Diphyllobothrium latum Spirometra spp. *Diphyllobothriidean cestodes are sometimes referred to as

• • Diphyllobothriidean cestodes have indirect life cycles that require two intermediate hosts before becoming infectious to the definitive host.
    • Adult tapeworms discharge operculated eggs from a midventral genital pore; these eggs are then passed in the feces. When the egg contacts water, a ciliated embryo hatches and infects the first intermediate host, a copepod that eats it.
    • In the copepod the first larval stage (procercoid) develops in 2-3 weeks.
    • When the copepod is consumed by a second intermediate host, the next larval form develops (plerocercoid also called sparganum). Dogs and cats are infected when they ingest these larval forms in the second intermediate host.
    • Transport (paratenic) hosts may also be important in the life cycle of Diphyllobothriidean tapeworms.
      • With D. latum, if a small infected fish is eaten by a larger fish, the tapeworm larva will transfer to the muscles of the larger fish and remain infective for the fish eating mammal.
      • With Spirometra, if an infected second intermediate host is eaten by another second intermediate host, the plerocercoid larvae transfers to the tissues of the new host and is capable of infecting the dog or cat that ingests these infected animals.

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  Stages
  • The egg of a diphyllobothriidean tapeworm shows the characteristic operculum (arrow).
  • The ciliated first stage larva, termed a coracidium, emerges from the egg.
  • The copepod is the first intermediate host..
  • The larval form that is infective for vertebrates, sometimes referred to as a sparganum, develops in the second intermediate host after it ingests the copepod.
  • The adult diphyllobothriidean cestode (see image) is found in the small intestine of dogs or cats and other fish-eating mammals including people.
  • The scolex of adult diphyllobothriidean tapeworms does not have hooks or suckers like those found in “true” (Cyclophyllidean) tapeworms. The worms attach to the gut with dorsal and ventral longitudinal groves that are on the scolex. These slits are called bothria.
  • Diphyllobothriidean eggs may be confused with trematodes eggs and eggs of D. latum and Spirometra species are hard to distinguish from each other because of similar size range (D. latum 67-71 x 40-50 um, Spirometra spp 65-70 x 35-37 um). However, Spirometra eggs are more likely to float in sugar or zinc solutions than D. latum, and Spirometra eggs are more narrow at the anterior end that D. latum eggs.