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:
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.
Adult tapeworms are so well-adapted to their host's gastrointestinal tract that they usually cause minimal symptoms. There are some exceptions. Heavy infections with Hymenolepis nana can cause abdominal discomfort, diarrhea, and weight loss; members of the family Diphyllobothriidae can cause vitamin B12 deficiency and megaloblastic anemia.
In contrast to adult tapeworms, larvae can cause severe and even lethal disease when they develop in extraintestinal sites, most importantly in the brain, but also in the liver, lungs, eyes, muscles, and subcutaneous tissues. In humans, T. solium causes cysticercosis, and Echinococcus granulosus and Echinococcus multilocularis cause cystic hydatid disease and alveolar disease, respectively. Rarely, larvae of Spirometra species, Sparganum proliferum, Taenia multiceps, Taenia serialis, Taenia brauni, and Taenia glomeratus can also infect humans producing mass lesions in subcutaneous tissue or muscle, and less commonly, brain or eye depending on the infecting species.
Adult tapeworm infections are diagnosed by identifying eggs or gravid proglottid segments in stool. Larval disease is best identified by imaging (eg, brain CT and/or MRI). Serologic tests may also be helpful.
The anthelmintic drug praziquantel is effective for intestinal tapeworm infections. Niclosamide is an alternative that is not available in the US. Nitazoxanide can be used for H. nana infections.
Some extraintestinal infections respond to anthelmintic treatment with albendazole and/or praziquantel; others require surgical intervention.
Prevention and control involve the following:
Thorough cooking of pork, beef, lamb, game meat, and fish (recommended temperatures and times vary)
Prolonged freezing of meat for some tapeworms (eg, fish tapeworm)
Regular deworming of dogs and cats
Prevention of recycling through hosts (eg, dogs eating dead game or livestock)
Reduction and avoidance of intermediate hosts such as rodents, fleas, and grain beetles
Sanitary treatment of human waste
Smoking and drying meat are ineffective in preventing infection.