Diphyllobothrium latum is a ribbon tapeworm belonging to the Cestode class, to the Diphyllobothriidea order and to the Diphyllobothriidae family. It can reach ten meters in length (Fig. 1) and can live several years. It is responsible of a parasite infection called the diphyllobothriosis.
Figure 1. Adult form of the fish tapeworm, Diphyllobothrium latum
The biological cycle (Fig. 2) of the parasite includes a definitive host: Man (and other fish-eating mammals) and at least two intermediate hosts: a planktonic crustacean and one or several freshwater fish. The eggs are excreted in freshwater with the feces of the definitive host. In favorable environment conditions, after 8 to 12 days of maturation, the eggs hatch and release a ciliated embryo that is ingested by a microscopical crustacean belonging to the Cyclops or Eudiaptomus genus. The embryo transforms into a procercoid larvae in the general cavity. Once ingested by a carnivorous fish, this larvae turns into a second type of larvae, called plerocercoid (of few millimeters in length, Fig. 3). The plercocercoid encysts into the fish musculature or viscera. Man and other fish-eating mammals contaminate themselves by ingesting flesh of freshwater fish that is raw or insufficiently cooked. Once in the intestine of the definitive host, the plerocercoid larvae grows of several centimeters per day and the first eggs are excreted in the feces approximately one month following infestation. Several species of this parasite are pathogenic to human beings but D. latum is the sole species to be contracted from metropolitan freshwater fish. Nevertheless, cases of diphyllobothriosis caused by D. nihonkaiense (species from the Pacific) have been notified in consumers of salmons (Onchorynchus sp.) imported from the Pacific ocean (Canada).
Figure 2. Life cycle of Diphyllobothrium latum .
The diphyllobothriosis is still encountered in Western Europe. It is decreasing in the historical areas of endemicity: the Scandinavian countries. On the contrary, it seems to emerge in French- and Italian-speaking areas of the subalpine lakes where a professional fishing activity often exists. Since 1987, more than 200 cases have been reported around the lakes of Geneva, Morat, Bienne, Majeur, Côme, Iseo and Garde. The lake of Geneva seems particularly endemic since 48 cases of contamination have been identified on the Swiss and French catchment basins in 2001 and 2002. Between 2002 and 2007, 44 cases have been identified in medical analysis laboratories in the Haute-Savoie region. The food involved is the raw flesh (marinated fillets, carpaccio, etc.) or raw eggs of freshwater fish: perch (Perca fluviatilis), pike (Esox lucius), char (Salvelinus alpinus), burbot (Lota lota), etc. Four to 10 % of the perch fillets consumed on the boarders of the lake of Geneva harbor the parasite. The Coregonidae (Féras) and probably, the European salmonids of the Salmo genus are refractory to D. latum. The Canadian salmonids of the Onchorynchus genus can harbor larvae of D. nihonkaiense.
Figure 3. A plerocercoid larvae of Diphyllobothrium latum in a fish fillet.
The parasitism becomes apparent with abdominal pain and excretion of proglottids resembling those of the beef or pork tapeworm. Rare cases of anemia due to lack of B12 vitamin have been described in case of prolonged infestation in undernourished populations.
The prevention is based on sufficient cooking of the fish (65°C) or freezing at - 20°C during 8 to 72 hours depending on the fish thickness. As far as general hygiene practice is concern, the treatment of sewage water in a modern water treatment plant should interrupt the transmission cycle.
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