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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.
=== Snail biology ===
[[Image:Lymnaea truncatula.jpg|thumb|right|150px|''Lymnaea truncatula'' - Wikimedia Commons]]
==== ''Lymnaea truncatula'' ====
*5-10mm long
*Brown-black shell with 5-6 spirals
*1st spiral is greater than half the total length
*The shell opens on the right (when held with the opening upwards)
*Feeds on green slime
*Multiplies rapidly when food is abundant
*Most die during the British winter (unless very mild)
*Survivors lay eggs in spring, which hatch in June
==== Habitats ====
*''Lymnaea'' are 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 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
=== Winter infection of the snail ===
Fluke eggs passed in '''late summer'''
→ infect snails
→ development halted when temperature <10°C (i.e. flukes trapped in hibernating snails through the winter)
→ development resumes when temperature >10°C
→ cercariae shed from '''July'''
→ disease from '''August'''
====References====
====References====
Taylor, M.A, Coop, R.L., Wall,R.L. (2007) '''Veterinary Parasitology''' ''Blackwell Publishing''
Taylor, M.A, Coop, R.L., Wall,R.L. (2007) '''Veterinary Parasitology''' ''Blackwell Publishing''
G.L. Pritchard et al., Emergence of fasciolosis in cattle in East Anglia, ''The Veterinary Record'', Novemeber 5, 2005.
G.L. Pritchard et al., Emergence of fasciolosis in cattle in East Anglia, ''The Veterinary Record'', November 5, 2005.