Helminth infections in production and companion animals can create serious health impacts for their hosts. Pharmacological intervention is a mainstay of prevention and treatment, and veterinary anthelmintic research has benefited parasite control in human health. Ongoing research into pharmacological properties of antiparasitic drugs endeavors to improve anthelmintic treatment efficiency.
However, investment in control measures does not always result in the expected therapeutic success. Factors responsible for therapeutic failure include the following:
inadequate integration between management strategies and chemotherapy (eg, suboptimal timing of treatment)
incorrect use of anthelmintic drugs due to insufficient knowledge of their pharmacological features
insufficient understanding of the relation between pharmacological properties and several host-related factors that could lead to modifications of pharmacokinetic behavior and decreased antiparasitic efficacy of the chosen drug
In addition, the availability of many compounds with a common mode of action and the indiscriminate use of these drugs have accounted for widespread development of drug resistance, mainly in parasites of sheep, goats, and horses, but also in helminth parasites of pigs and cattle.
Classes of anthelmintics include the following:
salicylanilides and substituted phenols
organophosphates
Many highly effective and selective anthelmintics are available; however, such compounds must be used correctly, judiciously, and with consideration of 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 must always be discouraged. Underdosing is likely to result in lowered efficacy and increased pressure for selection of resistance.
Most anthelmintics have a wide margin of safety, considerable activity against immature (larval) and mature stages of helminths, and a broad spectrum of activity. Nonetheless, any anthelmintic's usefulness is limited by the intrinsic efficacy of the drug, 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, location in the body, degree of hypobiosis, susceptibility of the life stage, and susceptibility to the anthelmintic).
