Updated: Dec 17, 2018
  • Author: Derek Ryan Linklater, MD; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Practice Essentials

Diphyllobothriasis is defined as infection with the cestode Diphyllobothrium latum (see the image below) or other Diphyllobothrium species (eg, Diphyllobothrium nihonkaiense). It is endemic in areas where humans frequently consume raw or pickled fish.

Egg of Diphyllobothrium latum with arrow pointing Egg of Diphyllobothrium latum with arrow pointing to operculum.

Signs and symptoms

Most persons with diphyllobothriasis are asymptomatic. In symptomatic persons, the following are the most common symptoms:

  • Abdominal pain

  • Indigestion or dyspepsia

  • Passage of proglottids

Other, less common, symptoms include the following:

  • Fatigue

  • Diarrhea

  • Dizziness

  • Weakness (rare)

  • Numbness of extremities

  • Sensation of hunger

  • Pruritus ani

Most patients with diphyllobothriasis have no signs of illness . Rare physical findings that may be noted include the following:

  • Pallor

  • Glossitis

  • Dyspnea

  • Tachycardia

  • Weakness

  • Hypoesthesia

  • Paresthesias

  • Disturbances of movement and coordination, loss of vibratory sense and proprioception

In patients who present with obstruction, the following physical findings may be noted:

  • Abdominal tenderness

  • Abdominal distention

  • Peritoneal signs

See Presentation for more detail.


Laboratory studies that may be helpful include the following:

  • Microscopic stool examination for ova and parasites (the usual basis for the diagnosis)

  • Complete blood count (CBC)

  • Hemoglobin level and hematocrit

  • Mean cell volume

  • Peripheral smear

  • Vitamin B-12 level

  • Folate level

In anemia produced by diphyllobothriasis, increased free hydrochloric acid may be present in gastric juice.

Other studies that may be considered are as follows:

  • Diagnostic imaging modalities - These are not typically required, except as clinically indicated by other aspects of the presentation (eg, obstruction)

  • Capsule endoscopy

See Workup for more detail.


Most patients with diphyllobothriasis, unless they have severe symptoms, can be safely treated as outpatients.

Treatment of the infection is pharmacologic, involving one of the following agents:

  • Praziquantel (drug of choice)

  • Niclosamide (acceptable alternative)

If the first course of treatment fails, a second identical course of therapy may be administered.

Other aspects of treatment include the following:

  • Surgical treatment is not required unless otherwise indicated (eg, in intestinal obstruction)

  • Vitamin supplementation may be required in severe cases of vitamin B-12 deficiency

  • As a rule, no activity limitations or restrictions are necessary

See Treatment and Medication for more detail.



Diphyllobothriasis is defined as human intestinal infection with the cestode Diphyllobothrium latum or other Diphyllobothrium species. It is endemic in areas where humans frequently consume raw or pickled fish.

Cestodes, more commonly called tapeworms, are symmetric flatworms that parasitize the intestinal tract of vertebrates. Tapeworms consist of a head (scolex), a neck, and a germinal region composed of a string of separate individual segments that have a full set of progressively maturing reproductive organs. The scolex attaches to the host’s intestinal mucosa, and tapeworms grow when segments bud from the scolex. The segments enlarge by developing large numbers of eggs that are subsequently shed in the stool.

Cestodes are hermaphroditic and capable of self-fertilization, but Diphyllobothrium eggs must be passed into an aquatic environment to complete their development and become infective. Cestodes do not have a digestive tract at any stage of their development; consequently, they exchange nutrients and waste through their body covering (tegument). The tegument is covered by minute projections called microtriches, which lie in proximity to the host’s intestinal villi and greatly increase the absorptive area of the flatworm.

The symptoms associated with diphyllobothriasis are nonspecific, but megaloblastic anemia is a well-described complication. Diphyllobothrium nihonkaiense also causes human diphyllobothriasis, which for the most part is clinically indistinguishable from that caused by D latum. The 2 organisms are morphologically indistinguishable but can be differentiated via polymerase chain reaction (PCR)–based DNA sequence analysis of specific genes. This organism is commonly found in salmon and likely is the cause of all diphyllobothriasis in Japan. [1]



The subclass Cestoda of the class Cestoidea, under the phylum Platyhelminthes, includes 2 orders that infect humans: Cyclophyllidae and Pseudophyllidea. One of the main differentiating points between the genera of the 2 orders is that those of the first order typically have a scolex with 4 suckers, whereas those of the second have a scolex with 2 opposing sucking grooves. Another important point of differentiation is in Cyclophyllidae, there are 2 hosts in the life cycle, whereas in Pseudophyllidea, 3 are required.

The order Pseudophyllidea includes the family Diphyllobothriidae, and Diphyllobothrium is one of the genera within this family. The species of this genus are all parasites of fish-eating vertebrates (hence the common use of the term fish tapeworm). This article focuses on the D latum, for which humans are the definitive host.

A full-grown Diphyllobothrium worm can range from 1-15 m in length and is the longest human tapeworm. It consists of up to 3000-4000 proglottids. The scolex, as noted, has 2 sucking grooves, also called bothria. Proglottids are typically wider than they are long, which is why D latum is called the broad tapeworm.

In the gravid state, the worms have a distinctive rosettelike uterus in the center. The uterus sheds eggs into the fecal stream via a ventral pore. The eggs are unembryonated when shed and are operculated. To complete their maturation, the eggs must reach fresh water that contains crustaceans and fish, which act as the intermediate host (see the image below).

Life cycle of diphyllobothrium. Life cycle of diphyllobothrium.

Over a 10-day to 14-day period, the eggs transform into ciliated embryos with 6 hooks (called coracidia) that are released from the open opercula. The coracidia must then be eaten within a short period by crustaceans (eg, copepods or water fleas) to reach the next stage of maturation, which is the first-stage larva or procercoid.

Copepods that contain the procercoid are then eaten by freshwater fish that function as the second intermediate host. Here, the procercoid matures into the plerocercoid or sparganum in the fish muscle fibers. Infected fish are then consumed by progressively larger fish, with the sparganum being passed on, until, finally, the fish is consumed by a human, the definitive host. Over the subsequent 3-5 weeks, the plerocercoid larva matures into an adult that can live for up to 10 years.

Thus, the plerocercoid larva infects humans who have ingested heated or frozen freshwater fish. Because of the requirement for intermediate hosts, direct human-to-human transmission does not occur; therefore, no isolation measures are required.

Although the actual results of infestation are not well studied, megaloblastic anemia is known to be a possible consequence. Some scientists believe that the mechanism is related to the site of the worm, its marked affinity for vitamin B-12, and, perhaps, an underlying vitamin B-12 deficiency in patients at the outset.

One study demonstrated that when a mixture of vitamin B-12 and gastric juice was placed in the ileum, a hematologic remission resulted; such a remission did not occur when the mixture was taken orally. These results suggest that D latum preferentially absorbs vitamin B-12 in the blood, preventing vitamin B-12-IF complexes from reaching receptors in the small bowel.



Diphyllobothriasis is caused by ingestion of raw or undercooked infected fish and subsequent intestinal infection. The main causative organisms are D latum and D nihonkaiense, but other Diphyllobothrium species have also been reported as infecting agents, albeit less frequently. Examples include the following:

  • Diphyllobothrium pacificum

  • Diphyllobothrium cordatum

  • Diphyllobothrium ursi

  • Diphyllobothrium dendriticum

  • Diphyllobothrium lanceolatum

  • Diphyllobothrium dalliae

  • Diphyllobothrium yonagoensis



United States statistics

In North America, D latum infections have been reported in fish from the Great Lakes; however, no infections have been reported in the past few years, and the worm may have ceased to reside in this area. Eskimos have also reported diphyllobothriasis, and 6 Diphyllobothrium species are known to reside in Alaskan lakes and rivers. Diphyllobothriasis is not a species-specific infection, and widespread reports have described infection in North American fish-eating birds and mammals.

The incidence of diphyllobothriasis in the United States has been declining, but the growing popularity of Japanese sushi [2] and sashimi has the potential to increase the frequency of infection. Pike, perch, and salmon (80% in a recent case series) are among the fish most commonly infected. Outbreaks associated with the increased popularity and availability of fresh salmon (as opposed to canned or frozen salmon) have been described.

International statistics

Diphyllobothriasis is a worldwide disease that affects people near fresh water and appropriate intermediate hosts. D latum commonly infects persons residing in Europe, Africa, and the Far East. Areas where consumption of raw or precooked fish is popular tend to have endemicity (eg, northern Europe and Scandinavia). Aside from dietary preferences, night soil (human excrement) fertilization practices and poor sanitation seem to play a role in the increased incidence of diphyllobothriasis in some countries.

Age-, sex-, and race-related demographics

Diphyllobothriasis has no reported age predilection or sexual predilection. In addition, it has no known racial predilection, except as would be expected on the basis of geographic and cultural factors.



In general, diphyllobothriasis carries an excellent prognosis. D latum is not invasive, and mortality due to diphyllobothriasis is rare. Single-dose therapy is usually effective, though some treatment failures have been reported and repeat treatment is occasionally needed.

Occasionally, infestation can lead to severe megaloblastic anemia or intestinal obstruction. Although it is well described, megaloblastic anemia is in fact very unusual. Gastrointestinal (GI) obstruction is also rare but may occur, especially when numerous worms are present and form a substantial bulk in the GI tract.


Patient Education

Because reinfection is possible, patients should be advised to modify their dietary habits so as to minimize the potential for reexposure. Proper food preparation and hygiene should be encouraged, particularly during travel within endemic areas.

It is imperative to educate people about the effective ways of making freshwater fish safe for consumption. Cooking fish to a temperature of 56°C or higher for longer than 5 minutes or freezing fish to −18°C for 24 hours kills the plerocercoid larvae. The larvae can also be killed by pickling fish in brine under very rigorous conditions. The fish must never be sampled before it is properly prepared.