Detection and measurement of benzimidazole resistance alleles in Haemonchus contortus using real-time PCR with locked nucleic acid Taqman probes

Advances in diagnosis of gastrointestinal nematodes in livestock and companion animals

Diagnosis of gastrointestinal nematodes in livestock and companion animals has been neglected for years and there has been an historical underinvestment in the development and improvement of diagnostic tools, undermining the undoubted utility of surveillance and control programmes. However, a new impetus by the scientific community and the quickening pace of technological innovations, are promoting a renaissance of interest in developing diagnostic capacity for nematode infections in veterinary parasitology. A cross-cutting priority for diagnostic tools is the development of pen-side tests and associated decision support tools that rapidly inform on the levels of infection and morbidity. This includes development of scalable, parasite detection using artificial intelligence for automated counting of parasitic elements and research towards establishing biomarkers using innovative molecular and proteomic methods. The aim of this review is to assess the state-of-the-art in the diagnosis of helminth infections in livestock and companion animals and presents the current advances of diagnostic methods for intestinal parasites harnessing (i) automated methods for copromicroscopy based on artificial intelligence, (ii) immunodiagnosis, and (iii) molecular- and proteome-based approaches. Regardless of the method used, multiple factors need to be considered before diagnostics test results can be interpreted in terms of control decisions. Guidelines on how to apply diagnostics and how to interpret test results in different animal species are increasingly requested and some were recently made available in veterinary parasitology for the different domestic species.

Anthelmintic resistance in ruminants: challenges and solutions

Anthelmintic resistance (AR) is a growing concern for effective parasite control in farmed ruminants globally. Combatting AR will require intensified and integrated research efforts in the development of innovative diagnostic tests to detect helminth infections and AR, sustainable anthelmintic treatment strategies and the development of complementary control approaches such as vaccination and plant-based control. It will also require a better understanding of socio-economic drivers of anthelmintic treatment decisions, in order to support a behavioural shift and develop targeted communication strategies that promote the uptake of evidence-based sustainable solutions. Here, we review the state-of-the-art in these different fields of research activity related to AR in helminths of livestock ruminants in Europe and beyond. We conclude that in the advent of new challenges and solutions emerging from continuing spread of AR and intensified research efforts, respectively, there is a strong need for transnational multi-actor initiatives. These should involve all key stakeholders to develop indicators of infection and sustainable control, set targets and promote good practices to achieve them.

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.

Molecular detection of benzimidazole resistance levels associated with F167Y and F200Y polymorphisms in Haemonchus contortus of goats from Mozambique

Benzimidazole (BZ) resistance of Haemonchus contortus has been associated with single nucleotide polymorphisms (SNPs) in codons 200 (F200Y) and 167 (F167Y) and, to a lesser extent, in codon E198A, of the β-tubulin isotype 1 gene. The present study was undertaken to survey the status of BZ resistance in naturally infected goats in smallholder farms in southern Mozambique by real-time PCR (qPCR) using TaqMan® assays. H. contortus-infective larvae (L3; n = 432) from 12 populations were individually genotyped for F200Y and F167Y SNPs to detect BZ resistance. For the F200Y SNP, the results revealed an overall mean percentages of 18.8% homozygous resistant (RR), 47.8% homozygous susceptible (SS) and 33.4% heterozygous (RS) H. contortus. For the F167Y SNP, the overall mean percentages were 1.6% RR, 94.9% SS and 3.5% RS. The percentage of resistant alleles (%R) for the F200Y and F167Y SNPs was 35.7 and 3.4%, respectively. Genotype combinations of the two mutations indicate resistant percentages ranging from 0.0 to 52.9%. From the four herds with high RR individuals, three farms dewormed the animals monthly, while the fourth farm dewormed the animals every 3 months. In farms where animals were dewormed every 6 months, low percentages of RR individuals were found, whereas no RR individuals were discovered in herds where animals were dewormed annually. These results suggest that the F200Y SNP is more significant in BZ resistance development of the surveyed population compared with the F167Y SNP.

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.

Benzimidazole resistance survey for Haemonchus, Teladorsagia and Trichostrongylus in three European countries using pyrosequencing including the development of new assays for Trichostrongylus

Resistance to benzimidazoles (BZs) in trichostrongyloid nematodes is a worldwide problem for livestock production, particularly regarding small ruminants. Sensitive and reliable methods are required to assess anthelmintic resistance status. Currently available methods for BZ resistance detection can be divided into three main groups, in vivo (e.g. faecal egg count reduction test), in vitro (e.g. egg hatch assay) and molecular tests. Three single nucleotide polymorphisms (SNPs) in the isotype-1 β-tubulin gene of various nematode species correlate with BZ resistance. While PCR-based methods have been reported for the three most economically important nematodes of sheep, namely, Trichostrongylus, Haemonchus and Teladorsagia, pyrosequencing assays are so far only available for the latter two. Here, the design and evaluation of pyrosequencing assays for isotype-1 and isotype-2 β-tubulin genes of Trichostrongylus colubriformis are described. PCR fragments carrying the susceptible and corresponding resistant genotype were combined in defined ratios to evaluate assay sensitivity and linearity. The correlation between the given and the measured allele frequencies of the respective SNPs (codons F167Y, E198A and F200Y) was very high. Pyrosequencing assays for Haemonchus, Teladorsagia and Trichostrongylus were subsequently used for a BZ resistance survey, carried out in the three European countries, namely Ireland, Italy and Switzerland. Larval cultures obtained from field survey samples in 2012 and 2013 were used for pyrosequencing. The test was applied when the target species represented at least 10% of the sample. Trichostrongylus and Teladorsagia were detected in all countries' samples whereas Haemonchus was not detected in samples from Ireland. SNPs in isotype-1 associated with resistance were detected for all three species, with frequencies at codon F200Y far exceeding those at codons F167Y and E198A. Elevated SNP frequencies in isotype-2 of Trichostrongylus were only rarely detected. Farms with BZ resistance-associated SNP frequencies above 10% were most often found in Switzerland followed by Ireland and Italy.

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Successful development of a pyrosequencing assay for Trichostrongylus.

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Resistance survey revealed high number of SNPs present in all three countries.

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SNP at codon 200 most prevalent in European trichostrongyloid populations.

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Low number of SNPs found in Italy corresponds to FECRT data.

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Pyrosequencing successfully employed as a tool for large scale surveys.

Benzimidazole resistance in helminths: From problem to diagnosis

Helminth parasites cause significant morbidity and mortality in endemic countries. Given the severity of symptoms that helminths may elicit in the host, intervention with prophylactic and therapeutic measures is imperative. Treatment with benzimidazoles is the most widely used means of combatting these parasites. However, widespread use of these drugs can select for drug-resistant parasite strains. In this review, we approach the problem of benzimidazole resistance in helminths in both humans and animals, focusing on the properties of the drug, the molecular mechanisms of drug resistance and how resistance is diagnosed.

Anthelmintic Resistance in Haemonchus contortus: History, Mechanisms and Diagnosis

Haemonchus contortus has shown a great ability to develop resistance to anthelmintic drugs. In many instances, resistance has appeared less than 10years after the introduction of a new drug class. Field populations of this species now show resistance to all major anthelmintic drug classes, including benzimidazoles (BZs), imidazothiazoles and macrocyclic lactones. In addition, resistance to the recently introduced amino-acetonitrile derivative class (monepantel) has already been reported. The existence of field populations showing resistance to all three major drug classes, and the early appearance of resistance to monepantel, threatens the sustainability of sheep and goat production systems worldwide. This chapter reviews the history of the development of resistance to the various anthelmintics in H. contortus and examines the mechanisms utilized by this species to resist the effects of these drugs. Some of these mechanisms are well understood, particularly for BZ drugs, while our knowledge and understanding of others are increasing. Finally, we summarize methods available for the diagnosis of resistance. While such diagnosis currently relies largely on the faecal egg count reduction test, which suffers from issues of expense and sensitivity, we describe past and current efforts to utilize cheaper and less laborious phenotypic assays with free-living life stages, and then describe progress on the development of molecular assays to provide sensitive resistance-detection tests.

Absence of detectable benzimidazole-resistance associated alleles in Haemonchus placei in cattle in Nigeria revealed by pyrosequencing of β-tubulin isotype 1

Trichostrongyles are gastrointestinal parasites that occur globally and can cause subclinical to severe, sometimes life-threatening, infections in ruminants, particularly young animals. Benzimidazoles (BZ) are commonly used for the treatment of gastrointestinal parasites in ruminants. Increasing spread of worm populations with anthelmintics resistance has been reported and is considered a consequence of highly frequent and longstanding use of anthelmintics. To obtain initial information regarding the occurrence of putatively BZ-resistant Nigerian Haemonchus populations, screening based on the molecular analysis of BZ-resistance-associated β-tubulin isotype 1 gene sequence polymorphisms was undertaken. Genomic DNA was isolated from pooled adult Haemonchus sp. from 35 animals from each of the six states of southwestern Nigeria. Sequencing of internal transcribed spacer 2 (ITS-2) and external transcribed spacer (ETS) regions was used to determine the Haemonchus species. Pyrosequencing assays were used for detection of single-nucleotide polymorphisms (SNPs) in the β-tubulin isotype 1 genes of the worms at codons 200 and 167 (TTC/TAC) or 198 (GAA/GCA). Exclusively, Haemonchus placei was detected and allele frequencies obtained at all three positions showed no evidence for the presence of resistance-related alleles. For Lagos State, pools of 10 worms from 30 different animals were analyzed separately for the codon 200 SNP, successfully excluding the presence of resistance-associated SNPs in very low frequencies. These positive findings, showing absence of elevated frequencies of BZ-resistance-associated β-tubulin alleles, have considerable significance since it suggests that farmers can still rely on the efficacy of this important drug class when used for controlling trichostrongyle infections in cattle in Nigeria.

Phylogenetic characterization of β-tubulins and development of pyrosequencing assays for benzimidazole resistance in cattle nematodes

Control of helminth infections is a major task in livestock production to prevent health constraints and economic losses. However, resistance to established anthelmintic substances already impedes effective anthelmintic treatment in many regions worldwide. Thus, there is an obvious need for sensitive and reliable methods to assess the resistance status of at least the most important nematode populations. Several single nucleotide polymorphisms (SNPs) in the β-tubulin isotype 1 gene of various nematodes correlate with resistance to benzimidazoles (BZ), a major anthelmintic class. Here we describe the full-length β-tubulin isotype 1 and 2 and α-tubulin coding sequences of the cattle nematode Ostertagia ostertagi. Additionally, the Cooperia oncophora α-tubulin coding sequence was identified. Phylogenetic maximum-likelihood analysis revealed that both isotype 1 and 2 are orthologs to the Caenorhabditis elegans ben-1 gene which is also associated with BZ resistance upon mutation. In contrast, a Trichuris trichiura cDNA, postulated to be β-tubulin isotype 1 involved in BZ resistance in this human parasite, turned out to be closely related to C. elegans β-tubulins tbb-4 and mec-7 and would therefore represent the first non-ben-1-like β-tubulin to be under selection through treatment with BZs. A pyrosequencing assay was established to detect BZ resistance associated SNPs in β-tubulin isotype 1 codons 167, 198 and 200 of C. oncophora and O. ostertagi. PCR-fragments representing either of the two alleles were combined in defined ratios to evaluate the pyrosequencing assay. The correlation between the given and the measured allele frequencies of the respective SNPs was very high. Subsequently laboratory isolates and field populations with known resistance status were analyzed. With the exception of codon 167 in Cooperia, increases of resistance associated alleles were detected for all codons in at least one of the phenotypically resistant population. Pyrosequencing provides a fast, inexpensive and sensitive alternative to conventional resistance detection methods.

Efficiency of a genetic test to detect benzimidazole resistant Haemonchus contortus nematodes in sheep farms in Quebec, Canada

Haemonchus contortus is a hemophilic nematode which infects sheep and causes anemia and death to lambs. Benzimidazole drugs are used to remove these parasites, but the phenomenon of resistance has arisen worldwide. A sensitive test to detect resistance before treatment would be a useful tool to enable farmers to anticipate the efficiency of the drug before drenching the flock. In this study, we compared a test for benzimidazole resistance based on detection of genetic markers in H. contortus before treatment with the common method of fecal egg count reduction test (FECRT). We recruited 11 farms from different regions of Quebec for this study. Fecal samples from animals were collected per rectum before and after treatment in control and treated groups (10 animals per group). The 10 sheep were treated with fenbendazole at the recommended dose rate. Among the 11 farms participating in the study, we found H. contortus in 8 of them and it was the most predominant nematode species detected by egg count. Using the genetic test, we found benzimidazole resistance in each of these 8 farms. In 5 of these 8 farms there were sufficient sheep with an egg count for H. contortus above 150 eggs per gram to allow the FECRT test to be conducted. Benzimidazole resistance was observed in each of these 5 farms by the FECRT. When we compared the results from the genetic test for samples off pasture and from individual sheep, with the results from the FECRT, we concluded that the genetic test can be applied to samples collected off pasture to estimate benzimidazole resistance levels before treatment for H. contortus infections.

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.

Anthelmintic resistance: markers for resistance, or susceptibility?

The Consortium for Anthelmintic Resistance and Susceptibility (CARS) brings together researchers worldwide, with a focus of advancing knowledge of resistance and providing information on detection methods and treatment strategies. Advances in this field suggest mechanisms and features of resistance that are shared among different classes of anthelmintic. Benzimidazole resistance is characterized by specific amino acid substitutions in beta-tubulin. If present, these substitutions increase in frequency upon drug treatment and lead to treatment failure. In the laboratory, sequence substitutions in ion-channels can contribute to macrocyclic lactone resistance, but there is little evidence that they are significant in the field. Changes in gene expression are associated with resistance to several different classes of anthelmintic. Increased P-glycoprotein expression may prevent drug access to its site of action. Decreased expression of ion-channel subunits and the loss of specific receptors may remove the drug target. Tools for the identification and genetic analysis of parasitic nematodes and a new online database will help to coordinate research efforts in this area. Resistance may result from a loss of sensitivity as well as the appearance of resistance. A focus on the presence of anthelmintic susceptibility may be as important as the detection of resistance.

Phenotypic and genotypic characterisation of benzimidazole susceptible and resistant isolates of Haemonchus contortus

The study was designed to compare the in vitro egg hatch test (EHT) and molecular allele-specific polymerase chain reaction PCR (AS-PCR) methods as tools for detection of benzimidazole resistance in Haemonchus contortus, a nematode parasite of small ruminants. Comparisons were made during the course of an experimental infection and changes in EHT and AS-PCR were monitored to measure the correlation between in vitro and molecular tests. Both methods were carried out according to World Association for the Advancement of Veterinary Parasitology (WAAVP) recommendations. The molecular test was used to discriminate TAC/TTC polymorphism in the beta-tubulin 200 codon of eight (four resistant and four susceptible) isolates of H. contortus. Using DNA from 100 third-stage larvae, the polymerase chain reaction (PCR)-based analysis revealed a decrease of the homozygous TTC/TTC genotype and an increase in heterozygous TTC/TAC and homozygous TAC/TAC individuals in all resistant isolates. Both methods showed comparable and reliable results with regard to detection of benzimidazole resistance. The molecular test has an advantage over the EHT because of its higher sensitivity. On the other hand, EHT is less time-consuming, allows reliable detection of <10% resistance allele frequency and is fairly reliable for the detection of benzimidazole resistance under field conditions.

Molecular detection of benzimidazole resistance in Haemonchus contortus using real-time PCR and pyrosequencing

Benzimidazoles (BZ) are widely used to treat parasitic nematode infections of humans and animals, but resistance is widespread in veterinary parasites. Several polymorphisms in β-tubulin genes have been associated with BZ-resistance. In the present study, we investigated β-tubulin isotype 1 sequences of 18 Haemonchus contortus isolates with varying levels of resistance to thiabendazole. The only polymorphism whose frequency was significantly increased in the resistant isolates was TTC to TAC at codon 200. Real-time PCR (using DNA from 100 third-stage larvae, L3s) and pyrosequencing (from DNA from 1000–10 000 L3s) were used to measure allele frequencies at codon 200 of these isolates, producing similar results; drug sensitivity decreased with increasing TAC frequency. Pyrosequencing was also used to measure allele frequencies at positions 167 and 198. We showed that such measurements are sufficient to assess the BZ-resistance status of most H. contortus isolates. The concordance between real-time PCR and pyrosequencing results carried out in different laboratories indicated that these tools are suitable for the routine diagnosis of BZ-resistance in H. contortus. The molecular methods were more sensitive than the ‘egg hatch test’, and less time-consuming than current in vivo- or in vitro-anthelmintic resistance detection methods. Thus, they provide a realistic option for routine molecular resistance testing on farms.

Anthelmintic resistance in Swedish sheep flocks based on a comparison of the results from the faecal egg count reduction test and resistant allele frequencies of the beta-tubulin gene

A faecal egg count reduction test (FECRT) survey was conducted during the grazing season 2006 and 2007 to provide an updated indication of the prevalence of anthelmintic resistance in sheep flocks in Sweden. A total of 1330 faecal samples from 90 flocks on 45 farms, with a minimum of 20 ewes each, was collected by local sheep veterinarians. Per treatment group, approximately 15 lambs were dewormed either with oral suspensions of ivermectin (Ivomec vet.) or albendazole (Valbazen vet.). The efficacy on each farm was investigated either in 2006 or 2007 by faecal egg counts collected on the day of treatment and in a new sample from the same animals 7-10 days later. Third-stage larvae (L3) were initially identified morphologically from pooled cultures. These were then used as the source of genomic DNA template for two molecular tests. The first was a PCR-based test for specific identification of Haemonchus contortus, and the second was a Pyrosequencing assay for the analysis of benzimidazole (BZ) resistance targeting the P200 mutation in the parasite's beta-tubulin gene. Larval cultures indicated that Teladorsagia and Trichostrongylus were the predominant genera, but Haemonchus was diagnosed in 37% of the flocks. The PCR results revealed an almost 100% agreement with those farms that had previously been shown to have Haemonchus present, even when the % prevalence was low (approximately 3%). Only two (4%) of the surveyed farms showed evidence of BZ-resistant worm populations, with H. contortus being the species implicated according to post-treatment larval culture results. The Pyrosequencing assay detected BZ resistant allele frequencies of >40% in the Haemonchus-positive farms and 100% resistant alleles in the clinically most resistant farms. These preliminary results suggest that the FECRT is less sensitive than the molecular test at detecting BZ resistance. However, both tests need to be interpreted carefully, bearing in mind the relative proportions of species present and the starting egg and/or larval counts. Parasitological diagnosis of "clinical" resistance was also found against ivermectin in two flocks. However, both the pre-treatment FECs and the reductions in these were low, and only three lambs that had between 100 and 450 EPG after treatment were involved.

In vitro detection of benzimidazole resistance in Haemonchus contortus: egg hatch test versus larval development test

The present study was designed to compare the egg hatch test (EHT) and the larval development test (LDT) as in vitro tools for detection of benzimidazole (BZ) resistance in Haemonchus contortus, a nematode parasite of small ruminants. Comparisons were made during a course of infection and changes in both EHT and LDT were monitored to measure the correlation between resistance and susceptibility in different parasite stages (eggs and larvae). In addition, mixed doses of known numbers of susceptible and BZ-resistant H. contortus eggs were used to assess the sensitivity of LDT for the detection of low levels of resistance. The degree of resistance for each test was expressed as resistance factor (RF). The LDT showed a greater ability to distinguish between four susceptible and four resistant isolates of H. contortus with higher resistance factors compared to the EHT. For the EHT the RF by using ED(50) criterion ranged from 3.2 to 13.3 and from 7.4 to 25.2 by using LC(99). For LDT the resistant isolates were 4.3-63.1 times more tolerant than the susceptible isolates using the ED(50) criterion and 91.1-1411.0 times more tolerant using the LC(99) criterion. The LDT was also able to clearly indicate the presence of low level (4%) of resistant larvae amongst a susceptible background population.

Single nucleotide polymorphism (SNP) markers for benzimidazole resistance in veterinary nematodes

Resistance to the benzimidazole class of anthelmintics in nematodes of veterinary importance has a long history. Research into the mechanisms responsible for this resistance is subsequently at a more advanced stage than for other classes of anthelmintics. The principal mechanism of resistance to benzimidazoles is likely to involve changes in the primary structure of β-tubulins, the building blocks of microtubules. Specifically, point mutations in the β-tubulin isotype 1 gene leading to amino acid substitutions in codons 167, 198, and 200 of the protein have been associated with resistance in nematodes. These single nucleotide polymorphisms offer a means of detecting the presence of resistance within populations. In this mini-review, we focus on the prevalence and importance of these polymorphisms in three groups of nematodes: trichostrongylids, cyathostomins, and hookworms. A brief overview of existing strategies for genotyping single nucleotide polymorphisms is also presented. The CARS initiative hopes to exploit these known polymorphisms to further our understanding of the phenomenon of BZ resistance.