Effect of benzimidazole under-dosing on the resistant allele frequency in Teladorsagia circumcincta (Nematoda)

Quantitative tests of albendazole resistance in beta-tubulin mutants

Benzimidazole (BZ) anthelmintics are among the most important treatments for parasitic nematode infections in the developing world. Widespread BZ resistance in veterinary parasites and emerging resistance in human parasites raise major concerns for the continued use of BZs. Knowledge of the mechanisms of resistance is necessary to make informed treatment decisions and circumvent resistance. Benzimidazole resistance has traditionally been associated with mutations and natural variants in the C. elegans beta-tubulin gene ben-1 and orthologs in parasitic species. However, variants in ben-1 alone do not explain the differences in BZ responses across parasite populations. Here, we examine the roles of five C. elegans beta-tubulin genes (tbb-1, mec-7, tbb-4, ben-1, and tbb-6) to identify the role each gene plays in BZ response. We generated C. elegans strains with a loss of each beta-tubulin gene, as well as strains with a loss of tbb-1, mec-7, tbb-4, or tbb-6 in a genetic background that also lacks ben-1 to test beta-tubulin redundancy in BZ response. We found that only the individual loss of ben-1 conferred a substantial level of BZ resistance, although the loss of tbb-1 was found to confer a small benefit in the presence of albendazole (ABZ). The loss of ben-1 was found to confer an almost complete rescue of animal development in the presence of 30 μM ABZ, likely explaining why no additive effects caused by the loss of a second beta-tubulin were observed. We demonstrate that ben-1 is the only beta-tubulin gene in C. elegans where loss confers substantial BZ resistance.

The turkey ascarid, Ascaridia dissimilis, as a model genetic system

Parasitic nematodes cause significant effects on humans each year, with the most prevalent being Ascaris lumbricoides. Benzimidazoles (BZ) are the most widely used anthelmintic drug in humans, and although the biology of resistance to this drug class is understood in some species, resistance is poorly characterized in ascarids. Models such as Caenorhabditis elegans were essential in developing our current understanding of BZ resistance, but more closely related model nematodes are needed to understand resistance in ascarids. Here, we propose a new ascarid model species that infects turkeys, Ascaridia dissimilis, to develop a better understanding of BZ resistance.

Natural variation in Caenorhabditis elegans responses to the anthelmintic emodepside

Treatment of parasitic nematode infections depends primarily on the use of anthelmintics. However, this drug arsenal is limited, and resistance against most anthelmintics is widespread. Emodepside is a new anthelmintic drug effective against gastrointestinal and filarial nematodes. Nematodes that are resistant to other anthelmintic drug classes are susceptible to emodepside, indicating that the emodepside mode of action is distinct from previous anthelmintics. The laboratory-adapted Caenorhabditis elegans strain N2 is sensitive to emodepside, and genetic selection and in vitro experiments implicated slo-1, a large K+ conductance (BK) channel gene, in emodepside mode of action. In an effort to understand how natural populations will respond to emodepside, we measured brood sizes and developmental rates of wild C. elegans strains after exposure to the drug and found natural variation across the species. Some of the observed variation in C. elegans emodepside responses correlates with amino acid substitutions in slo-1, but genetic mechanisms other than slo-1 coding variants likely underlie emodepside resistance in wild C. elegans strains. Additionally, the assayed strains have higher offspring production in low concentrations of emodepside (a hormetic effect). We find that natural variation affects emodepside sensitivity, supporting the suitability of C. elegans as a model system to study emodepside responses across natural nematode populations.

Spread of anthelmintic resistance in intestinal helminths of dogs and cats is currently less pronounced than in ruminants and horses - Yet it is of major concern

Anthelmintic resistance (AR) has thus far only rarely been reported for intestinal helminths of dogs and cats, in contrast to parasites of livestock and horses. We highlight possible reasons for this striking and important discrepancy, including ecological, biological and genetic factors and/or intervention regimens of key intestinal helminths concerning both host groups. In view of the current knowledge related to the genetics, mechanisms and principles of AR development, we point at issues which in our view contribute to a comparatively lower risk of AR development in intestinal helminths of dogs and cats. Finally, we specify research needs and provide recommendations by which, based on the available information about AR in ruminant and equine helminths, the development of AR in dog and cat helminths may best be documented, prevented or at least postponed.

Molecular evidence on the emergence of benzimidazole resistance SNPs in field isolates of Marshallagia marshalli (Nematoda: Trichostrongylidae) in sheep

The infection with members of the Trichostrongylid nematodes has been frequently reported from sheep and goats. Because of the widespread use of Benzimidazoles (BZs), the resistance suspected to occur in some worms populations. In this study, we focused on the prevalent nematode, Marshallagia marshalli, from the abomasa of sheep. Samples were obtained from at least 10 infected farms and diagnosed with morphological and molecular methods. For resistance analysis, genomic DNA from pooled adult samples of all farms were analysed for the beta tubulin gene to detect any polymorphisms at codon positions of F167Y, E198A and F200Y. According to the results, seven farms (70%) revealed resistance (R) allele at F200Y with relatively high frequency. No other mutations were identified at the other two positions. Also, except for one homozygous (RR) occasion, the isolates with R allele had heterozygous (RS) genotype. This finding indicates that the worm populations are still affected by drugs of the BZ class. However, the genetic data also notes on developing resistance mechanisms in M. marshalli populations in sheep.

Risk factor analysis of equine strongyle resistance to anthelmintics

Intestinal strongyles are the most problematic endoparasites of equids as a result of their wide distribution and the spread of resistant isolates throughout the world. While abundant literature can be found on the extent of anthelmintic resistance across continents, empirical knowledge about associated risk factors is missing. This study brought together results from anthelmintic efficacy testing and risk factor analysis to provide evidence-based guidelines in the field. It involved 688 horses from 39 French horse farms and riding schools to both estimate Faecal Egg Count Reduction (FECR) after anthelmintic treatment and to interview farm and riding school managers about their practices. Risk factors associated with reduced anthelmintic efficacy in equine strongyles were estimated across drugs using a marginal modelling approach. Results demonstrated ivermectin efficacy (96.3% ± 14.5% FECR), the inefficacy of fenbendazole (42.8% ± 33.4% FECR) and an intermediate profile for pyrantel (90.3% ± 19.6% FECR). Risk factor analysis provided support to advocate for FEC-based treatment regimens combined with individual anthelmintic dosage and the enforcement of tighter biosecurity around horse introduction. The combination of these measures resulted in a decreased risk of drug resistance (relative risk of 0.57, p = 0.02). Premises falling under this typology also relied more on their veterinarians suggesting practitionners play an important role in the sustainability of anthelmintic usage. Similarly, drug resistance risk was halved in premises with frequent pasture rotation and with stocking rate below five horses/ha (relative risk of 0.53, p < 0.01). This is the first empirical risk factor analysis for anthelmintic resistance in equids. Our findings should guide the implementation of more sustained strongyle management in the field.

•

688 horses have been enrolled for Egg reduction rate measure.

•

Ivermectin remains efficient but fendendazole is not any more.

•

Evidence-based drenching and individual dosing lower resistance risk.

•

Higher biosecurity is associated with higher drug efficacy.

•

Premises with the least pasture constraints show higher drug efficacy.

Genotypic profile of benzimidazole resistance associated with SNP F167Y and F200Y beta-tubulin gene in Brazilian populations of Haemonchus contortus of goats

Benzimidazoles are the most common anthelminthic used for control of gastrointestinal nematodes of goats in the Brazilian semi-arid region. Resistance to these compounds in the nematode Haemonchus contortus has been associated with single nucleotide polymorphisms (SNP) in codons 167 (F167Y) and 200 (F200Y) on the β-tubulin isotype 1 gene. To determine the resistance profile to benzimidazoles of populations of H. contortus of goats of Brazilian semi-arid region, larvae of 29 populations of these nematodes were individually genotyped by real time PCR using a Taqman assay. The percentage of larvae homozygous (RR) for SNP F200Y was relatively low (18.9%), particularly when compared to SNP F167Y (32.7%), indicating that the latter has more relevance in this region. However, the associations between these two SNP demonstrate percentages of resistance ranging from 34.7% to 100% between populations, being the highest percentages for homozygous individuals resistant for the mutation 167 and susceptible to mutation 200 (RR-F167Y/F200Y-SS: 26.7%), followed by combination of heterozygous for both mutations (F167Y-SR/F200Y-SR: 22.8%). These results indicate high levels of resistance in populations of H. contortus of goats in the Brazilian semi-arid region, and thus ineffective antiparasitic control with the use of benzimidazoles in the region.

Reduction of benzimidazole resistance in established Haemonchus contortus populations in goats using a single infection with a benzimidazole-susceptible isolate

An in vivo study in goats evaluated the effect of superimposing a single artificial infection with a benzimidazole (BZ)-susceptible Haemonchus contortus isolate upon established H. contortus populations of known BZ resistance by measuring the phenotypic BZ resistance of eggs collected from faeces before and after re-infection. Two H. contortus isolates, one benzimidazole resistant (BZR) and the other susceptible (BZS), were used to infect worm-free goats. Eight goats were initially infected with 2000 third-stage larvae (L3). In each case the inoculum contained a pre-determined proportion of the two isolates: 100% BZS (one goat), 75% BZS/25% BZR (two goats), 50% BZS/50% BZR (two goats), 25%BZS/75% BZR (two goats) and, finally, 100% BZR (one goat). The phenotypic BZ susceptibility of the H. contortus population formed in each goat after the first infection was determined on days 30 and 36 post-infection using an egg-hatch assay (EHA) that estimated the concentration of thiabendazole (TBZ) required for 95% inhibition of larval hatching (EC(95)) with a 95% confidence interval (95% CI). On day 49 post-infection, goats were re-infected with 2000 L3 of the BZS isolate alone. A second set of EHA bioassays was performed 28 days and 34 days after re-infection. The first infection protocol produced three populations classified as BZS (EC(95) 0.055-0.065 μg TBZ/ml) while four were categorized as BZR (EC(95) 0.122-0.344 μg TBZ/ml). The status of one other population could not be determined. After re-infection with BZS L3, the number of susceptible populations increased to six (EC(95) 0.043-0.074 μg TBZ/ml) while the remaining two were deemed resistant (EC(95) 0.114-119 μg TBZ/ml). Re-infection with BZS L3 thereby reduced the resistance status of most H. contortus populations.

Molecular diagnosis of infections and resistance in veterinary and human parasites

Since 1977, >2000 research papers described attempts to detect, identify and/or quantify parasites, or disease organisms carried by ecto-parasites, using DNA-based tests and 148 reviews of the topic were published. Despite this, only a few DNA-based tests for parasitic diseases are routinely available, and most of these are optional tests used occasionally in disease diagnosis. Malaria, trypanosomiasis, toxoplasmosis, leishmaniasis and cryptosporidiosis diagnosis may be assisted by DNA-based testing in some countries, but there are very few cases where the detection of veterinary parasites is assisted by DNA-based tests. The diagnoses of some bacterial (e.g. lyme disease) and viral diseases (e.g. tick borne encephalitis) which are transmitted by ecto-parasites more commonly use DNA-based tests, and research developing tests for these species makes up almost 20% of the literature. Other important uses of DNA-based tests are for epidemiological and risk assessment, quality control for food and water, forensic diagnosis and in parasite biology research. Some DNA-based tests for water-borne parasites, including Cryptosporidium and Giardia, are used in routine checks of water treatment, but forensic and food-testing applications have not been adopted in routine practice. Biological research, including epidemiological research, makes the widest use of DNA-based diagnostics, delivering enhanced understanding of parasites and guidelines for managing parasitic diseases. Despite the limited uptake of DNA-based tests to date, there is little doubt that they offer great potential to not only detect, identify and quantify parasites, but also to provide further information important for the implementation of parasite control strategies. For example, variant sequences within species of parasites and other organisms can be differentiated by tests in a manner similar to genetic testing in medicine or livestock breeding. If an association between DNA sequence and phenotype has been demonstrated, then qualities such as drug resistance, strain divergence, virulence, and origin of isolates could be inferred by DNA-based tests. No such tests are in clinical or commercial use in parasitology and few tests are available for other organisms. Why have DNA-based tests not had a bigger impact in veterinary and human medicine? To explore this question, technological, biological, economic and sociological factors must be considered. Additionally, a realistic expectation of research progress is needed. DNA-based tests could enhance parasite management in many ways, but patience, persistence and dedication will be needed to achieve this goal.

Absence of three known benzimidazole resistance mutations in Trichostrongylus tenuis, a nematode parasite of avian hosts

Benzimidazole (BZ) resistance is widespread in nematode parasites of livestock, but very little is known about the levels of BZ resistance in parasites with avian hosts. We investigated BZ resistance in Trichostrongylus tenuis, a nematode parasite of red grouse, Lagopus lagopus scotica. BZ anthelmintics had been in use in this system for up to 15 years, yet existing phenotypic evidence for resistance was inconclusive. We screened 1530 individuals from 14 populations at the principal beta-tubulin locus for BZ resistance (isotype 1, residue 200) and 940 of these at two further resistance sites (isotype 1, residue 167; isotype 2, residue 200). No BZ resistant genotypes were found. Alternative mechanisms may be responsible for BZ resistance in this system, or the method and timing of treatments may reduce selection pressure for BZ resistance by creating substantial refugia for susceptible genotypes.

Characterization of beta-tubulin genes in hookworms and investigation of resistance-associated mutations using real-time PCR

Human hookworms (Ancylostoma duodenale, Necator americanus) are a major cause of malnutrition and anemia, particularly in children, and high worm burdens can lead to stunted growth and mental retardation. Mass drug administration (MDA) with benzimidazole (BZ) anthelmintics has the potential to greatly reduce morbidity and infection prevalence. However, such treatment strategies may apply significant selection pressure on resistance alleles. In several Strongylid parasites of livestock, resistance to BZ drugs is associated with single nucleotide polymorphisms (SNPs) in the beta-tubulin isotype-1 gene at codons 167 and 200. As an initial investigation into the possible development of BZ resistance in hookworms, we have cloned and sequenced the beta-tubulin isotype-1 genes of the canine hookworm Ancylostoma caninum and the two human hookworm species A. duodenale and N. americanus. The genomic sequences are highly conserved as evidenced by a similar structure of exons and introns; the 10 exons are of the same length in all three species and code for the same amino acids. The genomic sequences were then used to develop a real-time PCR assay for detecting polymorphisms in codons 167 and 200 in all three species. Hookworm specimens previously obtained from Pemba Island school children who had demonstrated a reduced response to treatment with mebendazole were then examined using the real-time PCR assay. None of the samples revealed significant levels of polymorphisms at these loci. If BZ resistance is present in the hookworm populations examined, the results do not support the hypothesis that changes in codons 167 and 200 of beta-tubulin isotype-1 are responsible for any resistance.

Innovative restriction site created PCR-RFLP for detection of benzimidazole resistance in Teladorsagia circumcincta

Benzimidazole compounds, especially albendazole, are the most commonly used anthelmintics for deworming of small ruminants in Iran. It is believed that the therapeutic effects of the benzimidazoles (BZs) come through their binding capacity to the beta-tubulin isotype 1. Substitution of phenylalanine to tyrosine at position 200 of this polypeptide confers resistance to BZs. Several investigators developed different biological- and molecular-based techniques to demonstrate the occurrence of resistance in helminthes against BZs. To address the determination of resistance at position 200 of beta-tubulin isotype 1, we developed an innovative restriction site created polymerase chain reaction-restriction fragment length polymorphism, in which nucleotide A at the position of 637 upstream flanked by the first two coding sequences of the phenylalanine (TT) triplet was substituted through the nucleotide G. The introduced modification in forward primer (UTvet MF-primer) leads to the creation of restriction site (AACGTT) for PSP1. Therefore, in the case of normal allele only, PSP1 can cut the corresponding PCR product. In the first step, the genomic DNA was isolated from each single Teladorsagia circumcincta collected either from the abomasa of untreated (n = 35) or of 5 mg/kg BW 2.5% albendazole suspension-treated (n = 40) sheep. It was amplified with the primer pair, creating PCR product of 403 bp in length. In the second step, the PCR product was extracted from agarose gel and amplified with the modified forward primer (UTvet MF-primer) and the same reverse primer as in step 1, creating a PCR product of 222 bp. The PCR product was then cut with PSP1 to obtain in the case of normal allele two DNA products (183 and 39 bp). Eight of the 35 worms collected from the untreated sheep were BZ(SS) homozygotes, and the rest (27) were BZ(RS) heterozygotes. In our preliminary experiment, we could not find a BZ(RR) homozygote form within the examined samples. Five out of 40 worms collected from the albendazole-treated sheep were BZ(RR) homozygotes, whereas the rest (35) were BZ(RS) heterozygotes. No BZ(SS) homozygote form was detected within this group.

Will technology provide solutions for drug resistance in veterinary helminths?

Drug resistance in veterinary helminths affects a growing number of livestock producers on a global basis. The parasites infecting the major species of livestock are presently showing resistance in varying degrees to the commonly used classes of anthelmintics. The degree and extent of this problem especially with respect to multidrug resistance (MDR) in nematode populations is likely to increase. Finding solutions to the spread of resistance requires knowledge of the drugs' modes of action and mechanisms of resistance. This knowledge can then be applied to detect and monitor the state of resistance. Here we present a brief overview of resistance mechanisms and some of the technologies being used to study them. We also discuss some of the strategies for slowing the spread of resistance. The issue of reversal of drug resistance is analysed under consideration of recent progress in the field of MDR reversal in non-infectious diseases. Finally, we propose an application of currently available technologies that could assist in the detection and monitoring of anthelmintic resistance. Taking into account the significant complexity of the genetic mechanism of anthelmintic resistance in and between the various species, we suggest to undertake a co-ordinated effort to systematically identify anthelmintic-related single nucleotide polymorphisms (SNPs) in the most important helminth parasites. Monitoring the state of resistance in field populations could be achieved with a SNP-based protocol for genotyping the many genes known or suspected to contribute to the modes of action or mechanisms of resistance to the various classes of anthelmintics. If significant associations between genotypes and phenotypes exist within a species, then a single test with sufficient SNPs could potentially have universal applicability. These could then be explored for the development of new molecular diagnostic procedures. New classes of anthelmintics are needed, but until they are developed and available to the producers, technology can assist to achieve the goal of better sustainability in anthelmintic usage.

Benzimidazole-resistant beta-tubulin alleles in a population of parasitic nematodes (Cooperia oncophora) of cattle

Three anthelmintic classes with distinct mechanisms of action are commercially available. Selection of nematode populations resistant to all these drugs has occurred, particularly in trichostrongyloid parasites of sheep. Anthelmintic resistance in cattle parasites has only recently been recognized and appears to be less pronounced, even though very similar species infect both hosts. To understand the bases for differences in the rate of resistance development in sheep versus cattle parasites, it is important to first demonstrate that the same kinds of resistance alleles exist in both. The benzimidazoles (BZ), which have been used for more than 40 years, were chosen as an example. BZ-sensitive (BZ(S)) and BZ-resistant (BZ(R)) nematodes that parasitize sheep have been distinguished at the molecular level by a single nucleotide change in the codon for amino acid 200 of a beta-tubulin gene, a switch from TTC (phenylalanine) to TAC (tyrosine). PCR primers were designed to completely conserved regions of trichostrongyloid beta-tubulin genes and were used to amplify DNA fragments from Haemonchus contortus (cDNA from a BZ(S) and a BZ(R) library) as positive controls. The technique was then extended to the cattle parasites, Cooperia oncophora and Ostertagia ostertagi (from genomic DNA). Sequence analysis proved the presence of amplified BZ(S) alleles in all three species and BZ(R) alleles in the BZ(R) population of H. contortus. Based on these data, nested PCR primers using the diagnostic T or A as the most 3' nucleotide were designed for each species. Conditions for selective PCR were determined. To demonstrate feasibility, genomic DNA was recovered from individual H. contortus L(3) larvae from both BZ(S) and BZ(R) populations. Genomic DNA was also isolated from >70 individual adult male C. oncophora collected from a cattle farm in New Zealand with reported BZ resistance. Allele-specific PCR discriminated among heterozygotes and homozygotes in both species. This method could find utility in studying the molecular epidemiology of BZ resistance in cattle parasites and for defining the variables that limit the development and spread of anthelmintic resistance in this host.

Molecular approaches to studying benzimidazole resistance in trichostrongylid nematode parasites of small ruminants

Molecular techniques are of growing importance in the study of anthelmintic resistance in trichostrongylid worm populations. A knowledge of the genetic determinants of benzimidazole (BZ) resistance has made it possible to construct a molecular tool for genotyping individual worms, in respect of mutation of the beta-tubulin gene responsible for BZ resistance. This tool offers new possibilities in the diagnosis of BZ resistance, and also in the study of anthelmintic use and other breeding management factors that can affect the selection of BZ-resistant alleles in worm populations. New molecular methods have also made it possible to study the origin and diversity of BZ-resistant alleles in trichostrongylid populations. The results demonstrate the value of a multidisciplinary approach to the study of anthelmintic resistance, combining molecular, ecological and epidemiological techniques.

Mitochondrial DNA variation in benzimidazole-resistant and -susceptible populations of the small ruminant parasite Teladorsagia circumcincta

The genetic diversity of the mtDNA ND4 gene in 11 Teladorsagia circumcincta populations from France and Morocco was assessed by sequencing. Some of these nematode populations were resistant to benzimidazole (BZ) anthelmintics, while others were susceptible. The nucleotide diversity in all populations studied was very high, probably due to a high mutation rate in nematodes, but there was no significant difference between them. This suggests that no strong, recurrent bottlenecks occur during the acquisition of BZ resistance by a worm population. The conservation of genetic variations during the acquisition of BZ resistance is probably due to the fact that anthelmintic treatments do not kill all the susceptible adult worms and to the presence of numerous free-living larvae that are not submitted to this anthelmintic pressure. There was no genetic subdivision between worm populations on a small geographical scale (less than 200 km), but significant F(ST)s were found on a larger geographical scale. This kind of subdivision cannot be explained by different genetic flows between populations because all these populations were isolated from each other. This subdivision is probably due to the breeding management practices and the large size of these worm populations, which limit genetic drift.