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==Hosts==
''Fasciola hepatica'' has an indirect life cycle, meaning it has both intermediate and final hosts.
''Fasciola hepatica'' has an indirect life cycle, meaning it has both intermediate and final hosts.
The most important intermediate host within Europe is the snail of the genus '''Lymnaea'''. The most common, '''Lymnaea truncatula''', which is an amphibious snail found worldwide.
The most important intermediate host within Europe is the snail of the genus '''Lymnaea'''. The most common, '''Lymnaea truncatula''', which is an amphibious snail found worldwide.
==Life Cycle==
Adult flukes in the bile ducts shed '''eggs''' directly into the bile, which then subsequently enter the intestine. Eggs are then passed out in the faeces of the mammalian host, where they develop and hatch releasing motile ciliated '''miracidia'''. These require 9-10 days at optimal temperatures, of around 22-26 degrees. The miracidium have a short life and must locate a suitable snail, the intermediate host, within approximately 3 hours if they are to be effective and continue the life cycle.
Adult flukes in the bile ducts shed '''eggs''' directly into the bile, which then subsequently enter the intestine. Eggs are then passed out in the faeces of the mammalian host, where they develop and hatch releasing motile ciliated '''miracidia'''. These require 9-10 days at optimal temperatures, of around 22-26 degrees. The miracidium have a short life and must locate a suitable snail, the intermediate host, within approximately 3 hours if they are to be effective and continue the life cycle.
The prepatent period of ''Fasciola hepatica'' is 10-12 weeks. In untreated sheep it may survive and continue to infect for many years. In cattle it is usually less than 1 year.
The prepatent period of ''Fasciola hepatica'' is 10-12 weeks. In untreated sheep it may survive and continue to infect for many years. In cattle it is usually less than 1 year.
==Identification==
The egg is relatively large; around 140μm x 70μm. It is oval shaped, with a thin outer shell, and is browny-yellow.
The egg is relatively large; around 140μm x 70μm. It is oval shaped, with a thin outer shell, and is browny-yellow.
The fully mature adult fluke is a dark brown colour, and around 3cm in length.
The fully mature adult fluke is a dark brown colour, and around 3cm in length.
==Snail biology==
[[Image:Lymnaea truncatula.jpg|thumb|right|150px|''Lymnaea truncatula'' - Francisco Welter Schultes, Wikimedia Commons]]
[[Image:Lymnaea truncatula.jpg|thumb|right|150px|''Lymnaea truncatula'' - Francisco Welter Schultes, Wikimedia Commons]]
==== ''Lymnaea truncatula'' ====
==== ''Lymnaea truncatula'' ====
Most die in the British winter, due to the harsh, cold conditions, but they may survive in milder winters. Survivors will lay eggs in spring, which will hatch in June.
Most die in the British winter, due to the harsh, cold conditions, but they may survive in milder winters. Survivors will lay eggs in spring, which will hatch in June.
==Habitats==
''Lymnaea'' are found predominantly in muddy areas, but do not survive well in highly acidic soils. Habitats may be permanent; seen in dry summers or temporary; found in wet summers.
''Lymnaea'' are found predominantly in muddy areas, but do not survive well in highly acidic soils. Habitats may be permanent; seen in dry summers or temporary; found in wet summers.
==Epidemiology==
In temperate areas, there are two superimposed epidemiological cycles, known as the summer and winter infections of the snail. On mainland Britain, the summer cycle predominates as a high proportion of snails perish during the winter, but very occasionally, weather sequences allow the winter cycle to affect the pattern of disease. On the west coast of Ireland, the winter cycle of events determines the timing of clinical outbreaks.
In temperate areas, there are two superimposed epidemiological cycles, known as the summer and winter infections of the snail. On mainland Britain, the summer cycle predominates as a high proportion of snails perish during the winter, but very occasionally, weather sequences allow the winter cycle to affect the pattern of disease. On the west coast of Ireland, the winter cycle of events determines the timing of clinical outbreaks.
The cercariae are shed from late August onwards. The metacercariae develop and are ingested by a host; the sheep for example. The immature flukes then migrate through the liver, causing acute disease between the months of September and November, or chronic disease from January onwards.
The cercariae are shed from late August onwards. The metacercariae develop and are ingested by a host; the sheep for example. The immature flukes then migrate through the liver, causing acute disease between the months of September and November, or chronic disease from January onwards.
==Winter infection of the snail==
In this case the fluke eggs are passed in the late summmer, which then infect the snails. Environmental conditions play a vital role in the success of the fluke development. Temperatures below 10 degrees will see the development being haltered, and the flukes will remain trapped in the hibernating snails throughout the winter. Development will then resume when temperatures rise above 10 degrees. The cercariae are then shed from July, and disease may be seen from August onwards.
In this case the fluke eggs are passed in the late summmer, which then infect the snails. Environmental conditions play a vital role in the success of the fluke development. Temperatures below 10 degrees will see the development being haltered, and the flukes will remain trapped in the hibernating snails throughout the winter. Development will then resume when temperatures rise above 10 degrees. The cercariae are then shed from July, and disease may be seen from August onwards.
==Pathogenesis==
The severity of the infection is mainly dependent on the number of metacercariae ingested. The Pathogenesis is often described as two-fold. The first stage occurring when the parasite migrates through the liver parenchyma, causing liver damage and haemorrhage. The second phase occurs when the parasite is in the bile ducts, and damage is a result of the haematophagic activity of the adult flukes.
The severity of the infection is mainly dependent on the number of metacercariae ingested. The Pathogenesis is often described as two-fold. The first stage occurring when the parasite migrates through the liver parenchyma, causing liver damage and haemorrhage. The second phase occurs when the parasite is in the bile ducts, and damage is a result of the haematophagic activity of the adult flukes.