Cestodes, debojit

Presentation
on
“Cestodes”
By :
Debojit Dekari
AAHM
CIFE, Mumbai

INTRODUCTION
❖ Commonly known as tapeworm.
❖ Cestodes derived from latin word “cestus” means belt or girdle.
❖ Flat and ribbon shaped body.
❖ Symmetrical & Dorsoventrally flattened.
❖ Lack an anus, specialized skeletal, circulatory system.
❖ Exclusively Endoparasite .
❖ Need at least one intermediate host.
❖ Adults live in the digestive tract of vertebrates and juveniles in both
vertebrate and invertebrate hosts.
❖ Body segmented (except Caryophyllidea)

❖ Devoid of epidermis, mouth and digestive tract.
❖ Nourishment is obtained through the body surface from the intestine of the host.
❖ Indirect life cycle.
❖ Excretory organ flame cells(protonephridia).
❖ Predominantly hermaphrodites.
❖ Mostly exhibit a high degree of host specificity.
❖ All possess the same general body plan

Cestoda
Tapeworms
• all endoparasitic (nearly every species of vertebrate)
• no mouth, no digestive system (only vestigal sucker and pharynx remaining)
• large reproductive system!
• absorb nutrients through tegument
SUB –CLASS
OF
CESTODA
CESTODARIA
(MONOZOIC -
unsegmented)
EUCESTODA
(POLYZOIC –
SEGMENTED)

Taxonomic Classification
Phylum : Platyhelminthes
Class: Cestoda
Sub – class : Eucestoda (True cestodes)
Order: Caryophyllidea
Pseudophyllidea
Proteocephalidea
Cyclophyllidea

Some important pathogenic fish tapeworms

Morphology
➢ attachment organ (head),consist of suckers, grooves,
hooklets.
➢ 3 suckers
➢ hooklets: These hooks are usually grouped at the apical
end of the scolex on a protrusible rostellum
➢ germinal portion of the parasite, the area of
proliferation from which the Proglottids of the
Strobilla grow.
➢ composed of a series of immature,mature,gravid
Proglottids.
➢ Proglottids – each contain a complete set of male and
female reproductive organs.
➢ Each Proglottids is a complete reproductive unit.
✓ The body of the adult tapeworm is divided into
three regions

SCOLEX
 Scolex with holdfast organelles (suckers, grooves, hooks, spines) -
3 types of sucking depressions:
 I - Bothria: shallow sucking grooves (Pseudophyllidea)
 ii- Bothridia: 4 leaflike flexible structures (Tetraphyllidea)
 iii- True suckers/acetabula (Cyclophyllidea)
❖ No special attachment organ: eg. Caryophyllidea
❖ Rostellum: protruding dome-shaped area on anterior end of
scolex
BOTHRIA
BOTHRIDIA
ACETABULA

➢ Tapeworms are hermaphrodite each proglottids contains both male
and female gonads.
➢ Mature proglottids contains a genital pore laterally (on the side)

The anterior, convoluted tube, which branches in the middle
of the proglottid, is the sperm duct.
It branches to the different testes in the segments. The
posterior duct is the vagina, which connects to the ovaries via
the oviducts.
MORPHOLOGY

STROBILA
❑ As new proglottids are formed from the neck region, they push the older ones
progressively posteriad, creating a chain of proglottids - the strobila.
❑ The asexual process of forming segments is termed strobilation.
The stroblia can be loosely subdivided into 3 regions:
➢ Immature
➢ Mature
➢ Gravid proglottids

Eucestoda
True tapeworms
REPRODUCTIVE SYSTEM
Pseudophyllidae (Diphyllobothrium latum)Order: Cyclophyllidae (Taenia spp.);

REPRODUCTION SYSTEM
• Male organs: develop before the female organs and also degenerates before the
female organs mature.
• Male reproductive organs – testes, sperm ducts(vas deferens) and a muscular cirrus
• consists of an ovary, vitelline glands, a Mehlis’ gland, and uterus, all these together
forms egg
• The large, gravid proglottids with developing and matured eggs is at the posterior end
of the body
• Where as the immature proglottids are closer to the scolex(anterior part)
• These eggs passes into uterus after fertilisation
MALE SYSTEM
FEMALE SYSTEM

Eucestoda
True tapeworms
Order: Pseudophyllidae
• bothriate scolex (possessing dorsal and ventral groove = bothrium)
• Testes and vitellaria scattered throughout the proglottid
• The ovary is bilobed and posterior. Genital pores ventral
• An uterine pore present on dorsal or ventral surface
• Eggs are operculate and contain ciliated embryo, the coracidium
• Adult parasites of all classes of vertebrates, but fish are their primary host.
• The life cycle involves procercoid (larval stage often found in copepods) and plerocercoid
larval stage (found in fish)
→ Diphyllobothrium latum (broad fish tapeworm)
Can be a serious pathogen, causing a pernicious anemia in human due to the worm
absorbing
large amounts of vitamin B12

Eucestoda
True tapeworms
Order: Cyclophyllidea
• scolex that usually contains four suckers; rostellum is present or absent (may be armed)
• The genital pore is usually lateral; Vitelline gland is single, compact, and posterior to the ovary.
• Eggs develop into some form of bladder worm in invertebrate or vertebrate intermediate host.
• Common tapeworms of birds and mammals, although some are found in reptiles.
• Infective eggs contain an oncosphere larva that bears 6 hooks.
• Variety of intermediate host types, both invertebrate and vertebrate

Pseudophyllidean Cyclophyllidean
Head Bears 2 slit like grooves Bears 4 cup like suckers
Uterus No branching, rosette shaped convoluted
tubule
Branching may or may not
be present
Uterine pore Present Absent
Common genital pore Ventral, in the midline Lateral
Eggs Operculated giving rise to ciliated larvae Not operculated, do not give
rise to ciliated larvae
Life cycle stages Egg → Coracidium → procercoid →
pleroceroid(Sparganum) →adult worm
Egg → hexacanth →
metacestode stage →adult
worm
Pseudophyllidean vs Cyclophyllidean cestodes

Pseudophyllidean vs Cyclophyllidean cestodes

Parasite Morphology
❖ Only common features between the caryophyllids and cestodarians is their monozoic body
plan.
❖ Plerocercoids of Schistocephalus and ligula distinguished by lack of segmentation in
Ligula (Conspicuous in Dchistocephalus).
❖ Plerocercoids of Triaenophorus are unique in that they bear tricuspid hooks which vary in
size, depending on the species.
❖ External tegumental morphology i.e. wrinkled versus smooth tegument and the form /shape
of the bothrial grooves for identification of Diphyllobothrium spp.
❖ The tetrahynchideans are readily seprated from tetraphyllideans by the presence of spiny
proboscides in the former.
❖ Plerocercoids of Schistocephalus and Ligula develop thicker outer tegumental layers,
surface ridge systems and microtriches characteristics of each species.

The plerocercoid larva of the cestode Ligula
intestinalis complete with scale (in mm).

Parasite Morphology
❖ Pseudophyllidean plerocercoids have revealed gland cells in the anterior end of the body
(head and bothria) in Eubothrium and Triaenophorus which release their contents via ducts.
❖ Unique structures on the tegument of the holdfasts of some adult tetraphyllideans
(Echeneibothrium, Phyllobothrium and Acanthobothrium may be specialized microtriches
with osmiophilic ends and present evidence that the myzorhynchus apical pad of
echeneibothrium is involved in direct nutrient uptake from the host.

Life Cycle of Cestodes
✓ Tapeworms usually release eggs that complete embryonation in water and
commonly hatch into ciliated, motile and short – lived larvae called coracidia.
✓ Non-motile embryo possess egg envelopes modified to maintain position ,
thereby enhancing the chance of being ingested by invertebrates .
✓ In the hoemocoel (body cavity) of the invertebrate host the coracidium
differentiates and develops into a procercoid stage.
✓ Migration of the procercoid and its differentiation into a plerocercoid in fish
muscle.
✓ Transmission to the definitive host (fish, bird or mammal is through ingestion of
the infected intermediate host and can involve consumption of carrion (natural
mortality) or offal left by sports and commercial fishermen.

Life Cycle of Cestodes
✓ Some parasites , e.g Caryophyllidea , are more specific in their invertebrate hosts
than others , e.g . Pseudophyllidea.
✓ Tapeworms are more specific to their definitive hosts.
✓ Most infections of fish occur during the summer and early autumn and adult
tapeworms grow and mature during late spring and early summer.
✓ On spring and early summer most tapeworms reach sexual maturity and release
eggs; this is the time when populations of the invertebrate hosts reach their peak.
✓ Differentiation into procercoids is also rapid due to high water temperature and
there is sufficient time for infections of fish to occur prior to winter.
✓ Adult tapeworms may be shed during the summer with new recruitment in the late
summer and early autumn.

Possible life cycle of cestodes

Life Cycle of Important Cestode Species
Diphyllobothrium latum Phylum Platyhelminthes; Class Cestoidea ; Order
Pseudophyllidea; Family: Diphyllobothriidaea.

Eggs of D. latum in an iodine-
stained wet mount.
Carmine-stained proglottids of D.
latum, showing the rosette-shaped
ovaries.
Scolex of D. latum

The entire life cycle can be summed up in 9 stages with 4 hosts:
 1 : Immature D. latum eggs are passed in the feces of the human host.
 2. These eggs then complete development in fresh water.
Life Cycle continues in crustaceans:
 3. Small, ciliated coracidium larvae hatch from mature eggs, and swim about until
ingested by crustaceans.
 4. The second larval stage is completed in the crustacean with the development of
the procercoid.

Life Cycle then moves to fish:
5. Infected crustaceans are the ingested by small freshwater fish. The procercoid larva are then
released from the crustacean into the fish. The larvae continue to develop in the flesh of the
fish, developing into the plerocercoid stage, which is the infective stage for humans. If
humans ingest this fish, they will become infected. However, the fish ingesting an infected
crustacean is small and usually not prey for any mammals.
6. Thus, a larger predator fish ingests the smaller infected fish. The plerocercoid may infect the
larger fish, but will not continue to grow as the fish is only a transport host.

Life Cycle completes itself in Human or other suitable Mammal:
7. Human (or other mammal) ingests raw or undercooked infected fish.
8. Plerocerciod larva is not digested, but instead remains in the small intestine of its new host
and grows to adulthood.
9. Proglottids release immature eggs, completing the cycle.

 Copepods, fish and piscivorous birds, constitute the hosts necessary for complete
parasite development.
 The complex life cycle of the parasite requires three hosts: a copepod as the first
intermediate host, a cyprinid fish as the second intermediate host and an avian
predator as the final host.
 Free swimming coracidia larval stages are eaten by planktonic copepods and transform in
the gut of their first host into six-hooked oncospheres, which rapidly mature into
procercoid forms on entering the haemocoel cavity.
 Infected copepods are ingested by planktivorous freshwater fish, essentially cyprinids.
 The life cycle is then complete when the bird eats the tapeworm hosting fish.

Pathogenesis
 Cestodes do not have a digestive system, for this reason adult forms are usually found in host
intestine to absorb host nutrients through the integument.
 The attachment of cestode in fish intestine cause necrosis, hemorrhage and inflammation at
attachment points, space occupying distension of the intestine and possible perforation
 For example pseudophylliidean tapeworm, Bothriocephalus gowkongensis found to infect the
cyprinids.
 Larval stage of cestodes are also reported as parasite in fish.
 The larval cestode Diphyllobothrium dentriticum can cause severe granulomatous enteritis in
trout and lead to peritonitis with visceral adhesions and death.
 Larval tapeworm, Ligula is found in the peritoneal cavity, will cause pressure atrophy of
ovaries.

HOST-PARASITE RELATIONSHIPS
❖ Fish have a well developed immune system which is often temperature dependent.
❖ Helminth produce a strong cellular response with non specific inflammation while the main
immunoglobulin antibody is of the IgM class.
❖ Acute inflammation to heavy metazoan parasite infections commonly produces lesions
manifested by cell death and necrosis and distortion of tissues.
❖ Migrating parasites (larval forms) produce the most serious reactions; leucocytosis and
fibrosis, and more rarely haemorrhage , hyperaemia and necrosis (Arme et al.,
1983;Roberts , 1989)

✓ Host cellular response to tapeworm infections – total (as in some
Diphyllobothrium spp.) or partial (as in Ligula) encapsulation of the parasite.
✓ Cellular exudates – initially accompany visceral reactions consists primarily of
fibroblasts.
✓ Infiltration in tissues or in the body cavity by leucocytes dominated by –
macrophages, neutrophils and lymphocytes.
✓ Lymphoid organs of fish infected with Ligula undergo changes – increase in
melanomacrophage centres in the spleen and increase in ‘vacuolated activity
decreases as the parasite is encapsulated and isolated.
Cellular response and Tissue Pathology
Cellular response

❖ Growth retardation, abdominal distension, displacement and morphological/ physiological alteration
of internal organs.
❖ Inhibition of gametogenesis and reduced liver glycogen, muscle carbohydrates, proteins, blood
amino acids, and lipids are frequently associated with infections.
❖ Increase in serum proteins and alkaline phosphatase.
❖ Parasite increase its own phospholipids at the expense of the host muscle.
❖ Increase of certain enzymes due to T. nodulosus infection of the liver and lesion production.
❖ Outright mortalities – due to larval Diphyllobothrium and Triaenophorus , in aquaculture situations
due to destruction of tissue – water interface, extensive haemorrhaging and secondary microbial
infection.
❖ Liver is a favourite site of infection and the pathology may include haemorrhaging, necrosis,
fibrosis, oedema and discoloration (Arme et al.,1983;Pronina,1977).
❖ Mortality is often attributable to liver dysfunction and blood loss: in salmonids infected with
Diphyllobothrium spp.
Tissue Pathology

❖ In the gut may cause tissue alteration or destruction, mechanical blockage and nutrient absorption at
the expense of the expense of the host.
❖ With cestodes that penetrate the mucosal layer(Triaenophorus , and many tetrarhynchideans) , the
establishment of gut infections usually begins with an initial phase of acute inflammation in the
region of contact.
❖ Inflammation is characterized by subepithelial oedema, leucocyte infiltration and, in heavy
infections, epithelial erosion and necrosis.
❖ Vascularization is decreased to the infected areas and this ia accompanied by epithelial hyperplasia.
❖ Affects movement of food through the intestine.
❖ Pathology in the gut is often related to the morphology of the holdfast organs of the parasite.
❖ Species of Caryophyllidea that possess well – developed / specialized but non-invasive attachment
organs elicit little or no pathology, whereas those with developed holdfasts or possessing the
terminal introvert on the holdfast are associated with ulcers and nodules .
Pathology In Gut

❖ Heavy gut infections may cause intestinal distension, mechanical obstruction and perforation, as in
Bothriocephalus acheilognathi.
❖ Clinical signs in gut include emaciation and anaemia (Caryophyllidea, Bothriocephalus).
❖ Nutritional demands made by gut tapeworms may adversely affect growth, condition and fitness.
❖ Eubothrium salvelini in juvenile coho salmon (Boyce and Yamada, 1977;Boyce 1979) may cause a
change in blood composition, or may increase susceptibility to environmental stress such as zinc.
Pathology In Gut

➢ A complement factor and C-reactive protein have also been implicated in leucocyte
adherence to tapeworms (plerocercoid of Ligula).
➢ Precipitins to phosphorylcholine epitopes which are common to a variety of organisms
including microbial pathogens and metazoans.
➢ Recent studies with ELISA have demonstrated unequivocally, the antibody response of
rainbow trout to Diphyllobothrium spp.
➢ Increase of melamanomacrophages in the spleen of infected gudgeon , although cell counts
in the pronephrons remained unchanged.
Humoral Immunity to
Cestodes

Diphyllobothrium spp. infection in the peritoneal cavity of a brown trout.
A feature of such infections is the severe fibrinous peritonitis that is induced by the
plerocercoids.

Plerocercoids of Ligula intestinalis within the body cavity of a
roach. (By courtesy of Miss H. Flockhart.)

Plerocercoids of Triaenophorous spp. within the liver
of a brown trout. (By courtesy of Dr T. Hastein.)

Diagnosis
❑ For intestinal tapeworm infection, examination of a stool sample.
❑ Large worms can be identified grossly.
❑ Larval cestodes may not have segmentation, but a recognizable, although a
rudimentary scolex is usually present.
❑ Intestinal cestode infections can presumably also be made from wet mounts of fecal
contents having proglottids or eggs.
❑ Identification of adult cestodes to species uses features of the scolex and organs of
the mature proglottid; immature cestodes might only be classifiable to order.

❑ To check for cysticercosis, computed tomography or magnetic resonance imaging and sometimes
blood tests.
Stool sample analysis :
❑ Adult Worms:Taenia infections are diagnosed by finding gravid segments in stool specimens; the
eggs of these species are indistinguishable. Other species are diagnosed on the basis of eggs in
stool specimens.
❑ Larval Worms: Cysts in tissues may be identified in biopsy specimens, by radiography (calcified
cysts), and by computed tomography (brain cysts). Serology (indirect hemagglutination, ELISA)
is useful but of variable sensitivity and specificity.
❑ The ovoid, operculated eggs passed in abundance in the human stool are diagnostic.
Occasionally, strands of exhausted segments with the characteristic rosette-shaped uteri are also
passed

 Eggs observed in stool samples had the characteristic shape observed in Diphyllobothrium spp.
(Figure, A). The average length was 64–71 μm, and the average width was 48–51 μm.
A) Diphyllobothrium latum
egg. Note opercular
constriction. B) Genital
papillae of mature
proglottids as seen under
scanning electron
microscope. C) Uterine
loops of gravid proglottids
in fresh preparation. D)
Sagittal section of the
genital pore region stained
with hematoxylin-eosin.
Note seminal vesicle
(arrowhead) situated
dorsocaudal to the cirrus sac
(magnification 100×).

Fig. 2. (a) Histopathological section of Paradilepis scolecina encysted within the liver of Tinca tinca. The parasite,
exhibiting stained hooks (*), is separated from the hepatic parenchyma by a thin wall of connective tissue
(arrow). (b) Transverse section through the rostellum of P. scolecina showing concentric rings of ten small (*) and
ten large (**), hollow rostellar hooks. (c) Aggregation of four degenerate P. scolecina encapsulated in a fibrous
cyst within the liver of T. tinca. Remnants of the rostellar hooks (*) can be seen along with lymphocytes (arrows)
within the surrounding hepatic tissue. (d) Hook fragments (*) discharged from an encysted, degenerate P.
scolecina (P) located within the hepatic parenchyma of T. tinca. These structures are surrounded by a focus of
macrophages engorged with light brown pigment (arrows).

(a) Histopathological section of
parasitic nodules showing anterior
part of the parasite with tentacle (T)
and hooks (H). (b–d)
Histopathological section of fish
viscera showing: (b) Parasitic larvae
attached to intestinal serosa. Notice,
the anterior part of the parasite (A)
in the peritoneal cavity (PC) near
the intestinal and hepatic tissue
(HT). (c) The anterior part of the
parasite (P) in the abdominal cavity
surrounded with a thin layer of
fibrous connective tissue (F), notice,
the melanophores aggregation (M)
in the hepatic tissue. (d) Fish liver,
with a cross section of parasitic
larvae (CS) surrounded with a thin
layer of fibrous connective tissue
proliferation (F), melanophores
aggregation (M) and atrophied
hepatocytes (AT), (H&E stain).

When the PCR products were digested with Nco1 restriction enzyme, all the cases
(A, B, C, and D) revealed identical PCR-RFLP patterns which were consistent with
the known T. saginata (Fig. 1). Only 1 band (491 bp) was observed for T. saginata,
whereas 2 different bands (152 bp and 339 bp) were obtained for T. solium and T.
asiatica.

Figure 1: Amplification of the cox1 mitochondrial gene of Taenia tapeworms by
PCR-RFLP analysis. The amplified products of Taenia solium and Taenia asiatica
were digested with Nco1 restriction endonuclease forming 2 different banding
patterns (152 and 339 bp). Nonetheless, Taenia saginata and our 4 samples (491
bp) were not digested with the Nco1 enzyme. Lanes 1 to 7 are T. solium, T.
asiatica, T. saginata (positive samples), and cases A, B, C, and D (test samples),
respectively. M: 1 kb size maker (bp).

 Using immunological techniques , antigenic affinities were demonstrated between
D.dendriticum and D.ditremum.
 Molecular techniques utilizing ribosomal DNA probes hybridized against endonuclease
restricted genomic DNA on an agarose gel were used to identify Diphyllobothrium
plerocercoids.
 Recent development of oligonucleotide primers from sequenced intergenic spacer regions
of ribosomal DNA from D. latum and D.dendriticum have been used to identify
plerocercoids and adults of D. latum and D. dendriticum.

Prevention and Control of Tapeworms in Fish
 Role of drugs on fish tapeworms and some were effective ,
especially those against Bothriocephalus and Proteocephalus.
 Niclosamide is the active ingredient of commercial drugs such as –
Devermin, Radeverm, Phenasal, Mansonil, Yomesan and medicated
feeds such as zestocarp and cyprinocestin.
 Other synthetic anthelmintics include dibutyl tin oxide used
successfully against Eubothrium crassum.
 Niclosamide preparations have largely replaced herbal drugs such as
– Kamala, Filixan and tobacco dust.

Prevention and Control of Tapeworms in Fish
❖ Environmental manipulations include extirpation of :
a. fish-eating birds in the vicinity of an aquatic system.
b. pike in the case of T. crassus.
c. Ciscoe to reduce larval cysts in commercially valuable whitefish.
d. removal of invertebrate hosts (copepods) of T. crassus was
considered but was unacceptable as they are important components
of the food chain of many aquatic animals.

Standard Therapies
✓ There are several drugs available for the treatment of cestode disease.
✓ Praziquantel : is effective against the cysterici of many cestode species. It works by causing
severe spasms and paralysis of the tapeworm muscles. It is used to treat Taenia. saginata, T
solium, and Diphyllobothrium latum.
✓ Albendazole : is a broad spectrum anthelmintic. It eradicates cestodes by interfering with
their microtubule and/or energy generating systems Webbe (1994). It is used to treat E.
multilocularis.
✓ Niclosamide : works by killing tapeworms on contact. The killed worms are then passed in
the stool. It is used to treat diphyllobothriasis.
✓ Mebendazole : is an antiworm medication used for the treatment of Echinococcus
mulilocularis infections.

REFERENCE
 Dick,T.A.; and Choudhury,A.(1995)Cestoidea(Phylum Platyhelminthes);Fish Disease and
Disorder,391-414.

Cestodes, debojit

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