Class Cestoda. Diphyllobothrium latum

Class Cestoda. Diphyllobothrium latum

The tapeworms, flatworms without a gut that live as ribbons of segments in the intestine of a vertebrate, their body, their larvae and their hosts; then Diphyllobothrium latum, the broad fish tapeworm, from its grooved scolex to the copepod and the perch it passes through, and the vitamin B12 it takes from people who eat raw fish.

Diphyllobothrium latum
On the far left, a short length of an adult Diphyllobothrium latum: a small scolex at the top, a thin neck, then a ribbon of proglottids that widen as they go back until they are far broader than long, each with a dark spot in its middle, the rosette of its uterus. Beside it, the scolex and neck drawn large: a long, flat head with a deep groove, a bothrium, down its face, then the neck and the first young proglottids behind it. At the top right, a mature proglottid drawn large from its ventral side, broader than long, its hind edge overhanging the next: on the left the small golden yolk glands, on the right the larger blue testes, in the middle the pink uterus coiled in loops either side of the midline into a rosette full of amber eggs, the two-lobed ovary near the hind edge, and near the front edge the blue cirrus sac at the genital pore, with the uterine pore just behind it. Along the foot, its forms outside the definitive host: an oval egg with a lid at one end and a small knob at the other, holding its egg cell among yolk cells; a round coracidium covered in cilia, the oncosphere inside it with its six hooks; a procercoid, a long larva with a round tail, the cercomer, that holds the hooks; and a plerocercoid, a longer, flat, finely wrinkled larva with a groove at its front end.

Point at a name or a part to light it.

Diphyllobothrium latum (now Dibothriocephalus latus), the broad fish tapeworm, grows 2 to 15 m long with 2000 to 5000 proglottids. It holds on by two grooves on its scolex, not suckers, and its proglottids are broader than long. Its eggs, 55 to 75 µm long, have a lid; in fresh water a ciliated coracidium hatches, a copepod eats it, a fish eats the copepod, and people are infected by the plerocercoid in raw or undercooked fish.

Simplified: not to scale: the adult, metres long, is drawn as a short length beside an enlarged scolex, an enlarged proglottid and the larvae, each at its own size; the adult's proglottids are far more numerous than drawn, and the ribbon is a short piece of it; the testes and the yolk glands both fill both side fields, the testes deep and the yolk glands near the surface; drawn one on each side, as taxonomists draw them; the excretory canals, the nerves, the vagina, the seminal receptacle and Mehlis' gland left out; the larvae drawn as simple outlines, their glands and inner tissues left out; the coracidium's hooks drawn larger than they are.

4 sources
  1. Centers for Disease Control and Prevention. DPDx: Diphyllobothriasis. Last reviewed 14 May 2019.
  2. Hernández-Orts JS, Kuzmina TA, Gomez-Puerta LA, Kuchta R. Diphyllobothrium sprakeri n. sp. (Cestoda: Diphyllobothriidae): a hidden broad tapeworm from sea lions off North and South America. Parasit Vectors. 2021;14:219.
  3. Kuchta R, Phillips AJ, Scholz T. Diversity and biology of Spirometra tapeworms (Cestoda: Diphyllobothriidea), zoonotic parasites of wildlife: a review. Int J Parasitol Parasites Wildl. 2024;24:100947.
  4. Wikipedia. Cestoda.

At a glance

  1. Tapeworms (class Cestoda) are flatworms with no mouth or gut: they absorb their food through their surface. The adult lives in the small intestine of a vertebrate, its definitive host.
  2. A tapeworm is a scolex that holds on, a neck that keeps making new segments, and a strobila of proglottids, each with its own male and female organs, growing from immature to mature to gravid, a bag of eggs, as it moves back.
  3. The tapeworms of medicine hold on in two ways: Diphyllobothrium with two grooves (bothria); Taenia, Echinococcus and Hymenolepis with four suckers, often with a crown of hooks.
  4. Diphyllobothrium latum, the broad fish tapeworm, is 2 to 15 m long. Its eggs hatch in water, and its larvae pass through a copepod and a fish before a person eats the fish raw or undercooked.
  5. Most infections cause no symptoms, but the worm takes up vitamin B12 and can cause megaloblastic anaemia. A single dose of praziquantel cures it; cooking or freezing fish prevents it.

The tapeworms

The Cestoda, the tapeworms, are a class of the flatworms with about 6,000 species, all of them parasites. The adults live in the gut of vertebrates, often for years; the larvae live in the tissues of other animals, the intermediate hosts, which can be insects, crustaceans or other vertebrates. Probably every vertebrate can host at least one species.

They take the flatworm's life of a parasite further than the flukes do. A tapeworm has no mouth and no gut: it lies in the host's intestine and absorbs its food through its surface, the tegument, through which it also exchanges gases. The tegument protects it from the host's digestive enzymes.

The body

PartWhat it isWhat it does
Scolexthe head, often tiny beside the rest, with two grooves (bothria) or four suckers, and in many species a rostellum with hooksanchors the worm to the gut wall
Necka short, unsegmented region behind the scolexbuds off new proglottids all the time
Strobilathe chain of proglottids, from a few to several thousandmakes the eggs

The proglottids are not true segments, like those of an earthworm: they are units the neck keeps making, pushed back by the newer ones behind them. So the youngest, immature proglottids lie just behind the neck; further back, each one grows a complete set of male and female organs and becomes mature; at the end of the worm, the gravid proglottids are little more than a uterus full of eggs.

  • Hermaphrodites. Every mature proglottid has one or more testes with a vas deferens and a cirrus, and an ovary, yolk glands and a uterus, opening at a genital pore. Cross-fertilisation is the rule, but a worm alone in its host can fertilise itself.
  • Excretion. Flame cells (protonephridia) drain into four collecting canals that run the length of the worm.
  • Nerves. A cerebral ganglion in the scolex, and two lateral nerve cords along the strobila. Its senses are touch and a chemical sense, with the most nerve endings round the genital pore.
  • The neck regrows the worm. If treatment removes the strobila but leaves the scolex and neck, the whole worm can grow back.

Two kinds of tapeworm

The tapeworms of people belong to two groups, which a student should be able to tell apart:

Diphyllobothriids (Diphyllobothrium)Cyclophyllids (Taenia, Echinococcus, Hymenolepis)
Scolextwo long grooves, the bothriafour suckers, often a rostellum with hooks
Eggswith a lid (operculum), passed unembryonated; they must mature in waterwith a six-hooked oncosphere inside, infective as soon as they are passed
How the eggs leavethrough a uterine pore, from the living wormin gravid proglottids that break off, or as the proglottids break up
Life cyclein water, through a crustacean and a fishon land, through a mammal or an insect

The larvae and the hosts

Inside every tapeworm egg is the first larva, the oncosphere or hexacanth, an embryo with six hooks. What it becomes depends on the species:

LarvaWhereWhat it is likeWhose
Coracidiumwateran oncosphere in a ciliated coat, swimmingDiphyllobothrium
Procercoida copepod's body cavitya small, solid larva with a tail-like knob carrying the six hooksDiphyllobothrium
Plerocercoid (sparganum)a fish's muscle and organsa solid, elongated, unsegmented larva, its scolex at the front end: infectiveDiphyllobothrium
Cysticercusthe muscle of cattle or pigs, or a person's tissuesa fluid-filled bladder with one scolex turned in on itselfTaenia
Cysticercoida villus, or a beetlea tiny, thick-walled larva with its scolex turned in; tailed in the beetleHymenolepis
Hydatid cystthe liver or lungs of sheep, or of peoplea large cyst whose wall buds brood capsules with many protoscolicesEchinococcus

Adult tapeworms are so well suited to their host that they rarely cause much harm. The larvae are the danger: in people, a cysticercus in the brain or a hydatid cyst in the liver can kill.

Diphyllobothrium latum

Diphyllobothrium latum, the broad fish tapeworm, is one of the largest parasites of people. It has recently been renamed Dibothriocephalus latus, with several of its relatives, but most textbooks keep the older name. Besides people, bears, dogs, cats and other mammals that eat fish can carry it.

The adult

The adult is 2 to 15 m long, a chain of 2,000 to 5,000 proglottids, and lives in the small intestine. It can shed up to a million eggs a day.

  • Scolex: small, long and flattened, with two bothria, long sucking grooves, one on each flat side; no suckers and no hooks.
  • Neck: short and unsegmented, the source of the proglottids.
  • Proglottids: broader than long, which gave the worm its name, latum, "broad"; they overlap a little, each over the one behind.

The parts of a mature proglottid, which the figure shows:

  • Male organs: many small testes in two fields at the sides; a vas deferens leads to the cirrus sac in the middle of the proglottid.
  • Female organs: a bilobed ovary near the proglottid's hind margin; yolk glands, many small follicles at the sides; and a coiled uterus in the middle, shaped like a rosette, which tells the proglottid of Diphyllobothrium from that of Taenia.
  • Pores: the genital pore opens in the middle of the ventral surface, and the uterine pore, from which the eggs are laid, just behind it.

The egg

The egg is oval, 55 to 75 µm by 40 to 50 µm, with a lid (operculum) at one end and a small knob at the other. It is passed unembryonated, and cannot infect anyone: it must first mature in fresh water.

The life cycle

Diphyllobothrium latum: the life cycle
A map of the life cycle of Diphyllobothrium latum, the broad fish tapeworm. Across the top, the patient's pale panel, the definitive host: the adult tapeworm lives in the small intestine and its eggs appear in the faeces 5 to 6 weeks after infection. Down the left, on the white page, the eggs are passed unembryonated, mature in fresh water in 18 to 20 days, and a ciliated coracidium hatches and swims. Along the foot, three panels from left to right. A copepod, the first intermediate host, eats the coracidium, which becomes a procercoid in its body cavity. A perch or a ruffe, the second intermediate host, eats the copepod, and the procercoid becomes a plerocercoid in its muscle and organs. A pike or a burbot eats the perch and gathers its plerocercoids unchanged in its own muscle: a paratenic host. From both fish an arrow climbs to the patient, who eats them raw or undercooked.

Point at a name to light its pathway.

The broad fish tapeworm passes through two aquatic hosts before it reaches people: a copepod, then a fish. Big predatory fish such as pike carry plerocercoids they took from the small fish they ate, without the larvae developing further. People are infected by eating raw, marinated or undercooked freshwater fish.

Simplified: a map, not anatomy: the hosts drawn as icons and the parasites drawn small, nothing to scale; one copepod, one perch and one pike drawn for the several copepods and fish that can carry it; dogs, cats, bears and other fish-eating mammals are definitive hosts too; each larva's way out of the gut of the host that eats it, into that host's tissues, drawn as one arrow.

2 sources
  1. Centers for Disease Control and Prevention. DPDx: Diphyllobothriasis. Last reviewed 14 May 2019.
  2. Kovalov V, Trubenová B. The life cycle of the tapeworm Dibothriocephalus latus in the Alpine region: what we know, what we miss, and how it is maintained. Int J Parasitol Parasites Wildl. 2026;31:101294.

The eggs pass in the stool, and in fresh water each matures in 18 to 20 days into a coracidium, an oncosphere in a coat of cilia, which hatches through the lid and swims.

  1. The copepod, the first intermediate host. A small crustacean of the plankton, such as Eudiaptomus or Cyclops, eats the coracidium, which becomes a procercoid in its body cavity over 16 to 18 days.
  2. The small fish, the second intermediate host. A perch or a ruffe eats the copepods, and the procercoid moves into its muscle and organs, where it becomes a plerocercoid, the infective form.
  3. The large fish, a paratenic host. Small fish are not always eaten raw, though around the Alpine lakes raw marinated perch is a known source. A pike or a burbot that eats them takes the plerocercoids into its own muscle unchanged, and they build up there, fish after fish.
  4. The definitive host. A person eats the fish raw or undercooked; the plerocercoid attaches in the small intestine and grows into an adult. Eggs appear in the stool 5 to 6 weeks after infection.

D. latum is most common in the cold, circumpolar north. In Europe it is found in the north and around the lakes along the Alps, such as Lake Geneva, where raw marinated perch infected several people in 2006. Sewage treatment has made the infection less common than it was, but not everywhere: around the Alpine lakes the cycle goes on.

Diphyllobothriasis

Diphyllobothriasis may affect up to 20 million people worldwide, and can last as long as 25 years if it is not treated.

  • Most infections cause no symptoms. Some cause mild abdominal discomfort, diarrhoea or weight loss, and some patients see lengths of the worm in their stool.
  • Vitamin B12 deficiency. The worm takes up vitamin B12 from the food in the gut, competing with its host, and occasionally causes a deficiency of it, with megaloblastic anaemia.
  • Rarely: a heavy infection blocks the bowel, or proglottids wander into the bile ducts and cause cholecystitis or cholangitis.

Diagnosis, treatment and prevention

  • Stool microscopy: the operculated eggs, or broad proglottids in the stool. A blood count looks for megaloblastic anaemia.
  • Treatment: a single dose of praziquantel; niclosamide is the alternative where it is available. Vitamin B12 corrects the anaemia. A stool sample a month later confirms the cure.
  • Prevention: cook freshwater fish to at least 63 °C inside, or freeze it at −20 °C or below for 7 days. Cold-smoking, salting, drying and marinating do not make it safe, as the Lake Geneva cases show.

Check yourselfAnswer, then say how sure you were: a lucky guess still counts as a weak point.

1Which part of a tapeworm makes its new proglottids?

2How does a tapeworm feed?

3How does the scolex of Diphyllobothrium latum hold on to the gut wall?

4In which order does Diphyllobothrium latum pass through its hosts?

5Why can Diphyllobothrium latum cause megaloblastic anaemia?