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 intermediate host: the fish tapeworm (Diphyllobothrium latum), the beef tapeworm (Taenia saginata), and the pork tapeworm (Taenia solium). 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 larvae develop, enter the circulation, and encyst in the musculature or other organs. When the intermediate host is eaten, cysts develop into adult tapeworms in the intestines of the definitive host, restarting the cycle. With some cestode species (eg, T. solium), the definitive host can also serve as an intermediate host; tissue cysts develop instead of intestinal worms after eggs are ingested.
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 sp. 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.
In contrast to adult tapeworms, larvae can cause severe and even lethal disease, 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 E. multilocularis cause hydatid disease. Sparganum mansoni and T. multiceps larvae can also infect humans.
Symptoms and Signs
Adult tapeworms are so well adapted to their hosts that they cause minimal symptoms. Hymenolepis nana is an exception and can cause abdominal discomfort, diarrhea, and weight loss. However, larvae may elicit intense immunologic reactions as they travel through tissues (hence inducing immunity) and cause severe disease when they settle in extraintestinal sites.
Adult tapeworm infections are diagnosed by identifying eggs or gravid proglottid segments in stool. Larval disease is best identified by imaging (eg, brain CT or MRI) and, for some species, serologic tests.
The anthelmintic drugs, praziquantel and niclosamide, are effective for most intestinal tapeworm infections. Some extraintestinal infections respond to anthelmintic treatment; others require surgical intervention.
Prevention and control involve the following:
Prolonged freezing of meat is effective, pickling is variably effective, and smoking and drying are ineffective.
Last full review/revision December 2009 by Richard D. Pearson, MD