Life cycle of Fasciola Hepatica
(the Sheep Liver Fluke) and the Anatomy of the Sheep Liver
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Anatomy of the Sheep Liver
pointed caudate lobe
all edges are 'sharp'
deeper division between lobes
cigar shaped gall bladder
weight approx 500-700 g
Adult flukes lay eggs in the infected cattle or sheep's bile ducts in the liver. These eggs eventually enter the intestinal tract with the bile and are then expelled with the animal's feces (1). If the feces drop onto wet pasture, the eggs hatch, releasing free-living miracidiae (2).
Miracidiae swim around in water trying to find intermediate host snails. A few species of lymnaeid snails act as intermediate hosts of the liverfluke (3). Ideal conditions for the snails are poorly drained pastures, irrigation ditches and springs.
Once located, the miracidium infects the lymnaeid snail and multiplies asexually to great numbers, finally becoming cercariae(4). The cercariae leave the snail after about two months and swim around to attach themselves to grass. The attached cercariae develop a protective membrane, forming cysts called metacercariae that are able to survive dry and adverse climatic conditions for several months (5).
While grazing, the cattle and sheep ingest metacercariae. Following ingestion, metacercariae hatch and develop into early immature flukes in the gut, cross the wall of the gastrointestinal tract and migrate to the liver through the body cavity (6).
Upon reaching and penetrating the liver, the early immature flukes migrate through the liver tissue, causing acute fasciolosis. These flukes gradually grow and become immatures. The immature stages cause sub-acute fasciolosis. After approximately eight weeks, the flukes eventually enter the bile ducts where they develop into adults. Adult flukes feed on blood and start laying eggs. They are the cause of chronic fasciolosis.
Picture of actual sheep liver taken by Skeet Keller
This page was last updated on: October 26, 2008
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Fasciola hepatica infects numerous mammals, especially cows, sheep and other ruminants, existing in the bile passages of the liver. The egg capsules are passed via bile to the small intestine, travel through the large intestine to ultimately be deposited into the environment through feces. Should the feces be deposited in water, the egg capsules will embryonate and in about 9-10 days in warm weather, hatch, liberating the miracidium. This ciliated, non-feeding larvae will seek out snails of Stagnicola bulimoides or Fossaria modicella in the US but other related snails in other parts of the world. Within the snail host, the miracidium transforms into a sporocyst which in turn produces redia known as mother redia since they will produce daughter redia. These intramolluscan stages will give rise to cercaria of the gymnocephalus type which contain cystogenous glands that enable them to form encysted metacercaria on vegetation or in the water. Should the metacercaria-infected vegetation end up "high and dry" and be eaten or should the host drink the metacercaria-contaminated water, these minute, immature worms excyst in the small intestine, penetrate the lining of the intestine and creep over internal organs till they find the liver. There the young worms will invade the capsule of the liver and wander about through the hepatic cells, feeding and growing until they enter the bile ducts about 2 months post-infection. About one month after entering the liver, the worms attain adulthood and begin producing egg capsules.