Life Cycle of Trematoda
There are about 9,000 trematode species--also known as flukes--that have been described. All are parasitic. Mammals are host to the adults of most species. They are members of the scientific phylum Platyhelminthes, which includes various types of flatworms.
Trematodes have complex life cycles. The larvae stage of the trematode acts as a parasite to different species that the adult trematode will use as host. Among the most recognized trematodes are the liver fluke and the blood fluke, both of which are parasites of humans.
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Eggs
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The adult fluke, living in the organ of a mammal specific to its species, will release immature eggs that are passed from the body in the stool. The eggs develop in water and each egg releases a larva that seeks out a snail as an intermediate host. Rather than living in an organ, the blood fluke lives within the blood vessels of its host. Eggs are passed into the blood vessels until they find their way to the host's tissue and are passed from the body in urine or stool. Female flukes may discharge up to 300 eggs per day, depending upon species.
Larvae
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Within the snail, the fluke will go through several stages of development, including the sporocyst stage. In this stage, cells begin dividing to produce additional sporocysts that will develop into larvae. Eventually, the parasitic larvae will emerge from the snail and will enclose themselves in a cyst on a surface in the water. The blood fluke does not go through this part of the cycle.
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Infection
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A mammal will eat the vegetation, including the cyst. After ingestion, the larvae will leave the cyst and migrate through the intestinal wall to the organ where its specific species lives. Development will continue to the adult stage within the organ. From ingestion to maturity usually takes 3 to 4 months. Again, the exception is the blood fluke. Blood flukes emerge from the snail and seek out their primary host, penetrating the skin and finding their way to the bloodstream.
Adult
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The adult fluke will be flat and leaf-shaped with an unsegmented body. It will live in the organ of the mammal that is specific to its species. Blood flukes live in the veins, attaching themselves to the blood vessel walls.
Reproduction
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Most trematode species are hermaphroditic, meaning both sexes are contained in one individual fluke and the individual performs self-fertilization. An exception is the blood fluke. In this species, the male and female attach permanently and mate for life.
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References
- Photo Credit dpd.cdc.gov
