''Diphyllobothrium'' is a genus of

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cesto ...

s which can cause diphyllobothriasis in humans through consumption of raw or undercooked fish. The principal

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

causing diphyllobothriasis is ''D. latum'', known as the broad or fish tapeworm, or broad fish tapeworm. ''D. latum'' is a pseudophyllid

cestode Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...

that infects

fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...

and mammals. ''D. latum'' is native to Scandinavia, western Russia, and the Baltics, though it is now also present in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

, especially the Pacific Northwest. In Far East Russia, ''D. klebanovskii'', having Pacific salmon as its second intermediate host, was identified. Other members of the genus ''Diphyllobothrium'' include '' D. dendriticum'' (the salmon tapeworm), which has a much larger range (the whole northern hemisphere), ''D. pacificum'', ''D. cordatum'', ''D. ursi'', ''D. lanceolatum'', ''D. dalliae'', and ''D. yonagoensis'', all of which infect humans only infrequently. In Japan, the most common species in human infection is ''D. nihonkaiense'', which was only identified as a separate species from ''D. latum'' in 1986. More recently, a molecular study found ''D. nihonkaiense'' and ''D. klebanovskii'' to be a single species.

Morphology

The adult worm is composed of three fairly distinct morphological segments: the scolex (head), the neck, and the lower body. Each side of the scolex has a slit-like groove, which is a bothrium for attachment to the intestine. The scolex attaches to the neck, or proliferative region. From the neck grow many proglottid segments which contain the reproductive organs of the worm. ''D. latum'' is the longest

in humans, averaging ten meters long. Unlike many other tapeworms, ''Diphyllobothrium'' eggs are typically unembryonated when passed in human feces. In adults,

proglottid Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...

s are wider than they are long (hence the name ''broad tapeworm''). As in all pseudophyllid cestodes, the genital pores open midventrally.

Life cycle

Adult tapeworms may infect humans, canids, felines,

bear Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the No ...

pinniped Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the ...

s, and

mustelid The Mustelidae (; from Latin ''mustela'', weasel) are a family of carnivorous mammals, including weasels, badgers, otters, ferrets, martens, minks and wolverines, among others. Mustelids () are a diverse group and form the largest family in t ...

s, though the accuracy of the records for some of the nonhuman species is disputed. Immature eggs are passed in feces of the mammal host (the definitive host, where the worms reproduce). After ingestion by a suitable freshwater

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

such as a

copepod Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have ...

(the first

intermediate host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' ( symbiont). The guest is typically provided with nourishment and shelter. Examples include ...

), the coracidia develop into procercoid

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e. Following ingestion of the copepod by a suitable second intermediate host, typically a minnow or other small freshwater fish, the procercoid larvae are released from the crustacean and migrate into the fish's flesh where they develop into a plerocercoid larvae (sparganum). The plerocercoid larvae are the infective stage for the definitive host (including humans). Because humans do not generally eat undercooked minnows and similar small freshwater fish, these do not represent an important source of infection. Nevertheless, these small second intermediate hosts can be eaten by larger predator species, for example

trout Trout are species of freshwater fish belonging to the genera '' Oncorhynchus'', '' Salmo'' and '' Salvelinus'', all of the subfamily Salmoninae of the family Salmonidae. The word ''trout'' is also used as part of the name of some non-salm ...

perch Perch is a common name for fish of the genus ''Perca'', freshwater gamefish belonging to the family Percidae. The perch, of which three species occur in different geographical areas, lend their name to a large order of vertebrates: the Per ...

walleye The walleye (''Sander vitreus'', synonym ''Stizostedion vitreum''), also called the yellow pike or yellow pickerel, is a freshwater perciform fish native to most of Canada and to the Northern United States. It is a North American close relat ...

, and pike. In this case, the sparganum can migrate to the musculature of the larger predator fish and mammals can acquire the disease by eating these later intermediate infected host fish raw or undercooked. After ingestion of the infected fish, the plerocercoids develop into immature adults and then into mature adult tapeworms which will reside in the

small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

. The adults attach to the intestinal

mucosa A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...

by means of the two bilateral grooves ( bothria) of their scolices. The adults can reach more than 10 m (up to 30 ft) in length in some species such as ''D. latum,'' with more than 3,000 proglottids. One or several of the tape-like proglottid segments (hence the name tapeworm) regularly detach from the main body of the worm and release immature eggs in freshwater to start the cycle over again. Immature eggs are discharged from the proglottids (up to 1,000,000 eggs per day per worm) and are passed in the feces. The incubation period in humans, after which eggs begin to appear in the feces is typically 4–6 weeks, but can vary from as short as 2 weeks to as long as 2 years.http://web.gideononline.com/web/epidemiology/

Disease

Diphyllobothriasis is considered a parasitic, zoonotic infection. ''D. latum'' causes a wide spectrum of disease and severity. The tapeworm induces changes in the concentration of several immunomodulators in the host. It can also cause structural changes in the GI tract as it modulates neuroendocrine responses and enhances secretion and gut motility. Damage may also come from the body's immune response against the worm and its millions of eggs (around 1 million/day) mediated by mast cells, eosinophilic cell degranulations resulting to inflammatory

cytokine Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in au ...

s. Diphyllobothriosis is considered as the most important fish-borne

zoonosis A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...

with up to 20 million individuals infected. ''D. latum'' causes B12 deficiency in humans leading to megaloblastic or

pernicious ''Pernicious'' is a Thai-American supernatural horror film directed by James Cullen Bressack, who also wrote the story along with co-writer Taryn Hillin. The film stars Ciara Hanna, Emily O'Brien, and Jackie Moore. Cast * Ciara Hanna as A ...

anemia. The worm absorbs around 80% of dietary B12 and prolonged infection can also cause abdominal pain, mechanical obstruction, and symptoms of iron deficiency anemia. Patients with prolonged infection of D. latum must also undergo B12 supplementations along with anti-parasitics such as

niclosamide Niclosamide, sold under the brand name Niclocide among others, is an anthelmintic medication used to treat tapeworm infestations, including diphyllobothriasis, hymenolepiasis, and taeniasis. It is not effective against other worms such as flukes ...

or praziquantel.Vuylsteke P, Bertrand C, Verhoef GE, Vandenberghe P. Case of megaloblastic anemia caused by intestinal taeniasis. Ann Hematol. 2004 Jul;83(7):487-8. doi: 10.1007/s00277-003-0839-2. Epub 2004 Jan 17. PMID 14730392.

References

Sources

* * * * Bonsdorff, B von: Diphyllobothriasis in Man. Academic Press, London, 1977 *Keas, B. E: Microscopy - Diphyllobothrium latum. Michigan State University, East Lancing, 1999

External links