The impact of repeated doses of anthelmintic on the intensity of infection and age structure of Fasciola hepatica populations in sheep.

A mathematical model of ovine fascioliasis is used to assess the likely impact of repeated doses of 2 anthelmintic types on the intensity of infection in the ewes. When chemotherapy is applied in the autumn and early winter, the type of anthelmintic that kills proportionately more of the immature parasites brings about a greater reduction in the predicted overall mean intensity of infection than the type of anthelmintic that kills proportionately more of the mature parasites, and leads to a lower predicted burden of mature flukes. In both cases, each dose of flukicide has proportionately less effect on the predicted mean intensity of infection than the preceding one. It is argued that analytical models of the kind described here provide a more informative pre-trial comparison of anthelmintic properties than tabulation of age-dependent kill rates.