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Trematode Life Cycle
"The complex life-cycle of the parasitic trematode Riberoria ondatrae", ink on paper April-May 2002
Trematode Life Cycle

Digenetic trematodes have  complex life cycles  with a primary host and one or more intermediate hosts.  The primary host is usually a vertebrate carnivore, in which the adult trematode (or fluke) lives.  In the case of the multilegged amphibians that we have examined the trematode appears to be a species of the genus Ribeiroia, Family Psilostomidae, a trematode that uses aquatic birds as its primary, or definitive, host and amphibians as its second intermediate host (Schell, 1985;  Sessions, et al., 2000).  A diagnostic feature of this trematode is a pair of diverticulae just posterior to the muscular pharynx;  no other North America species of digenetic trematode has this feature ( picture of the trematode; Schell, 1985). The trematode eggs are released from the primary host into the environment, including ponds, where they hatch into a free swimming stage called a miracidum, which penetrates aquatic snails (we have found it in both planorbids and physids).  Once in a snail, each miracidium produces numerous embryos of the next larval stage, or redia.  Each redia, in turn, contains numerous embryos of a third larval stage, which is either another redia or a swimming larval stage called a cercaria.

This results in a kind of  embryonic amplification  in which hundreds, thousands, or even hundreds of thousands of cercariae are ultimately produced (Schell, 1985).  Thus the snails serve as a kind of "incubator" for embryonic amplificiation.  The cercariae exit the snail, in response to particular environmental stimuli (e.g. light and/or temperature), and swim vigorously around until they encounter a suitable second intermediate host, in this case a frog tadpole or salamander larva.  The cercariae form cysts (metacercariae) on the surface of the tadpoles' skin which then penetrate into the tissues over the next few hours.  The result is extensive inflammation, swelling, and tissue outgrowths at the sites of cercaria infestation.   Clearing and staining  can be used to visualize these cysts in close physical association with deformities ( picture ).  If such a tadpole is eaten by the primary host (in this case, a bird), the metacercariae excyst to form adult trematode worms.  Thus, the amphibians are used by the trematodes as vehicles to transport their offspring from the snails back into their primary hosts!  At least 41 species of trematodes utilize amphibians as second intermediate hosts in North America alone (Schell, 1985). We have found Ribeiroia infected snails in New York as well as in Oregon and California, so this trematode species is very widespread.

(Source of Information: Update on Deformed Amphibian Research at Hartwick College)

Research at Hartwick College

Deformed Amphibian Research at Hartwick College being done in the laboratory of Stanley K. Sessions in collaboration with undergraduate Hartwick students and various other colleagues across the country.

website: Trematode Life Cycles

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