第七章 Chapter 2 Heterogeneities in macroparasite infections patterns and processes 1.pdf

Chapter 2: Heterogeneities in macroparasite infections: patterns and processes1 K. Wilson, O.N. Bjørnstad, A.P. Dobson, S. Merler, G. Poglayen, S.E. Randolph, A.F. Read and A. Skorping. Animals vary markedly in the number of parasites they harbour - most have just a few, but some have many. In this chapter, we ask why there is so much variation between individuals, how do we quantify this variation and what are the consequences of these heterogeneities for the dynamics of the host- parasite interaction? 2.1 Background Exhaustive empirical surveys have shown that, almost without exception, macroparasites (parasitic helminths and arthropods) are aggregated across their host populations, with most individuals harbouring low numbers of parasites, but a few individuals playing host to many (Shaw and Dobson 1995). Heterogeneities such as these are generated by variation between individuals in their exposure to parasite infective stages and by differences in their susceptibility once an infectious agent has been encountered. Experimental studies have shown that the extent of spatial aggregation in the infective stage distribution is reflected in the level of parasite aggregation across hosts (Keymer and Anderson 1979). Moreover, in the absence of any heterogeneity in exposure, even small differences in susceptibility between hosts can rapidly produce non-random, aggregated distributions of parasites (Anderson and May 1978). What is unclear at present, is the relative significance of these different mechanisms, and the importance of interactions between mechanisms in accentuating individual