Section 2: Cestodes

what are cestodes

flatworm in phylum platyhelminthes

• most segmented; all lack digestive system (must absorb nutrients through tegument)

tapeworm structure

strobila (body): proglottids (individual segments)

• each proglottid consists of both M/F reproductive organs

new strobila: occurs in neck region (strobilation)

• each proglottid moves to poster end

• end: matured, copulated, and produced eggs

• gravid: proglottid contains fully developed shelled embryos

proglottids: detach and pass out of body with feces

• spent/rotten: said to be senile

scolex: head at anterior end (help maintain position of worm within gut of host)

• grooves, hooks, suckers, spines, glands, tentacles

protrusible: dome-shaped area on the apex of scolex

• rostellum (1-2 rows of hooks)

tapeworm life cycle

indirect (2+ hosts)

• adult in intestinal tract

cestode aquatic life cycle

involve 3 host life cycles

• eggs shed unembryonated usually in water

• 6 hooked larval stage (coracidium) develops

  • - coracidium: composed of oncospheres (embryo) and outer ciliated embryo

• mature coracidium emerge and eaten by aquatic arthropod

  • - entire egg eaten; larva hatch in digestive tract

• procercoid develops

• infection arthropod eaten by 2nd intermediate host (usually fish)

• procercoid penetrate intestine, migrate to skeletal muscle, develop into plerocercoid

  • - pleurocercoid: elongate, scolex, some strobilation

• pleurocercoid past paratenically through predation

• 2nd intermediate host eaten by appropriate vertebrate final host

  • - worm attaches to intestinal wall, develops into adult

cestode terrestrial life cycle

involve 2 host life cycles

• eggs passed out fully embryonated

• ingested by arthropod/herbivore: oncosphere penetrates gut; develops into larval sage

  • - may be cysticercoid (solid body, invaginated, in arthropods)

  • - cysticercus (scolex, invaginated, introverted)

several types of cysticerci:

• coenurus (few/many scolex, protoscolices, arise from cyst)

• uniocular hydatid cyst (several million protoscolices)

• multilocular hydatid cysts (infiltration of tissues like a cancer)

• ingestion by final host: worm will grow and mature

cestode simple structure

scolex (attachment; suckers/hooks) → neck (growth zone; new proglottids formed continuously) → immature proglottids (organs form) → mature proglottids (reproductive; genital pore/gamete exchange) → gravid proglottids (egg filled)

strobila: all proglottids (excluding head, neck, scolex)

Diphyllobothrium eggs (slide 13)

• structure, eggs

structure: operculate, no shoulders, tiny knob opposite operculum

eggs: similar to trematodes, pass in feces, oval eggs

Taenia pisiformis immature proglottids (slide 14)

• adults, larvae, immature proglottids, infect

infect: tapeworm of canids (dogs/wild canids, foxes/coyotes)

adults: live in small intestine of definitive host

• rabbits/hares: intermediate host; infected by ingesting eggs sheds in canids feces

larvae: develop into cysticerci (in abdominal cavity)

immature proglottids: difficult to distinguish reproductive structure

• segmented organization of strobila; genital pore

Taenia pisiformis eggs (slide 15)

spherical/subspherical

oncosphere surrounded by thick embryosphere (inner sphere)

numerous pores (striated look)

Echinococcus granulosus adult (slide 16)

• structures, infect

worm structures: 4 suckers/several circles of hooks

• tiny worm with only 3-4 segements only

infect: do not infect humans

• canines/carnivores

• terrestrial tapeworm

Echinococcus granulosus hydatid cyst (slide 17)

• hydatid cyst, humans, structures

hydatid cysts: larval stage; develops in intermediate host after eggs from canid feces ingested

• wild ungulated common intermediate hosts

humans: accidental intermediate hosts (can develop hydatid cysts)

• liver: common site of infection; followed by lungs (cysts can develop in other organs)

Structure: cyst wall, brood capsules, protoscolices

• each protoscolex = potential future worm (if cyst eaten, protoscolices envaginate, attach to small intestine, develop into adult worms)

hydatid cysts structure: large, can cause disease by compressing/disrupting surrounding tissue

• leakage/rupture: cause severe allergic reaction

Dipylidium four regions (slide 18)

• infect, structures, slide portions

infect: dog/cat tapeworm, occasionally seen in kids

• ingestion of arthropod (flea) containing the cysticercoid

structures: retractable rostellum, several circles of hooks, proglottids (two genital pores on each side)

slide portions: portions of strobila, immature/mature/gravid proglottids

• mature: contain M/F reproductive structures, genital pore on each lateral margin

• gravid: egg packets containing eggs