Parasites In Humans
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Diphyllobothrium Latum - Fish Tapeworm

Diphyllobothrium latum, the fish tapeworm, is the biggest tapeworm in humans. It causes a parasitic infection called diphyllobothriasis which is acquired by eating raw fish infected with the parasite. Diphyllobothriasis is found in Chile, Peru, Uganda and in the Northern Hemisphere (northern Asia, Europe and America) in areas of rivers and lakes. Diphyllobothrium species include: D. cordatum, D. dalliae, D. dendriticum, D. lanceolatum, D. latum, D. pacificum, D. ursi and D. yonagoensis. Diphyllobothrium latum is the most common and mostly found in Scandinavia, the Baltics and western Russia.

The life cycle of Diphyllobothrium latum starts, when immature eggs are passed in the feces of an infected human. The eggs mature in water within three weeks and form oncospheres. Larvae called coracidia hatch and get eaten by freshwater crustaceans such as copepod. After ingestion coracidia develop into procercoid larvae. If the copepod is eaten by a small fish (second intermediate host), the procercoid larvae penetrate the gut and migrate to muscle tissue where they develop into plerocercoid larvae (sparganum), the infective stage for humans. Usually a third intermediate host is needed because humans do not usually eat raw fish this small. If a trout, walleyed pike or perch eats the smaller fish, the plerocercoid larvae once again penetrate the gut and migrate to fish flesh. If a human eats the infected fish raw or undercooked the plerocercoid larvae develop into adults in the small intestine. The adults attach to the intestinal mucosa with two shallow, bilateral grooves (bothria) of their scolex. The scolex is 3 mm long and 1 mm wide. The long, flat body consists of segments, proglottids, that are produced by the neck. Full grown proglottids are about 10 mm wide and 3 mm long. As proglottids mature, they release eggs and eventually break off from the body. A Diphyllobothrium latum proglottid is characterized by a rosette-shaped uterus at its center. The eggs are ellipsoidal or oval measuring 55–75 µm by 40–50 µm. They are passed in the feces unembryonated (immature). From the start of the infection it takes about six weeks for the eggs to appear in the feces. One adult tapeworm can shed up to a million eggs per day. It can grow over 10 meters long and live up to 20 years.

Diphyllobothrium latum life cycle

People who eat raw fish are at risk of getting infected with the larvae that are sometimes present in the fish flesh. Some fishermen eat their catches raw using salt to kill parasites. Salting the fish is usually not enough to kill larval Diphyllobothrium latum. Some housewives taste fish as they are preparing it and might ingest the tiny larva. Women get infected more often than men because they usually prepare meals for the family. Raw fish is included in many regional cuisines, for example, carpaccio di persico (Italy), ceviche (Latin America), marinated herring (Scandinavia), sashimi and sushi (Japan) and tartare maison in (France).

Diphyllobothriasis is usually asymptomatic. In some cases it causes severe vitamin B12 deficiency because D. latum can absorb most of the B12 intake. In some cases it can lead to neurological symptoms. Diphyllobothriasis symptoms include:

  • constipation
  • diarrhea
  • fatigue
  • obstruction of the bowel
  • pernicious anemia (caused by vitamin B12 deficiency) which can lead to
  • subacute combined degeneration of spinal cord
  • stomach pain
  • vomiting
  • weight loss.

Migrating proglottids can cause inflammation of the bile duct or the gall bladder.

Diphyllobothriasis is diagnosed by examining a stool sample to find eggs or sometimes proglottids. Identification is restricted to genus level which actually does not matter when treating the disease since all Diphyllobothrium species respond to the same drugs. When doing research the specific worm species can be identified by performing PCR on purified eggs.

Diphyllobothriasis was previously treated with praziquantel or niclosamide. Praziquantel has some side effects that are very similar to diphyllobothriasis symptoms. Niclosamide is not absorbed in the gastrointestinal tract and has usually no side effects. However, these drugs are not reliable in cutting the scolex loose. If the head is left in the intestinal wall it will regrow. The best method is a diatrizoic acid injection into the duodenal wall. The diatrizoic acid causes all worms to detach leaving nothing behind. Niclosamide or praziquantel should only be used, if endoscopic treatment is not available.

To prevent infection:

  • Cook fish properly. If you eat sashimi or sushi, freeze it first at -10 °C (or below) for two days to kill the tapeworm larvae.
  • Do not defecate in water (rivers, lakes etc.). If the fish tapeworm larvae cannot get in touch with the intermediate hosts, they cannot infect humans.

Also check out the Diphyllobothrium latum pictures and videos.