Parasite life cycles facilitate dispersal and overcome host defenses

Animals that live as endoparasites are bathed in the nutritious tissues of their host or in the digested food that fills their host’s digestive tract. Thus they may not need to exert much energy to obtain food, but to survive they must overcome the host’s defenses. Furthermore, either they or their offspring must disperse to new hosts while their host is still living, because they die when their host dies.

The fertilized eggs of some parasites are voided with the host’s feces and later ingested directly by other host individuals. Most parasite species, however, have complex life cycles involving one or more intermediate hosts and several larval stages (Figure 30.12). Some intermediate hosts transport individual parasites directly between other host species. Others house and support the parasite until another host ingests them. Complex life cycles may thus facilitate the transfer of individual parasites among hosts.

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Figure 30.12 Reaching a New Host by a Complex Route The broad fish tapeworm Diphyllobothrium latum must pass through the bodies of a copepod (a type of crustacean) and at least one fish before it can reinfect its primary host, a mammal. Such complex life cycles assist the parasite’s colonization of new host individuals, but they also provide opportunities for humans to break the cycle with hygienic measures (such as thoroughly cooking food to kill the parasites).