Trematode life cycle stages

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Life-history stages of the trematode flatworm Fasciola hepatica from 1911 Encyclopædia Britannica

Trematodes are parasitic flatworms of the class Trematoda, specifically parasitic flukes with two suckers, one ventral and the other oral. Trematodes are covered by a tegument, which simultaneously protects the organism from the environment and provides secretory and absorptive functions.

The life cycle of a typical trematode begins with an egg. Some trematode eggs hatch directly in the environment, while others are eaten and hatch within a host, typically a mollusc. The hatchling is called a miracidium, a free-swimming, ciliated larva. Miracidia may grow and develop within the intermediate host into a sac-like structure known as a sporocyst or into rediae, either of which may give rise to free-swimming, motile cercariae larvae. The cercariae then may either definitively infect a vertebrate host or a second intermediate host. Adult metacercariae or mesocercariae, depending on the individual trematode's life cycle, can then infect the vertebrate host or be rejected and excreted through faeces or urine.[1]

Typical life cycle stages[edit]

Bucephalid cercaria larva from Ernst Haeckel's Kunstformen der Natur (1904): The tail's furcae give the impression of horns, hence the genus name Bucephalus meaning "ox head".

While the details vary with each species, the general life cycle stages are:

Egg[edit]

The egg is found in the faeces, sputum, or urine of the definitive host. Depending on the species, it will be either non-embryonated (immature) or embryonated (ready to hatch). The eggs of all trematodes except schistosomes are operculated. Some eggs are eaten by the intermediate host (snail) or hatch in the environment (water).

Miracidium (plural miracidia)[edit]

Miracidia hatch from eggs either in the environment or in the intermediate host. They do not have a mouth, therefore, they cannot eat and need to find a host quickly if they hatch in the environment. Energy is needed to develop into a sporocyst. The first intermediate host can differ for different trematodes.[3]

Sporocyst[edit]

An elongated sac that produces either more sporocyst or rediae. This is where larva can develop.[4]

  • Mother Sporocyst: These have loose plates (cilia) and migrate to gonas.
  • Daughter Sporocyst: These are an asexual production of cercariae, they absorb nutrients while having no mouth.

Redia (plural rediae)[edit]

After the sporocyst form the larva, the first development from it forms the redia. [5] They have a mouth which allows them to have an advantage to their competitors because they can just consume them and will either produce more rediae or start to form cercariae. The amount of rediae, or daughter sporocyst, varies a lot in the representatives of different deign taxa. [6]

Parasite Competition in Snail Hosts[edit]

Co-infections of different parasite species within the same post could occur and cause competition between the rediae and sporocysts. Not all trematode species have a rediae stage, some may just have a sporocyst stage depending on the life cycle. The rediae are dominant to sporocyst because they have a mouth and are able to either eat their competitors' food or their competitor.[citation needed]

Cercaria (plural cercariae)[edit]

The larval form of the parasite, develops within the germinal cells of the sporocyst or redia.[7] A cercaria has a tapering head with large penetration glands.[8] It may or may not have a long swimming "tail", depending on the species.[7] 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: They are involved in an encysted stage either on vegetation or in a host tissue on the second intermediate host. They have a hard shell and are also involved in the trophic transmission. This is where the parasite is able to infect the definitive host because it consumes the second intermediate host that has metacercariae on/in it.[citation needed]
  • Metacercaria: A cercaria encysted and resting.They are only involved when there are 3 intermediate host life cycles.

Cercaria is also used as a genus of trematodes, when adult forms are not known.[9] The usage dates back to Müller, in 1773.[10]

Adult[edit]

The fully developed mature stage, it is capable of sexual reproduction.

Deviations from the typical life cycle[edit]

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.

Most trematodes are hermaphroditic, but members of the family Schistosomatidae are dioecious. Males are shorter and stouter than the females.[8]

Representations of life cycles of several different trematode species[edit]

Life cycle stages of a digenean human parasite, Schistosoma japonicum
Life cycle stages of a digenean fish parasite, Bucephalus polymorphus
Life cycle stages of trematode species that cause "swimmer's itch"

See also[edit]

References[edit]

  1. ^ Poulin, Robert; Cribb, Thomas H (2002). "Trematode life cycles: Short is sweet?". Trends in Parasitology. 18 (4): 176–83. doi:10.1016/S1471-4922(02)02262-6. PMID 11998706.
  2. ^ Caffara, Monica; Davidovich, Nadav; Falk, Rama; Smirnov, Margarita; Ofek, Tamir; Cummings, David; Gustinelli, Andrea; Fioravanti, Maria L (2014). "Redescription of Clinostomum phalacrocoracis metacercariae (Digenea: Clinostomidae) in cichlids from Lake Kinneret, Israel". Parasite. 21: 32. doi:10.1051/parasite/2014034. PMC 4078730. PMID 24986336.
  3. ^ Galaktionov, K. V., & Dobrovolʹskiĭ, A. A. (2003). The biology and evolution of trematodes: An essay on the biology, morphology, life cycles, transmission, and evolution of digenetic trematodes. Dordrecht: Kluwer Academic.[page needed]
  4. ^ Fried, Bernard, and Thaddeus K. Graczyk. Echinostomes as Experimental Models for Biological Research. Springer, 2011.[page needed]
  5. ^ Fried, Bernard, and Thaddeus K. Graczyk. Echinostomes as Experimental Models for Biological Research. Springer, 2011.[page needed]
  6. ^ Galaktionov, K. V., & Dobrovolʹskiĭ, A. A. (2003). The biology and evolution of trematodes: An essay on the biology, morphology, life cycles, transmission, and evolution of digenetic trematodes. Dordrecht: Kluwer Academic.[page needed]
  7. ^ a b "Glossary". VPTH 603 Veterinary Parasitology. University of Pennsylvania School of Veterinary Medicine.
  8. ^ a b "Schistosoma". Australian Society for Parasitology. Retrieved 19 February 2011.
  9. ^ Cercaria at WoRMS
  10. ^ Vermium terrestrium et fluviatile seu animalium infusorium, helminthicorum et testaceorum, non marinorum, succinct historia. OF Müller, 1773

External links[edit]