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.

Laboratory Diagnosis

Both eggs and proglottids may be found in patient’s feces. Visualization of the eggs is enhanced using a wet preparation of the patient’s stool sample. Diagnosis is made by identification of the ovoid, operculated, yellow-brown eggs (58 to 75 µm by 40 to 50 µm) passed in abundance in the stool. They are sometimes confused with the eggs of Paragonimus. The mature gravid proglottids are wider than long (3 × 11 mm), often in chains, and contain a rosette-shaped central uterus (Figure 54-3). Species identification is through assessment of the morphologic characteristics of the proglottids as previously described (Tables 54-2 and 54-3).

TABLE 54-2

Common Human Parasites, Diagnostic Specimens, Tests, and Positive Findings

Organism	Acquire Infection from:	Location in Host	Diagnostic Specimen	Diagnostic Test*	Positive Specimen	Comments
Intestinal cestodes Taenia saginata (beef)	Ingestion of raw beef	Intestine	Stool and/or proglottids	O&P, India ink proglottids	Eggs, proglottid branches	Both Taenia eggs look alike; need gravid proglottid or scolex for definitive identification.
Taenia solium (pork)	Ingestion of raw pork		Stool and/or proglottids	O&P, India ink proglottids	Eggs, proglottid branches
Diphyllobothrium latum	Ingestion of raw freshwater fish		Stool and/or proglottids	O&P	Eggs, proglottid shape
Hymenolepis nana	Tapeworm eggs		Stool	O&P	Eggs
Hymenolepis diminuta	Grain beetles		Stool	O&P	Eggs
Dipylidium caninum	Fleas from dogs/cats		Stool and/or proglottids	O&P	Eggs, proglottid shape

^*Although serologic tests are not always mentioned, they are available for a number of parasitic infections. Unfortunately, most are not routinely available. Contact your state public health laboratory or the Centers for Disease Control and Prevention in Atlanta, Georgia.

TABLE 54-3

Cestode Parasites of Humans (Intestinal)

	Diphyllobothrium latum	Taenia saginata	Taenia solium	Hymenolepis nana	Hymenolepis diminuta	Dipylidium caninum
Intermediate hosts (common)	Two: copepods and fish	One: cattle	One: pig	One: various arthropods (beetles, fleas); or none	One: various arthropods (beetles, fleas)	One: various arthropods (fleas, dog lice)
Mode of infection	Ingestion of plerocercoid (sparganum) in flesh of infected fish	Ingestion of cysticercus in infected beef	Ingestion of cysticercus in infected pork	Ingestion of cysticercoid in infected arthropod or by direct ingestion of egg; autoinfection may also occur	Ingestion of cysticercoid in infected arthropod	Ingestion of cysticercoid in fleas, lice
Prepatent period	3-5 weeks	10-12 weeks	5-12 weeks	2-3 weeks	≈3 weeks	3-4 weeks
Normal life span	Up to 25 years	Up to 25 years	Up to 25 years	Perhaps many years as a result of autoinfection	Usually <1 year	Usually <1 year
Length	4-10 m	4-12 m	1.5-8 m	2.5-4.0 cm	20-60 cm	10-70 cm
Scolex	Spatulate, 3 × 1 mm; no rostellum or hooklets; has 2 shallow grooves (bothria)	Quadrate, 1-2 mm in diameter; no rostellum or hooklets; 4 suckers	Quadrate, 1-mm diameter; has rostellum and hooklets; 4 suckers	Knoblike but not usually seen; has rostellum and hooklets; 4 suckers	Knoblike but not usually seen; has rostellum but no hooklets; 4 suckers	0.2-0.5 mm in diameter; has conical/retractile rostellum armed with 4-7 rows of small hooklets; 4 suckers
Usual means of diagnosis	Ovoid, operculate yellow-brown eggs (58-75 µm × 40-50 µm) in feces; egg usually has small knob at abopercular end; proglottids may be passed, usually in chain of segments (few cm to 0.5 m long); proglottids wider than long (3 × 11 mm) and have rosette-shaped central uterus	Gravid proglottids in feces; they are longer than wide (19 × 17 mm) and have 15-20 lateral branches on each side of central uterine stem; they usually appear singly; spheroidal yellow-brown, thick-shelled eggs (31-43 µm in diameter) containing an oncosphere may be found in feces	Gravid proglottids in feces; they are longer than wide (11 × 5 mm) and have 7-13 lateral branches on each side of central uterine stem; usually appear in chain of 5-6 segments; spheroidal, yellow-brown, thick-shelled eggs (31-43 µm) containing an oncosphere may be found in feces	Nearly spheroidal, pale, thin-shelled eggs (30-47 µm in diameter) in feces; oncosphere surrounded by rigid membrane, which has two polar thickenings from which 4-8 filaments extend into the space between the oncosphere and thin, outer shell	Large, ovoid, yellowish, moderately thick-shelled eggs (70-85 µm × 60-80 µm) in feces; egg contains oncosphere	Gravid proglottids (8-23 µm long) containing compartmented cluster of eggs in feces; proglottids have genital pores at both lateral margins; occasionally may see individual oncospheres (20-33 mm in diameter) in feces
Diagnostic problems or notes	Eggs are sometimes confused with eggs of Paragonimus; eggs are unembryonated when passed in feces	Eggs are identical to those of Taenia solium; ordinarily can distinguish between species only by examination of gravid proglottids; eggs can be confused with pollen grains (handle all proglottids with extreme care)	Eggs are identical to those of T. saginata; one is less likely to find eggs in feces than with T. saginata (handle all proglottids with extreme care since T. solium eggs are infective to humans)	Sometimes confused with eggs of Hymenolepis diminuta; rodents serve as reservoir hosts	Should not be confused with H. nana because eggs lack polar filaments; rodents serve as reservoir hosts	Gravid proglottids resemble rice grains (dry) or cucumber seeds (moist); dogs and cats serve as reservoir hosts

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