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Host and Pathogen Coexistence in Sexually Transmitted and Vector-Borne Diseases Characterized by Frequency-Dependent Disease Transmission

There is likely to be a fundamental difference between the dynamics of diseases that are transmitted by random contact among individuals (or by aerial dissemination) and the dynamics of sexually transmitted iseases (or vector-borne diseases in which vectors actively search out their hosts) (May and Anderson 1979; Getz and Pickering 1983; Anderson and May 1991; Antonovics 1992). In the case of random-contact ransmission, the probability of a healthy individual's becoming diseased is expected to be a function of the absolute density of diseased individuals (regardless of the number of healthy individuals), whereas in the case of sexually transmitted and vector-borne diseases, this probability is a function of the frequency (or relative density) of diseased individuals. Getz and Pickering (1983) showed that, in simple models of host-pathogen systems incorporating frequency-dependent transmission, pathogens cannot stably coexist with their hosts and regulate population size. There is no threshold density for disease spread, and therefore the disease either spreads and results in extinction of both host and pathogen or the production of healthy individuals outstrips disease transmission and the disease is "washed out" of the population. Clearly, this raises the question of what factors may lead to host-pathogen coexistence in venereal and vector-borne diseases. In diseases in which transmission is density-dependent, coexistence of host and pathogen is facilitated if there is host regulation by some factor that acts independently of the disease (e.g., resources, safe sites) (Anderson and May 1978). Getz and Pickering (1983) demonstrated that density-dependent population regulation due to factors other than the disease can also lead to host-pathogen coexistence when there is frequency-dependent transmission. Here we explore the Getz-Pickering model further and in particular consider the case in which nondisease density-dependent factors act more severely on healthy than on diseased classes. Such differential density dependence is likely to be characteristic of sexually transmitted iseases in which population regulation is by density dependence acting primarily on the juvenile stage. Under such situations (and assuming only horizontal disease transmission) recruitment into the healthy class will be density-dependent, but recruitment into the diseased class