Appearance and Morphology
• Body divided into scolex, neck, strobila.
• Scolex may contain holdfast organs in the form of suckers or slits and/or rostellum armed with hooks.
• Neck is the germinal region from which the strobila arises.
• Strobila is the rest of the body; composed of proglottids each of which contains one set of reproductive organs; proglottids mature as they move away from the neck with gravid (egg-containing) proglottids at end of body.
The tapeworms of veterinary importance are divided among two groups — the Eucestoda and the Pseudophyllidea. Table Classification of Cestodes Encountered in Veterinary Medicine presents the families and species of these two groups of tapeworms encountered in North America.
Table Classification of Cestodes Encountered in Veterinary Medicine
|Order||Family||Species||Intermediate Hosts||Metacestode Type||Definitive Hosts|
|Pseudophyllidea||Diphyllobothriidae||Diphyllobothrium latum||Fish||Procercoid, plerocercoid||Dogs|
|Spirometra manson oides||Procercoid, plerocercoid||Dogs, cats|
|Cyclophyllidea||Taeniidae||Taenia spp.||Pigs, ruminants, rodents||Cysticercus, coenurus, strobilocercus||Carnivores, humans|
|Echinococcus spp.||Ruminants, rodents, humans||Hydatid cyst||Canids, felids|
|Dilepididae||Dipylidium can in urn||Fleas, lice||Cystercoid||Dogs, cats, humans|
|Mesocestoididae||Mesocestoides spp||First IH unknown; Rodents||First metacestode unknown; Tetrathyridium||Canids|
All cestodes have an indirect life cycle; however, the stages vary according to the group involved. Adult tapeworms are found in the intestinal tract (or ducts of associated organs) while larval stages (metacestodes) can be found in a variety of organs.
• Stages include egg, hexacanth embryo, metacestode, adult.
• Gravid proglottid usually passes out of host with feces; ruptures releasing eggs.
• Eggs contain embryo with six hooks (hexacanth embryo); must be ingested by intermediate host to develop to metacestode stage.
• Metacestode can take several forms; if ingested by arthropod, develops into cysticercoid; if ingested by mammalian host, develops into cysticercus, strobilocercus, coenurus, hydatid cyst, or tetrathyridium; metacestode type is characteristic of the cestode species involved.
• Definitive host becomes infected by ingesting the infected intermediate host; extra-intestinal migration in the definitive host does not occur.
• Stages include operculated egg, coracidium (ciliated hexacanth embryo), metacestode, adult.
• Eggs are expelled from the gravid proglottid; pass out with feces.
• Coracidium hatches in water; first intermediate host is a copepod; ingested coracidium develops to procercoid (metacestode).
• Copepod is ingested by the second intermediate host; procercoid develops into another metacestode stage called the plerocercoid.
• Definitive host becomes infected by ingesting the intermediate host containing the plerocercoid; extra-intestinal migration in the definitive host does not occur.