Nematode parasites of sheep: a survey of epidemiological parameters and their application in a simple model

We review the literature on parameter values relevant to the epidemiology o f strongyle nematode infections of domestic sheep. Information is subdivide d by parasite genus, country of origin and climate type. While field observ ations have been made in a large number of countries, the bulk of studies u nder controlled conditions have been conducted in Australia, New Zealand an d the UX. For these countries, experiments and parameters are interpreted i n terms of a previously published model of nematode dynamics, and are used to calculate the basic reproduction number. Average values range from less than 6 for Haemonchus contortus in New Zealand and a winter rainfall region of Australia, to more than 16 for Ostertagia circumcincta in New Zealand a nd the UK. Additional considerations of the effects of climate and the annu al replacement of host stock show that for conditions favourable for parasi te transmission this is a robust indicator of parasite epidemiology. When c limate variation and annual replacement are added to the model, it is shown to reasonably describe the qualitative behaviour of an experimental data s et, indicating it to be a useful tool for further investigation of some of the underlying assumptions of sheep-nematode dynamics.