Combined effects modeling of ionizing radiation and biological agent exposures.

A simple two-species population model is used to simulate the interactions of a replicating pathogen and the immune system response. This deterministic model is flexible enough to reproduce the salient population-dynamic features, chiefly the tendency toward an equilibrium state in which the pathogen population is reduced to zero or a negligible level after some finite time. The specific immune system species that can be taken initially at zero increases to a finite equilibrium level that represents an immune system "memory" of a given pathogen. The coefficients in the model equations represent production, interaction, and loss rates. Their relative sizes determine the qualitative behavior of the solution in its phase space. The effect on the immune system of exposure to ionizing radiation is modeled by reducing the production rate coefficients; the reduction is proportional to the level of ionizing radiation. According to the model and based on historical hematopoietic data from retrospective accident studies, exposures to ionizing radiation can cause pathogen levels to rise an order of magnitude or more than the baseline (no radiation exposure) case and, for some simulated levels, escape from the bounded region of the phase space. A means is offered to link the deterministic model of time-varying pathogen level in an individual host to the corresponding probability of lethality (increasing with time) of an individual in a population.