Seasonal availability of Fasciola hepatica metacercariae in a temperate Mediterranean area (Madrid, Spain).

In this experiment, conducted over a 3-year period (1988-1990), the seasonal availability of Fasciola hepatica metacercariae in a temperate Mediterranean area (Madrid, central Spain) was analysed according to the rhythms of snail infection, the periods of cercarial emission, and the resistance of metacercariae. In this area, snails could be infected from late April to early November. Cercarial emission by spring infection started at the beginning of summer and terminated 3-4 weeks later. Mid-summer temperatures led to the extinction of shedding populations and of the metacercariae emitted by them, but were well tolerated by the snails with undeveloped infections. No aestivation was observed and summer infection led to a progressive shedding wave from August to late December, when it was interrupted. These populations became extinguished in the colder winter of 1988, but overwintered in the mild and belated winter of 1989. Only the snails infected from late summer onwards resumed activity and shed cercariae in spring, from the second half of March to late June. Risk of infection for grazing animals during winter will depend on survival of metacercariae. With a mean mortality of 23% during December and January, and of 38% during the following 2 months, no more than 30% of metacercariae reach the spring alive. Accordingly, risk of infection for grazing animals in spring will depend on overwintering infection of snails. These results indicate that temperature more than humidity could be responsible for different transmission patterns from year to year as winter thermic profiles could be the key for the transmission of fasciolosis during spring.