Simulation of pasture larval populations of Haemonchus contortus.

Studies at Armidale on the ecology of helminthoses have provided information on the populations of both parasitic and free-living stages of sheep nematodes under varying seasonal conditions. The complexity of the climatic, nutritional, physiological and immunological factors which interact in the epidemiology of parasitic disease is such that even given this information, it is difficult to predict the occurrence or likely consequences of worm infections in grazing sheep. The technique of computer simulation appeared to offer an attractive method for combining this diverse collection of data in such a way that it not only summarized our present knowledge of the system but also possessed some predictive value. This first attempt at the analysis of the system has been restricted to the freeliving stages of the abomasal parasite Haemonchus contortus. This parasite is of major economic importance in the New England environment, and there is probably more information available on its climatic requirements than for any other nematode infecting sheep. Ractliffe et al ( 1969) have constructed a model of the parasitic stages of the life cycle which they used to simulate host-parasite interactions in real sheep. The significance of their work was not only that the simulations were successful, but also that they drew attention to some hitherto unsuspected physiological differences between sheep which may be of great importance in the response to H. contortus infection. The aims of the present study were to simulate the fluctuations in the population of infective N. contortus larvae on the pasture, and to compare the simulated results with actual results obtained from a Phalaris-white clover pasture grazed by infected sheep over a two-year period.