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Overview of Tapeworm Infections

by Richard D. Pearson, MD

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, 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 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.

Symptoms and Signs

Adult tapeworms are so well-adapted to their host's GI tract that they usually cause minimal symptoms. There are some exceptions. Heavy infections with Hymenolepis nana can cause abdominal discomfort, diarrhea, and weight loss; Diphyllobothrium latum can cause vitamin B 12 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 E. multilocularis cause hydatid disease. Larvae of Spirometra spp, Sparganum proliferum, T. multiceps, and T. serialis can also infect humans.

Diagnosis

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). Serologic tests may also be helpful.

Treatment

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 or praziquantel; others require surgical intervention.

Prevention

Prevention and control involve the following:

  • Thorough cooking (to a temperature > 57° C [> 135° F]) of pork, beef, lamb, game meat, and fish

  • Prolonged freezing of meat (effective) and pickling (variably effective)

  • Regular worming 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

  • Meat inspection

  • Sanitary treatment of human waste

Smoking and drying meat are ineffective in preventing infection.

Drugs Mentioned In This Article

  • Drug Name
    Select Trade
  • BILTRICIDE
  • ALINIA
  • ALBENZA

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