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→Summer Infection of the Snail
→ '''acute''' disease '''September-November'''; or '''chronic''' diesease '''January''' onwards.
→ '''acute''' disease '''September-November'''; or '''chronic''' diesease '''January''' onwards.
=== Winter Infection of the Snail ===
Eggs dropped 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.
=== Geographical Distribution of Disease ===
Transmission is dependent on the snail, and therefore associated with snail habitats. These are more plentiful in high rainfall areas such as found on the western side of the British Isles, or alongside drainage canals in low-lying areas such as the Romney Marsh (or the Netherlands). It is important to remember that sheep and cattle are often bred in Ireland or western Britain and transported to the east of th country for fattening. Infected animals may therefore be found in areas where no transmission occurs.
=== Weather Patterns and Disease Risk ===
'''Wet summer''':
Many fluke and snail eggs hatching and snail habitats expanded, therefore:
→ snail population increases rapidly
→ many infected snails
→ many cercariae shed
→ high density of metacercariae on herbage
→ '''increased''' risk of disease.
'''Dry summer''':
Few fluke and snail eggs hatching and snails restricted to permanent habitats, therefore:
→ snail populations small and overcrowded
→ few infected snails, high mortality
→ few cercariae shed
→ few metacercariae confined to restricted areas
→ '''reduced''' risk of disease.