Biology 625
ANIMAL PARASITOLOGY
Fall semester lecture note outline

Updated: 24 September 1999

1. Embryogenesis within egg to form larva termed an oncosphere (6 or 10 hooked larva)
2. Basic aquatic life-cycle
   1. adults in gut
   2. eggs (usually unembryonated) into environment; mature; hatch (many eggs operculated)
   3. coracidium (ciliated membranes around oncosphere) emerges
   4. usually eaten by first intermediate host (i.e. crustacean)
   5. uses larval hooks to penetrate gut
   6. procercoid forms in hemocoel
      1. larval hooks
      2. hooks relegated to posterior structure termed a cercomer
      3. usually no scolex/holdfast organ
      4. only rarely do species have any sort of strobilation
   7. eaten by second intermediate host (usually a fish)
   8. procercoid crosses gut; into muscles
   9. pleurocercoid
      1. strobila often present
      2. scolex/holdfast organ often present
   10. eaten by definitive host
   11. matures
3. Basic terrestrial life-cycle
   1. adults in gut
   2. eggs usually passed fully embryonated and non-operculate
   3. eaten
   4. oncosphere unciliated; hatches
   5. crosses gut wall
   6. cysticercoid or cysticercus forms (larval stages)
   7. eaten by definitive host
   8. attaches to gut wall; matures
4. terrestrial larval types
   1. cysticercoid
      1. solid bodied larva
      2. fully developed scolex invaginated in body
      3. cercomer exterior to cyst and with larval hooks
      4. in invertebrates
   2. cysticerci
      1. fluid filled bladders; not a solid body
      2. scolex invaginated and introverted
      3. scolex forms on germinal membrane enclosing the fluid filled bladder
      4. types
         1. simple cysticercus with one scolex; sometimes some strobilzation oocurs and termed a strobilocercus
         2. coenurus with multiple (few to many) protoscolices, each on a simple stalk
   3. hydatid cysts (also fluid filled bladders, but with brood capsules)
      1. unilocular hydatid cyst, with up to several million protoscolices; often endogenous (rarely exogenous) budding brood cysts each with many protoscolices; when many protoscolices break free and fall to the bottom of the cyst, termed hydatid sand. Genus Echinococcus only.
      2. multilocular or alveolar hydatid cyst, known only from Echinococcus multilocularis. Also with million of protoscolices; often with extensive exogenous budding in abnormal host