Diphyllobothrium Latum: Morphology, Life Cycle, Symptoms

The  Diphyllobothrium latum is a flat class Cestoda parasite that can cause infections in humans. The disease it produces goes by various names: botryocephalus, diphyllobothriasis or botryocephalus, but all refer to the same intestinal parasitic disease.

Infection with this flatworm occurs when eating raw or poorly cooked fish. This characteristic limited the pathology to those regions with culinary habits that include raw fish, such as Asia, the Arctic and America, but the globalization of sushi and ceviche as common dishes has spread the parasite worldwide.

These parasites have a really interesting morphology and life cycle. The form of infection in humans —their main host— and other mammals and birds occurs orally, although getting to this point is a long and complex process with many edges and variables.

The symptoms it produces are very nonspecific, mostly related to the gastrointestinal tract. Reaching the diagnosis is not so easy because this possibility is not usually considered and it is often achieved thanks to fortuitous findings. Treatment can be somewhat complicated, but it is almost always effective.

Morphology

From a taxonomic point of view, like any member of the phylum flatworm and the class cestoda, Diphyllobothrium latum is a flat, tapered worm. It has a more elongated scolex (head) than most of the other members of its class and has suction discs instead of the usual suction cups.

These parasites have a proliferation zone or neck just after the scolex and the rest of the body is made up of numerous segments or proglottids, each with its own set of genital organs of both sexes; that is, they are hermaphrodites. Some authors have described specimens with up to 4000 segments in their extension.

The Diphyllobothrium latum is one of the longest parasites that can affect human: they can grow in the intestine from 2 to 15 meters.

Its maximum length reached has been 25 meters. The growth rate can reach 22 cm per day (that is, almost 1 cm per hour) and they survive up to 25 years inside the body.

Biological cycle

The development of these parasites involves up to two intermediate hosts and several evolutionary stages before reaching the definitive host: humans.

Egg and heart

Eggs that travel in human feces are not embryonated and have an operculum in their narrowest portion. When the feces reach the water they become first stage larvae (oncosphere), which are covered with a ciliated outer envelope, thus forming a coracidium that opens on contact with water, becoming embryonated.

First guest

The mobile heart swims in the water, attracting potential first intermediate hosts. These initial hosts are crustaceans of the copepod subclass, which are part of the plankton in most bodies of water on the planet (oceans, seas, rivers, lakes, among others).

Coracidia penetrate the intestinal walls of copepods and transform into procercoids, which lack scolexes and genitalia, but have a posterior appendix containing embryonic hooks.

Second guest

Procercoid-infected copepods are ingested by freshwater or saltwater fish; salmon have a real predilection for these crustaceans.

Once inside, the procercoids move to the muscle tissues, organs and abdominal cavity of the fish and there they become plerocercoids.

These plerocercoids can be found without capsules inside the fish, although surrounded by cystic connective tissue. Some are automatically encapsulated by being located in the muscles of the fish, this being the part most ingested by the final hosts of the parasite.

Definitive host

Humans, as well as certain mammals or fish birds, are the definitive hosts. The contaminated fish meat is consumed by the host and the plerocercoids rapidly develop into adult worms within the intestine. There they lay their first eggs after 2 to 6 weeks of infection and start a new biological cycle.

The  Diphyllobothrium latum , as well as most members of their species, has low specificity for the host. This means that humans can be infected by species that normally affect other mammals or birds and vice versa.

Symptoms it produces

Despite the large size of these parasites and the large areas they occupy in the host’s gastrointestinal tract, many infections are asymptomatic. About 20% of patients present with nonspecific symptoms such as abdominal pain or discomfort, diarrhea, and constipation.

Other symptoms can be fatigue, headache, allergic reactions, and a sore tongue when eating. Massive infestations can cause intestinal obstruction, cholangitis, and cholecystitis, especially because of small segments of the parasite that break off and migrate into the common bile duct and gallbladder.

Prolonged or severe Diphyllobothrium latum infection  can cause megaloblastic anemia due to parasite-mediated dissociation of intrinsic factor vitamin B12 within the intestinal lumen, rendering the vitamin unavailable to the host. About 80% of the vitamin B12 intake is absorbed by the worm.

Treatment

Antiparasitic

Diphyllobothrium latum adult worms  are easily treatable with Praziquantel, an anthelmintic drug that affects calcium within the parasite, paralyzing it and preventing it from attaching to the walls of the intestine.

This drug also alters the absorption of adenosine, so the worm cannot synthesize purines, being unable to grow and reproduce.

A single dose of 25 mg / kg has been shown to be highly effective against Diphyllobothrium latum . Another anthelmintic drug, niclosamide, is also effective against this parasite in its usual single dose of 2 grams by mouth, which can be administered in patients older than 6 years.

The adverse effects of these two drugs are not very severe and can be treated without major complications. The most important are: general malaise, dizziness, abdominal pain with or without nausea, fever and hives. However, all these symptoms are caused by the infection itself, so it is difficult to tell them apart.

Other treatments

The administration of vitamin B12 is necessary in those patients with megaloblastic anemia. Other supportive measures such as nutritional support and dietary recommendations are welcome; the symptomatic treatment is permanent with antipyretics, anti-inflammatories and gastric protectors.

Preventive measures are also essential. Sewage treatment plants and the use of adequate toilets and sanitary facilities represent the most effective sanitary measures to avoid water contamination.

The best prophylactic therapy is to avoid consuming raw, smoked, or pickled fish. Another alternative is freezing the fish.

Some authors suggest keeping fish for 24 to 48 hours at -18 ° C, and more stringent ones recommend -20 ° C for 7 days or -35 ° C for 15 hours to kill parasites.

References

  1. Scholz, Tomás and collaborators (2009). Update on the Human Broad Tapeworm (Genus Diphyllobothrium), Including Clinical Relevance. Clinical Microbiology Reviews, 22 (1): 146-160.
  2. Guttowa A. and Moskwa, B. (2005). The history of the exploration of the Diphyllobothrium latum life cycle. Wiadomosci parazytologiczne, 51 (4): 359-364.
  3. Von Bonsdorff, B. and Bylund, G. (1982). The ecology of Diphyllobothrium latum. Ecology of disease, 1 (1): 21-26.
  4. Rosas, Reinaldo and Weitzel, Thomas (2014). Diphyllobothrium latum. Chilean journal of infectology, 31 (2).
  5. Escobedo, Angel (2015). Diphyllobothrium.  Medical Microbiology and Parasitology, First Edition, Chapter 117, 361-364.
  6. Wikipedia (2018). Diphyllobothrium latum. Recovered from: es.wikipedia.org

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