What's Eating You: 12 Common Intestinal Parasites - Medscape

Cestodes

The life cycle of Echinococcus is shown above.

Tapeworms are long, segmented worms of the class Cestoda, which includes the genera Taenia, Diphyllobothrium, Hymenolepis, Dipylidium, Echinococcus, and Spirometra.^[1,2] Adult tapeworms lack an intestinal tract and absorb all nutrients through their integument. The adults have a head (termed a scolex), a neck, and a segmented body with both male and female gonads.^[1]

Tapeworms require one or more intermediate hosts in their life cycle. Typically, the eggs are passed from the host into the environment, where they are ingested by an intermediate host. In the intermediate host, the eggs hatch and the larvae enter the host tissues and encyst. The primary host then ingests the cysts by consuming the flesh of the intermediate host.^[1]

Humans are the primary hosts for Taenia, Diphyllobothrium, and Hymenolepis,^[1] but they may be intermediate hosts for Echinococcus^[3] and Spirometra.^[2] Infection is typically from either fecal-oral transmission or the ingestion of contaminated, undercooked meat.^[1] Most cestode infections are uncommon in the United States, with Hymenolepis nana being the most commonly diagnosed.^[3]

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