Overview of tapeworm infections

[Couch Potatoe]

All tapeworms (cestodes) cycle through 3 stages—eggs, larvae, and adults. Adults inhabit the intestines of definitive hosts, mammalian carnivores. Several of the adult tapeworms that infect humans are named after their main intermediate host:

The fish tapeworms (members of the family Diphyllobothriidae)
The beef tapeworm (Taenia saginata)
The pork tapeworm (Taenia solium)

An exception is the Asian tapeworm (Taenia asiatica), which is similar to T. saginata in many respects, but it is acquired by eating pork in Asia.
Eggs laid by adult tapeworms living in the intestines of definitive hosts are excreted with feces into the environment and ingested by an intermediate host (typically another species), in which the eggs hatch into larvae, which develop, enter the circulation, and encyst in the musculature or other organs. When the intermediate host is eaten raw or undercooked, the parasites are released from the ingested cysts in the intestines and develop into adult tapeworms in the definitive host, restarting the cycle. With some cestode species (eg, T. solium), the definitive host can also serve as an intermediate host; that is, if eggs rather than tissue cysts are ingested, the eggs develop into larvae, which enter the circulation and encyst in various tissues.
Adult tapeworms are multisegmented flat worms that lack a digestive tract and absorb nutrients directly from the host’s small bowel. In the host’s digestive tract, adult tapeworms can become large; the longest parasite in the world is the 40-m whale tapeworm, Polygonoporus species.
Tapeworms have 3 recognizable portions:

The scolex (head) functions as an anchoring organ that attaches to intestinal mucosa.
The neck is an unsegmented region with high regenerative capacity. If treatment does not eliminate the neck and scolex, the entire worm may regenerate.
The rest of the worm consists of numerous proglottids (segments). Proglottids closest to the neck are undifferentiated. As proglottids move caudally, each develops hermaphroditic sex organs. Distal proglottids are gravid and contain eggs in a uterus.