Trematodes

• Cestodes are a type of helminth (parasitic worm) that belong to the phylum Platyhelminthes
• Types of Cestodes


  • Diphyllobothrium latum
  • Taenia saginata
  • Taenia solium
  • Taenia saginata spp. Asiatica
  • Hymenolepis nana
  • Hymenolepis diminuta
  • Dipylidium caninum
  • Raillietina garrisoni
• Cestodes can be classified into

  • Intestinal Cestodes
  • Extraintestinal Cestodes
• Cestodes


  • Segmented, ribbon-like appearance, absent digestive tract
• Cestode head (scolex)


  • Organ of attachment to the intestinal mucosa
• Cyclophyllidean scolex


  • Possesses 4 suckers, may or may not have a rostellum
• Pseudophyllidean scolex


  • Possesses a pair of bothria (spatulae with sucking grooves)
• Neck


  Region of growth from where new proglottids are formed
• Strobila


  Consists of immature, mature and gravid proglottids
• Proglottids


  • Contain both male and female reproductive organs, do not have a body cavity or alimentary canal, have rudimentary excretory and nervous systems
• Structures that differ between Pseudophyllidea and Cyclophyllidea


  • Scolex
  • Segments
  • Eggs
• Pseudophyllidean eggs


  Operculated and immature, require aquatic development to coracidium
• Cyclophyllidean eggs


  Non-operculated, passed out readily, contain hexacanth embryo/oncosphere
• Intermediate hosts required


  • Pseudophyllidea: 2 hosts

  Cyclophyllidea: 1 host
• Pseudophyllidean larval stages


  Procercoid in 1st intermediate host, plerocercoid in 2nd intermediate host
• Intestinal Cestodes


  • Diphyllobothrium latum
  • Taenia saginata
  • Taenia solium
  • Taenia saginata asiatica
  • Hymenolepis nana
  • Hymenolepis diminuta
  • Dipylidium caninum
  • Raillietina garrisoni
• Extraintestinal Cestodes


  • Echinococcus spp.
  • Spirometra spp.
• Diphyllobothrium latum


  Also known as fish tapeworm or broad tapeworm, longest cestode infecting humans, inhabits small intestine (ileum)
• Diphyllobothrium latum adult


  • Measures 3-10 meters in length, may have 4,000 proglottids, scolex is spatulate with 2 bothria, proglottids are wider than long, contains one set of reproductive organs
• Diphyllobothrium latum eggs


  Operculated, unembryonated, released when uterus is distended, approximately 1 million eggs per day, broadly ovoid with thick shell, abundant yolk granules, inconspicuous operculum, knob-like thickening on opposite side
• Diphyllobothrium latum life cycle

  1. Release of operculated eggs

  2. Ingestion by freshwater copepod (1st intermediate host)

  3. Development into procercoid larvae

  4. Ingestion of copepod by fish (2nd intermediate host)

  5. Development into plerocercoid larvae (sparganum)

  6. Ingestion of infected fish by definitive host (humans)
• Diphyllobothriasis


  May be asymptomatic, can cause nervous disturbances, digestive disorders, abdominal discomfort, weight loss, weakness, and megaloblastic anemia due to competition with host for vitamin B12
• Diagnosis of diphyllobothriasis is by stool examination for operculated eggs or gravid proglottids, and blood examination showing megaloblastic anemia
• Treatment for diphyllobothriasis


  Drug of choice is praziquantel, proper cooking or deep freezing of fish to kill larvae
• Taenia saginata


  Also known as beef tapeworm, adult inhabits small intestine of humans, measures 4-10 meters in length
• Taenia saginata scolex


  • Has 4 suckers, lacks a rostellum, neck is long and narrow
• Taenia saginata strobila


  • Consists of 1000-2000 proglottids, gravid segments are longer than broad with a median club-shaped uterus with >15 lateral branches, proglottids break away and are expelled singly
• Taenia saginata eggs


  Spherical or sub-spherical, brownish with thick embryophore, oncosphere with 6 hooklets
• Taenia saginata larvae (cysticercus bovis)


  Infective stage to humans, ovoid, fluid-filled vesicle with single invaginated scolex, affects cattle muscles (measly beef)
• Taenia solium


  Also known as pork tapeworm, adult inhabits small intestine of humans, measures 2-3 meters in length
• Taenia solium scolex


  • Has 4 suckers, possesses a cushion-like rostellum with double crown of 25-30 hooks, neck is shorter
• Taenia solium strobila


  • Consists of <1000 proglottids, gravid segments are longer than broad with <15 lateral branches, proglottids are expelled in chains
• Taenia solium eggs


  Indistinguishable from T. saginata, have thick brown striated embryophore surrounding hexacanth embryo
• Taenia solium larvae (cysticercus cellulosae)


  Infective stage to humans, develop in various organs of pigs and humans, similar morphology to cysticercus bovis
• Taenia saginata/Taenia solium life cycle

  1. Ingestion of eggs by intermediate host

  2. Oncosphere penetrates intestinal mucosa and develops into cysticercus in muscle

  3. Ingestion of infected meat by definitive host (humans)

  4. Cysticercus attaches to intestinal mucosa and develops into adult
• Double crown


  25 to 30 hooks
• Strobila


  Consists of <1000 proglottids
• Gravid proglottids


  • Longer than they are broad
  • Contain <15 lateral branches
  • Are expelled in chains
  • Are hermaphrodites
• Eggs

  • Indistinguishable from that of T. saginata
  • Have a thick brown striated embryophore surrounding a hexacanth embryo
• Infective stage to humans

  • Develop in various organs of pigs and humans
  • Morphology is similar to cysticercus bovis
  • Scolex appears similar to adult T. solium