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Transmission of F. hepatica under natural conditions was analysed in a three year programme. The variables used were the indirect haemagglutination (IHA) technique, worm establishment in tracer lambs and the population dynamics, infection rate and shedding pattern of Lymnaea truncatula. It is concluded that fluke eggs, infected snails and metacercariae on herbage can survive the winter in the Netherlands. Metacercarial availability was positively correlated to the amount of rainfall in the grazing period. The role developed eggs that survive the winter is important, because this results in earlier infections in the herd. The use of the serological diagnosis method IHA is important to detect F. hepatica infection in an early stage. Use of cellophane paper on floats is a useful method for determining the shedding pattern of cercariae from L. truncatula. It is concluded that collection of metacercariae on cellophane floats, inventarization of L. truncatula and its infection level are useful tools for the prediction of liverfluke infections.

The effectiveness of the molluscicide N-tritylmorpholine (Frescon, Shell) in the control of fascioliasis was tested in various field conditions. The effect was not only measured by the study of populations of L. truncatula but its use in prevention was also determined by turning tracer lambs out to grass in treated and control field. The studies showed that Frescon applied at the recommended dose rate is a reliable agent, even in the prevention of acute outbreaks of fascioliasis. The preventive effect was found to be most marked on treatment in the spring (preferable to single treatment during the summer), which is of particular importance in view of the fact that infections may occur in the southern Netherlands during the early grazing season.