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→Disease Manifestations
*'''Feed intake''' - fluke infections cause reduced food intake. Note: chronic fasciolosis occurs at a time of year when animals are on a low plane of nutrition. This combined with the reduced food intake causes a significant effect on the development and severity of clinical and subclinical fasciolosis.
*'''Feed intake''' - fluke infections cause reduced food intake. Note: chronic fasciolosis occurs at a time of year when animals are on a low plane of nutrition. This combined with the reduced food intake causes a significant effect on the development and severity of clinical and subclinical fasciolosis.
*'''Species susceptibility''' - the proportion of flukes that reach the bile ducts is determined mainly by the fibroplastic potential of the liver and the effectiveness of the protective immune responses (which are ineffective in sheep). Therefore, establishment rate: '''sheep>cattle>pig'''.
*'''Species susceptibility''' - the proportion of flukes that reach the bile ducts is determined mainly by the fibroplastic potential of the liver and the effectiveness of the protective immune responses (which are ineffective in sheep). Therefore, establishment rate: '''sheep>cattle>pig'''.
=== Snail Biology ===
==== ''Lymnaea truncatula'' =====
*is 5-10mm long
*has a brown-black shell with 5-6spirals
*first spiral is greater than half the total length
*the shell opens on the right (when held with the opening upwards)
*feeds on green slime
*can multiply rapidly if food is abundant
*most die during the British winter (unless very mild)
*survivors lay eggs in spring, which hatch in June.
==== Habitats ====
*''Lymnaea'' is found in muddy areas (but not on highly acidic soils)
*habitats may be permanent (dry summer) or temporary (wet summer).
=== 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.
=== Summer Infection of the Snail ===
Fluke eggs that are passed in spring:
→ hatch in June (i.e. coincident with snail hatch)
→ miracidia infect newly hatched snails
→ develop and multiply in snail hepatopancreas during summer
→ cercariae shed from late August onwards
→ metacercariae ingested by sheep
→ immature flukes migrate through liver
→ '''acute''' disease '''September-November'''; or '''chronic''' diesease '''January''' onwards.