerequisite for genetic mapping. This review will discuss the current progress on developing H. contortus as a model system in which to study anthelmintic resistance.

A single sex-linked dominant gene does not fully explain the codling moth's resistance to granulovirus

BACKGROUND

In 2004, resistance to a commercial formulation of the Cydia pomonella granulovirus (CpGV) was identified in a field population of Cydia pomonella from an organic orchard in southern France. The genetic inheritance of this resistance was analysed in the resistant laboratory strain RGV. This strain was obtained using successive crosses between the resistant field population and a susceptible laboratory strain, SV, with selection for CpGV resistance at each generation.

RESULTS

After eight generations of introgression of the resistant trait into SV, the RGV-8 strain exhibited 7000-fold higher resistance than SV. Mass-crossing experiments showed that resistance to CpGV is strongly dominant, sex dependent and under the control of a single major gene. However, the contribution of other genes is required to explain all of the data obtained in this study. These additional genes do not follow the laws of classical Mendelian transmission.

CONCLUSION

Transmission of granulovirus resistance in the RGV-8 strain of C. pomonella cannot be fully explained by the effect of a locus located on the Z chromosome. The action of other factors needs to be considered.

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

•

Ivermectin and its two components contribute to action and resistance.

•

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

•

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

•

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.

Detection of emamectin benzoate tolerance emergence in different life stages of sea lice, Lepeophtheirus salmonis, on farmed Atlantic salmon, Salmo salar L

Emamectin benzoate has been used to treat sea lice, Lepeophtheirus salmonis, infestations on farmed Atlantic salmon, Salmo salar. Recent evidence suggests a reduction in effectiveness in some locations. A major challenge in the detection of tolerance emergence can be the typically low proportion of resistant individuals in a population during the early phases. The objectives of this study were to develop a method for determining differences in temporal development of tolerance between sea lice life stages and to explore how these differences might be used to improve the monitoring of treatment effectiveness in a clinical setting. This study examined two data sets based on records of sea lice abundance following emamectin benzoate treatments from the west coast of Scotland (2002-2006) and from New Brunswick, Canada (2004-2008). Life stages were categorized into two groups (adult females and the remaining mobile stages) to examine the trends in mean abundance and treatment effectiveness. Differences in emamectin benzoate effectiveness were found between the two groups by year and location, suggesting that an important part of monitoring drug resistance development in aquatic ectoparasites may be the need to focus on key life stages.

Single nucleotide polymorphisms in β-tubulin selected in Onchocerca volvulus following repeated ivermectin treatment: possible indication of resistance selection

The control of onchocerciasis or river blindness by mass treatment of the population with ivermectin (IVM) has been a great success until now, so that in certain foci its elimination has become feasible. However, after more than 20 years of repeated IVM mass treatment, the disease still persists in many endemic countries. Sub-optimal responses and genetic changes have been reported in Onchocerca volvulus populations under high IVM pressure but more work is needed to determine whether resistance is developing. The situation needs to be urgently clarified to preserve the achievements of onchocerciasis control programs. In this study, O. volvulus adult worms were collected from the same individuals, before IVM exposure and following three years of annual or three-monthly treatments at 150 μg/kg or 800 μg/kg. Four single nucleotide polymorphisms (SNPs) occurring in the β-tubulin gene of these parasites were investigated. We found changes in genotype frequencies in O. volvulus β-tubulin gene associated with IVM treatments. The SNP at position 1545 (A/G) showed a significant increase in frequency of the less common nucleotide in the female worms following treatment. After 13 three-monthly treatments, female worm homozygotes with the less common genotype, prior to treatment, increased in frequency. The selected homozygotes, as well as heterozygotes, appeared to be less fertile (without or with very few embryonic stages in their uteri) than the wild-type homozygotes. These results provide additional evidence for genetic selection and strengthen the warning that selection for IVM resistance may be occurring in some O. volvulus populations.

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

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

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

Resistance to macrocyclic lactone anthelmintics byHaemonchus contortusandOstertagia circumcinctain sheep in New Zealand

AIM

To establish the efficacy of oral formulations of ivermectin and moxidectin against naturally acquired abomasal nematode infections on a North Island sheep farm.

METHODS

Two controlled slaughter trials were undertaken. In the first, 30 sheep on pasture were randomly allocated on the basis of faecal egg count to 1 of 3 groups, comprising an untreated control group and 2 treatment groups. One treatment group was given a single oral dose of ivermectin and the other a single oral dose of moxidectin, both at the manufacturer's recommended dose rates of 0.2 mg/kg liveweight. Six days after treatment, all animals were slaughtered and their abomasa recovered for worm counting. The second trial, which involved 47 animals, was essentially the same as the first except that, as well as involving the slaughter of 30 sheep from all 3 groups, 6 days after treatment, it also included a further 8 untreated control animals and 9 moxidectin treated animals which were slaughtered 27 days after treatment.

RESULTS

At 6 days after treatment, moxidectin was highly effective against all 3 of the abomasal nematodes present. While ivermectin was similarly effective against Trichostrongylus axei 6 days after treatment, it was not effective against either Ostertagia circumcinta or Haemonchus contortus, against which average efficacies of only 63.6% and 61.6%, respectively, were recorded. At 27 days after treatment, moxidectin, was also highly effective against T. axei (97.3% reduction) but not against either H. contortus (71.4% reduction) or O. circumcinta (61.0% reduction).

CONCLUSIONS

These results provide the first record of macrocyclic lactone resistance in H. contortus in sheep or in any other host in New Zealand, and the first case where such resistance has been exhibited in more than one parasite species at a time. Although the therapeutic efficacy of moxidectin was high against these resistant H. contortus and O. circumcincta strains, resistance to moxidectin was indicated by its diminished prophylactic activity against them. It is suggested that this reduction in the prophylactic activity of moxidectin is also likely to reduce its apparent current high therapeutic efficacy.

CLINICAL RELEVANCE

As well as providing further evidence that it can no longer be automatically assumed that macrocylic lactone anthelmintics will be effective on sheep farms in this country, these findings also present a warning that increasingly complex parasite control options may have to be faced in the future.

The use of DNA markers to map anthelmintic resistance loci in an intraspecific cross of Haemonchus contortus

The use of DNA markers to track the development of anthelmintic resistance in parasites of livestock would allow informed choices for the management of this important problem. We describe a genetic mapping approach for the discovery of DNA markers for anthelmintic resistance in Haemonchus contortus. We crossed a multi-drug resistant field isolate of H. contortus with a well-characterized laboratory strain susceptible to 4 drug classes. The F2 were separately selected with 5 anthelmintics from 4 drug classes, producing drug-resistant populations carrying gene variants derived from both the field isolate and the laboratory strain. Individual F2 worms were analysed using amplicon length polymorphisms (ALPs). We looked for field isolate alleles over- or under-represented in F2 populations compared to the unselected F2 and/or the laboratory strain. The data we obtained suggest that marker association can be used to link neutral markers with resistance, but also that more markers and perhaps more inbred laboratory strains would make the procedure more likely to succeed.

Genetics of mating and sex determination in the parasitic nematode Haemonchus contortus

Genetic analysis of parasitic nematodes has been a neglected area of research and the basic genetics of this important group of pathogens are poorly understood. Haemonchus contortus is one of the most economically significant livestock parasites worldwide and is a key experimental model for the strongylid nematode group that includes many important human and animal pathogens. We have undertaken a study of the genetics and the mode of mating of this parasite using microsatellite markers. Inheritance studies with autosomal markers demonstrated obligate dioecious sexual reproduction and polyandrous mating that are reported here for the first time in a parasitic helminth and provide the parasite with a mechanism of increasing genetic diversity. The karyotype of the H. contortus, MHco3(ISE) isolate was determined as 2n = 11 or 12. We have developed a panel of microsatellite markers that are tightly linked on the X chromosome and have used them to determine the sex chromosomal karyotype as XO male and XX female. Haplotype analysis using the X-chromosomal markers also demonstrated polyandry, independent of the autosomal marker analysis, and enabled a more direct estimate of the number of male parental genotypes contributing to each brood. This work provides a basis for future forward genetic analysis on H. contortus and related parasitic nematodes.

Nematode ligand-gated chloride channels: an appraisal of their involvement in macrocyclic lactone resistance and prospects for developing molecular markers

Ligand-gated chloride channels, including the glutamate-(GluCl) and GABA-gated channels, are the targets of the macrocyclic lactone (ML) family of anthelmintics. Changes in the sequence and expression of these channels can cause resistance to the ML in laboratory models, such as Caenorhabditis elegans and Drosophila melanogaster. Mutations in multiple GluCl subunit genes are required for high-level ML resistance in C. elegans, and this can be influenced by additional mutations in gap junction and amphid genes. Parasitic nematodes have a different complement of channel subunit genes from C. elegans, but a few genes, including avr-14, are widely present. A polymorphism in an avr-14 orthologue, which makes the subunit less sensitive to ivermectin and glutamate, has been identified in Cooperia oncophora, and polymorphisms in several subunits have been reported from resistant isolates of Haemonchus contortus. This has led to suggestions that ML resistance may be polygenic. Possible reasons for this, and its consequences for the development of molecular tests for resistance, are explored.

An analysis of the population genetics of potential multi-drug resistance in Wuchereria bancrofti due to combination chemotherapy

Currently, annual mass treatments with albendazole (ABZ) plus ivermectin (IVM) or diethylcarbamazine (DEC) are administered under the Global Programme to Eliminate Lymphatic Filariasis (GPELF). Drug resistance against both ABZ and IVM is prevalent in nematodes of veterinary importance, raising awareness that if anthelmintic resistance were to develop among Wuchereria bancrofti populations, this would jeopardize GPELF's goals. Genetic structure was incorporated into an existing transmission dynamics model for lymphatic filariasis (LF) to investigate the potential development of concurrent resistance to ABZ and IVM. The resultant models explore the impact of different inheritance modes of resistance to ABZ and IVM on the likely risk of treatment failure under our model assumptions. Results indicate that under ABZ+IVM combination, selection for resistance to one drug is enhanced if resistance to the other drug is already present. Excess parasite homozygosity may increase selection for dominant IVM resistance via enhancing the frequency of recessive ABZ resistance. The model predicts that if multiple resistance genes are associated with different efficacy properties of a drug combination, then examining changes at single loci may be misleading. Sampling schemes in genetic epidemiological surveys investigating the frequency of an allele under selection should consider host age, as individuals of different ages may acquire parasites at different rates.

Evidence for changes in the transcription levels of two putative P-glycoprotein genes in sea lice (Lepeophtheirus salmonis) in response to emamectin benzoate exposure

Overexpression of P-glycoproteins (Pgps) is assumed to be a principal mechanism of resistance of nematodes and arthropods to macrocyclic lactones. Quantitative RT-PCR (Q-RT-PCR) was used to demonstrate changes in transcription levels of two putative P-glycoprotein genes, designated here as SL0525 and SL-Pgp1, in sea lice (Lepeophtheirus salmonis) following exposure to emamectin benzoate (EMB). Pre-adult L. salmonis were challenged in an EMB bioassay for 24h and gene expression was studied from lice surviving EMB concentrations of 0, 10, and 30ppb. Gene expression was measured using Q-RT-PCR with elongation factor 1 (eEF1alpha) as an internal reference gene. The results show that both target genes, SL0525 and SL-Pgp1, had significantly increased levels of expression with exposure to 10ppb EMB (p=0.11 and p=0.17, respectively) whereas the group exposed to 30ppb was on the verge of being significant (p=0.053) only in the expression of SL-Pgp1. Gene expression for SL0525 and SL-Pgp1 were increased over five-fold at 10ppb EMB. Therefore, the upregulation of these target genes may offer protection by increasing Pgp expression when lice are exposed to EMB. Our optimized Q-RT-PCR can be used to determine if over-expression of these genes could be the basis for development of resistance in sea lice and thus allow suitable alternative chemotherapeutic options to be assessed.

Understanding anthelmintic resistance: The need for genomics and genetics

Anthelmintic resistance is a major problem for the control of many parasitic nematode species and has become a major constraint to livestock production in many parts of the world. In spite of its increasing importance, there is still a poor understanding of the molecular and genetic basis of resistance. It is unclear which mutations contribute most to the resistance phenotype and how resistance alleles arise, are selected and spread in parasite populations. The main strategy used to identify mutations responsible for anthelmintic resistance has been to undertake experimental studies on candidate genes. These genes have been chosen predominantly on the basis of our knowledge of drug mode-of-action and the identification of mutations that can confer resistance in model organisms. The application of these approaches to the analysis of benzimidazole and ivermectin resistance is reviewed and the reasons for their relative success or failure are discussed. The inherent limitation of candidate gene studies is that they rely on very specific and narrow assumptions about the likely identity of resistance-associated genes. In contrast, forward genetic and functional genomic approaches do not make such assumptions, as illustrated by the successful application of these techniques in the study of insecticide resistance. Although there is an urgent need to apply these powerful approaches to anthelmintic resistance research, the basic methodologies and resources are still lacking. However, these are now being developed for the trichostrongylid nematode Haemonchus contortus and the current progress and research priorities in this area are discussed.

A computer model to simulate control of parasitic gastroenteritis in sheep on UK farms

A computer model that simulates the population dynamics and epidemiology of three major species of parasitic nematodes of sheep found in the UK (Telodorsagia [Ostertagia] spp., Haemonchus spp. and Trichostrongylus spp.) is described. The model has been developed as a tool for veterinarians and advisors to aid in the implementation of integrated parasite control strategies designed to optimise anthelmintic usage and delay the development of resistance on UK farms. The model represents the parasite life cycle, flock dynamics and the response of individuals with different susceptible and resistant genotypes to the major broad-spectrum classes of anthelmintic available in the UK. Where possible, UK data have been used for the model parameters. The model allows worm control simulations on individual UK farms. Inputs include environmental and farm management variables which impact on the epidemiology of the disease, e.g. regional weather data; flock stocking rates; initial pasture larval contamination levels and species proportions; lambing dates; timing of flock movements to clean pastures; and removal of lambs during the year. Farm management data, as well as nematode egg outputs and grass larval counts, were collected from eight UK farms over a 1-year period for initial validation of the model outputs. The management data for each farm were used as inputs for each model run and model outputs for nematode egg counts from ewes and lambs were compared to the observed data for each farm. Statistical analysis of results shows a positive correlation for observed and simulated counts and regression analysis suggests an acceptable fit between the data. Comparison of observed and simulated outputs for resistance were possible for only one farm due to low numbers of worms developing in the laboratory tests. Additional studies will be necessary before resistance data can be reliably compared. Further validation studies are proposed to ensure that the model is robust and applicable across a diverse range of farm types. The model will be used to demonstrate the advantage, in terms of delaying resistance development, of current guidelines for anthelmintic use and management practices for worm control in sheep.

Eradication of targeted species of internal parasites

Eradication can be defined as the permanent reduction to zero of the incidence of infection caused by a specific agent as a result of deliberate efforts. Eradication strategies must be designed to increase access to and utilization of veterinary and animal health support services beyond that normally achieved by routine services. Consequently, before undertaking an eradication program, proponents must be certain that they can be sure of a commitment of resources sufficient to complete the project. A helminth eradication campaign would require the application of anthelmintics to all hosts at a level aimed at killing heterozygote resistant individuals. This strategy runs counter to the prevailing opinion that resistance to chemicals will always occur and control schemes should accept some production loss by letting a proportion of parasites escape selection (in refugia) and thereby increase the useful life of the chemical. Under certain circumstances, environmental constraints may make some species of parasitic nematodes vulnerable to an eradication program. Anthelmintics have been used to exploit these constraints in past control programs and have lead to local eradication of several species of gastro-intestinal parasitic nematodes. Also, there are at present eradication programs for nematode parasites of human that are based on anthelmintic treatment. However, the suppressive pesticide treatment required to bring about the virtual elimination of species is a high-risk strategy and should only be undertaken if eradication is an achievable goal. It is also important to plan an exit strategy if eradication fails.

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.

Is anthelmintic resistance a concern for heartworm control? What can we learn from the human filariasis control programs?

Heartworm prophylaxis is currently largely dependent on the ability of avermectins and milbemycins to arrest the development of third and fourth stages of Dirofilaria immitis for prolonged periods, without producing adulticidal effects. Major control programs, dependent on the activity of ivermectin, are being implemented for human onchocerciasis and lymphatic filariasis. The avermectins and milbemycins act on glutamate-gated and gamma-aminobutyrate-gated chloride channel subunit proteins in nematodes. Ivermectin resistance has been widely described in trichostrongylid nematodes of ruminants. There is evidence that when ivermectin resistance occurs in nematodes, there may be selection on some, but not all of the genes that code for ligand-gated chloride channel subunit proteins as well as on some ABC-transporter genes, whose products may be involved in regulating macrocyclic lactone drug concentrations at receptors, and on some structural protein genes of amphidial neurones. Although ivermectin resistance has not been reported in filarial nematodes, there have recently been reports of suboptimal responses to ivermectin in Onchocerca volvulus. Evidence has been found of ivermectin selection on at least ABC-transporter genes and some neuronal structural protein genes in O. volvulus. To date, there is no evidence of avermectin/milbemycin resistance in D. immitis, also a filarial nematode. Chemotherapy against trichostrongylids of animals, human filariae, and D. immitis, relies on avermectins or milbemycins. However, control involves targeting different stages or processes in the nematode life cycles, different control strategies, different proportions of the nematode population in refugia, and different drug dosage rates. Consideration of the proportion of the D. immitis population normally in refugia, the life cycle stage targeted, and the anthelmintic dosages used suggest that it is unlikely that significant avermectin/milbemycin resistance will be selected in D. immitis with current treatment strategies.

Anthelmintic resistance – looking to the future: a UK perspective

The full extent of anthelmintic resistance in nematodes of farm animals is not known. Resistance can be detected with a faecal egg count reduction test and two in vitro tests, the egg hatch and larval development tests. The sensitivity of these two in vitro tests can be increased by using discriminating doses rather than calculating LD50 values. Only benzimidazole resistance can be detected with PCR based tests because the molecular mechanisms of resistance to levamisole and the macrocyclic lactones remain unknown. Resistance detection is important because it enables the appropriate management strategies to be put in place. The development of resistance is delayed by keeping sufficient parasites in refugia (not exposed to anthelmintic), but the necessary management details have not yet been validated in the field. It is probably too late to use combination products to delay the development of resistance, except in cattle but quarantining animals to prevent introduction of resistant helminths onto a farm is important. Dilution of resistant worms with susceptible ones is only at the preliminary research stage and the application of non-chemical methods of control to delay resistance is not yet a practical option. Extensive research is required to manage resistance, especially in the control of resistance in Fasciola hepatica.

Characterization of moxidectin resistant Trichostrongylus colubriformis and Haemonchus contortus

The development of moxidectin resistance (MOX-R) in sheep parasitic gastrointestinal nematodes already carrying multiple resistances to other anthelmintic groups has made control of these strains very difficult. The anthelmintic resistance patterns of MOX-R strains of Trichostrongylus colubriformis and Haemonchus contortus were characterized to provide an insight into the remaining role of anthelmintics in the control of such strains. Homozygous MOX-R individuals of both genera were unaffected by moxidectin. For MOX-R heterozygotes a dose rate of 200 microg/kg abamectin (ABA) given orally removed 25% of H. contortus while 200 microg/kg MOX given orally achieved a 72% reduction. Doubling the dose rate of ABA improved the mean efficacy to 37%. Consequently, in H. contortus, the degree of dominance differs markedly between the two anthelmintics. A dose rate of 8 mg/kg levamisole and 185 mg/kg napthalophos achieved >95% reduction in worm count of the MOX-R homozygous H. contortus but only 85 and 7%, respectively against the MOX-R homozygous T. colubriformis.

Drug resistance in veterinary helminths

At present, there is no effective alternative to chemical control of parasitic helminths where livestock are grazed intensively. Resistance to anthelmintics has become a major problem in veterinary medicine, and threatens both agricultural income and animal welfare. The molecular and biochemical basis of this resistance is not well understood. The lack of reliable biological and molecular tests means that we are not able to follow the emergence and spread of resistance alleles and clinical resistance as well as we need. This review summarizes some of the recent findings on resistance mechanisms, puts forward some recommendations for limiting its impact and suggests some priorities for research in this area.

Profiling of gender-specific gene expression for Trichostrongylus vitrinus (Nematoda: Strongylida) by microarray analysis of expressed sequence tag libraries constructed by suppressive-subtractive hybridisation

Gender-specific gene expression in Trichostrongylus vitrinus (order Strongylida) was investigated by constructing male- and female-specific gene archives using a suppressive-subtractive hybridisation approach, sequencing of expressed sequence tags from these archives, comparison with genes of Caenorhabditis elegans and other organisms, and expression profiling of a representative subset of 716 expressed sequence tags by microarray and macroarray analysis. Of these T. vitrinus expressed sequence tags, 391 had sequence homology to C. elegans genes. Of the remaining expressed sequence tags, 62 had homology to genes of other species of parasitic nematodes, and 263 expressed sequence tags had no significant homology. Expression profiling showed gender-specific expression for 561 of the 716 T. vitrinus expressed sequence tags. Male-specific protein kinases and protein phosphatases, major sperm proteins and enzymes involved in carbohydrate metabolism were abundant in the cDNA archive. Female-specific vitellogenins, heat-shock proteins and chaperonins were also highly represented. Genes involved in a number of cellular processes, such as ubiquitination and proteasome function, gene transcription, cell signalling, protein-protein interactions and chromatin assembly and function were also expressed in a gender-specific manner. The potential roles of these genes in gametogenesis, embryogenesis and reproduction in the parasitic nematode are discussed in relation to the known roles of their homologues in C. elegans.

Scabies: New Future for a Neglected Disease

Scabies is a disease of global proportions in both human and animal populations, resulting from infestation of the skin with the "itch" mite Sarcoptes scabiei. Despite the availability of effective chemotherapy the intensely itching lesions engender significant morbidity primarily due to secondary sepsis and post-infective complications. Some patients experience an extreme form of the disease, crusted scabies, in which many hundreds of mites may infest the skin causin severe crusting and hyperkeratosis. Overcrowded living conditions and poverty have been identified as significant confounding factors in transmission of the mite in humans. Control is hindered by difficulties with diagnosis, the cost of treatment, evidence for emerging resistance and lack of effective vaccines. Historically research on scabies has been extremely limited because of the difficulty in obtaining sufficient quantities of the organism. Recent molecular approaches have enabled considerable advances in the study of population genetics and transmission dynamics of S. scabiei. However, the most exciting and promising development is the potential exploitation of newly available data from S. scabiei cDNA libraries and EST projects. Ultimately this knowledge may aid early identification of disease, novel forms of chemotherapy, vaccine development and new treatment possibilities for this important but neglected parasite.

Sustainable use of anthelmintics in grazing animals

It is suggested that the major factor in avoiding the development of anthelmintic resistance is the percentage of worms that do not encounter the anthelmintics (worms in refugia). This in turn is determined by the numbers of larvae on pasture, the percentage of animals treated and whether any stages in the host can avoid the action of anthelmintic. To maintain anthelmintic efficacy the percentage of worms in refugia must be sufficiently large. In cattle, this should involve treating only first-year animals and using a different pasture each year for calves. For sheep, only animals that have to be treated should be dosed with anthelmintic and clean grazing strategies that involve the use of anthelmintics should be avoided. For horses, reliance should be placed on the removal of faeces from pasture and only treating when the animals' condition requires it. Without a change in anthelmintic use there is the likelihood of increasing numbers of cases for which no anthelmintic is effective and animal welfare may be compromised.

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.

Anthelmintic resistance in New Zealand

Anthelmintic resistance was first confirmed in New Zealand in 1979 and since then has become common-place; more than 50% of sheep farms now have detectable levels of resistance to one or more chemical classes of anthelmintic. Farmer drenching practices have changed little over the last 15-20 years and are clearly exerting a significant level of selection for resistance. In the absence of new chemical classes of anthelmintics, current parasite control practices will be unsustainable in the long-term. Once substantial resistance has developed, significant reversion to susceptibility is unlikely and re-introduction of failed drugs is likely to result in the re-emergence of control problems. The number of anthelmintic treatments applied is not necessarily a reliable indicator of selection pressure and should not be the only factor considered in strategies for minimising the development of resistance. The relative potential of the different anthelmintics now available, particularly the long-acting products, to select for resistance varies with the way they are used and with other epidemiological and management factors; generalisations about their respective roles in the development of resistance are often unreliable. In many cases, literal extrapolation of recommendations for the management of resistance from Australia to New Zealand is unsupportable, given the differences in climate, parasite ecology and farming practices between the 2 countries. In the absence of a refuge for susceptible genotypes, as occurs when anthelmintic treatments are used as a means of generating low-contamination 'safe' pasture for young stock, the rapid development of resistance is likely. Anthelmintic treatments applied to animals with a high level of immunity, or which become immune while the anthelmintic is active, are likely to select for resistance faster than treatments applied to non-immune stock.

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.

Principles for the use of macrocyclic lactones to minimise selection for resistance

OBJECTIVE

To provide principles for the appropriate use of avermectin/milbemycin or macrocyclic lactone (ML) anthelmintics in sheep, to ensure effective worm control and to minimise selection for ML resistance.

STRATEGY

The principles were based on an assessment of the information currently available. The MLs were categorised into three groups (ivermectin [IVM], abamectin [ABA] and moxidectin [MOX]) based on structural differences, persistence and efficacy against ML resistant strains. The reported order of activity or efficacy against ML resistant worm strains was IVM<ABA<MOX. General treatment schemes were considered for Australian conditions and were divided into the following situations: 1. quarantine treatment, 2. treatment on/to clean pasture, 3. treatment on/to safe pasture, 4. treatment on/to moderate/heavily contaminated pasture. For each of these situations a strategy was considered for farms where ML resistance was present or absent. It was assumed that resistance commonly occurs in some or all other broad spectrum anthelmintics, and even where ML resistance has been detected, the ML group remains the most effective. The guidelines provided are general and it is expected that state agencies and sheep/veterinary advisers would give specific advice to suit their environments and drug resistance/worm problems.

CONCLUSIONS

The primary recommendation is to use a mixture of effective drugs when treating sheep. However, unless the combination treatment is highly effective it is unlikely to delay selection for ML resistance if sheep are being treated and moved to a clean or safe pasture. Where possible, reliance on the ML anthelmintics should be reduced by not using them every year, not using them in low risk stock or by using narrow spectrum and low efficacy drugs such as naphthalophos when appropriate. Anthelmintic treatment should be given as part of a strategic worm control program. It is suggested that IVM-oral and IVM-capsules should not be used when ML resistance is present. In this situation MOX or ABA should be used in combination with other drugs, provided that the chosen ML is effective against the resistant parasite. It is essential to monitor the efficacy of ML and drug combinations by post-treatment worm egg counts, particularly when ML resistance has been detected.

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.