Platyhelminthes (platy means flat; helminth means worm) are divided into two classes: Cestoda (tapeworms) and Trematoda (flukes). The trematodes are described in Chapter 55.
Tapeworms consist of two main parts: a rounded head called a scolex and a flat body consisting of multiple segments. Each segment is called a proglottid. The scolex has specialized means of attaching to the intestinal wall, namely, suckers, hooks, or sucking grooves. The worm grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts such as cattle, pigs, and fish.
Humans usually acquire the infection when undercooked meat or fish containing the larvae is ingested. However, in two important human diseases, cysticercosis and hydatid disease, it is the eggs that are ingested and the resulting larvae cause the disease.
There are four medically important cestodes: Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus. Their features are summarized in Table 54–1, and the medically important stages in the life cycle of these organisms are described in Table 54–2. Three cestodes of lesser importance, Echinococcus multilocularis, Hymenolepis nana, and Dipylidium caninum, are described at the end of this chapter.
TABLE 54–1Features of Medically Important Cestodes (Tapeworms) |Favorite Table|Download (.pdf) TABLE 54–1 Features of Medically Important Cestodes (Tapeworms)
|Cestode ||Mode of Transmission ||Intermediate Host(s) ||Main Sites Affected in Human Body ||Diagnosis ||Treatment |
|Taenia solium ||(A) Ingest larvae in undercooked pork ||Pigs ||Intestine ||Proglottids in stool ||Praziquantel |
| ||(B) Ingest eggs in food or water contaminated with human feces || ||Brain and eyes (cysticerci) ||Biopsy, computed tomography (CT) scan ||Praziquantel, albendazole, or surgical removal of cysticerci |
|Taenia saginata ||Ingest larvae in undercooked beef ||Cattle ||Intestine ||Proglottids in stool ||Praziquantel |
|Diphyllobothrium latum ||Ingest larvae in undercooked fish ||Copepods and fish ||Intestine ||Operculated eggs in stool ||Praziquantel |
|Echinococcus granulosus ||Ingest eggs in food contaminated with dog feces ||Sheep ||Liver, lungs, and brain (hydatid cysts) ||Biopsy, CT scan, serology ||Albendazole or surgical removal of cyst |
TABLE 54–2Medically Important Stages in Life Cycle of Cestodes (Tapeworms) |Favorite Table|Download (.pdf) TABLE 54–2 Medically Important Stages in Life Cycle of Cestodes (Tapeworms)
|Organism ||Insect Vector ||Stage That Infects Humans ||Stage(s) in Humans Most Associated with Disease ||Important Stage(s) Outside of Humans |
|Taenia solium ||None ||1. Larvae in undercooked pork ||Adult tapeworm in intestine ||Larvae in muscle of pig |
| || ||2. Eggs in food or water contaminated with human feces ||Cysticercus, especially in brain ||None |
|Taenia saginata ||None ||Larvae in undercooked beef ||Adult tapeworm in intestine ||Larvae in muscle of pig |
|Diphyllobothrium latum ||None ||Larvae in undercooked fish ||Adult tapeworm in intestine can cause vitamin B12...|