Intro to Cestodes

• Phylum Platyhelminthes (Flatworms)

  • Multicellular animals characterized by a flat, bilaterally symmetric body
  • Dorsoventrally flattened and solid (no body cavity)
  • Hermaphroditic, having both male and female reproductive organs in the same individual
  • Overall size varies greatly, some members are as small as 1 mm, and others may be 20 meters or longer
  • The classes Cestoda (tapeworms) and Trematoda (flukes) contain species parasitic to humans

  
• Cestodes

  • Flat, segmented body with various length (several mm – several meters)
  • Hermaphroditic with each proglottid developing both male and female reproductive organs, and mature eggs developing in the most distal proglottids
  • No circulatory system
  • No digestive system
  • Microtriches – outward projections present in the tegument, similar to microvilli of our gut mucosal cells, increase the surface area for nutrient absorption, interdigitate with host microvilli

  
• Cestodes are diagnosed by

  Finding eggs or proglottids (diagnostic stage) in the feces
• Habitat of adult tapeworm

  Intestine of the definitive host

  
• Intermediate host

  Larval stage encysted in the tissue
• Mnemonics for Taenia species

  • Adult worms are seen in the intestine
  • Proglottids contain male and female parts
  • Eggs have 6 hooked embryos
  • World-wide distribution
  • No respiratory, vascular, or digestive system
  • Ribbon-like body, flat
  • Middle (intermediate host is required)

  
• Cestode structure

  • Scolex (head)
  • Neck (germinal portion)
  • Strobila (immature, mature, gravid proglottids/segments)

  
• Scolex

  • Similar to the head, knobby-looking and has either suckers or sucking groove for attachment to the mucosa
  • Rostellum: armed with hooks for attachment to the intestinal wall, particularly important during digestion
  • Suckers for the absorption of nutrients once the hooklets have penetrated the intestinal wall
• Neck

  Germinal portion that produces new proglottids, the segment of proglottids become mature as they move away from the scolex or the neck portion
• Strobila

  Immature, mature, gravid proglottids (segments) that bud from behind the scolex, mature and fill with eggs, and become gravid proglottids
• Anatomy of proglottids

  • Immature: reproductive organs are undifferentiated
  • Mature: reproductive organs are differentiated (kidney-shaped ovaries, surrounding testes)
  • Gravid: uterus with lateral branches filled with eggs, reproductive organs start to disintegrate
• Classification of Cestodes

  • Species which require a vertebrate intermediate host (Taenia solium, Taenia saginata, Echinococcus granulosus)
  • Species which require an invertebrate intermediate host (Dipylidium caninum, Hymenolepis diminuta, Raillietina garrisoni)
  • Species which may or may not require intermediate host (Hymenolepis nana)
  • Species which infect man in their larval stages (Echinococcus granulosus, Echinococcus multilocularis, Taenia solium, Diphyllobothrium latum)
• Binomial names and common names of Cestodes

  • Taenia saginata (Beef tapeworm)
  • Taenia solium (Pork tapeworm)
  • Taenia multiceps (Coenurus tapeworm)
  • Echinococcus granulosus (Hydatid tapeworm)
  • Echinococcus multilocularis (The multiloculate or Alveolar Hydatid tapeworm)
  • Hymenolepis nana (Dwarf tapeworm)
  • Hymenolepis diminuta (Rat tapeworm)
  • Dipylidium caninum (Double-pored tapeworm)
  • Diphyllobothrium latum (Broad Fish tapeworm)

  
• Comparison of Pseudophyllidean and Cyclophyllidean Cestodes

  • Scolex: Pseudophyllidean - Almond/Spatulate, 2 slit-like grooves (bothria); Cyclophyllidean - Globular, 4 cup-like suckers
  • Uterus: Pseudophyllidean - No branching, with pores; Cyclophyllidean - Branching, blind sac
  • Uterine pore: Pseudophyllidean - Present; Cyclophyllidean - Absent
  • Genital pore: Pseudophyllidean - Ventral, in the midline; Cyclophyllidean - Lateral
  • Eggs: Pseudophyllidean - Operculated, gives rise to ciliated larvae; Cyclophyllidean - Not operculated, do not give rise to ciliated larvae
  • Intermediate host: Pseudophyllidean - 2; Cyclophyllidean - 1 or none
  • Apolysis: Pseudophyllidean - Anapolytic (proglottids are not shed off because they have uterine pore); Cyclophyllidean - Apolytic (proglottids are shed off because they do not have uterine pore)