Heterogeneities in macroparasite infections: patterns and processes

Abstract

Abstract 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 differences in parasite loads. Mathematical models that examine these problems rapidly become intractable (Grenfell et al. 1995), while experimental studies and computer simulations also become rather complex.