eMedicine Specialties > Pediatrics: General Medicine > Parasitology
Diphyllobothrium Latum Infection
Updated: Jan 21, 2009
Introduction
Background
Human intestinal infection with the fish tapeworm, or broad tapeworm, Diphyllobothrium latum is called diphyllobothriasis. It is endemic in areas where humans frequently consume raw or pickled fish. The associated symptomatology is nonspecific, but megaloblastic anemia is a well-described complication.
Pathophysiology
Cestodes, a subclass of the phylum Platyhelminthes, has 2 orders that infect humans: Cyclophyllidae and Pseudophyllidea. One of the main differentiating points between the genera of the 2 orders is that the genera of the first order typically have a scolex with 4 suckers whereas the genera of the second order have a scolex with 2 opposing sucking grooves. Another important point of differentiation is that Cyclophyllidae has 2 hosts in its life cycle whereas Pseudophyllidea requires 3.
Diphyllobothrium is an example of the genera under Pseudophyllidea. The species of this genus are all parasites of fish-eating vertebrates. This article focuses on the organism D latum, for which humans are the definitive host.
A full-grown worm can vary in size from 1-15 m and is the longest human tapeworm. It consists of up to 3000-4000 proglottids. The scolex, as mentioned, has 2 sucking grooves, also called bothria. Proglottids are typically wider than they are long (see Media file 3), which is why D latum is called the broad tapeworm.
In the gravid state, the worms have a distinctive rosette-like 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. The eggs (see Media file 2) must reach fresh water that contains crustaceans and fish to act as the intermediate host to complete their maturation.
During a 10-day to 14-day period, they 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 crustacea (eg, copepods or water fleas) to undergo the next stage of maturation, which is the first-stage larva or procercoid (see Media file 1).
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. During the next 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 actual results of infestation are not well studied, megaloblastic anemia can result. Some scientists believe 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 were placed in the ileum, a hematological remission resulted. This did not occur when the mixture was taken orally. This suggests that D latum preferentially absorbs vitamin B-12 in the blood, preventing vitamin B-12-IF complexes from reaching receptors in the small bowel.
Frequency
United States
Outbreaks associated with the increased popularity and availability of fresh salmon (as opposed to canned or frozen salmon) have been described. Also, infestation is related to the popularity of delicacies such as sushi.
International
This is a worldwide disease that affects people near freshwater and appropriate intermediate hosts. Areas where consumption of raw/precooked fish is popular tend to have endemicity (eg, northern Europe, Scandinavia).
Mortality/Morbidity
Occasionally, infestation can lead to severe megaloblastic anemia or intestinal obstruction. No figures are available to assess morbidity from this disease. Outcomes with single-dose therapy are excellent.
Race
No racial predilection has been reported.
Sex
No sex predilection has been documented.
Age
No age predilection has been reported.
Clinical
History
Diphyllobothrium latum infection is typically an asymptomatic condition. Patients do not usually observe passage of segments in the stool as noted in other tapeworm infections. A few reports have documented patients who presented after vomiting a ball of worms that were determined to be fish tapeworms.
One study looked at a selection of symptoms in a group of patients who were affected but nonanemic. Increased symptoms of fatigue, extremity numbness, and anorexia occurred in the affected group. Abdominal pain was not a significant symptom. GI obstruction is rare but may occur, especially when numerous worms are present. If this occurs, patients present with acute abdominal pain associated with vomiting, distension, or both.
Megaloblastic anemia is well described although very unusual. Studies note that about 40% of patients have decreased serum vitamin B-12 levels, but less than 2% of patients are anemic.1 Patients with an underlying problem (eg, celiac disease or congenital malabsorption) are at higher risk of developing a symptomatic infection.
Anemic patients can present with pallor, breathlessness, and neurological symptoms such as weakness, numbness, and disturbances in coordination. The symptoms depend on the severity.
Physical
No typical examination findings are noted except those found in severe cases of anemia. These can include glossitis, signs of degeneration of the posterior columns of the spinal cord (which include loss of vibratory sense, proprioception, and coordination), tachycardia, pallor, and dyspnea.
If the patient presents with obstruction, abdominal tenderness, distension, and peritoneal signs may be present.
Causes
The main causative organism is D latum, but, less frequently, other Diphyllobothrium species have been reported as infecting agents. Examples include Diphyllobothrium pacificum, Diphyllobothrium cordatum, Diphyllobothrium ursi, Diphyllobothrium dendriticum, Diphyllobothrium lanceolatum, Diphyllobothrium dalliae, and Diphyllobothrium yonagoensis.
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References
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Further Reading
Keywords
Diphyllobothrium latum infection, abdominal pain, broad tapeworm, celiac disease, cestodes, congenital malabsorption, diphyllobothriasis, Diphyllobothrium latum, D latum, dyspnea, fish tapeworm, gastrointestinal obstruction, GI obstruction, glossitis, megaloblastic anemia, sparganosis, tachycardia, vitamin B-12 deficiency, Diphyllobothrium pacificum, Diphyllobothrium cordatum, Diphyllobothrium ursi, Diphyllobothrium dendriticum, Diphyllobothrium lanceolatum, Diphyllobothrium dalliae, Diphyllobothrium yonagoensis
