Many highly effective and selective anthelmintics are available, but such compounds must be used correctly, judiciously, and with consideration of the parasite/host interaction to obtain a favorable clinical response, accomplish good control, and minimize selection for anthelmintic resistance. Any decrease or increase of the recommended dose rate must always be discouraged. Underdosing is likely to result in lowered efficacy and possibly increased pressure for development of resistance. Overdosing may result in toxicity without necessarily increasing product efficacy.
Most anthelmintics generally have a wide margin of safety, considerable activity against immature (larval) and mature stages of helminths, and a broad spectrum of activity. Nonetheless, the usefulness of any anthelmintic is limited by the intrinsic efficacy of the drug itself, its mechanism of action, its pharmacokinetic properties, characteristics of the host animal (eg, operation of the esophageal groove reflex), and characteristics of the parasite (eg, its location in the body, its degree of hypobiosis, susceptibility of the life stage, or susceptibility to the anthelmintic).
There are several classes of anthelmintics: benzimidazoles and probenzimidazoles, salicylanilides and substituted phenols, imidazothiazoles, tetrahydropyrimidines, organophosphates, macrocyclic lactones and, more recently introduced, the amino-acetonitrile derivatives, the cyclic octadepsipeptides, and the spiroindoles. Although it may be thought that chemotherapeutic control of helminth infections is currently satisfactory, selection for parasite resistance is an increasing concern.