Diphyllobothrium latum, the freshwater broad fish tapeworm, is the largest human tapeworm. Adults have been known to reach up to 10 m in length, with more than 3000 to 4000 proglottids, and reside within a host for 30 years or more. The proglottids are characteristically wider than long with a central rosette-shaped uterine structure (Figure 54-1). The scolex is spatulate and contains two shallow sucking grooves referred to as bothria (Figure 54-1, B). D. latum has unembryonated eggs. The eggs are operculated with a terminal knob, similar to trematode eggs (Figure 54-1, C). The intermediate hosts include crustaceans and freshwater fish.

Pathogenesis and Spectrum of Disease

Diphyllobothrium latum is the only cestode to have an aquatic life cycle (Figure 54-2). Fish serve as the reservoir host, with humans serving as the definitive host. D. latum eggs are found in the feces of infected humans and other fish-eating mammals. Once passed into a water source, such as a lake, the life cycle requires two intermediate hosts. After incubation in freshwater for approximately 2 weeks, the mature eggs release the first larval stage (coracidium). The coracidia are ingested by copepods. The fish feed upon the small crustaceans ingesting the procercoid larvae. Within the freshwater fish, the larvae develop into the infective larvae. D. latum infection occurs through the ingestion of poorly cooked freshwater fish containing the plerocercoid larval form. D. latum matures to an adult tapeworm within the human small intestine. Infection is usually asymptomatic, but mild gastrointestinal symptoms may occur such as diarrhea, abdominal pain, fatigue, vomiting, or dizziness. Symptoms vary depending on the worm burden and the host’s immune response to the organism. The tapeworm nutritional requirements may decrease the host’s vitamin B₁₂ level, resulting in megaloblastic anemia (Table 54-1).

TABLE 54-1

Epidemiology of the Intestinal Cestodes That Cause Disease in Humans

Parasite	Habitat (Reservoir)	Mode of Transmission
Diphyllobothrium latum	Adult tapeworms can be found in a number of wild animals, the most important being dogs, bears, seals, and walrus, that serve as reservoir hosts; the human serves as the definitive host.	Occurs via ingestion of poorly cooked freshwater fish containing the sparganum or plerocercoid larval form.
Dipylidium caninum	Humans serve as an accidental host for the dog tapeworm. Dogs, cats, and wild animals serve as a reservoir host. Arthropods such as the dog and cat flea serve as an intermediate host.	Transmitted to humans through the ingestion of dog/cat fleas.
Hymenolepis nana	The dwarf tapeworm can also occur in rodents; the human can serve as both intermediate and definitive host, with development from the egg to adult worm occurring in the human intestine.	Primarily acquired from accidental ingestion of eggs from an adult tapeworm, most commonly via fecal-oral exposure.
Hymenolepis diminuta	The rat tapeworm frequently infects rodents, and rarely infects humans.	Infects humans after infected mice and rats contaminate food with their feces.
Taenia solium	Humans serve as the definitive host for the pork tapeworm, whereas pigs and humans serve as intermediate hosts.	Humans become infected when cyst-infected pork is ingested.
Taenia saginata	Humans serve as the definitive host for the beef tapeworm, whereas cows/camels serve as intermediate hosts.	Humans become infected when cyst-infected beef is ingested.