Fasciola hepatica
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| Fasciola hepatica (Linnaeus, 1758) |
Fasciola hepatica, commonly known as the liver fluke, is a parasitic flatworm of the class Trematoda, phylum Platyhelminthes that infects the hepatic bile ducts of sheep and cattle, sometimes also humans, causing fascioliasis also known as fasciolosis.
[edit] Life cycle
In order to complete its life cycle, F. hepatica requires an aquatic snail as an intermediate host such as Lymnaea truncatula, in which the parasite can reproduce asexually. From the snail, minute cercariae emerge and swim through pools of water in pasture, and encyst as metacercariae on near-by vegetation. From here, the metacercariae are ingested by the ruminant, or in some cases, by humans eating un-cooked foods such as water-cress. Contact with low pH in the stomach causes the early immature juvenile to begin the process of excystment. In the duodenum, the parasite breaks free of the metacercariae and burrows through the intestinal lining into the peritoneal cavity. The newly excysted juveile does not feed at this stage, but once it finds the liver parenchyma after a period of days, feeding will start. This immature stage in the liver tissue is the pathogenic stage, causing anaemia and clinical signs sometimes observed in infected animals. The parasite browses on liver tissue for a period of up to 5-6 weeks and eventually finds its way to the bile duct where it matures into an adult and begins to produce eggs. Up to 25,000 eggs per day per fluke can be produced, and in a light infection, up to 500,000 eggs per day can be deposited onto pasture by a single sheep.
[edit] Disease biology
In the United Kingdom, Fasciola hepatica is a frequent cause of disease in ruminants - this is most common between March and December. Cattle and sheep are infected when they consume the infectious stage of the parasite from low-lying, marshy pasture. The effects of liver fluke are referred to as fascioliasis, and include anaemia, weight loss and sub-mandibular oedema. Diarrhoea is only an occasional consequence of liver fluke. Liver fluke is diagnosed by yellow-white eggs in the faeces. They are distinguishable from the eggs of Fascioloides magna, although the eggs of F. magna are rarely passed in sheep, goats or cattle.
A serious consequence of the liver damage caused by fascioliasis is that latent Clostridium novyi spores can be activated by the low oxygen conditions in the damaged tracts the parasite forms in the liver - this can lead to "black disease", caused by Clostridium novyi type B or immune-mediated haemolytic anaemia (IMHA) leading to haemoglobinuria caused by Clostridium novyi type D.
[edit] Treatment
The drug of choice in the treatment of fasciolosis is triclabendazole, a member of the benzimidazole family of anthelmintics. The drug works by preventing the polymerisation of the molecule tubulin into the cytoskeletal structures, microtubules. Unfortunately, resistance to this drug is rising worldwide. Some populations of liver fluke, for example in Australia, the Netherlands, southern Ireland and the UK have shown to be persistent after treatment with this drug, which is administered orally at a dose rate of 10mg per kg weight of the animal. Albendazole and praziquantel have been used.
