title: Altered avr-14B gene transcription patterns in ivermectin-resistant isolates of the cattle parasites, Cooperia oncophora and Ostertagia ostertagi.
abstract: Ivermectin (IVM) resistance is an emerging problem for the control of gastrointestinal nematodes of cattle such as Cooperia oncophora and Ostertagia ostertagi. Although there is still a poor understanding of the molecular basis of macrocyclic lactone (ML)-resistance, it is clear that IVM exerts its activity by binding to glutamate-gated chloride (GluCl) channels within the parasite's neuromuscular system. One of the GluCl genes (avr-14) encodes, via alternative splicing, two subunits, AVR-14A and AVR-14B; the latter is suggested to be the main target for IVM. The genomic DNA (gDNA) sequence of avr-14 in C. oncophora contains 21 exons separated by 20 introns and spans approximately 10 kb of gDNA. Intron 13 contains a sequence with high homology to a mammalian mariner transposase. The L256F polymorphism in the avr-14 gene, which was shown to be associated with IVM resistance in a UK isolate of C. oncophora, was not found in the IVM-resistant C. oncophora and O. ostertagi isolates investigated in this study. However, genetic analyses on C. oncophora indicated a loss in allelic diversity of the avr-14 gene in the resistant isolates compared with the susceptible isolate. This suggests that the avr-14 gene, or another genetically linked locus, is under selection in these Belgian C. oncophora isolates. Comparison of the full-length avr-14B coding sequence in the susceptible and resistant C. oncophora isolates did not show any polymorphisms specifically linked to IVM resistance, although a decrease in the number of avr-14B isoforms was observed in the resistant isolates compared with the susceptible one. Measuring the transcription levels of avr-14B in adult male and female C. oncophora and O. ostertagi worms showed significantly lower levels in resistant worms compared with susceptible ones. Whether the down-regulation of this IVM target actually contributes to the resistance mechanism in these worms remains unclear.
authors.affiliation: Department of Virology, Parasitology and Immunology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium.
authors.forename: A
authors.initials: A
authors.lastname: El-Abdellati
authors.forename: J
authors.initials: J
authors.lastname: De Graef
authors.forename: A
authors.initials: A
authors.lastname: Van Zeveren
authors.forename: A
authors.initials: A
authors.lastname: Donnan
authors.forename: P
authors.initials: P
authors.lastname: Skuce
authors.forename: T
authors.initials: T
authors.lastname: Walsh
authors.forename: A
authors.initials: A
authors.lastname: Wolstenholme
authors.forename: A
authors.initials: A
authors.lastname: Tait
authors.forename: J
authors.initials: J
authors.lastname: Vercruysse
authors.forename: E
authors.initials: E
authors.lastname: Claerebout
authors.forename: P
authors.initials: P
authors.lastname: Geldhof
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000969:Antinematodal Agents / Q000494:pharmacology*; D002417:Cattle; D002418:Cattle Diseases / Q000188:drug therapy / Q000469:parasitology*; D004351:Drug Resistance; D005260:Female; D015801:Helminth Proteins / Q000235:genetics* / Q000378:metabolism; D007559:Ivermectin / Q000494:pharmacology*; D008297:Male; D008969:Molecular Sequence Data; D010028:Ostertagia / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014251:Trichostrongyloidea / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D014252:Trichostrongyloidiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*
chemical_list: D000969:Antinematodal Agents; D015801:Helminth Proteins; D007559:Ivermectin