Analysis of anthelmintic trial protocols using sheep experimentally or naturally infected with Fasciola hepatica.

Analytical models of the common anthelmintic trial protocols for ovine fascioliasis are used to investigate the way in which the conventional measure of anthelmintic efficiency (percentage reduction in mean fluke burden) depends on factors other than the additional fluke death rate due to chemotherapy. In prophylactic protocols where infection and dosing are coincident, the natural death rate of the parasites (both flukes and metacercariae) is an important factor. Numerical simulations indicate that the between-trial variations in anthelmintic efficiency due to fluctuations in natural parasite mortality may be substantial, especially for anthelmintic trials in the field.