©2004 by MEDIMOND S.r.l. 245 E719C0610 Management and Control of Parasitic Gastroenteritis in Sheep Using Computer Model Simulations M.A. Taylor, J. Learmount, C. Morgan and G. Smith Central Science Laboratory, Sand Hutton, York, United Kingdom Summary Parasitic gastroenteritis (PGE) is a complex helminthological disease syndrome resulting from the interplay of a large number of factors, including climate, nematode bionomics, animal management and hus- bandry practices. This paper reports on the development of a computer model built around extensive published data for the major species of parasitic nematodes of sheep found in the UK. The model can be used to describe the dynamics of anthelmintic treatments with the 3 major anthelmintic classes and the interaction between treatments and resist- ant and susceptible worms within the parasite lifecycle. It has the po- tential to be used to examine the effects of different treatment strategies on levels of infections and predict the effect that they have on the development of resistance. The model is unique in that it has been constructed to allow a wide range of user inputs that can be used to define flock management systems and treatment strategies for individual farms in differing regions of the UK. Introduction Helminth diseases are a major cause of ill-thrift, loss of production and death in cattle and sheep, and remain major welfare issues. The epidemiology of these diseases is directly influenced by climate, an- thelmintic usage and patterns of dosing, management and husbandry systems, and these in turn influence the development of natural immu- nity and the selection pressures that increase the potential for anthelmintic resistance to develop.