The fish tapeworm Diphyllobothrium latum (Dibothriocephalus latus) is only rarely a problem in the United States. The organisms are ingested with raw pike fish from the Great Lakes area. Usually very few symptoms are produced, but occasionally the syndrome of megaloblastic anemia may result from ingestion of dietary vitamin B12 by the parasite. Diagnosis consists of stool examination for ova or detached tapeworm proglottid segments. The beef tapeworm (Taenia saginata) and the pork tapeworm (Taenia solium) also are diagnosed by stool examination for ova or proglottids.

Tapeworm infestation in a human ordinarily occurs when the intermediate host (animal or fish) ingests tapeworm eggs or prolarvae, the egg (or prolarva) evolves to a larval form within the intermediate host, the person eats flesh from the intermediate host that contains the larva, and the larva develops into an adult worm in the lumen of the individual’s intestine. If someone ingests ova or prolarva rather than the larva, larvae may develop within the person’s intestine, proceed through the intestinal wall, and reach the bloodstream, whence they are capable of producing abnormality in various organs or tissues.

The most publicized tapeworm larval diseases are Echinococcus or hydatid cyst infection (Echinococcus granulosus, the larva of the dog tapeworm), Cysticercus cellulosa (infection by the larva of T. solium, the swine tapeworm), and sparganosis (infection by the larva of a dog or cat tapeworm of the Spirometra genus related to the fish tapeworm D. latum). No good laboratory method for diagnosis of sparganosis is available. Enzyme immunoassays have been reported for IgM and IgG antibodies to cysticercosis, but these are available only in some reference laboratories. The laboratory may be helpful in Echinococcus (hydatid) disease, although this condition is rare in the United States. The primary host is the dog. The dog sheds ova in the feces, and material contaminated by dog feces is ingested by humans or sheep (or other animals) who act as intermediate hosts. Larvae emerge from the ova, penetrate the intestinal wall, and travel to the liver. Cystic structures (hydatid cysts) containing brood capsules filled with scolices grow in the liver (75% of cases) but may appear in the lungs or other locations. Diagnostic aids include imaging procedures such as the radioisotope liver scan, ultrasound, or computerized tomography; a skin test known as the Casoni test; a hemagglutination test; and immunofluorescent procedures. Results of the Casoni test and the hemagglutination tests are said to be positive in 90% of those patients with hepatic lesions but abnormal in less than one half of patients with cysts elsewhere. False positive results in the Casoni test are said to occur with some frequency. Therefore the Casoni test has mostly been replaced by serologic tests, such as immunofluorescence and enzyme immunoassay. These are available in reference laboratories.