Cestodes

• Cestode (tapeworm)


  • Long, sluggish parasite-like a tape measure
  • Rely on host for warmth production and nutrients
  • No mouth or gut, absorbs nutrients through body surface
  • Scolex (head) with attachment organ
  • Proglottids (segments) that can stretch
• Cestode lifecycle


  1. Egg consumed by intermediate host

  2. Oncosphere (embryo) hatches, penetrates tissues, develops into larval stage

  3. Larval stage (cysticercus) consumed by definitive host

  4. Develops into adult tapeworm in definitive host's intestines
• Cestode species

  • Anoplocephala perfoliata (in horses)
  • Dipylidium caninum (in dogs and cats)
• Oncosphere has hooks to penetrate tissues of intermediate host
• Larval stages of cestodes include cysticercus (fluid-filled cyst with single larva)
• Cysticercus can cause issues like unthriftiness, diarrhoea, intestinal obstruction in intermediate hosts
• Definitive hosts develop resistance to adult tapeworms over time
• Adult tapeworms can grow up to 15 m long and 1.5 cm wide
• Proglottids are easily recognisable as rice grain-like segments in faeces of infected hosts
• Fleas/lice ingest oncospheres which develop into cysticercoids in their bodies, then infect the definitive host when consumed
• Treatment and control of cestodes must target both the adult worms in the definitive host and the larval stages in intermediate hosts
• Cestodes


  Long sluggish parasite-like a tape measure
• Cestodes


  • Rely on host for warmth, protection and nutrients
  • No mouth or gut, absorb nutrients through body surface
• Scolex


  Attachment organ
• Rostellum


  Part of scolex
• Strobila


  Each segment (end) is a proglottid
• Proglottids


  Bud from neck region, those near scolex are youngest
• Cestode structure


  • Outermost layer tegument
  • 4 muscle layers underneath
• Coenurus


  • Consisting of many scolices
• Strobilocercus


  • Scolex connected to a chain of asexual proglottids
• Hydatid


  • Large fluid filled cyst within which scolices form
• Cestode lifecycle


  1. Cyst must be consumed by definitive host

  2. Larvae released from cyst when intermediate host is digested

  3. Develop to adult tapeworm within definitive host's gut
• Cestodes are hermaphrodites
• Cestode reproduction


  1. Fertilisation between worms or between proglottids of the same worm, self fertilisation of proglottids

  2. Gravid proglottid segments shed from strobila

  3. Segment disintegrates in environment and eggs liberated or may be shed through genital pore
• Cestode lifecycle


  1. Eggs ingested by intermediate host

  2. Oncosphere hatches and migrates to site

  3. Develops into metacestode larval stage
• Adult tapeworms within definitive host can cause illness, the intermediate cyst stage is more likely to cause disease in the intermediate host
• Tapeworm eggs have a thick shell (embryosphere)
• Cestode lifecycle


  1. Adult tapeworm sheds proglottids in faeces

  2. Eggs released from proglottid into environment

  3. Intermediate host ingests egg (embryophore)

  4. Oncosphere released and activated

  5. Oncosphere hooks tear through mucosa to reach blood or lymph and then migrate to predilection site

  6. Loses hooks and develops into metacestode larval stage

  7. Metacestode (cysticercoid) forms within intermediate host (forage mite)

  8. Forage mite eaten by definitive host
• Metacestode


  Larval stage of cestodes, a cyst containing viable larvae
• Metacestode types


  • Cysticercus (fluid filled cyst with single scolex)
  • Coenurus (fluid filled cyst with multiple scolices)
  • Hydatid (large fluid filled cyst)
• Heavy cestode infestation can cause unthriftiness
• Cestode infections can cause inflammation of the intestine, ulceration of mucosa, diarrhoea or intestinal obstruction
• Diagnosis is by faecal examination, saliva and blood tests
• Moniezin


  Anthelmintic treatment for cestode infections
• Echinococcus granulosus


  • Very small (6mm)
  • Adult tapeworm present in small intestine of ruminants (sheep, goats, cattle)
  • Intermediate host is forage mite
  • Eggs are irregularly triangular in shape
• Echinococcus granulosus lifecycle


  1. Eggs ingested by intermediate host

  2. Oncosphere hatches and migrates to site

  3. Develops into hydatid cyst in liver or lungs of intermediate host

  4. Hydatid cyst eaten by definitive host (dog)
• Echinococcus granulosus infections tend to be short lived in definitive hosts
• Echinococcus granulosus hydatid cysts can grow up to 20cm in diameter and cause severe illness or death in humans
• Each dog can carry thousands of adult Echinococcus granulosus tapeworms
• Dipylidium caninum is the most common tapeworm of dogs and cats