CHANGES IN PARASITE AGGREGATION WITH AGE - A DISCRETE INFECTION MODEL

We present a discrete time model for age-related changes in the mean a nd variance of the number of helminth parasites per host. We assess th e degree of aggregation as the negative binomial parameter, k, and use the model to examine the influence of various factors on changes in a ggregation with host age: discrete versus continuous infection; the de gree of predisposition to infection; infection rate; parasite survival rate; and the variance in exposure to infective stages. The model can produce both increases and decreases in k with host age. However, wit h parameter values typical of many human helminth infections, a monoto nic increase in k (decrease in aggregation) with age is predicted. Wit h an age-dependent infection rate, convex relationships between K and age are possible. These predictions are consistent with data from fiel d studies, but differ from those of previous models which have suggest ed that k is independent of host age in the absence of density depende nce in parasite population dynamics. Differences between the models, a nd some difficulties in the interpretation of field data, are discusse d.