Candidate anthelmintic resistance-associated gene expression and sequence polymorphisms in a triple-resistant field...

An isolate of Haemonchus contortus, UGA/2004, highly resistant to benzimidazoles, levamisole, and ivermectin was isolated from sheep at the University of Georgia, and passaged through experimentally infected goats. We measured the expression of twenty-nine mRNAs encoding drug targets and P-glycoproteins (P-gps), comparing the results to a fully susceptible laboratory passaged isolate. Expression levels of some nicotinic acetylcholine receptor mRNAs were markedly different in UGA/2004. Levels of the Hco-acr-8b mRNA, encoding a truncated subunit, were very high in resistant L3, but undetectable in susceptible larvae, with expression of the full-length Hco-acr-8a mRNA also significant increased. Expression of Hco-unc-63 and Hco-unc-29.3 mRNAs was significantly reduced in the resistant larvae. Expression of the Hco-glc-3 and Hco-glc-5 mRNAs, encoding glutamate-gated chloride channel subunits, were slightly reduced in resistant larvae. We observed significant increases in the expression of the Hco-pgp-2 and Hco-pgp-9 mRNAs in the UGA/2004 larvae, consistent with previous reports; we also saw a decrease in the levels of Hco-pgp-1 mRNA. Treatment of the larvae with ivermectin and moxidectin in vitro produced variable and inconsistent changes in P-gp mRNA levels. The sequences of the β-tubulin isotype 1 mRNAs showed that the resistant larvae had a resistance-associated allele frequency of >95% at codon 200 and ∼40% and codon 167. No changes at codon 198 were present. The presence of the truncated acr-8b mRNA may be a reliable indicator of levamisole resistance, but complex changes in gene expression associated with macrocyclic lactone resistance make the identification of a single genetic marker for this resistance difficult.