Summary CHAPTER 4 - cestoda

CHAPTER 4: CESTODA
(TAPEWORMS)
We are focussing on eucestoda, a subclass of cestode characterized by its segmentation, present in man and
animals. The cestodes are also multicellular parasites that can be larger in size than the trematodes. Typical for
tapeworms is that they are segmented Platyhelminthes. All tapeworms as adults can be found in the intestine,
but at a certain point some cestodes can also be present extra-intestinal, this is the case when the human acts
as an intermediate host (parasite in larval stage in the human host).

GENERAL CHARACTERISTICS

o Parasitic in vertebrates
o Global distribution
o As adult: always in the gastrointestinal system (small intestine)
o At least 2 different hosts: indirect, heteroxene cycle
o Characteristics first stage larva: hexacanth (3x2 hooks)
o No free-living stages, all stages are endoparasites
o Host specificity: (<-> trematodes)
 High specificity for vertebrate final host
 Low specificity for intermediate hosts
o Variable size: few mm to several meters

Morphology
o Dorso-ventrally flattened
o Segmented (<-> trematodes): each segment has similar organisation as trematodes
 Coelome and intestinal system is absent: no pharynx and caeca
 Excretion system based on flame cells and protonephridia: excretion of electrolytes +
osmoregulation
 Hermaphrodite: male and female reproductive organs repeating per segment
 Rule is self-fertilization (<-> trematodes: cross-fertilization)
o Scolex – neck – storbila (proglottids)
 Scolex: head of the parasite
 Strobila: part where you start having the different segments being formed
 Segments = proglottids

General structure:

o Scolex – neck – strobulae
 Scolex: different possibilities according to species (identification)
 Bothriate: kind of 2-membranous fold with cavity, enables attachment
 Rostellum: anterior part of parasite has hooks and suckers (actebula)
o Oral and ventral suckers important for motility and attachment
o Hooks can cause tissue damage
 Armed scolex: hooks
o Ladder-type neurological system + scolex has neuronal ganglion that controls the muscular
movements of the suckers of the rostellum

Organs and structures:

, o Tegument: cytoplasmic syncytium
 Constantly proliferating and renewing: fast turnover
 Important for nutrient uptake
 Lot of microvilli (<-> trematodes): increase of surface with factor of 20, important for
uptake of nutrients through the tegument
 Microtrichen on top of villi: small spines
 Nutrient uptake through tegument: glucose, amino-acids
o Nervous system: ladder-type with ganglion in scolex
 innervation of muscle for attachment
 important for strobilation process: process of making the different proglottids
 immature proglottids
 mature proglottids: both male and female reproductive organs (hermaphrodit)
o first male organs are produced
o later female organs are produced
 gravid proglottids: after fertilization, contain in the uterus the presence of eggs
o no digestive system: no pharynx, no caeca
 uptake of nutrients (glycose and AA) through tegument
 can store glucose in form of glycogen
o excretion of electrolytes: based on protonephridia and flame cells
 excretion ducts: transversal + longitudinal, removal of ammonia
 high capacity to re-absorb glucose, low excretion of glucose  can be fully used for
metabolism
o reproduction: hermaphrodites
 mostly self-fertilization, in some cases when people are infected with multiple tape-worms
cross-fertilization is also a possibility
 but protandry: first development of male reproductive organs, afterwards female
 2 major types of tape worms:
 Pseudophilidae: continuous excretion of eggs
 Cyclophilidae: eggs generally in proglottide, they will just release the proglottid

Proglottid structure:
The proglottid structure is species-specific and thus can be used for the identification of different cestode
species.




2 different strategies:
o Apolytic: remove or excrete entire proglottid where the eggs are stored in the uterus and as such
also are protected from the environment

, o Anapolytic: eggs from the uterus are excreted through the genital pore, the eggs have an egg shell
which protects the eggs better in the environment

Comparative morphology
o Diphylolobothrium
 Scolex: bothriate
 Eggs: dark egg shell + operculum
o Taenia: pork and beef tapeworm
 Scolex: armed rostellum, has hooks and suckers (acetubula) important for attachment
 Taenia saginata: hooks absent
 Taenia solium: hooks present
 Eggs: no egg shell (apolytic) but thick multi-layered wall of the egg (embryophore) and inner
envelope
o Dipylidium
 Scolex: rostellum with very small hooks, other line-up of the hooks
 Proglottids:
 2 female and 2 male reproductive organs: clearly see 2 genital pores on both sides
 Gravid proglottids: eggs stored in egg packages in the uterus
 Eggs: egg shell + thick embryophore and inner envelope
 Most of time: eggs are excreted as egg packages

Conclusion: based on the morphology of the scolex, the proglottids and the eggs, differential diagnosis is
possible.

GENERAL CLASSIFICATION OF EUCESTODA




The Eucestoda consists of 2 different orders, Pseudophilidea and Cyclophilidea, that differ in the life stages in
the intermediate host.

o Pseudophilidea: adaptations that enable them for life cycle in water environment
 Diphyllobothrium latum
 Spirometra: extra-intestinal parasites (larval stages), normally not pathogen of humans but
cats/dogs (fish-eating carnivores)
o Cyclophilidea: life cycle typical on land, bladder-type of intermediate stages
 Dipylidium caninum: infection through flees of dogs
 Hymenolepis nana/diminuta: infection through eating contaminated cereals
 Taenia solum/saginata: beef and pork tapeworm
 Echinococcus granulosus/multilocularis: fox tapeworm