A model of encounters between host and parasite populations.

A simulation model of the encounter between host and parasite populations is described. The model is two-dimensional in that it represents hosts and parasites as sums of random numbers. It allows for the manipulation of host and parasite numbers, areas of interaction, congruity of geographic ranges, parasite infectivity, and reproduction, or non-reproduction, of the parasite. The model generates parasite distributions (number of hosts vs. parasite/host classes) and their parameters (prevalence, mean number of parasites/host, variance/mean ratio as a measure of aggregation), and thus reveals the manner in which these parameters vary under different encounter conditions, i.e. their "behavior". Simulation results indicated that the behavior of parasite population mean, prevalence, and degree of aggregation was primarily a function of the rate at which infective stages were supplied to the system. In cases in which infective stages were continuously available, prevalence rose rapidly to nearly 100%, with increasing infectivity and parasite numbers, and the populations were not particularly aggregated. When infective stages were introduced in single large waves, both mean and prevalence remained low and the parasite populations were highly aggregated. Model results were compared with published data sets. The latter were also seen to fall into the two general categories of parameter behavior.