MODELING PATTERNS OF PARASITE AGGREGATION IN NATURAL-POPULATIONS - TRICHOSTRONGYLID NEMATODE-RUMINANT INTERACTIONS AS A CASE-STUDY

The characteristically aggregated frequency distribution of macroparas ites in their hosts is a key feature of host-parasite population biolo gy. We begin with a brief review of the theoretical literature concern ing parasite aggregation. Though this work has illustrated much about both the sources and impact of parasite aggregation, there is still no definitive analysis of both these aspects. We then go on to illustrat e the use of one approach to this problem - the construction of Moment Closure Equations (MCEs), which can be used to represent both the mea n and second moments (variances and covariances) of the distribution o f different parasite stages and phenomenological measures of host immu nity. We apply these models to one of the best documented interactions involving free-living animal hosts - the interaction between trichost rongylid nematodes and ruminants. The analysis compares patterns of va riability in experimental infections of Teladorsagia circumcincta in s heep with the equivalent wildlife situation - the epidemiology of T. c ircumcincta in a feral population of Soay sheep on St Kilda, Outer Heb rides. We focus on the relationship between mean parasite load and agg regation (inversely measured by the negative binomial parameter, k) fo r cohorts of hosts. The analysis and empirical data indicate that k tr acks the increase and subsequent decline in the mean burden with host age. We discuss this result in terms of the degree of heterogeneity in the impact of host immunity or parasite-induced mortality required to shorten the tail of the parasite distribution (and therefore increase k) in older animals. The model is also used to analyse the relationsh ip between estimated worm and egg counts (since only the latter are of ten available for wildlife hosts). Finally, we use these results to re view directions for future work on the nature and impact of parasite a ggregation.