The trematodes Fasciola gigantica and Fasciola hepatica (the Sheep Liver Fluke) are parasites of herbivores that can infect humans accidentally, causing a condition known as fascioliasis. Fascioliasis occurs worldwide . Human infections with F. hepatica are found in areas where sheep and cattle are raised, and where humans consume raw watercress (see life cycle), including Europe, the Middle East, and Asia. Infections with F. gigantica have been reported, more rarely, in Asia, Africa, and Hawaii. Fascioliasis in Europe, the Americas, and Oceania involves only F. hepatica, but both F. hepatica and F. gigantica occur in many parts of Africa and Asia and there is evedince that hybridization occurs (Mas-Coma et al. 2005). (Centers for Disease Control Parasites and Health Website)
Immature eggs are discharged in the biliary ducts and in the stool. Eggs become embryonated in water and release miracidia, which invade a suitable snail intermediate host, including snails in the genera Galba, Fossaria and Pseudosuccinea. In the snail, the parasites pass through several developmental stages: sporocyst, redia, and cercaria. The cercariae are released from the snail and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals acquire the infection by eating vegetation containing metacercariae. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress. After ingestion, the metacercariae excyst in the duodenum and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults. In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months. The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75 mm) reside in the large biliary ducts of the mammalian host. (Centers for Disease Control Parasites and Health Website)
Two hosts are needed for these parasites to complete their life cycle. The deﬁnitive host range is very broad and includes many herbivorous mammals, including humans. Intermediate hosts are freshwater snail species of the family Lymnaeidae (Gastropoda: Basommatophora). Fasciola hepatica has spread to other continents from Europe through the exportation of European livestock to other continents, where it has adapted to new hosts such as camelids in Africa and South America and marsupials in Australia. This expansion is also related to the geographic expansion of the original European lymnaeid intermediate host species of F. hepatica, G. truncatula, the spread of the American intermediate host species Pseudosuccinea columella, and the parasite's adaptation to lymnaeid species occurring in new areas. The more limited geographic distribution of F. gigantica seems to be related to the weaker diffusion capacity of its intermediate snail hosts, the African Radix natalensis and the European Radix auricularia. Mas-Coma et al. (2005) reviewed the biology, diagnosis, treatment, and epidemiology of fascioliasis.(Mas-Coma et al. 2005 and references therein) Young et al. (2011) reported on a transcriptome analysis of F. gigantica, which should facilitate genetic and physiological studies that could lead to effective interventions against this parasite.