ALLOCATION TO SEXUAL VERSUS NONSEXUAL DISEASE TRANSMISSION

Many diseases have both sexual and nonsexual transmission routes, and closely related diseases often differ in their degree of sexual transm ission. We investigate the evolution of transmission mode as a functio n of host social and mating structure using a model in which disease t ransmission is explicitly dependent on the numbers of sexual and nonse xual contacts (which are themselves a function of population density) and per-contact infection probabilities. Most generally, and in the ab sence of tradeoffs between the degree of sexual transmission and effec ts on host fecundity and mortality, nonsexual transmission is favored above the social-sexual crossover point (the host density at which the number of nonsexual contacts exceeds the number of sexual contacts), while sexual transmission is favored below this point. When changes in allocation to the two transmission modes are accompanied by changes i n mortality or fecundity, both mixed and pure transmission strategies can be favored. If invading genotypes differ substantially from reside nt genotypes, genetic polymorphism in transmission mode is possible. T he evolutionary outcomes are predictable from a knowledge of the equil ibrium population sizes in relation to the social-sexual crossover poi nt. Our results also show that predictions about dynamic outcomes, bas ed on rates of invasion for single pathogens into healthy populations, do not adequately describe the resulting disease prevalence nor predi ct the subsequent evolutionary dynamics; once invasion of a pathogen h as occurred, the conditions for spread of a second pathogen are themse lves altered. If the host is considered as a single resource, our resu lts show that two pathogens may coexist on a single resource ii they u se that resource differentially and have differential feedbacks on res ource abundance; such resource feedback effects map be present in othe r biological systems.