Modelling anthelmintic resistance

Anthelmintic resistance in nematode parasites of sheep and goats is now a firmly established phenomenon, particularly in warm temperate or tropical regions of the world. The evolution of resistance by nematodes to broad-spectrum anthelmintics is of particular concern, as there are currently only three different chemical families of such drugs. Resistance to two of these is already ubiquitous, with several reports of resistance to the third. With no new broad-spectrum anthelmintics on the horizon, it is vital that we leant to manage resistance and conserve susceptibility so these valuable drugs for longer than we have in the past. A crucial factor in the evolution of anthelmintic resistance in a worm population is the extent to which survivors of drug treatment contribute their genes to future generations of worms. This contribution is influenced by frequency and timing of anthelmintic treatment, drug efficacy, life-expectancy and fecundity of adult worms, and current and future rates of larval intake. Larval intake, in torn, is determined by previous egg deposition, grazing management and weather. The acquired immune response of the host is of central importance through its effects on worm establishment, fecundity and death rate, and provides density-dependent regulation of worm populations. Physical experimentation with such a complex system is difficult, expensive and above all time-consuming. Further, results arc usually specific to the site and type of animal management used in the experiment. Because of the large number of climatic, biological and management variables that interact to determine the size and genetic constitution of a worm population, we believe that the only practical way to explore the system is with the aid of a model. Examples of the types of models used to investigate anthelmintic resistance are presented. The CSIRO model UNIVERSE is used to examine the consequences of some common management practices (or evolution of anthelmintic resistance in the ruminant parasite Trichostrongylus colubriformis