Diphyllobothriasis
Encyclopedia
Diphyllobothrium is a genus of tapeworm which can cause Diphyllobothriasis
in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriosis is Diphyllobothrium latum, known as the broad or fish tapeworm, or broad fish tapeworm. D. latum is a pseudophyllid cestode that infects fish
and mammals. D. latum is native to Scandinavia, western Russia, and the Baltics, though it is now also present in North America
, 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 Diphyllobothrium 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. It was indicated to be synonymous to D. klebanovskii from the molecular study.
In adults, proglottids are wider than they are long (hence the name broad tapeworm). As in all pseudophyllid cestodes, the genital pores open midventrally.
s, bear
s, pinniped
s, and mustelids, 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
such as a copepod
(the first intermediate host
), the coracidia develop into procercoid larva
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
, perch
, and walleyed 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 adults attach to the intestinal mucosa by means of the two bilateral grooves (bothria
) of their scolex. 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 tape-worm) regularly detach from the main body of the worm and release immature eggs in fresh water 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. The tapeworm can live up to 20 years.
deficiency due to the parasite absorbing 80% or more of the host’s B12 intake, and a megaloblastic anemia indistinguishable from pernicious anemia
. The anemia can also lead to subtle demyelinative neurological symptoms (subacute combined degeneration of spinal cord
). Infection for many years is ordinarily required to deplete the human body of vitamin B-12 to the point that neurological symptoms appear.
can be performed on samples of purified eggs, or native fecal samples following sonication
of the eggs to release their contents.
, 5–10 mg/kg PO once for both adults and children. An alternative treatment is Niclosamide
, 2 g PO once for adults or 50 mg/kg PO once. Another interesting potential diagnostic tool and treatment is the contrast medium, Gastrografin, introduced into the duodenum, which allows both visualization of the parasite, and has also been shown to cause detachment and passing of the whole worm.
Diphyllobothriasis
Diphyllobothrium is a genus of tapeworm which can cause Diphyllobothriasis in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriosis is Diphyllobothrium latum, known as the broad or fish tapeworm, or broad fish tapeworm. D. latum is a pseudophyllid...
in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriosis is Diphyllobothrium latum, known as the broad or fish tapeworm, or broad fish tapeworm. D. latum is a pseudophyllid cestode that infects fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and mammals. D. latum is native to Scandinavia, western Russia, and the Baltics, though it is now also present in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, 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 Diphyllobothrium 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. It was indicated to be synonymous to D. klebanovskii from the molecular study.
History
The fish tapeworm has a long documented history of infecting people who regularly consume fish and especially those whose customs include the consumption of raw or undercooked fish. In the 1970s, most of the known cases of diphyllobothriasis came from Europe (5 million cases), and Asia (4 million cases) with fewer cases coming from North America and South America, and no reliable data on cases from Africa or Australia. Interestingly, despite the relatively small number of cases seen today in South America, some of the earliest archeological evidence of diphyllobothriasis comes from sites in South America. Evidence of Diphyllobothrium spp. has been found in 4,000-10,000 year old human remains on the western coast of South America. There is no clear point in time when Diphyllobothrium latum and related species were “discovered” in humans, but it is clear that diphyllobothriasis has been endemic in human populations for a very long time. Due to the changing dietary habits in many parts of the world, autochthonous, or locally-acquired, cases of diphyllobothriasis have recently been documented in previously non-endemic areas, such as Brazil. In this way, diphyllobothriasis represents an emerging infectious disease in certain parts of the world where cultural practices involving eating raw or undercooked fish are being introduced.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 (tentacle) for attachment to the intestine. The scolex attaches to the neck, or proliferative region. From the neck, grows many proglottid segments which contain the reproductive organs of the worm. D. latum is the longest tapeworm in humans, averaging ten meters long. Adults can shed up to a million eggs a day.In adults, proglottids 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, felineFelidae
Felidae is the biological family of the cats; a member of this family is called a felid. Felids are the strictest carnivores of the thirteen terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily pinnipedia are as carnivorous as the...
s, bear
Bear
Bears are mammals of the family Ursidae. Bears are classified as caniforms, or doglike carnivorans, with the pinnipeds being their closest living relatives. Although there are only eight living species of bear, they are widespread, appearing in a wide variety of habitats throughout the Northern...
s, pinniped
Pinniped
Pinnipeds or fin-footed mammals are a widely distributed and diverse group of semiaquatic marine mammals comprising the families Odobenidae , Otariidae , and Phocidae .-Overview: Pinnipeds are typically sleek-bodied and barrel-shaped...
s, and mustelids, 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
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
such as a copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
(the first intermediate host
Intermediate host
A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which some developmental stage is completed. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host...
), the coracidia develop into procercoid larva
Larva
A larva 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
Plerocercoid
Plerocercoid refers to last larval form found in the second intermediate host of many Cestoda with aquatic life cycles....
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
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
, perch
Perch
Perch is a common name for fish of the genus Perca, freshwater gamefish belonging to the family Percidae. The perch, of which there are three species in different geographical areas, lend their name to a large order of vertebrates: the Perciformes, from the Greek perke meaning spotted, and the...
, and walleyed 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
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
. The adults attach to the intestinal mucosa by means of the two bilateral grooves (bothria
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
) of their scolex. 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 tape-worm) regularly detach from the main body of the worm and release immature eggs in fresh water 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. The tapeworm can live up to 20 years.
Clinical symptoms, including occasional parasite-induced B12 deficiency
Symptoms of diphyllobothriasis are generally mild, and can include diarrhea, abdominal pain, vomiting, weight loss, fatigue, constipation and discomfort. Approximately four out of five cases are asymptomatic and may go many years without being detected. In a small number of cases, this leads to severe vitamin B12Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...
deficiency due to the parasite absorbing 80% or more of the host’s B12 intake, and a megaloblastic anemia indistinguishable from pernicious anemia
Pernicious anemia
Pernicious anemia is one of many types of the larger family of megaloblastic anemias...
. The anemia can also lead to subtle demyelinative neurological symptoms (subacute combined degeneration of spinal cord
Subacute combined degeneration of spinal cord
Subacute combined degeneration of spinal cord, also known as Lichtheim's disease, refers to degeneration of the posterior and lateral columns of the spinal cord as a result of vitamin B12 deficiency , vitamin E deficiency or Friedrich's ataxia...
). Infection for many years is ordinarily required to deplete the human body of vitamin B-12 to the point that neurological symptoms appear.
Diagnosis
Diagnosis is usually made by identifying proglottid segments, or characteristic eggs in the feces. These simple diagnostic techniques are able to identify the nature of the infection to the genus level, which is usually sufficient in a clinical setting. However, when the species needs to be determined (in epidemiological studies, for example), restriction fragment length polymorphisms can be effectively used. PCRPolymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
can be performed on samples of purified eggs, or native fecal samples following sonication
Sonication
thumb|right|A sonicator at the [[Weizmann Institute of Science]] during sonicationSonication is the act of applying sound energy to agitate particles in a sample, for various purposes. In the laboratory, it is usually applied using an ultrasonic bath or an ultrasonic probe, colloquially known as...
of the eggs to release their contents.
Treatment
Upon diagnosis, treatment is quite simple and effective. The standard treatment for diphyllobothriasis, as well as many other tapeworm infections is a single dose of PraziquantelPraziquantel
Praziquantel is an anthelmintic effective against flatworms. Praziquantel is not licensed for use in humans in the UK; it is, however, available as a veterinary anthelmintic, and is available for use in humans on a named-patient basis....
, 5–10 mg/kg PO once for both adults and children. An alternative treatment is Niclosamide
Niclosamide
Niclosamide is a teniacide in the anthelmintic family especially effective against cestodes that infect humans...
, 2 g PO once for adults or 50 mg/kg PO once. Another interesting potential diagnostic tool and treatment is the contrast medium, Gastrografin, introduced into the duodenum, which allows both visualization of the parasite, and has also been shown to cause detachment and passing of the whole worm.