The problem of anthelmintic resistance prevents efficient control of parasites of livestock and may soon compromise human parasite control. Research into the mechanisms of resistance and the quest for diagnostic tools to aid control has required research that focuses on field resistance. On the other hand, resistant worms, including those kept in the laboratory, provide useful tools for studying drug action, especially at neuromuscular targets in worms. While the needs and directions of these research aims overlap, this review concentrates on research on drug targets. In this context, resistance is a useful tool for site of action confirmation. For example, correlations between molecular expression studies and resistance assays conducted on whole worms can strengthen claims for sites of anthelmintic action. Model systems such as Caenorhabditis elegans have been very useful in understanding targets but give a limited picture as it is now clear that resistance mechanisms in this worm are different from those in parasites. Accordingly, research on parasites themselves must also be performed. Resistant isolates of the sheep nematode parasite Haemonchus contortus are the most widely used for this purpose as in vivo, in vitro, physiological and molecular studies can be performed with this species. Neuromuscular target sites for the anthelmintics levamisole and ivermectin are the best studied and have benefited most from the use of resistant worm isolates. Resistance to praziquantel and the newer chemical groups should provide new tools to explore targets in the future.

The actions of known platyhelminth FaRPs on the contractility of whole-worm preparations of the monogenean, Diclidophora merlangi have been examined in vitro for the first time. All of the peptides tested had excitatory effects on the motor activity of the worm. The order of potency for the peptides tested was: YIRFamide > GYIRFamide = RYIRFamide > GNFFRFamide = FLRFamide. However, although YIRFamide was more potent than GYIRFamide, the latter was the most efficacious on each of the motility parameters (tension, contraction amplitude and contraction frequency) examined at concentrations [ges ]0·1 μm. Serotonin, which stimulates contractility in the worm was used as a positive control. The excitatory activity of turbellarian and cestode neuropeptides on a monogenean indicates at least some structural similarities in the neuropeptide receptors of these classes of flatworm.