Trematode life cycle stages
Trematodes or flukes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs (usually freshwater snails or land snails) as their intermediate host. To accomplish this, they have several varied life cycle stages.
The life cycle of a typical digenean trematode can be thought to begin when its egg is immersed in water. Following this, a miracidium hatches, which swims to find a mollusc host. The miracidia go through several stages in the mollusc host, eventually emerging as motile cercarial larvae. The cercaria either infects vertebrates through the skin or is ingested. In its vertebrate host, the cercaria matures to an adult form, the fluke, and lays eggs that are discharged with the host faeces or urine. In the presence of open water, the eggs hatch and the miracidium stage of life is reached again.
Typical life cyle stages
While the details vary with each species, the general life cycle stages are:
- Egg – discharged either in open water or in intestine of definitive host
- Miracidium (plural miracidia) – a free-living motile form, it is covered with cilia, and settles in the mollusc to become a sporocyst.
- Sporocyst – an elongated sac, it produces either rediae or more sporocysts.
- Redia (plural rediae) – a larval form with an oral sucker, it will produce either more rediae, or cercariae.
- Cercaria (plural cercariae) – the larval form of the parasite, it develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long swimming "tail", depending on the species. The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.
- Mesocercaria – a cercaria little modified but resting
- Metacercaria – a cercaria encysted and resting
- Adult – the fully developed mature stage, it is capable of sexual reproduction.
Deviations from the typical life cycle
Not all trematodes follow the typical sequence of eggs, miracidia, sporocysts, rediae, cercariae, and adults. In some species, the redial stage is omitted, and sporocysts produce cercariae. In some species, the cercaria develops into an adult within the same host.
Many digenean trematodes require two hosts, one (typically a snail) where asexual reproduction occurs in sporocysts, the other a vertebrate (typically a fish) where the adult form engages in sexual reproduction to produce eggs. In some species (for example Ribeiroia) the cercaria encysts, and waits until the host is eaten by a third host, in whose gut it emerges and develops into an adult.
Representations of life cycles of several different trematode species
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