cestodes

• Cyclophyllidean tapeworms


  • Definitive hosts are carnivores, herbivores, birds
  • Examples: Dipylidium caninum, Taenia spp., Mesocestoides spp., Echinococcus spp., Moniezia spp., Anoplocephala spp., Thysanosoma sp., Hymenolepis spp.

  
• Pseudophyllidean tapeworms

  • Examples: Diphylobothrium spp., Spirometra spp.

  
• Definitive Host (DH)


  Highly host specific, each species has its own DH
• Intermediate Host (IH)


  Cyclophyllideans have only one IH, Pseudophyllideans have two IH
• Hymenolepis nana is a tapeworm of birds, humans and rodents that is unique as it does not require an IH

  
• Adult tapeworms

  • Principally found in the small intestine, but also bile ducts, pancreatic ducts and large intestine
  • Immature forms found in abdominal cavity, liver, muscle, cranial cavity and eyes

  
• Adult tapeworms

  • Some a few mm long, but usually 10 - 100 cm, with a scolex, neck and strobilia (chain of segments)

  
• Scolex of Cyclophyllideans

  • Has four suckers and sometimes an anterior projection the rostellum, which may be armed i.e. with hooks, and may be retractable into the body of the scolex

  
• Scolex of Pseudophyllideans


  • Has two longitudinal groves (bothria) for attachment
• Neck


  • Small unsegmented germinal area which joins the scolex to the strobila
• Strobila


  • Chain of segments, with anterior segments immature, middle segments reproductively active, and terminal segments egg laden or gravid with degenerating reproductive elements

  
• Tapeworms

  • No mouth or alimentary system, nutrients absorbed directly through the tegument
  • Each segment has one or two sets of reproductive organs

  
• Cyclophyllidean eggs

  • Spherical hexacanth (6 hooked) embryo (1st larval stage) or oncosphere, with a thick, dark, radially striated shell or embryophore, and other membranes forming the true shell

  
• Pseudophyllidean eggs

  • Oval, brown, operculate with a thick shell and contain discrete, fat-like globules (yolk), the coracidium is the ciliated hexacanth onchosphere with an embyophore

  
• Differentiating features between Pseudophyllidea and Cyclophyllidea

  • Scolex: Pseudophyllidea has two longitudinal grooves (bothria), Cyclophyllidea has four muscular suckers (acetabula)
  • Genital pore: Pseudophyllidea is on the center of ventral surface of each proglottid, Cyclophyllidea is on the margin(s) of each proglottid
  • Uterus (gravid): Pseudophyllidea is relatively long and coiled, Cyclophyllidea is sac-like and highly branched
  • Eggs: Pseudophyllidea are operculated, Cyclophyllidea are non-operculated
  • Oncosphere: Pseudophyllidea is ciliated (coracidium), Cyclophyllidea is non-ciliated
  • Larvae: Pseudophyllidea has procercoid & plerocercoid forms which are both solid, Cyclophyllidea has cysticercoid, cysticercus, hydatid forms which are all cystic

  
• Genus Taenia - Cysticercus


  • A white fluid-filled cyst, the size of a pea, with an invaginated scolex, surrounded by a connective tissue capsule formed by the host

  
• Genus Taenia - Coenurus

  • A fluid-filled cyst, up to 5 cm, with many invaginated scolices surrounded by a connective tissue capsule formed by the host. Each scolex can give rise to a mature tapeworm

  
• Genus Taenia - Strobilocercus

  • An elongated form consisting of a cysticercus with the scolex evaginated and a small strobila approximately 1cm long

  
• Alveolar Hydatid


  • Similar to the hydatid cyst but not confined by a capsule with exogenous budding, the daughter cysts bud off from the internal and external surface of the germinal layer which enable the cyst to invade into the tissue of the organ like a tumor
• Genera Dipylidium, Moniezia, Anoplocephala - Cysticercoid


  • A single evaginated scolex embedded in a small, solid cyst with little or no fluid

  
• Genera Diphyllobothrium, Spirometra - Procercoid and Plerocercoid


  • Solid and ribbon-like larval stages, with the procercoid in a crustacean, and the plerocercoid in the 2nd IH

  
• Cyclophyllidean life cycle

  Gravid segments, rather than free eggs, may be passed in the faeces. Segments dehydrate, and rupture to liberate the eggs, each containing a hexacanth larva. These eggs are immediately infective when passed in faeces and are ingested by the IH. The larva in the IH migrates to a specific site and matures to the second stage larva. The DH ingests the IH or part of it that contains the infective form, the IH disintegrates, the scolex of the tapeworm evaginates, the bladder sloughs off, and the tapeworm matures to an adult in the intestine.

  
• Pseudophyllidean life cycle


  Eggs are liberated from the uterine pore of many gravid segments to appear in faeces. The eggs are not immediately infective. The ciliated hexacanth larva (coracidium) hatches from the egg, and swims to penetrate the first IH, and matures to the second stage. The infected first IH is ingested by the second IH and the second stage immature tapeworm matures to the third stage in the second IH. There may be a series of paratenic hosts until a suitable DH is found.

  
• Some adult tapeworms may be highly pathogenic but for many there is controversy over the pathological significance. Few adult tapeworms in the intestine of an animal have no clinical signs. Some immature tapeworms can have tremendous pathological significance in domestic animals and humans.

  
• Faecal flotation

  Can be negative for tapeworms eggs, since many common tapeworms in domestic animals are Cyclophyllideans that generally pass segments rather than eggs in the faeces

  
• Examine faeces for segments, but these are shed intermittently, and may need several faecal samples. Segments of some tapeworms are motile, and can move away from faeces.