Table 2.

Summary of mechanisms for the resistance of H. contortus against the various anthelmintic drugs.

Class of Anthelmintic Drugs	Target in Helminth	Mechanism
Benzimidazoles	β-tubulin (isotype-1)	Point mutation at codon 200 leading to a phenylalanine to tyrosine substitution
		Point mutation at codon 167 leading to a phenylalanine to tyrosine substitution
		Point mutation at codon 198 leading to an alanine to glutamic acid substitution
	β-tubulin (isotype-2)	Attribution to polymorphism(s)
Imidazothiazoles	Nicotinic acetylcholine receptor genes	Mutated genes (Hco-unc-63, Hco-acr-8) expressing production of a protein, binding to the nicotinic acetyl-choline receptors, preventing binding of drug on them
		Under-expression of certain genes (Hco-unc-63a, Hco-unc-29.3, etc.) encoding the nicotinic acetylcholine receptors
Macrocyclic lactones	Glutamate-gated chloride ion channels	Presence of glycine residue in the gene encoding these channels enhancing susceptibility of H. contortus; mutation of this glycine residue on HEK293 cells, resulting in loss of H. contortus drug-sensitivity
	P-glycoprotein gene	Polymorphisms or over-expression of P-glycoprotein genes
Closantel		Resistance attributed to reduced closantel intake by resistant helminths, to strong binding of the drug to albumins in the intestine of helminths and to increased excretion of the drug from resistant helminths
Monepantel	Nicotinic acetylcholine receptor genes	Mutated genes (Hco-des-2H, Hco-acr-23H, Hco-MPTL-1) associated with resistance