Fasciola LifeCycle 2013
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''Fasciola'', commonly known as the liver fluke, is a genus of parasitic
trematodes Trematoda is a class of flatworms known as trematodes, and commonly as flukes. They are obligate internal parasites with a complex life cycle requiring at least two hosts. The intermediate host, in which asexual reproduction occurs, is a moll ...
. There are three species within the genus ''Fasciola'': ''Fasciola nyanzae,'' ''Fasciola'' ''hepatica'' and ''Fasciola'' ''gigantica''. ''Fasciola hepatica'' and ''F. gigantica'' are known to form hybrids. Both ''F. hepatica'' and ''F. gigantica'' and their hybrids infect the liver tissue of a wide variety of mammals, including humans, in a condition known as fascioliasis. ''F. hepatica'' measures up to 30 mm by 15 mm, while ''F. gigantica'' measures up to 75 mm by 15 mm. ''Fasciola nyanzae'' is thought to exclusively infect the common hippopotamus, ''Hippopotamus amphibius''.


Species

*''Fasciola nyanzae'' Leiper, 1910 *''
Fasciola hepatica ''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitism, parasitic trematode (fluke (flatworm), fluke or flatworm, a type of helminth) of the class (biology), class Trematoda, phylum Platyhelminthes. It ...
''
Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
,
1758 Events January–March * January 1 – Swedish biologist Carl Linnaeus (Carl von Linné) publishes in Stockholm the first volume (''Animalia'') of the 10th edition of ''Systema Naturae'', the starting point of modern zoologic ...
*'' Fasciola gigantica'' Cobbold, 1855
  • Also published separately as:
  • Abstract published as:
* Hybrid or introgressed populations of ''Fasciola gigantica'' × ''Fasciola hepatica''


Life cycle

''Fasciola'' pass through five phases in their life cycle: egg, miracidium, cercaria, metacercaria, and adult fluke. The eggs are passed in the feces of mammalian hosts and, if they enter freshwater, the eggs hatch into miracidia. Miracidia are free-swimming. The miracidia then infect
gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...
intermediate hosts and develop into cercariae, which erupt from the body of the snail host and find and attach to aquatic plants. The cercariae then develop into metacercarial cysts. When these cysts are ingested along with the aquatic plants by a mammalian host, they mature into adult flukes and migrate to the bile ducts. The adults can live for 5–10 years in a mammalian host.


Animal hosts

The intermediate hosts, where ''Fasciola'' reproduce asexually, are
gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...
from the family
Lymnaeidae Lymnaeidae, common name the pond snails, is a Taxonomy (biology), taxonomic family (biology), family of small to large air-breathing freshwater snails, Aquatic animal, aquatic pulmonate gastropod mollusks, that belong to the clade Hygrophila (cl ...
, also known as pond snails. A wide variety of mammals can be
definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...
s, where ''Fasciola'' reach adulthood and reproduce, including pigs, rodents,
ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...
s, and humans. The most important animal reservoir hosts for human infections are sheep and cattle.


Geographic distribution

''Fasciola'' are widespread and inhabit 70 countries and parts of all continents except for Antarctica. It is most common in areas with sheep and cattle are raised. The regions most impacted by ''Fasciola'' are the northern
Andes The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...
and the Mediterranean region.


History and discovery

Evidence of fascioliasis in humans exists dating back to Egyptian mummies that have been found there are with Fasciola eggs. Cercariae of ''F. hepatica'' in a snail and flukes infecting sheep were first observed in 1379 by Jehan De Brie. The life cycle and hatching of an egg were first described in 1803 by Zeder.


Prevention and treatment

Fascioliasis is treated with
triclabendazole Triclabendazole, sold under the brand name Egaten among others, is a medication used to treat fascioliasis and paragonimiasis. It is very effective for both conditions. Treatment in hospital may be required. It is taken by mouth with typicall ...
. There is no vaccine for ''Fasciola'' currently available. In severe cases of biliary tract obstruction, surgery is an option to remove adult flukes. The most common way that humans are infected is through the consumption of undercooked vegetables, often
watercress Watercress or yellowcress (''Nasturtium officinale'') is a species of aquatic flowering plant in the cabbage family, Brassicaceae. Watercress is a rapidly growing perennial plant native to Eurasia. It is one of the oldest known leaf vegetabl ...
, that are contaminated with metacercariae. This is of particular concern in areas where animal waste is used as fertilizer for the cultivation of watercress, as the full life cycle of ''Fasciola'' can sustained while contaminating crops intended for human consumption. Additionally, in rare cases, ingestion of the raw liver of an infected animal can lead to infection. This is primarily in the Middle East and is known as halzoun. One prevention method is to kill off the snail hosts in a water body using molluscicides. Another method is treating entire communities that are at risk for contracting fascioliasis with triclabendazole. This is a time efficient method in impoverished rural communities, as it does not require testing the entire community for fascioliasis.


References

{{Authority control Veterinary helminthology