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.

Foreword: Towards markers for anthelmintic resistance in helminths of importance in animal and human health

Anthelmintic resistance is a serious problem in veterinary medicine and appears to be developing in some helminths of importance to human health. Anthelmintic drugs remain the principal means of control of helminth infections in animals and humans and the continued dependence on these pharmaceuticals will continue to impose selection pressure for resistance development. Our ability to detect anthelmintic resistance before control breaks down and to monitor the spread of anthelmintic resistance is quite limited. We are currently dependent on biological methods which are not sufficiently sensitive to detect low levels of drug resistance and are particularly difficult to perform on helminth parasites of humans. There is a serious need for new molecular markers for detecting and monitoring for anthelmintic resistance. The problem of anthelmintic resistance is already very serious in nematode parasites of livestock. In addition, there should be great concern about possible anthelmintic resistance development and the lack of tools and efforts for monitoring it as part of the major worldwide programmes to control helminth parasites in people. An international Consortium has been formed to develop Anthelmintic Resistance Single nucleotide polymorphism markers (CARS). Discussions within the Consortium have addressed the need for such markers, the current state of knowledge about them, possible mechanisms of anthelmintic resistance and approaches and constraints to the development of markers. Summaries of the state of the art in these areas are presented in a series of papers in this Special Issue of Parasitology.

Ivermectin selection on beta-tubulin: evidence in Onchocerca volvulus and Haemonchus contortus

Ivermectin resistance is common in trichostrongylid nematodes of livestock, such as Haemonchus contortus. This anthelmintic is the only drug approved for mass administration to control onchocerciasis caused by the nematode parasite, Onchocerca volvulus. In parts of West Africa up to 18 rounds of ivermectin treatment have been administered to communities and there are reports of poor parasitological responses to treatment. Understanding ivermectin resistance and ivermectin selection is an important step to reduce selection pressure for resistance, and to develop molecular markers which can be used to monitor the development of resistance and its spread. Here we report evidence that ivermectin selection changes the frequency of beta-tubulin alleles in both the sheep parasite, H. contortus, and the human parasite, O. volvulus. In O. volvulus we have been able to look at the frequency of beta-tubulin alleles in O. volvulus obtained before any ivermectin was used in humans in Africa, and following its widespread use. In H. contortus, we have been able to look at the frequency of beta-tubulin alleles in a strain which has not seen any anthelmintic selection and in an ivermectin selected strain derived from the unselected strain. We have found ivermectin selects on beta-tubulin in both of these nematode species. In the case of O. volvulus, we had previously reported that ivermectin selects for specific single nucleotide polymorphisms in the O. volvulus beta-tubulin gene. This polymorphism results in three amino acid changes in the H3 helix of beta-tubulin, as well as deletions in an associated intron. We report a simple PCR assay to detect the amplicon length polymorphism, resulting from these intronic deletions, which can be used to monitor the frequency of the beta-tubulin allele selected for by ivermectin in O. volvulus.

Selection at a P-glycoprotein gene in ivermectin- and moxidectin-selected strains of Haemonchus contortus

Resistance to anthelmintics that are used to control parasite populations in domestic animals has become a serious problem worldwide. The development of resistance is an evolutionary process that leads to genetic changes in parasite populations in response to drug exposure. The anthelmintic ivermectin is known to bind to the human membrane transport protein, P-glycoprotein, and P-glycoprotein-deficient mice treated with ivermectin have shown signs of neurotoxicity. P-glycoprotein is believed to be involved in the multidrug resistance phenotype seen in some human cancers and for drug resistance in some protists. We have examined the genetic variation of a P-glycoprotein homologue from the nematode Haemonchus contortus to see if an association exists between specific alleles of this gene and survival to exposure to ivermectin or moxidectin. Two parasite strains passaged without drug treatment and three strains, subjected to anthelmintic selection and derived from the unselected strains, were examined. Allelic variation in the unselected strains showed this locus to be highly polymorphic. chi 2 analyses of allele frequencies showed significant differences between the unselected and the drug-selected derived strains. In all three drug-selected strains, an apparent selection for the same allele was observed. These findings suggest that P-glycoprotein may be involved in resistance to both ivermectin and moxidectin in H. contortus.

Haemonchus contortus: selection at a glutamate-gated chloride channel gene in ivermectin- and moxidectin-selected strains

Anthelmintic resistance in nematode parasites of livestock is a serious problem worldwide. Ivermectin, an avermectin, and moxidectin, a milbemycin, are potent endectocides commonly used to control these parasites. The proposed mode of action of avermectins and possibly the milbemycins involves the binding of the drug to the alpha-subunit of a glutamate-gated chloride channel, which opens or potentiates gating of the channel and leads to the hyperpolarization of the target neuromuscular cell. Glutamate gates the channel by binding to the beta-subunit. We have cloned a fragment of a putative alpha-subunit gene from Haemonchus contortus. The sequence of the beta subunit is available from GenBank. Genetic variability of this fragment was analysed by single-strand conformation polymorphism in five strains of H. contortus: two strains passaged without drug selection, two strains selected with ivermectin, and one strain selected with moxidectin. One allele of the putative alpha-subunit gene appeared to be associated with resistance to the drugs, increasing in frequency in the three drug-selected strains relative to the unselected strains. Another allele appeared to be associated with susceptibility, decreasing in frequency in the three drug-selected strains relative to the unselected strains. A similar analysis of the beta-subunit gene showed no significant differences in allele frequencies between the unselected and drug-selected strains. Our findings suggest a correlation between changes in allele frequencies of the putative alpha-subunit gene and resistance to ivermectin and moxidectin.