Common name. broad tapeworm, fish tapeworm

Disease. diphyllobothriasis

Geographic distribution. The distribution of this worm is worldwide, especially in northern Italy, Switzerland, Germany, Baltic countries, Finland, Sweden, central Siberia, North America, northern Manchuria, Japan, Korea, and Chile.

Infection rate. The case reports or increased outbreaks have been reported in the above countries.

Life cycle. Infection with the adult worm is acquired by the ingestion of raw, poorly cooked, or pickled salmon, trout, perch, pike, white fish, grayling, ruff, eel, etc., harboring the plerocercoid larvae. After five or six weeks, the larva matures to the adult worm. Both eggs and proglottids are passed in the stool. The eggs develop in 2 weeks, and hatch to become ciliated coracidium larvae, and are ingested by the first intermediate host, the copepod. The copepods, containing the procercoid larvae, are ingested by fish, the second intermediate host, contains the plerocercoid larvae.

Morphology. The adult worm reaches up to 10 m with 3,000 proglottids. The scolex is elongated and spoon shaped with two long sucking grooves. It measures 1 mm in width by 2.5 mm in length. The mature and gravid proglottids are broader than long, with the typical rosette-shaped uterus. They measure up to 2 cm in width. The eggs are oval and operculated. They measure 65～70 by 45～50 ㎛.

Pathology and clinical symptoms. Symptoms depend on the mass of the worm, amount of its by-products absorbed by the host, and the host's susceptibility to the foreign substances. Some infected people show no symptoms. There may be intestinal obstruction, diarrhea, abdominal pain, or anemia.

Diagnosis. Diagnosis is based on the recovery of the characteristic eggs or proglottids. Proglottids are often passed in chains in a few cm or longer. The proglottid morphology with the rosette-shaped uterus confirms the species.

Prevention. Avoid ingestion of raw or undercooked salmon, trout, perch, and pike.

Egg of D. latum from human.

Egg of Diphyllobothrium latum from human.

SEM view of D. latum egg.

SEM view of D. latum egg.

Scolex of Diphyllobothrium latum.

Note the bothria, two deep dorsoventral suctorial grooves, which are typical structures of the pseudophyllidean tapeworm.

Diphyllobothrium latum: Rosette shape uteri in the gravid proglittids stained with Aceto-carmine solution, X20

Diphyllobothrium latum : Rosette-shape uteri in the gravid proglottids stained with Aceto-carmine solution, X 40

Three scolices of D. latum from human.

Mature or gravid proglottid of D. latum from human. Semichon's acetocarmine stained.

Mature or gravid proglottid of D. latum from human. Semichon's acetocarmine stained.

Mature or gravid proglottids of D. latum from a human case, Semichon's acetocarmine stained.

Sectional view of D. latum mature proglottid, H&E stained.

Chum salmon, Onchorhynchus keta, which is one of the second intermediate host of D. latum.