Drug resistance mechanisms in helminths: is it survival of the fittest?

Evidence from two independent backcross experiments supports genetic linkage of microsatellite Hcms8a20, but not other candidate loci, to a major ivermectin resistance locus in Haemonchus contortus

Haemonchus contortus is the leading parasitic nematode species used to study anthelmintic drug resistance. A variety of candidate loci have been implicated as being associated with ivermectin resistance in this parasite but definitive evidence of their importance is still lacking. We have previously performed two independent serial backcross experiments to introgress ivermectin resistance loci from two H. contortus ivermectin-resistant strains - MHco4(WRS) and MHco10(CAVR) - into the genetic background of the ivermectin-susceptible genome reference strain MHco3(ISE). We have interrogated a number of candidate ivermectin resistance loci in the resulting backcross populations and assessed the evidence for their genetic linkage to an ivermectin resistance locus. These include the microsatellite marker Hcms8a20 and six candidate genes Hco-glc-5, Hco-avr-14, Hco-lgc-37 (previously designated Hco-hg-1), Hco-pgp-9 (previously designated Hco-pgp-1), Hco-pgp-2 and Hco-dyf-7. We have sampled the haplotype diversity of amplicon markers within, or adjacent to, each of these loci in the parental strains and fourth generation backcross populations to assess the evidence for haplotype introgression from the resistant parental strain into the genomic background of the susceptible parental strain in each backcross. The microsatellite Hcms8a20 locus showed strong evidence of such introgression in both independent backcrosses, suggesting it is linked to an important ivermectin resistance mutation in both the MHco4(WRS) and MHco10(CAVR) strains. In contrast, Hco-glc-5, Hco-avr-14, Hco-pgp-9 and Hco-dyf-7 showed no evidence of introgression in either backcross. Hco-lgc-37 and Hco-pgp-2 showed only weak evidence of introgression in the MHco3/4 backcross but not in the MHco3/10 backcross. Overall, these results suggest that microsatellite marker Hcms8a20, but not the other candidate genes tested, is linked to a major ivermectin resistance locus in the MHco4(WRS) and MHco10(CAVR) strains. This work also emphasises the need for genome-wide approaches to identify mutations responsible for the ivermectin resistance in this parasite.

Oxidative stress plays major role in mediating apoptosis in filarial nematode Setaria cervi in the presence of trans-stilbene derivatives

Lymphatic filariasis, affecting around 120 million people in 80 countries worldwide, is an extremely painful disease and caused permanent and long term disability. Owing to its alarming prevalence there is immediate need for development of new therapeutics. A series of trans-stilbene derivatives were synthesized using aqueous reaction condition showing potential as antifilarial agents demonstrated in vitro. MTT reduction assay and dye exclusion test were performed to evaluate the micro and macrofilaricidal potential of these compounds. Amid 20 trans-stilbene derivatives together with Resveratrol (RSV), a multifunctional natural product was screened; nine compounds (28, 29, 33, 35, 36, 38, 39, 41 and 42) have showed promising micro and macrofilaricidal activities and four of them (28, 39, 41 and 42) showed better effectiveness than RSV. In the treated parasites apoptosis was established by DNA laddering, in situ DNA fragmentation and FACS analysis. The generation of ROS in the treated parasites was indicated by the depletion in the level of GSH, GR and GST activity and elevation of SOD, catalase, GPx activity and superoxide anion and H2O2 level. Along with the ROS generation and oxidative stress, the decreased expression of anti-apoptotic ced-9 gene and increased expression of nematode specific pro-apoptotic genes, egl-1, ced-4 and ced-3 at the level of transcription and translation level; the up-regulation of caspase-3 activity and involvement of caspase-8,9,3, cytochrome-c and PARP were also observed and which denotes the probable existence of both extrinsic and intrinsic pathways apoptosis in parasitic nematodes. This observation is reported first time and thus it confirmed the mode of action and effectiveness of the compounds. Further, the comparative bioavailability-pharmacokinetics studies showed that compound 28 possesses comparable properties with Ivermectin. This study will certainly intensify our understanding of the pharmacological importance of trans-stilbenes as an anti-filarial agent.

Anthelmintic resistance in gastrointestinal nematodes of beef cattle in the state of Rio Grande do Sul, Brazil

Gastrointestinal nematodes resistant to anthelmintics have been reported in several regions of Brazil, and they may be associated with economic losses for the cattle industry. This study aimed to evaluate the resistance status of gastrointestinal nematodes from naturally infected beef cattle to several commercially available anthelmintics, as well as to test the efficacy of combinations of anthelmintics against multi-resistant gastrointestinal nematodes. Ten farms located in Rio Grande do Sul state were selected by: farmers' consent; extensive raising system; availability of calves aged from 7 to 9 months naturally infected by gastrointestinal nematodes; absence of anthelmintic treatment for 60 days before the study; and presence of 70-100 calves or more of both genders with ≥ 200 eggs per gram of feces (EPG) (sensitivity of 50 EPG). These calves were distributed into 10 groups (of 7-10 animals) per farm and treated with ivermectin, doramectin, eprinomectin, fenbendazole, closantel, nitroxynil, disophenol, levamisole, albendazole, or moxidectin. Feces were collected 2 days before treatment and 14 days after treatment. Additional groups of 7-10 calves were used to test six different two-drug combinations at four of the studied farms. In general terms, fenbendazole was the most effective drug, followed by levamisole, disophenol, and moxidectin. However, parasite resistance to multiple drugs was found in all herds, especially in the genera Cooperia spp., Trichostrongylus spp., and Haemonchus spp.. Some of the two-drug combinations were effective against nematode populations identified as resistant to the same compounds when used as single drugs. The most effective combinations were moxidectin + levamisole, doramectin + fenbendazole, and levamisole + closantel. In this study, parasites resistant to the main commercially available anthelmintics were found in all herds, and some combinations of two active components belonging to different chemical groups were effective against multi-drug resistant gastrointestinal nematodes.

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

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

The Biochemistry of Haemonchus contortus and Other Parasitic Nematodes

Different life cycle stages of Haemonchus contortus adapt to different ecosystems. This adaptation is accompanied by alterations in gene transcription and expression associated with the energy, amino acid, nitrogen, lipid and/or nucleic acid metabolism of the respective stages. For example, the aerobic metabolism of larvae depends on an efficient citric acid cycle, whereas the anaerobic metabolism of adults requires glycolysis, resulting in the production of volatile fatty acids, such as acetic acid and propionic acid. There are only few anthelmintics targeting nematode energy metabolism. In addition, H. contortus has reduced pathways for amino acid metabolism, polyamine metabolism and nitrogen excretion pathways. Moreover, nucleic acid metabolism comprising purine and pyrimidine salvage pathways as well as lipid metabolism are reduced. In addition, nematodes possess a particular composition of their cuticle. Energy production of adult worms is mainly linked to egg production and complex regulation of the neuromuscular system in both females and males. In this context, microtubules consisting of α- and β-tubulin heterodimers play a crucial role in the presynaptic vesicle transport. Due to the significant distinction of its quarternary structure in nematodes in comparison to other organisms, β-tubulin was identified as a major target for benzimidazoles used for anthelmintic treatment. Concerning the function of the neuromuscular system, acetylcholine, a ligand of the nicotinic acetylcholine receptor (nAChR), is the major excitatory neurotransmitter in H. contortus. In contrast, glutamate-gated chloride channels, calcium- and voltage-dependent potassium channels as well as γ-aminobutyric acid (GABA)A and its receptors act as inhibitory neurotransmitters and thus opponents to nAChR. For example, the calcium- and voltage-dependent potassium channel SLO-1 is an important target of emodepside, which is involved in the sensitive regulation of activatory and inhibitory receptors of the nervous system. Most of the modern anthelmintics target these different neuromuscular receptors. The mechanisms of resistance to anthelmintics, either specific or non-specific, are associated with changes in the molecular targets of the drugs, changes in metabolism of the drug (inactivation, removal or prevention of its activation) and/or increased efflux systems. The biochemical and molecular analyses of key developmental, metabolic and structural process of H. contortus still require substantial efforts. The nAChR, glutamate-gated chloride channel and calcium- and voltage-dependent potassium channel SLO-1 have long been known as being essential for nematode survival. Therefore, future research should be intensified to fully resolve the three-dimensional structures of these receptors, as has already been started for glutamate-gated chloride channel. With this knowledge, it should be possible to design new anthelmintics, which possess improved binding capacities to corresponding receptors.

Overcoming drug resistance for macro parasites

Helminth infections impose burden on human and livestock populations, and their control predominantly relies on periodic mass administration of anthelmintic drugs. However, recent emergence of drug resistance among parasites to currently available drugs raises serious problems for continuation of control strategies and achievement of elimination of parasitic diseases. This review discusses the problem of anthelmintic resistance in humans and livestock, and suggests steps that can be taken to overcome this problem. To achieve the goals of morbidity reduction or elimination of infection we need to develop novel tools, including more efficacious drugs, vaccines and/or antivectorial agents; new diagnostics for infection and assessment of drug efficacy; and markers for possible anthelmintic resistance. Harnessing the knowledge generated from sequencing of parasite genome sequences is the key to identify genes responsible for drug resistance, which can be used as a starting point for discovery of target-specific pharmacological or genetic modulation to test the functional importance of individual genes and pathways. Involvement of chemical genetic screens and Caenorhabditis elegans as a model system for drug discovery needs to be explored in greater detail. Collective effort from several quarters is needed to think of a world that is free of parasitic infections.

Viability of developmental stages of Schistosoma mansoni quantified with xCELLigence worm real-time motility assay (xWORM)

Infection with helminth parasites causes morbidity and mortality in billions of people and livestock worldwide. Where anthelmintic drugs are available, drug resistance is a major problem in livestock parasites, and a looming threat to public health. Monitoring the efficacy of these medicines and screening for new drugs has been hindered by the lack of objective, high-throughput approaches. Several cell monitoring technologies have been adapted for parasitic worms, including video-, fluorescence-, metabolism enzyme- and impedance-based tools that minimize the screening bottleneck. Using the xCELLigence impedance-based system we previously developed a motility-viability assay that is applicable for a range of helminth parasites. Here we have improved substantially the assay by using diverse frequency settings, and have named it the xCELLigence worm real-time motility assay (xWORM). By utilizing strictly standardized mean difference analysis we compared the xWORM output measured with 10, 25 and 50 kHz frequencies to quantify the motility of schistosome adults (human blood flukes) and hatching of schistosome eggs. Furthermore, we have described a novel application of xWORM to monitor movement of schistosome cercariae, the developmental stage that is infectious to humans. For all three stages, 25 kHz was either optimal or near-optimal for monitoring and quantifying schistosome motility. These improvements in methodology sensitivity should enhance the capacity to screen small compound libraries for new drugs both for schistosomes and other helminth pathogens at large.

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25 kHz on the xCELLigence system dramatically improves the schistosome xWORM assay.

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xWORM assay can efficiently determine viability of Schistome adults or eggs.

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First time cercariae have been incorporated into an automated viability assay.

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Other helminth monitoring may benefit from alternate xCELLigence frequency options.

Cracking the nodule worm code advances knowledge of parasite biology and biotechnology to tackle major diseases of livestock

Many infectious diseases caused by eukaryotic pathogens have a devastating, long-term impact on animal health and welfare. Hundreds of millions of animals are affected by parasitic nematodes of the order Strongylida. Unlocking the molecular biology of representatives of this order, and understanding nematode-host interactions, drug resistance and disease using advanced technologies could lead to entirely new ways of controlling the diseases that they cause. Oesophagostomum dentatum (nodule worm; superfamily Strongyloidea) is an economically important strongylid nematode parasite of swine worldwide. The present article reports recent advances made in biology and animal biotechnology through the draft genome and developmental transcriptome of O. dentatum, in order to support biological research of this and related parasitic nematodes as well as the search for new and improved interventions. This first genome of any member of the Strongyloidea is 443 Mb in size and predicted to encode 25,291 protein-coding genes. Here, we review the dynamics of transcription throughout the life cycle of O. dentatum, describe double-stranded RNA interference (RNAi) machinery and infer molecules involved in development and reproduction, and in inducing or modulating immune responses or disease. The secretome predicted for O. dentatum is particularly rich in peptidases linked to interactions with host tissues and/or feeding activity, and a diverse array of molecules likely involved in immune responses. This research progress provides an important resource for future comparative genomic and molecular biological investigations as well as for biotechnological research toward new anthelmintics, vaccines and diagnostic tests.

Profiling of differentially expressed genes in sheep T lymphocytes response to an artificial primary Haemonchus contortus infection

BACKGROUND

Haemonchus contortus is a common bloodsucking nematode causing widespread economic loss in agriculture. Upon H. contortus infection, a series of host responses is elicited, especially those related to T lymphocyte immunity. Existing studies mainly focus on the general immune responses of sheep T lymphocyte to H. contortus, lacking investigations at the molecular level. The objective of this study was to obtain a systematic transcriptional profiling of the T lymphocytes in H. contortus primary-infected sheep.

METHODS

Nematode-free sheep were orally infected once with H. contortus L3s. T lymphocyte samples were collected from the peripheral blood of 0, 3, 30 and 60 days post infection (dpi) infected sheep. Microarrays were used to compare gene transcription levels between samples. Quantitative RT-PCR was employed to validate the microarray data. Gene Ontology and KEGG pathway analysis were utilized for the annotation of differentially expressed genes.

RESULTS

Our microarray data was consistent with qPCR results. From microarrays, 853, 242 and 42 differentially expressed genes were obtained in the 3d vs. 0d, 30d vs. 0d and 60d vs. 0d comparison groups, respectively. Gene Ontology and KEGG pathway analysis indicated that these genes were involved in metabolism, signaling, cell growth and immune system processes. Functional analysis of significant differentially expressed genes, such as SLC9A3R2, ABCB9, COMMD4, SUGT1, FCER1G, GSK3A, PAK4 and FCER2, revealed a crucial association with cellular homeostasis maintenance and immune response. Our data suggested that maintaining both effective immunological response and natural cellular activity are important for T lymphocytes in fighting against H. contortus infection.

CONCLUSIONS

Our results provide a substantial list of candidate genes in sheep T lymphocytes response to H. contortus infection, and contribute novel insights into a general immune response upon infection.

An antioxidant response is involved in resistance of Giardia duodenalis to albendazole

Albendazole (ABZ) is a therapeutic benzimidazole used to treat giardiasis that targets β-tubulin. However, the molecular bases of ABZ resistance in Giardia duodenalis are not understood because β-tubulin in ABZ-resistant clones lacks mutations explaining drug resistance. In previous work we compared ABZ-resistant (1.35, 8, and 250 μM) and ABZ-susceptible clones by proteomic analysis and eight proteins involved in energy metabolism, cytoskeleton dynamics, and antioxidant response were found as differentially expressed among the clones. Since ABZ is converted into sulphoxide (ABZ-SO) and sulphone (ABZ-SOO) metabolites we measured the levels of these metabolites, the antioxidant enzymes and free thiols in the susceptible and resistant clones. Production of reactive oxygen species (ROS) and levels of ABZ-SO/ABZ-SOO induced by ABZ were determined by fluorescein diacetate-based fluorescence and liquid chromatography respectively. The mRNA and protein levels of antioxidant enzymes (NADH oxidase, peroxiredoxin 1a, superoxide dismutase and flavodiiron protein) in these clones were determined by RT-PCR and proteomic analysis. The intracellular sulfhydryl (R-SH) pool was quantified using dinitrobenzoic acid. The results showed that ABZ induced ROS accumulation in the ABZ-susceptible Giardia cultures but not in the resistant ones whilst the accumulation of ABZ-SO and ABZ-SOO was lower in all ABZ-resistant cultures. Consistent with these findings, all the antioxidant enzymes detected and analyzed were upregulated in ABZ-resistant clones. Likewise the R-SH pool increased concomitantly to the degree of ABZ-resistance. These results indicate an association between accumulation of ABZ metabolites and a pro-oxidant effect of ABZ in Giardia-susceptible clones. Furthermore the antioxidant response involving ROS-metabolizing enzymes and intracellular free thiols in ABZ-resistant parasites suggest that this response may contribute to overcome the pro-oxidant cytotoxicity of ABZ.

Effect of alternate and simultaneous grazing on endoparasite infection in sheep and cattle

This experiment was carried out on 8 ha of Panicum maximum cv. Tanzania pastures, with rotational grazing consisting of 7 days of occupation and 21 days of rest. Four treatments were evaluated: cattle grazing alone (BOV), sheep grazing alone (OVI), cattle and sheep grazing simultaneously (SIM) and cattle grazing followed by sheep (alternate - ALT). Twenty heifers and 30 male Santa Inês lambs were used. Fecal egg count (FEC) and fecal cultures were carried out. Blood was also collected to examine red and white cell series, total plasma protein (TPP), albumin and hemoglobin. FEC and estimated nematode pathogenicity index in sheep were lower in the SIM treatment. The Haemonchus spp. proportion was higher in isolated grazing systems. For sheep, mixed grazing was shown to reduce endoparasite infection, and SIM was better than ALT. For cattle, no difference between grazing systems was seen. Therefore, simultaneous grazing (sheep and cattle) may be a tool for reducing the need for anthelmintic treatments in sheep.

A new methodology for evaluation of nematode viability

Nematodes infections are responsible for debilitating conditions and economic losses in domestic animals as well as livestock and are considered an important public health problem due to the high prevalence in humans. The nematode resistance for drugs has been reported for livestock, highlighting the importance for development of new anthelmintic compounds. The aim of the current study was to apply and compare fluorimetric techniques using Sytox and propidium iodide for evaluating the viability of C. elegans larvae after treatment with anthelmintic drugs. These fluorescent markers were efficient to stain larvae treated with ivermectin and albendazole sulfoxide. We observed that densitometric values were proportional to the concentration of dead larvae stained with both markers. Furthermore, data on motility test presented an inverse correlation with fluorimetric data when ivermectin was used. Our results showed that lower concentrations of drugs were effective to interfere in the processes of cellular transport while higher drugs concentrations were necessary in order to result in any damage to cell integrity. The methodology described in this work might be useful for studies that aim to evaluate the viability of nematodes, particularly for testing of new anthelminthic compounds using an easy, economic, reproducible, and no time-consuming technique.

Ion channels and drug transporters as targets for anthelmintics

Infections with parasitic helminths such as schistosomes and soil-transmitted nematodes are hugely prevalent and responsible for a major portion of the global health and economic burdens associated with neglected tropical diseases. In addition, many of these parasites infect livestock and plants used in agriculture, resulting in further impoverishment. Treatment and control of these pathogens rely on anthelmintic drugs, which are few in number, and against which drug resistance can develop rapidly. The neuromuscular system of the parasite, and in particular, the ion channels and associated receptors underlying excitation and signaling, have proven to be outstanding targets for anthelmintics. This review will survey the different ion channels found in helminths, focusing on their unique characteristics and pharmacological sensitivities. It will also briefly review the literature on helminth multidrug efflux that may modulate parasite susceptibility to anthelmintics and may prove useful targets for new or repurposed agents that can enhance parasite drug susceptibility and perhaps overcome drug resistance.

Inhibition or knockdown of ABC transporters enhances susceptibility of adult and juvenile schistosomes to Praziquantel

Parasitic flatworms of the genus Schistosoma cause schistosomiasis, a neglected tropical disease that affects hundreds of millions. Treatment of schistosomiasis depends almost entirely on the drug praziquantel (PZQ). Though essential to treating and controlling schistosomiasis, a major limitation of PZQ is that it is not active against immature mammalian-stage schistosomes. Furthermore, there are reports of field isolates with heritable reductions in PZQ susceptibility, and researchers have selected for PZQ-resistant schistosomes in the laboratory. P-glycoprotein (Pgp; ABCB1) and other ATP binding cassette (ABC) transporters remove a wide variety of toxins and xenobiotics from cells, and have been implicated in multidrug resistance (MDR). Changes in ABC transporter structure or expression levels are also associated with reduced drug susceptibility in parasitic helminths, including schistosomes. Here, we show that the activity of PZQ against schistosome adults and juveniles ex vivo is potentiated by co-administration of either the highly potent Pgp inhibitor tariquidar or combinations of inhibitors targeting multiple ABC multidrug transporters. Adult worms exposed to sublethal PZQ concentrations remain active, but co-administration of ABC transporter inhibitors results in complete loss of motility and disruption of the tegument. Notably, juvenile schistosomes (3–4 weeks post infection), normally refractory to 2 µM PZQ, become paralyzed when transporter inhibitors are added in combination with the PZQ. Experiments using the fluorescent PZQ derivative (R)-PZQ-BODIPY are consistent with the transporter inhibitors increasing effective intraworm concentrations of PZQ. Adult worms in which expression of ABC transporters has been suppressed by RNA interference show increased responsiveness to PZQ and increased retention of (R)-PZQ-BODIPY consistent with an important role for these proteins in setting levels of PZQ susceptibility. These results indicate that parasite ABC multidrug transporters might serve as important targets for enhancing the action of PZQ. They also suggest a potentially novel and readily-available strategy for overcoming reduced PZQ susceptibility of schistosomes.

Schistosomes are parasitic flatworms that cause schistosomiasis, a tropical disease affecting hundreds of millions worldwide. Praziquantel (PZQ) is the current drug of choice against schistosomiasis, and, indeed, is the only approved antischistosomal treatment available in most parts of the world. Though effective overall, PZQ has limitations, including its lack of activity against immature schistosomes. Furthermore, reported cure rates in the field are often below optimal levels, and there is increasing evidence that schistosomes can become resistant to the drug. ABC transporters such as P-glycoprotein are efflux transporters that mediate detoxification of cells via removal of toxins and xenobiotics, including drugs. They underlie multidrug resistance in mammalian cells, and are also associated with drug resistance in parasitic worms, including schistosomes. Here, we show that compounds that inhibit these efflux transporters potentiate the activity of PZQ against schistosomes, including normally PZQ-insensitive juvenile worms. Similarly, suppressing expression of these transporters also increases adult worm responsiveness to PZQ. Our experiments may provide insights into the role of these drug transporters in PZQ action, and could also translate into new therapeutic strategies for augmenting treatment of schistosome infections and overcoming drug resistance.

Schistosome ABC multidrug transporters: From pharmacology to physiology

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The genuine and hypothesized roles of schistosome ABC transporters are reviewed.

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Evidence suggesting a role for transporters in schistosome drug susceptibility is discussed.

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Potential roles of ABC transporters in normal schistosome biology are outlined.

Praziquantel (PZQ) is essentially the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions worldwide. Though highly effective overall, PZQ has limitations, most notably its significant lack of activity against immature schistosomes. Furthermore, the availability of only a single drug for a disease of this magnitude makes reports of PZQ-resistant isolates particularly troubling. ATP-binding cassette (ABC) multidrug transporters such as P-glycoprotein (Pgp; ABCB1) are efflux transporters that underlie multidrug resistance (MDR); changes in their expression or structure are also associated with drug resistance in parasites, including helminths. This review will discuss the role these transporters might play in modulating schistosome susceptibility to PZQ, and the implications for developing new or repurposed treatments that enhance the efficacy of PZQ. However, in addition to influencing drug susceptibility, ABC transporters play important roles in several critical physiological functions such as excretion and maintenance of permeability barriers. They also transport signaling molecules with high affinity, and several lines of evidence implicate mammalian transporters in a diverse array of physiological functions, including regulation of immune responses. Like their mammalian counterparts, schistosome ABC transporters appear to be involved in functions critical to the parasite, including excretory activity and reproduction, and we hypothesize that they underlie at least some aspects of parasite–host interactions. Thus, in addition to their potential as targets for enhancers of PZQ susceptibility, these transporters might also serve as candidate targets for agents that disrupt the parasite life cycle and act as antischistosomals on their own.

Efficacy of Essential Oils of Thymus vulgaris and Origanum vulgare on Echinococcus granulosus

The aim of the present work was to determine the in vitro effect of T. vulgaris and O. vulgare essential oils against E. granulosus protoscoleces and cysts. Essential oils were added to the medium resulting in thymol final concentrations of 10 μg/mL. The essential oils had a time-dependent effect provoking the complete loss of protoscolex viability after 72 days of postincubation. The results were confirmed at the ultrastructure level. Loss of infectivity in protoscoleces incubated with O. vulgare after 60 days was observed. On the other hand, the weight of cysts recorded in mice inoculated with T. vulgaris treated protoscoleces was significantly lower than that obtained in control group. Gamma-glutamyl-transpeptidase activity was readily detected in the culture supernatant of protoscoleces treated either with the essential oils or thymol. T. vulgaris and O. vulgare essential oils and thymol can induce cell apoptosis of protoscoleces after short incubation times. The efficacy of T. vulgaris and O. vulgare essential oils was also demonstrated in vitro on E. granulosus murine cysts. Our data suggest that essential oils of T. vulgaris and O. vulgare have anthelmintic effect against protoscoleces and cysts of E. granulosus.

Cloning, expression and characterization of UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) from Wolbachia endosymbiont of human lymphatic filarial parasite Brugia malayi

Wolbachia, an endosymbiont of filarial nematode, is considered a promising target for treatment of lymphatic filariasis. Although functional characterization of the Wolbachia peptidoglycan assembly has not been fully explored, the Wolbachia genome provides evidence for coding all of the genes involved in lipid II biosynthesis, a part of peptidoglycan biosynthesis pathway. UDP-N-acetylglucosamine enolpyruvyl transferase (MurA) is one of the lipid II biosynthesis pathway enzymes and it has inevitably been recognized as an antibiotic target. In view of the vital role of MurA in bacterial viability and survival, MurA ortholog from Wolbachia endosymbiont of Brugia malayi (wBm-MurA) was cloned, expressed and purified for further molecular characterization. The enzyme kinetics and inhibition studies were undertaken using fosfomycin. wBm-MurA was found to be expressed in all the major life stages of B. malayi and was immunolocalized in Wolbachia within the microfilariae and female adults by the confocal microscopy. Sequence analysis suggests that the amino acids crucial for enzymatic activity are conserved. The purified wBm-MurA was shown to possess the EPSP synthase (3-phosphoshikimate 1-carboxyvinyltransferase) like activity at a broad pH range with optimal activity at pH 7.5 and 37°C temperature. The apparent affinity constant (K_m) for the substrate UDP-N-acetylglucosamine was found to be 0.03149 mM and for phosphoenolpyruvate 0.009198 mM. The relative enzymatic activity was inhibited ∼2 fold in presence of fosfomycin. Superimposition of the wBm-MurA homology model with the structural model of Haemophilus influenzae (Hi-MurA) suggests binding of fosfomycin at the same active site. The findings suggest wBm-MurA to be a putative antifilarial drug target for screening of novel compounds.

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.

Transcriptomic evaluation of the nicotinic acetylcholine receptor pathway in levamisole-resistant and -sensitive Oesophagostomum dentatum

Nematode anthelminthic resistance is widespread for the 3 major drug classes commonly used in agriculture: benzamidazoles, macrocyclic lactones, and nicotinic agonists e.g. levamisole. In parasitic nematodes the genetics of resistance is unknown other than to the benzimidazoles which primarily involve a single gene. In previous work with a levamisole resistant Oesophagostomum dentatum isolate, the nicotinic acetylcholine receptor (nAChR) exhibited decreased levamisole sensitivity. Here, using a transcriptomic approach on the same isolate, we investigate whether that decreased nAChR sensitivity is achieved via a 1-gene mechanism involving 1 of 27 nAChR pathway genes. 3 nAChR receptor subunit genes exhibited ≥2-fold change in transcript abundance: acr-21 and acr-25 increased, and unc-63 decreased. 4 SNPs having a ≥2-fold change in frequency were also identified. These data suggest that resistance is likely polygenic, involving modulated abundance of multiple subunits comprising the heteropentameric nAChR, and is not due to a simple 1-gene mechanism.

Transcriptome analysis of a parasitic clade V nematode: comparative analysis of potential molecular anthelmintic targets in Cylicostephanus goldi

Clade V nematodes comprise several parasitic species that include the cyathostomins, primary helminth pathogens of horses. Next generation transcriptome datasets are available for eight parasitic clade V nematodes, although no equine parasites are included in this group. Here, we report next generation transcriptome sequencing analysis for the common cyathostomin species, Cylicostephanus goldi. A cDNA library was generated from RNA extracted from 17 C. goldi male and female adult parasites. Following sequencing using a 454 GS FLX pyrosequencer, a total of 475,215 sequencing reads were generated, which were assembled into 26,910 contigs. Using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes databases, 27% of the transcriptome was annotated. Further in-depth analysis was carried out by comparing the C. goldi dataset with the next generation transcriptomes and genomes of other clade V nematodes, with the Oesophagostomum dentatum transcriptome and the Haemonchus contortus genome showing the highest levels of sequence identity with the cyathostomin dataset (45%). The C. goldi transcriptome was mined for genes associated with anthelmintic mode of action and/or resistance. Sequences encoding proteins previously associated with the three major anthelmintic classes used in horses were identified, with the exception of the P-glycoprotein group. Targeted resequencing of the glutamate gated chloride channel α4 subunit (glc-3), one of the primary targets of the macrocyclic lactone anthelmintics, was performed for several cyathostomin species. We believe this study reports the first transcriptome dataset for an equine helminth parasite, providing the opportunity for in-depth analysis of these important parasites at the molecular level. Sequences encoding enzymes involved in key processes and genes associated with levamisole/pyrantel and macrocyclic lactone resistance, in particular the glutamate gated chloride channels, were identified. This novel data will inform cyathostomin biology and anthelmintic resistance studies in future.

Insights into the immuno-molecular biology of Angiostrongylus vasorum through transcriptomics--prospects for new interventions

Angiostrongylus vasorum is a metastrongyloid nematode of dogs and other canids of major clinical importance in many countries. In order to gain first insights into the molecular biology of this worm, we conducted the first large-scale exploration of its transcriptome, and predicted essential molecules linked to metabolic and biological processes as well as host immune responses. We also predicted and prioritized drug targets and drug candidates. Following Illumina sequencing (RNA-seq), 52.3 million sequence reads representing adult A. vasorum were assembled and annotated. The assembly yielded 20,033 contigs, which encoded proteins with 11,505 homologues in Caenorhabditis elegans, and additional 2252 homologues in various other parasitic helminths for which curated data sets were publicly available. Functional annotation was achieved for 11,752 (58.6%) proteins predicted for A. vasorum, including peptidases (4.5%) and peptidase inhibitors (1.6%), protein kinases (1.7%), G protein-coupled receptors (GPCRs) (1.5%) and phosphatases (1.2%). Contigs encoding excretory/secretory and immuno-modulatory proteins represented some of the most highly transcribed molecules, and encoded enzymes that digest haemoglobin were conserved between A. vasorum and other blood-feeding nematodes. Using an essentiality-based approach, drug targets, including neurotransmitter receptors, an important chemosensory ion channel and cysteine proteinase-3 were predicted in A. vasorum, as were associated small molecular inhibitors/activators. Future transcriptomic analyses of all developmental stages of A. vasorum should facilitate deep explorations of the molecular biology of this important parasitic nematode and support the sequencing of its genome. These advances will provide a foundation for exploring immuno-molecular aspects of angiostrongylosis and have the potential to underpin the discovery of new methods of intervention.

Drug resistance in vectorborne parasites: multiple actors and scenarios for an evolutionary arms race

Drug-resistant pathogens emerge faster than new drugs come out of drug discovery pipelines. Current and future drug options should therefore be better protected, requiring a clear understanding of the factors that contribute to the natural history of drug resistance. Although many of these factors are relatively well understood for most bacteria, this proves to be more complex for vectorborne parasites. In this review, we discuss considering three key models (Plasmodium, Leishmania and Schistosoma) how drug resistance can emerge, spread and persist. We demonstrate a multiplicity of scenarios, clearly resulting from the biological diversity of the different organisms, but also from the different modes of action of the drugs used, the specific within- and between-host ecology of the parasites, and environmental factors that may have direct or indirect effects. We conclude that integrated control of drug-resistant vectorborne parasites is not dependent upon chemotherapy only, but also requires a better insight into the ecology of these parasites and how their transmission can be impaired.

Computational cloning of drug target genes of a parasitic nematode, Oesophagostomum dentatum

Background

Gene identification and sequence determination are critical requirements for many biological, genomic, and bioinformatic studies. With the advent of next generation sequencing (NGS) technologies, such determinations are predominantly accomplished in silico for organisms for which the genome is known or for which there exists substantial gene sequence information. Without detailed genomic/gene information, in silico sequence determination is not straightforward, and full coding sequence determination typically involves time- and labor-intensive PCR-based amplification and cloning methods.

Results

An improved method was developed with which to determine full length gene coding sequences in silico using de novo assembly of RNA-Seq data. The scheme improves upon initial contigs through contig-to-gene identification by BLAST nearest–neighbor comparison, and through single-contig refinement by iterative-binning and -assembly of reads. Application of the iterative method produced the gene identification and full coding sequence for 9 of 12 genes and improved the sequence of 3 of the 12 genes targeted by benzimidazole, macrocyclic lactone, and nicotinic agonist classes of anthelminthic drugs in the swine nodular parasite Oesophagostomum dentatum. The approach improved upon the initial optimized assembly with Velvet that only identified full coding sequences for 2 genes.

Conclusions

Our reiterative methodology represents a simplified pipeline with which to determine longer gene sequences in silico from next generation sequence data for any nematode for which detailed genetic/gene information is lacking. The method significantly improved upon an initial Velvet assembly of RNA-Seq data that yielded only 2 full length sequences. The identified coding sequences for the 11 target genes enables further future examinations including: (i) the use of recombinant target protein in functional assays seeking a better understanding of the mechanism of drug resistance, and (ii) seeking comparative genomic and transcriptomic assessments between parasite isolates that exhibit varied drug sensitivities.

Molecular characterization of an rsmD-like rRNA methyltransferase from the Wolbachia endosymbiont of Brugia malayi and antifilarial activity of specific inhibitors of the enzyme

The endosymbiotic organism Wolbachia is an attractive antifilarial drug target. Here we report on the cloning and expression of an rsmD-like rRNA methyltransferase from the Wolbachia endosymbiont of Brugia malayi, its molecular properties, and assays for specific inhibitors. The gene was found to be expressed in all the major life stages of B. malayi. The purified enzyme expressed in Escherichia coli was found to be in monomer form in its native state. The activities of the specific inhibitors (heteroaryl compounds) against the enzyme were tested with B. malayi adult and microfilariae for 7 days in vitro at various concentrations, and NSC-659390 proved to be the most potent compound (50% inhibitory concentration [IC50], 0.32 μM), followed by NSC-658343 (IC50, 4.13 μM) and NSC-657589 (IC50, 7.5 μM). On intraperitoneal administration at 5 mg/kg of body weight for 7 days to adult jirds into which B. malayi had been transplanted intraperitoneally, all the compounds killed a significant proportion of the implanted worms. A very similar result was observed in infected mastomys when inhibitors were administered. Docking studies of enzyme and inhibitors and an in vitro tryptophan quenching experiment were also performed to understand the binding mode and affinity. The specific inhibitors of the enzyme showed a higher affinity for the catalytic site of the enzyme than the nonspecific inhibitors and were found to be potent enough to kill the worm (both adults and microfilariae) in vitro as well as in vivo in a matter of days at micromolar concentrations. The findings suggest that these compounds be evaluated against other pathogens possessing a methyltransferase with a DPPY motif and warrant the design and synthesis of more such inhibitors.

New approaches for understanding mechanisms of drug resistance in schistosomes

Schistosomes are parasitic flatworms that cause schistosomiasis, a neglected tropical disease that affects hundreds of millions worldwide. Treatment and control of schistosomiasis relies almost entirely on the single drug praziquantel (PZQ), making the prospect of emerging drug resistance particularly worrisome. This review will survey reports of PZQ (and other drug) resistance in schistosomes and other platyhelminths, and explore mechanisms by which drug resistance might develop. Newer genomic and post-genomic strategies that offer the promise of better understanding of how drug resistance might arise in these organisms will be discussed. These approaches could also lead to insights into the mode of action of these drugs and potentially provide markers for monitoring the emergence of resistance.

Transcriptional responses of in vivo praziquantel exposure in schistosomes identifies a functional role for calcium signalling pathway member CamKII

Treatment for clinical schistosomiasis has relied centrally on the broad spectrum anthelmintic praziquantel; however, there is limited information on its mode of action or the molecular response of the parasite. This paper presents a transcriptional and functional approach to defining the molecular responses of schistosomes to praziquantel. Differential gene expression in Schistosoma japonicum was investigated by transcriptome-wide microarray analysis of adult worms perfused from infected mice after 0.5 to 24 hours after oral administration of sub-lethal doses of praziquantel. Genes up-regulated initially in male parasites were associated with "Tegument/Muscle Repair" and "Lipid/Ion Regulation" functions and were followed by "Drug Resistance" and "Ion Regulation" associated genes. Prominent responses induced in female worms included up-regulation of "Ca(2+) Regulation" and "Drug Resistance" genes and later by transcripts of "Detoxification" and "Pathogen Defense" mechanisms. A subset of highly over-expressed genes, with putative drug resistance/detoxification roles or Ca(2+)-dependant/modulatory functions, were validated by qPCR. The leading candidate among these was CamKII, a putative calcium/calmodulin-dependent protein kinase type II delta chain. RNA interference was employed to knockdown CamKII in S. japonicum to determine the role of CamKII in the response to praziquantel. After partial-knockdown, schistosomes were analysed using IC50 concentrations (50% worm motility) and quantitative monitoring of parasite movement. When CamKII transcription was reduced by 50-69% in S. japonicum, the subsequent effect of an IC50 dosage of praziquantel was exacerbated, reducing motility from 47% to 27% in female worms and from 61% to 23% in males. These observations indicated that CamKII mitigates the effects of praziquantel, probably through stabilising Ca(2+) fluxes within parasite muscles and tegument. Together, these studies comprehensively charted transcriptional changes upon exposure to praziquantel and, notably, identified CamKII as potentially central to the, as yet undefined, mode of action of praziquantel.

Current drug targets for helminthic diseases

More than 2 billion people are infected with helminth parasites across the globe. The burgeoning drug resistance against current anthelmintics in parasitic worms of humans and livestock requires urgent attention to tackle these recalcitrant worms. This review focuses on the advancements made in the area of helminth drug target discovery especially from the last few couple of decades. It highlights various approaches made in this field and enlists the potential drug targets currently being pursued to target economically important helminth species both from human as well as livestock to combat disease pathology of schistosomiasis, onchocerciasis, lymphatic filariasis, and other important macroparasitic diseases. Research in the helminths study is trending to identify potential and druggable targets through genomic, proteomic, biochemical, biophysical, in vitro experiments, and in vivo experiments in animal models. The availability of major helminths genome sequences and the subsequent availability of genome-scale functional datasets through in silico search and prioritization are expected to guide the experimental work necessary for target-based drug discovery. Organized and documented list of drug targets from various helminths of economic importance have been systematically covered in this review for further exploring their use and applications, which can give physicians and veterinarians effective drugs in hand to enable them control worm infections.

Derquantel and abamectin: effects and interactions on isolated tissues of Ascaris suum

Startect(®) is a novel anthelmintic combination of derquantel and abamectin. It is hypothesized that derquantel and abamectin interact pharmacologically. We investigated the effects of derquantel, abamectin and their combination on somatic muscle nicotinic acetylcholine receptors and pharyngeal muscle glutamate gated chloride receptor channels of Ascaris suum. We used muscle-strips to test the effects of abamectin, derquantel, and abamectin+derquantel together on the contraction responses to different concentrations of acetylcholine. We found that abamectin reduced the response to acetylcholine, as did derquantel. In combination (abamectin+derquantel), inhibition of the higher acetylcholine concentration response was greater than the predicted additive effect. A two-micropipette current-clamp technique was used to study electrophysiological effects of the anthelmintics on: (1) acetylcholine responses in somatic muscle and; (2) on l-glutamate responses in pharyngeal preparations. On somatic muscle, derquantel (0.1-30μM) produced a potent (IC50 0.22, CI 0.18-0.28μM) reversible antagonism of acetylcholine depolarizations. Abamectin (0.3μM) produced a slow onset inhibition of acetylcholine depolarizations. We compared effects of abamectin and derquantel on muscle preparations pretreated for 30min with these drugs. The effect of the combination was significantly greater than the predicted additive effect of both drugs at higher acetylcholine concentrations. On the pharynx, application of derquantel produced no significant effect by itself or on responses to abamectin and l-glutamate. Abamectin increased the input conductance of the pharynx (EC50 0.42, CI 0.13-1.36μM). Our study demonstrates that abamectin and derquantel interact at nicotinic acetylcholine receptors on the somatic muscle and suggested synergism can occur.

ABC multidrug transporters in schistosomes and other parasitic flatworms

Schistosomiasis, a neglected tropical disease affecting hundreds of millions, is caused by parasitic flatworms of the genus Schistosoma. Treatment and control of schistosomiasis relies almost exclusively on a single drug, praziquantel (PZQ), a dangerous situation for a disease of this magnitude. Though PZQ is highly effective overall, it has drawbacks, and reports of worms showing PZQ resistance, either induced in the laboratory or isolated from the field, are disconcerting. Multidrug transporters underlie multidrug resistance (MDR), a phenomenon in which resistance to a single drug is accompanied by unexpected cross-resistance to several structurally unrelated compounds. Some of the best studied multidrug transporters are members of the ancient and very large ATP-binding cassette (ABC) superfamily of efflux transporters. ABC multidrug transporters such as P-glycoprotein (Pgp; ABCB1) are also associated with drug resistance in parasites, including helminths such as schistosomes. In addition to their association with drug resistance, however, ABC transporters also function in a wide variety of physiological processes in metazoans. In this review, we examine recent studies that help define the role of schistosome ABC transporters in regulating drug susceptibility, and in normal schistosome physiology, including reproduction and excretory activity. We postulate that schistosome ABC transporters could be useful targets for compounds that enhance the effectiveness of current therapeutics as well as for agents that act as antischistosomals on their own.

The metabolic fate of ivermectin in host (Ovis aries) and parasite (Haemonchus contortus)

Ivermectin (IVE), one of the most important anthelmintics, is often used in the treatment of haemonchosis in ruminants. The objective of our work was (1) to find and identify phase I and II metabolites of IVE formed by the Barber's pole worm (Haemonchus contortus), and (2) to compare IVE metabolites in helminths with IVE biotransformation in sheep (Ovis aries) as host species. Ultrahigh-performance liquid chromatography/tandem mass spectrometry (UHPLC/MS/MS) was used for this purpose. During in vitro incubations, microsomes (from adult worms or from ovine liver) and a primary culture of ovine hepatocytes were incubated with IVE. In the ex vivo study, living H. contortus adults were incubated in the presence of 1 μM IVE for 24 h. The results showed that the H. contortus enzymatic system is not able to metabolize IVE. On the other hand, 7 different phase I as well as 9 phase II IVE metabolites were detected in ovine samples using UHPLC/MS/MS analyses. Most of these metabolites have not been described before. Haemonchus contortus is not able to deactivate IVE through biotransformation; therefore, biotransformation does not contribute to the development of IVE-resistance in the Barber's pole worm.

Transcriptional analysis of Schistosoma mansoni treated with praziquantel in vitro

Schistosomiasis is one of the foremost health problems in developing countries and has been estimated to account for the loss of up to 56 million annual disability-adjusted life years. Control of the disease relies almost exclusively on praziquantel (PZQ) but this drug does not kill juvenile worms during the early stages of infection or prevent post-treatment reinfection. As the use of PZQ continues to grow, there are fears that drug resistance may become problematic thus there is a need to develop a new generation of more broadly effective anti-schistosomal drugs, a task that will be made easier by having an understanding of why PZQ kills sexually mature worms but fails to kill juveniles. Here, we describe the exposure of mixed-sex juvenile and sexually mature male and female Schistosoma mansoni to 1 μg/mL PZQ in vitro and the use of microarrays to observe changes to the transcriptome associated with drug treatment. Although there was no significant difference in the total number of genes expressed by adult and juvenile schistosomes after treatment, juveniles differentially regulated a greater proportion of their genes. These included genes encoding multiple drug transporter as well as calcium regulatory, stress and apoptosis-related proteins. We propose that it is the greater transcriptomic flexibility of juvenile schistosomes that allows them to respond to and survive exposure to PZQ in vivo.

Unique structure and regulation of the nematode detoxification gene regulator, SKN-1: implications to understanding and controlling drug resistance

Nematodes parasitize an alarming number of people and agricultural animals globally and cause debilitating morbidity and mortality. Anthelmintics have been the primary tools used to control parasitic nematodes for the past several decades, but drug resistance is becoming a major obstacle. Xenobiotic detoxification pathways defend against drugs and other foreign chemicals in diverse organisms, and evidence is accumulating that they play a role in mediating resistance to anthelmintics in nematodes. Related antioxidation pathways may also provide filarial parasites with protection against host free-radical-mediated immune responses. Upstream regulatory pathways have received almost no attention in nematode parasites, despite their potential to coregulate multiple detoxification and antioxidation genes. The nuclear eurythroid 2-related factor 2 (NRF2) transcription factor mediates inducible detoxification and antioxidation defenses in mammals, and recent studies have demonstrated that it promotes multidrug resistance in some human tumors. Recent studies in the free-living model nematode, Caenorhabditis elegans, have defined the homologous transcription factor, SKN-1, as a master regulator of detoxification and antioxidation genes. Despite similar functions, SKN-1 and NRF2 have important differences in structure and regulatory pathways. Protein alignment and phylogenetic analyses indicate that these differences are shared among many nematodes, making SKN-1 a candidate for specifically targeting nematode detoxification and antioxidation.

Anthelmintic tolerance in free-living and facultative parasitic isolates of Halicephalobus (Panagrolaimidae)

Studies on anthelmintic resistance in equine parasites do not include facultative parasites. Halicephalobus gingivalis is a free-living bacterivorous nematode and a known facultative parasite of horses with a strong indication of some form of tolerance to common anthelmintic drugs. This research presents the results of an in vitro study on the anthelmintic tolerance of several isolates of Halicephalobus to thiabendazole and ivermectin using an adaptation of the Micro-Agar Larval Development Test hereby focusing on egg hatching and larval development. Panagrellus redivivus and Panagrolaimus superbus were included as a positive control. The results generally show that the anthelmintic tolerance of Halicephalobus to both thiabendazole and ivermectin was considerably higher than that of the closely related Panagrolaimidae and, compared to other studies, than that of obligatory equine parasites. Our results further reveal a remarkable trend of increasing tolerance from fully free-living isolates towards horse-associated isolates. In vitro anthelmintic testing with free-living and facultative parasitic nematodes offers the advantage of observing drug effect on the complete life cycle as opposed to obligatory parasites that can only be followed until the third larval stage. We therefore propose Halicephalobus gingivalis as an experimental tool to deepen our understanding of the biology of anthelmintic tolerance.

Low efficacy of single-dose albendazole and mebendazole against hookworm and effect on concomitant helminth infection in Lao PDR

Background

Albendazole and mebendazole are increasingly deployed for preventive chemotherapy targeting soil-transmitted helminth (STH) infections. We assessed the efficacy of single oral doses of albendazole (400 mg) and mebendazole (500 mg) for the treatment of hookworm infection in school-aged children in Lao PDR. Since Opisthorchis viverrini is co-endemic in our study setting, the effect of the two drugs could also be determined against this liver fluke.

Methodology

We conducted a randomized, open-label, two-arm trial. In total, 200 children infected with hookworm (determined by quadruplicate Kato-Katz thick smears derived from two stool samples) were randomly assigned to albendazole (n = 100) and mebendazole (n = 100). Cure rate (CR; percentage of children who became egg-negative after treatment), and egg reduction rate (ERR; reduction in the geometric mean fecal egg count at treatment follow-up compared to baseline) at 21–23 days posttreatment were used as primary outcome measures. Adverse events were monitored 3 hours post treatment.

Principal Findings

Single-dose albendazole and mebendazole resulted in CRs of 36.0% and 17.6% (odds ratio: 0.4; 95% confidence interval: 0.2–0.8; P = 0.01), and ERRs of 86.7% and 76.3%, respectively. In children co-infected with O. viverrini, albendazole and mebendazole showed low CRs (33.3% and 24.2%, respectively) and moderate ERRs (82.1% and 78.2%, respectively).

Conclusions/Significance

Both albendazole and mebendazole showed disappointing CRs against hookworm, but albendazole cured infection and reduced intensity of infection with a higher efficacy than mebendazole. Single-dose administrations showed an effect against O. viverrini, and hence it will be interesting to monitor potential ancillary benefits of a preventive chemotherapy strategy that targets STHs in areas where opisthorchiasis is co-endemic.

Clinical Trial Registration

Current Controlled Trials ISRCTN29126001

Parasitic worms remain a public health problem in developing countries. Regular deworming with the drugs albendazole and mebendazole is the current global control strategy. We assessed the efficacies of a single tablet of albendazole (400 mg) and mebendazole (500 mg) against hookworm in children of southern Lao PDR. From each child, two stool samples were examined for the presence and number of hookworm eggs. Two hundred children were found to be infected. They were randomly assigned to albendazole (n = 100) or mebendazole (n = 100) treatment. Three weeks later, another two stool samples were analyzed for hookworm eggs. Thirty-two children who were given albendazole had no hookworm eggs anymore in their stool, while only 15 children who received mebendazole were found egg-negative. The total number of hookworm eggs was reduced by 85.3% in the albendazole and 74.5% in the mebendazole group. About one third of the children who were co-infected with the Asian liver fluke Opisthorchis viverrini were cleared from this infection following albendazole treatment and about one forth in the mebendazole group. Concluding, both albendazole and mebendazole showed disappointingly low cure rates against hookworm, with albendazole performing somewhat better. The effect of these two drugs against O. viverrini should be studied in greater detail.

Genetic knockdown and pharmacological inhibition of parasite multidrug resistance transporters disrupts egg production in Schistosoma mansoni

P-glycoprotein (Pgp) and multidrug resistance-associated proteins (MRPs) are ATP-dependent transporters involved in efflux of toxins and xenobiotics from cells. When overexpressed, these transporters can mediate multidrug resistance (MDR) in mammalian cells, and changes in Pgp expression and sequence are associated with drug resistance in helminths. In addition to the role they play in drug efflux, MDR transporters are essential components of normal cellular physiology, and targeting them may prove a useful strategy for development of new therapeutics or of compounds that enhance the efficacy of current anthelmintics. We previously showed that expression of Schistosoma mansoni MDR transporters increases in response to praziquantel (PZQ), the current drug of choice against schistosomiasis, and that reduced PZQ sensitivity correlates with higher levels of these parasite transporters. We have also shown that PZQ inhibits transport by SMDR2, a Pgp orthologue from S. mansoni, and that PZQ is a likely substrate of SMDR2. Here, we examine the physiological roles of SMDR2 and SmMRP1 (the S. mansoni orthologue of MRP1) in S. mansoni adults, using RNAi to knock down expression, and pharmacological agents to inhibit transporter function. We find that both types of treatments disrupt parasite egg deposition by worms in culture. Furthermore, administration of different MDR inhibitors to S. mansoni-infected mice results in a reduction in egg burden in host liver. These schistosome MDR transporters therefore appear to play essential roles in parasite egg production, and can be targeted genetically and pharmacologically. Since eggs are responsible for the major pathophysiological consequences of schistosomiasis, and since they are also the agents for transmission of the disease, these results suggest a potential strategy for reducing disease pathology and spread.

Schistosomes are parasitic flatworms that are the causative agents of schistosomiasis, a major tropical disease. As adults, schistosomes reside within the host vasculature, taking up nutrients, evading host defenses, and expelling wastes and toxins. Multidrug resistance transporters are involved in removal of toxins and foreign compounds, including drugs, from cells. These transporters have broad selectivity, and when upregulated or mutated, can confer resistance to a wide spectrum of drugs against mammalian tumor cells. They are also associated with drug resistance in various parasites, including helminths. In this report, we have used knockdown of expression of these proteins and pharmacological inhibition of their transport function to dissect their physiological role in the schistosome life cycle. We find that either reducing transporter expression or pharmacologically inhibiting transporter function leads to disruption of egg production by adult worms. Eggs deposited within the host are the major cause of disease pathology, and eggs excreted by the host are the means of continuation of the life cycle and transmission of the disease. The capability to interfere with schistosome egg production could have major implications for development of new treatment strategies.

In silico approach to screen compounds active against parasitic nematodes of major socio-economic importance

Background

Infections due to parasitic nematodes are common causes of morbidity and fatality around the world especially in developing nations. At present however, there are only three major classes of drugs for treating human nematode infections. Additionally the scientific knowledge on the mechanism of action and the reason for the resistance to these drugs is poorly understood. Commercial incentives to design drugs that are endemic to developing countries are limited therefore, virtual screening in academic settings can play a vital role is discovering novel drugs useful against neglected diseases. In this study we propose to build robust machine learning model to classify and screen compounds active against parasitic nematodes.

Results

A set of compounds active against parasitic nematodes were collated from various literature sources including PubChem while the inactive set was derived from DrugBank database. The support vector machine (SVM) algorithm was used for model development, and stratified ten-fold cross validation was used to evaluate the performance of each classifier. The best results were obtained using the radial basis function kernel. The SVM method achieved an accuracy of 81.79% on an independent test set. Using the model developed above, we were able to indentify novel compounds with potential anthelmintic activity.

Conclusion

In this study, we successfully present the SVM approach for predicting compounds active against parasitic nematodes which suggests the effectiveness of computational approaches for antiparasitic drug discovery. Although, the accuracy obtained is lower than the previously reported in a similar study but we believe that our model is more robust because we intentionally employed stringent criteria to select inactive dataset thus making it difficult for the model to classify compounds. The method presents an alternative approach to the existing traditional methods and may be useful for predicting hitherto novel anthelmintic compounds.

On the mode of action of emodepside: slow effects on membrane potential and voltage-activated currents in Ascaris suum

BACKGROUND AND PURPOSE

Anthelmintics are required for treatment and prophylaxis of nematode parasites of humans and domestic animals. Emodepside, a cyclooctadepsipeptide, is a modern anthelmintic that has a novel mode of action involving a Ca-activated K channel (SLO-1) in Caenorhabditis elegans, sometimes mediated by a latrophilin (LAT) receptor. We examined mechanisms of action of emodepside in a parasitic nematode, Ascaris suum.

EXPERIMENTAL APPROACH

RT-PCR was used to investigate expression of slo-1 and lat-1 in A. suum muscle flaps, and two-micropipette current-clamp and voltage-clamp techniques were used to record electrophysiological effects of emodepside.

KEY RESULTS

Expression of slo-1 and lat-1 were detected. Emodepside produced a slow time-dependent (20 min), 4-aminopyridine sensitive, concentration-dependent hyperpolarization and increase in voltage-activated K currents. Sodium nitroprusside increased the hyperpolarizations and K currents. N-nitro-L-arginine inhibited the hyperpolarizations and K currents. Phorbol-12-myristate-13 acetate increased the K currents, while staurosporine inhibited the hyperpolarizations and K currents. Iberiotoxin reduced these emodepside K currents. The effect of emodepside was reduced in Ca-free solutions. Emodepside had no effect on voltage-activated Ca currents.

CONCLUSIONS AND IMPLICATIONS

Asu-slo-1 and Asu-lat-1 are expressed in adult A. suum muscle flaps and emodepside produces slow activation of voltage-activated Ca-dependent SLO-1-like K channels. The effect of emodepside was enhanced by stimulation of protein kinase C and NO pathways. The data are consistent with a model in which NO, PKC and emodepside signalling pathways are separate and converge on the K channels, or in which emodepside activates NO and PKC signalling pathways to increase opening of the K channels.

Gene expression changes in a P-glycoprotein (Tci-pgp-9) putatively associated with ivermectin resistance in Teladorsagia circumcincta

Anthelmintic resistance in parasitic nematodes of small ruminants is widespread and, in some parts of the world, threatens the sustainability of sheep production. The genetic changes underlying resistance to anthelmintics, particularly ivermectin (IVM), remain to be determined. The majority of studies to date have investigated target site mutations; relatively little attention has been paid to the role of changes in gene expression. In this study, we investigated the expression of putative drug transporter molecules, P-glycoproteins (Pgps), in Teladorsagia circumcincta, the predominant parasitic nematode species of sheep in the UK and the major anthelmintic resistant species. Utilising a degenerate PCR approach, 11 partial Pgp sequences were identified. Constitutive differences in gene expression between an IVM-susceptible (MTci2) and a multidrug-resistant (MTci5) isolate were determined for 10 of the Pgps using the ΔΔCt TaqMan® real-time PCR method. Gene expression differences were particularly marked in one of these genes, namely Tci-pgp-9. In the MTci5 isolate, statistically significant increases in Tci-pgp-9 expression, at the mRNA level, were observed across all life-cycle stages and most notably in eggs (55-fold increase). Comparison of the partial Tci-pgp-9 nucleotide sequences from MTci2 and MTci5 also identified high levels of polymorphism. This work has shown that constitutively increased expression in Tci-pgp-9, coupled with increased sequence polymorphism, could play a role in allowing multidrug-resistant T. circumcincta to survive IVM exposure. The genetic changes underpinning these gene expression changes remain to be elucidated and need to be investigated in other isolates. These changes could form the basis of an IVM resistance marker to monitor the spread of resistance and to evaluate management practices aimed at delaying its spread.

Advances in helminth immunology: optimism for future vaccine design?

Intestinal helminths infect approximately 2 billion people worldwide. Worm burdens correlate with disease morbidity and children generally harbor the largest numbers. The majority of intestinal helminths do not replicate within their host, and worm burdens increase through constant reinfection. Current strategies of worm control involve drug administration to school-aged children. Yet the rapid rate of reinfection and the appearance of drug resistant strains in livestock raise concerns over the sustainable nature of this strategy. A combined strategy of drug treatment for the expulsion of adult worms and vaccination designed to halt reinfection would offer the most effective means of control. Before successful vaccines can be developed our knowledge of the initiation and implementation of host immunity must be improved.

Pharmacology and potential physiological significance of schistosome multidrug resistance transporters

Schistosomes are the causative agents of schistosomiasis, a neglected tropical disease affecting hundreds of millions worldwide and a major global health burden. Current control of schistosomiasis depends largely on a single drug, praziquantel (PZQ). One potential physiological target for new antischistosomal drugs is the parasite's excretory system, which removes wastes and xenobiotics. Multidrug resistance (MDR) transporters that are members of the ATP-binding cassette (ABC) superfamily of proteins are ATP-dependent efflux pumps involved in removal of toxins and xenobiotics from cells. They mediate the phenomenon of multidrug resistance, in which cells resistant to one drug show cross-resistance to a broad range of other agents, and are also associated with reduced drug susceptibility in parasitic helminths. In this review, we survey the different types of ABC transporter genes present within the schistosome genome, and examine recent evidence indicating that at least some of these transporters may play a role in fine-tuning susceptibility of schistosomes to PZQ. Disruption of their function may therefore provide a strategy for enhancing drug action or overcoming or attenuating drug resistance. Furthermore, dissection of the roles these transporters may play in normal schistosome physiology could potentially lead to identification of highly "druggable" targets for new antischistosomals.