Much of our understanding of the population dynamics of host-pathogen interactions is based upon mathematical theory. This theory is in turn based upon particular assumptions about how the density of host and pathogen populations affects the rate at which new infections accrue. Specifically, the instantaneous rate of horizontal transmission is assumed to be proportional to the product of the densities of healthy and infected hosts, or the densities of healthy hosts and the infectious pathogen particles. There is, however, little evidence that this assumption is correct. and in fact, alternative models typically have very different dynamics. Most such alternatives, however, are either ad hoc, or were developed for human diseases, for which experimental tests of model assumptions are not ethical. In this research, the PI's will explore the conditions under which this basic assumption of the mathematical theory of disease is violated. These researchers will use the gypsy moth-nuclear polyhedrosis virus (NPV) system as a model system because the simplicity of this system makes it ideal for performing ecological experiments. The approach builds from earlier work in which the PI's demonstrated that a model using estimates of transmission from small-scale field experiments provides a reasonable approximation of the dynamics of disease at a large scale. Both the small-scale experiments and the model suggest that there is significant heterogeneity in transmission with respect to host and pathogen density, and that this heterogeneity is essential to understanding the dynamics of the gypsy moth-NPV system. The PI's will develop a set of alternative models that incorporate this transmission. The plan is to parameterize alternative models using small-scale transmission experiments, and test the different models using large-scale experimental epidemics. %%% The type of heterogeneity in disease spread is believed to be of fundamental importance for understanding the nature of infectious diseases, but this a spect has been rarely studied and is poorly understood. Ultimately, this research will not only provide details of the dynamics of disease spread in this particular system, but will also explore the foundations of the mathematical theory of infectious diseases, in general, bringing together theory and experiment in a way that is impossible for most host-pathogen systems.
