Diphyllobothrium latum (or fish tapeworm) has a complex cycle of infection. If sewage that is contaminated with eggs from humans who carry the fish tapeworm gets ingested by fish in a freshwater lake, the fish get infected with larvae that hatched out of the eggs in the water.
The exact cycle is even more complicated, as the larvae first infect the plankton, which in turn is ingested by the fish (see life cycle).
The meat of the infected fish is full of the cysticerci, which is the storage form of the tapeworm. When raw fish (such as infested sushi) is consumed by humans, these transform into fish tape worms in the intestinal wall of the human host within about 1 month. One fish tapeworm produces 1 million eggs per day in the human gut.
Signs and symptoms
As the fish tapeworm is very small compared to other tapeworms, it usually is asymptomatic. However, with its hyperactive metabolism to support the enormous egg production, it uses up all of the dietary vitamin B-12 of the human host. This leads to pernicious anemia in about 1% of affected patients.
The characteristic eggs of the fish tapeworm can easily be detected from stool samples or clear self-adhesive tape samples from skin around the anal area.
A single oral dose of praziquantel (brand name: Biltricide) is the treatment of choice. Pernicious anemia, if present, is treated with an intramuscular dosage of Vitamin B-12.
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