THE OVERWINTERING OF EGGS, INTRAMOLLUSCAL STAGES AND METACERCARIAE OFFASCIOLA-HEPATICA UNDER THE TEMPERATURES OF A MEDITERRANEAN AREA (MADRID, SPAIN)

The survival of embryonated and unembryonated eggs, of snails with mat ure or immature infection and of metacercariae over the winter of a Me diterranean area was analyzed. Embryonated eggs were more resistant th an unembryonated eggs to cold weather, leaving a residual contaminatio n which was responsible for earlier spring infection of the snail. Ove rwintering was similar in snail populations with both mature and immat ure infections although the lifespan was shorter in the former. Both s nail populations survived in the mildest winter but not in the coldest . Only metacercariae from mid autumn were able to overwinter in an sig nificant proportion (45%) but they were non-viable by mid spring. Our results suggest that in very cold winters no risk for grazing animals should be expected in spring.