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.
Blackhall
W J
WJ
McGill University, 21,111 Lakeshore Road, Ste.-Anne-de-Bellevue, Quebec, Canada, H9X 3V9.
Pouliot
J F
JF
Prichard
R K
RK
Beech
R N
RN
D000595:Amino Acid Sequence; D000818:Animals; D000900:Anti-Bacterial Agents; D018118:Chloride Channels / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics*; D017931:DNA Primers; D004351:Drug Resistance / Q000235:genetics; D005787:Gene Frequency; D017238:Genes, Helminth; D018698:Glutamic Acid / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics*; D006189:Haemonchus / Q000187:drug effects* / Q000235:genetics*; D007559:Ivermectin / Q000494:pharmacology*; D018942:Macrolides / Q000494:pharmacology; D046911:Macromolecular Substances; D008297:Male; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D012641:Selection, Genetic; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity
D000900:Anti-Bacterial Agents; D018118:Chloride Channels; D017931:DNA Primers; D018942:Macrolides; D046911:Macromolecular Substances; C109777:glutamate-gated chloride channels; D018698:Glutamic Acid; D007559:Ivermectin; C027837:moxidectin