Lab 4: Cestodes
Phylum: Platyhelminthes
The simplest animals that are bilaterally symmetrical and triploblastic (composed of three
fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body
cavity other than the gut (and the smallest free-living forms may even lack that!) and lack
an anus; the same pharyngeal opening both takes in food and expels waste. Because of
the lack of any other body cavity, in larger flatworms the gut is often very highly
branched in order to transport food to all parts of the body. The lack of a cavity also
constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far
from the outside, making a flattened shape necessary.
The greatest number of platyhelminthes are hermaphroditic or monoecious. The sexes
are separate in a few cases, such as blood flukes and a small number of tapeworms. The
reproductive structures are used more than any other structures for identification and
classification of parasitic flatworms.
There are currently four classes within the Phylum Platyhelminthes; Tubellarians
(Tubellarian flat worms), Digeneans (parasitic flukes), Monogeneans (parasitic flukes),
and the Cestodes (tapeworms).
The tapeworms:
Class: Cestoda
Tapeworms (Class Cestoda) are all endoparasitic
in nearly every species of vertebrate. This
adaptation to parasitism has resulted in the
complete loss of the intestine, with only a
vestigial oral sucker and pharynx remaining and a
tremendous increase in the capacity of the
reproductive system. In other words, tapeworms
lack a mouth and digestive system, and absorb
nutrients through the tegument.
Subclass Cestodaria
The subclass Cestodaria consists of monozoic (unsegmented) tapeworms, with a single
set of reproductive organs. No scolex is present and the shelled embryo contains ten
hooks. All are parasites of fish. The Cestodaria will not be studied in this course.
Subclass: Eucestoda
General tapeworm form and function
The true tapeworms are usually polyzoic (segmented), comprised of a scolex which
functions as a holdfast and a strobila that is divided into many proglottids. The scolex,
or head is the anterior end of the tapeworm and can be equipped with a variety of
attachment structures. The scolex may bear suckers, grooves, hooks, spines or any
combination of the above. Become familiar with the different attachment organs
associated with the tapeworm scolex.
In the strobila, each proglottid potentially contains one or more sets of both male and
female reproductive organs, making the tapeworms hermaphroditic. Proglottids are added
on in the “neck” region (by a process called strobillation) between the scolex and
strobila, and mature as they move down the body. The male system develops first and
anterior proglottids will often contain only male organs. Older proglottids will contain
both reproductive systems. The anatomy of gravid proglottids is obscured by developing
eggs. Eggs may be released through the uterine pore or the entire gravid proglottid may
break off (apolysis) releasing eggs as it breaks up. True tapeworms usually produce a
hexacanth embryo (6-hooked). You have slides and whole body preparations representing
2 orders of tapeworms: Pseudophyllidae and Cyclophyllidea.
Order Cyclophyllidea
Members of this order are common parasites in the
intestines of amphibians, reptiles, birds and mammals.
They are characterized by:
• a scolex that usually contains four suckers.
• a rostellum that is present or absent, and may
or may not be armed.
• a lateral genital pore
• a uterine pore is absent.
• vitelline gland is single, compact, and usually
posterior to the ovary.
• proglottids are often wider than long
Gravid segments generally leave the host’s body individually, but may also be released in
groups. Non-operculate eggs escape by rupture or disintegration of gravid proglottids
and develop into some form of bladderworm in an invertebrate or vertebrate intermediate
host. Infective eggs contain an oncosphere larva that bears 6 hooks.
They have a variety of intermediate host types, both invertebrate and vertebrate.
Depending on the cestode species, the larval stage in the vertebrate intermediate host may
be a cysticercus, strobilocercus, coenurus (multiceps), or hydatid. Know these!
• a cysticercus has a bladder-like form with an invaginated scolex located at one
end
• a coenerus is similar but bears multiple scoleces.
• In a hydatid, brood pouches develop within the bladder and gives rise to multiple
scolices (20 or so) and each hydatid cyst may contain thousands of scolices.
Larvae in invertebrate intermediate hosts (usually arthropods) are cystercercoids; these
have no bladder but a simple invaginated scolex. The scolex has 4 prominent suckers and
a terminal rostellum, which may or may not be armed with hooks.
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Family Taeniidae
The largest cyclophyllideans are in the family Taeniidae, and are the most medically
important tapeworms of humans. Most tapeworms in this family are large parasites of
mammals, although some species are found in birds.
These parasites are characterized by:
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a scolex consisting of 4 large suckers and a non-retractable rostellum that may be
armed with hooks.
The genital pores are irregularly alternating and the compact vitelline gland is
located posterior to the ovary.
The gravid uterus is median with lateral branches.
The eggshell is thick with radial striations.
The larval stage may be either a cysticercus, coenurus, strobilocercus, or hydatid.
Genus Taenia
There are several species of Taenia that humans are likely to encounter. These include
two species for which humans serve as the definitive host: Taenia saginata, the beef
tapeworm; and T. solium, the pork tapeworm. Several species of Taenia also infect dogs
and cats (e.g.,T. pisiformis), and humans are likely to encounter these when they note the
presence of these tapeworms' proglottids in their pets' feces.
All species of Taenia have similar life cycles. The adult tapeworm lives in the definitive
host's small intestine. Proglottids, which contain eggs, break off the posterior end of the
tapeworm, and these proglottids are either passed intact in the host's feces or they
dissolve in the host's intestine and eggs are passed in the feces. When a suitable
intermediate host ingests the eggs, the oncosphere larva is released and, with the aid of
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the embryonic hooks, penetrates the intestinal wall and enters the bloodstream. Upon
reaching the liver the oncosphere begins to develop into a cysticercus. Bladderworms
break out of the liver and attach to the mesenteries throughout the abdominal cavity. The
definitive host is infected when it eats an intermediate host infected with
cysticerci. Upon ingestion the scolex evaginates, attaches to the intestinal lining, the
bladder disintegrates, and the strobila is formed by the budding of the neck region.
As adults in the definitive host's small intestine, tapeworms rarely cause problems; in
exceptional cases the tapeworms might physically block the intestinal tract, due to their
large size, or proglottids might become lodged in the appendix and result in
appendicitis. The proglottids of Taenia are large and muscular. Occasionally single
proglottids or long chains of proglottids might crawl out of the anus of an infected
human. Understandably, most humans would find this quite disturbing, and it surely
would not be an appropriate topic for
"polite conversation."
Slide: Taenia pisiformis is a
cosmopolitan parasite of the dog and
other canids. Study the whole mount
of this species and note the scolex
with rostellum, four suckers, and
double row of hooks. Note the
difference in the general shape of the
immature, mature, and gravid
proglottids. Study a mature segment
and note the following: excretory
vessels, ovary, vitellaria, testes,
genital pore, cirrus pouch, vas
deferens, uterus, and vagina. All structures may not be visible in your specimen. Study
the immature segments and identify as many structures as possible. Note the branched
uterus in gravid proglottids.
Slide - Taenia eggs. These
are non-operculate so they have
no little door for the larvae to
exit through. They rupture to
release eggs. Since tapeworms
are parasites of terrestrial hosts,
a thick shell surrounds the
eggs. Note the radial lines.
Slide: Taenia solium cysticercus in muscle tissue
(cross-section). Taenia solium, the human pork
tapeworm, has humans as its only natural definitive
host. Humans can be the host of the cysticercus as well
as the adult. Humans become infected by ingesting
poorly cooked pork containing cysticerci. Once
ingested, the scolex evaginates in the small intestine
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and attaches to the wall of the gut where it strobilates. Gravid proglottids develop within
about 5 weeks. T. solium is the most dangerous tapeworms of humans because of the
possibility of self-infection with cysticerci.
Slide: Taenia sp. Cysticercus
Tapeworm (Taenia pisiformis)
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Genus Echinococcus
The genus Echinococcus contains the smallest tapeworms in the Taeniidae.
Echinococcus granulous is a parasite of carnivores, particularly canids, with the adult
living in the small intestine of the definitive host. It is one of the smallest tapeworms,
measuring less than 10 mm in total length.
Many mammals may serve as intermediate hosts, but herbivores, such as sheep, cattle,
and man, are most likely to become infected by eating the eggs on contaminated foliage.
Echinococcus eggs contain an oncosphere. Oncosphere larvae are released from the
eggs once in the stomach of an herbivore. Larval stages of this parasite represent the most
unusual of asexual types of reproduction among cestodes. Once the eggs hatch, the
oncosphere larvae is released and goes on to penetrate the gut wall, enter the host blood
stream and migrate to other organs (the liver and lungs being the usual sites for
development). By a very slow process, the oncosphere develops into a type of bladder
worm called a hydatid. The hydatid is a large tumor-like bladder filled with developing
scolices. Over the course of infection, the hydatid will develop a thick outer non-cellular
layer and a thin inner germinal layer and become a hydatid cyst. The inner layer
eventually produces the protoscolices that are infective to the definitive host.
Protoscolices are usually produced individually and accumulate within the fluid of the
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cyst, producing what is commonly called hydatid sand. Scolices may also be produced
within brood capsules. Brood capsules are small cysts that contain 20-30 protoscolices.
Each hydatid cyst is capable of producing thousands of individual scolices, each capable
of developing into an adult worm. In addition, daughter cysts, which also produce
scolices, may be produced either internally or externally. Hydatid cysts may attain the
size of softballs and in some cases even larger.
The hydatid cyst of E. granulosus is unilocular and can be surgically removed. When a
carnivore eats a hydatid, the cyst wall is digested releasing the protoscolices, which go on
to attach to the intestinal wall.
Slide – E. granulosus adult. Note that the body
consists of a scolex, one immature proglottid, one
mature proglottid, and one gravid proglottid.
Structures in the reproductive system are typical of
those found in other taeniids.
Slide – Tissue section of Echinococcus
granulosus hydatid cyst. Here you see the
outer wall of the cyst, 6 brood capsules each
containing several protoscolices.
Slide – Tissue section of Echinococcus
granulosus hydatid sand. These are
scolices.
Echinococcus multilocularis life cycle involves carnivores as definitive hosts and
rodents as intermediate hosts. The hydatid cyst of associated with Echinococcus
multilocularis is called a multilocular cyst. Instead of developing a thick laminated
layer and growing into large single cysts, this parasite has a thin outer wall that grows
and infiltrates into the surrounding tissues forming an irregular growth consisting of
many loculi, each with budding scolices. In addition, the protoscolices do not remain
with the cyst making it difficult to treat. Prognosis for those infected with multilocular
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hydatids is never very good. In humans and other non-natural hosts the multilocular
pockets often do not contain protoscolices, making the disease difficult to diagnose.
Slide: Tissue section of E. multilocularis
multilocular cyst. The cyst can resemble a
mass of bubbles all linked together. Human
infection with multilocular cysts is rare as
because the normal cycle is sylvatic rather
than urban and because humans do not seem
to be very good hosts.
Preserved Specimen – Rodent with
E. multilocularis.
Compare the level of organization of a
multilocular cyst with that of a unilocular
cyst. You are responsible for knowing the difference between unilocular and
multilocular cysts.
Family Dilepididae
Dilepididae tapeworms are parasites in birds, mammals, and occasionally reptiles.
Members of this family are characterized by:
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a retractable armed rostellum with circlets of hooks often similar in shape to rose
thorns.
reproductive organs may be single or double in each proglottid.
genital pores are lateral
gravid segments are longer than broad.
In many species the uterus is sacculate and when gravid, breaks up into egg sacks or
capsules. The larval stage is a cysticercoid and the intermediate host is some type of
insect.
Dipylidium caninum is a common parasite of dogs, cats, foxes, and occasionally humans.
It is probably the most common tapeworm of dogs, and is worldwide in distribution.
Gravid proglottids leave the definitive host with the feces. Eggs are released and
ingested by larval fleas, the usual intermediate host, where the cysticercoid larval stage
develops. Cysticercoids remain through the metamorphosis of the flea to the adult stage.
Definitive hosts become infected by ingesting the adult flea contain cysticercoids.
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Slide: Dipylidium caninum proglottid. Observe the
double set of reproductive organs in the mature proglottid. A
genital pore occurs on each side of the proglottid, hence the
name double-pored dog tapeworm. A bilobed ovary occurs
on each slide with two postovarian vitelline glands. Testes
are numerous and scattered throughout the middle of the
segment. Mature and gravid proglottids have a characteristic
“cucumber-seed or pumpkinseed shape.” Study a gravid
proglottid and note how the uterus fills the
entire segment and breaks down into
numerous egg capsules, each containing 8 to
15 eggs enclosed in a membrane.
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Slide: Dipylidium caninum scolex Note the several circles of
rose-thorn shape hooks on the rostellum.
Slide – Dipylidium
caninum eggs. Note that
theses are egg capsules containing many eggs.
Family Hymenolepididae
Hymenolepididae are common tapeworms in birds, but occur in mammals as well.
Larval stages of hymenolepid cestodes are cysticercoids and a number of arthropods
function as intermediate hosts.
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Hymenolepis spp.
Slide: Hymenolepis sp. Cystercercoids in
Hydra (intermediate host).
Preserved sample - Hymenolepis
diminuta is a cosmopolitan parasite of
rodents. The scolex has four suckers and an
unarmed rostellum. A long neck leads from
the scolex to the proglottids. Study the
diagram of a mature Hymenolepis proglottid
and note the three testes, lateral genital pore,
and the well-developed circus pouch. Again
the vitelline gland is postovarian in location.
Slide – Hymenolepis diminuta eggs are available for
study.
Family Anoplocephalidae
The anoplocephalid cestodes are large muscular worms found primarily in the intestine of
mammals, but also in reptiles and birds. Many of these tapeworms are of economic
importance.
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The scolex lacks a rostellum and the suckers are unarmed.
Proglottids are usually much wider than long, and sometimes contain two sets of
reproductive systems.
Preserved specimen - Moniezia expansa is a common tapeworm in sheep. They
have a simple scolex and wide proglottids, each with two sets of reproductive structures.
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The genital pores are marginal and the testes are spread throughout the segment, but
mainly posterior to and between the ovaries. These glands are arranged in a short row at
the posterior of each segment and stain rather intensely. Mites serve as intermediate
hosts.
Order Pseudophyllidae
Adult pseudophyllidean tapeworms are parasites of all classes of vertebrates, but fish are
their primary host. The life cycle involves a procercoid (larval stage often found in
copepods) and plerocercoid larval stage (found in fish).
Tapeworms in this order are characterized by:
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A bothriate scolex, i.e., possessing a dorsal and a ventral groove or bothrium.
A neck may be present or absent.
Testes and vitellaria are follicular and scattered throughout the proglottid.
The ovary is bilobed and posterior.
Genital pores are usually ventral and may be lateral or dorsal in some groups.
A uterine pore is present on the dorsal or ventral surface.
The eggs are usually operculate and contain a ciliated embryo, the coracidium.
Diphyllobothrium latum is commonly called the broad fish tapeworm. This cestode is
common in fish-eating carnivores in northern Europe, Russia, the Arctic, and the Great
Lakes areas of North America. There appears to be little host specificity for adults have
been recovered from the intestine of many canines, felines, mustelids, pinnipeds, bears,
and humans. In humans D. latum can be a serious pathogen, causing a pernicious
anemia. This is due to the worm absorbing large amounts of vitamin B12.
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Life cycle: Eggs escape as they are produced, and after a period of embryonation, the
operculum opens and the free-swimming coracidium emerges. Coracidia are ingested
by crustaceans (primarily copepods), the oncosphere penetrates the intestine, and the
procercoid develops in the hemocoel. When the crustacean is ingested by a fish, the
procercoid escapes and migrates to the muscles where the plerocercoid develops. The
definitive host becomes infected by eating raw or improperly cooked fish. A life cycle
diagram has been included on the previous page.
Slides: Study the slides showing the scolex and proglottids of D. latum. Note that
segments are both mature and gravid at the same time. The uterus containing eggs has a
characteristic rosette shape. Find the cirrus pouch and cirrus aperture that opens on the
anterior, midventral surface, and is surrounded by small papillae. The uterine pore is
posterior to this. The testes and vitellaria are both scattered throughout the parenchyma.
The vitellaria are smaller, darker, and obscure the larger, lighter-staining testes. The
ovary is bilobed and located at the posterior of the proglottid. Study the scolex, noting
the two bothria.
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Diagrams and Life cycles:
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Learning Objectives
1. Plathelmintes - Flatworms
- Why are they flat? Why are they special?
- Know that reproductive structures are used in id!
2. Subclass Eucestoda
- Polyzoic; body parts; Strobilation
- Hermaphrodites or monoecious; compare eggs
- Difference between Cyclo- & Pseudophyllidae
3. Order Cyclophyllidae
- General characteristics
4. Family Taeniidae
- General charcterristics
5. Genus Taenia
- Life cycle, hosts, stages, transmission, etc.
- Taenia pisiformis (scolex, immature, mature, gravid proglottid)
- Anatomy of mature proglottid
- Taenia eggs: non-operculate, radial lines (striated appearance), thick-shelled
- Cysticercus: cross section + whole mount; invaginated scolex
6. Genus Echinococcus
- Echinococcus granulosus
- Life cycle, host, transmission, pathology
- Visual id
- Tissue section: unilocular cyst, brood caspules, protoscolices
- Hydatid sand: protoscolices
- Echinococcus multilocularis: multilocular cyst (Why worse?)
7. Family Dilepididae
- General characteristics
8. Dipylidium caninum
- Life cycle, hosts, transmission, etc.
- Double-pored tapeworm!
- Double reproductive system, egg capsules containing many eggs, scolex
9. Family Hymenolepididae
- Life cycle
- Cysticercoid slide in invertebrate Hydra host
- Eggs
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10. Order Pseudophyllidae
- General characteristics
11. Diphyllobothrium latum
- Life cycle, transmission, host, stages, etc.
- Pathology
- Scolex: bothria
- Eggs: operculate, contain ciliated embryo (coracidium)
Vocabulary
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Monoecious
Dioecious
Endoparasitic
Polyzoic
Scolex
Strobila
Proglottids
Operculum
Coracidium
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Strobillation
Metacestode
Cysticercus
Coenurus
Hydatid cyst and sand
Cysticercoid
Evagination
Bothria
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Vitellaria
Gravid
Brood capsules
Unilocular cyst
Multilocular cyst
Protoscolex
Procercoid
Plerocercoid