Diphyllobothrium latum – Fish-borne Parasite
Broad or Fresh Water Fish Tapeworm of Humans
Jul 28, 2009
The broad fish tapeworm, or fresh water fish tapeworm, Diphyllobothrium latum, infects an estimated nine million people in temperate and subarctic regions.
Life Cycle of Diphyllobothrium latum
A human with an intestinal broad fish tapeworm passes stools containing hundreds of thousands of eggs. The eggs are too small to be seen without a microscope. Magnified, they are similar to a chicken’s egg with a flip lid at the narrow end and a knob at the broad end.
In fresh water between 4 and 25 °C (39 to 77 °F), and in the presence of light, the embryo develops to a larval stage called a coracidium. The coracidium escapes through the flip lid after about two weeks.
- Fresh water copepods (tiny planktonic crustaceans) ingest coracidia whereupon the tiny larvae break through into the body cavity. Now a parasite of the copepod, the coracidium develops to the next stage called a procercoid.
- The parasite gets eaten again when a fish swallows the copepod. Migrating out of the copepod’s body, the procercoid moves to the muscles of the fish. There, it develops to the next stage, called a plerocercoid.
- Now it’s ready to infect a human once more, and if the fish is eaten raw by a human, the plerocercoid attaches to the lining of the small intestine and quickly grows into a long frilly-looking tapeworm.
- In about a month, the tapeworm is mature enough to produce eggs. It will grow to an average length of about ten metres (30.5 feet) — worms of 25 metres have been reported — and live for years.
FAQ – Frequently Asked Questions About the Fresh Water Fish Tapeworm
How do people catch broad fish tapeworm?
People acquire the parasite by eating fish that contains the live plerocercoid. Raw fish dishes such as sushi or sashimi; lightly salted, smoked, or pickled raw fish; and tasting fish during cooking are all possible sources.
What kinds of fish are infected with Diphyllobothrium latum?
Many different species of fish can carry the parasite. The most common are pike, burbot, and perch.
How can one make sure the fish is safe to eat?
Thorough cooking, thorough smoking, or freezing fish for forty-eight hours at -10°C (14°F) kills the parasite.
Can you see a plerocercoid larva in fish?
A plerocercoid larva is up to a centimeter and a half long (a bit more than half an inch) and is usually coiled up. It looks like a white mass in raw fish. When fish is even partially cooked, the parasite is difficult to see.
What are the symptoms of fresh water fish tapeworm infection?
Many infections with D. latum (diphyllobothriasis or diphyllobothriosis) are without symptoms and the patient may only become aware of the worm when a long portion of it is passed in the toilet. Some people complain of mild abdominal discomfort and may suffer some diarrhea. Other complaints include tiredness, dizziness, hunger, and a craving for salt.
In rare cases, mostly in Scandinavians, the worm robs the body of vitamin B12 causing B12 deficiency and pernicious anemia. Symptoms include tingling burning or itching skin, loss of coordination, and vision problems.
Why can’t we get rid of Diphyllobothrium latum?
Breaking the life cycle of D. latum is possible in three different ways: treat all infected people, prevent human feces from contaminating water sources where people fish, and prevent people from eating fish that may contain live plerocercoid larvae. Although other animals, including cats and dogs, can harbour the worm, they don’t appear to keep the life cycle going where there are no infected humans.
Interesting Facts About Fresh Water Fish Tapeworm
The complicated natural history of D. latum can be even more confusing:
- Diphyllobothrium latum is the usual species to infect humans; however, various closely related species also do so. Normally parasites of birds, whales, seals, even bears, these species would be much harder to eradicate.
- Despite the name, the fresh water fish tapeworm can be acquired from fish caught in low salinity sea water, and from ocean-going fish like salmon.
- If a human swallows a copepod infected with various Diphyllobothrium spp. or the very similar Spirometra spp. a disease called sparganosis results. The procercoid migrates into human tissues and develops to the plerocercoid stage.
Sources
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
“Diphyllobothrium latum.” Bylund, B. Göran. In: Parasites of the Colder Climates. Akuffo, Hannah, Ewert Linder, Inger Ljungström, and Mats Wahlgren, eds. London: Taylor and Francis, 2003, pp. 169 – 176.
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
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