Infection by helminths (worms) may be limited solely to the intestinal lumen or may involve a complex process with migration of the adult or immature worm through the body before localization in a particular tissue. Complicating our understanding of the host– parasite relationship and the role of chemotherapy in helminth-induced infections is the complex life cycle of many of these organisms. Whereas some helminths have a simple cycle of egg deposition and development of the egg to produce a mature worm, others must progress through one or more hosts and one or more morpho-logical stages, each metabolically distinct from the other, before emerging as an adult. Furthermore, an in-fective form may be either an adult worm or an imma-ture worm. Treatment may be further complicated by infection with more than one genus of helminth. Pathogenic helminths can be divided into the following major groups: cestodes (flatworms), nematodes (round-worms), trematodes (flukes) and less frequently, Acanthocephala (thorny-headed worms).
The complex life cycle and host–parasite relation-ship means that treatment is sometimes difficult and may have to be protracted. Most available anthelmintic drugs exert their antiparasitic effects by interference with (1) energy metabolism, (2) neuromuscular coordi-nation, (3) microtubular function, and (4) cellular per-meability. The mode of action of most drugs used in the treatment of helminthic infections is summarized in Table 54.1. Some of the drugs used in the treatment of diseases caused by helminths also are used in the treat-ment of specific protozoal diseases.