Emerging infectious diseases can be classified by whether they arise from novel pathogen exposure or declines in host susceptibility. Identifying the relative roles of exposure and susceptibility provides important information for both predicting and managing disease. For many natural systems, pathogen exposure is challenging to measure and is frequently estimated using indirect correlates. Moreover, intraspecific variation in host susceptibility is often neglected and treated as a fixed parameter. Direct and refined measures of exposure and susceptibility may be powerful tools for understanding the processes that regulate disease. From June to December 2017, I tracked epidemics of a virulent aquatic pathogen in six host populations and directly measured pathogen exposure and host susceptibility to identify their importance in driving epidemic emergence. Exposure was measured by observing the prevalence of early, non-transmitting infections and by counting the number of infectious propagules found attacking individual hosts. Susceptibility of wild-caught hosts was measured experimentally using both dose response curves and a suite of immunological assays.
Results/Conclusions
I found that epidemic emergence cannot be predicted from exposure or susceptibility alone, but depends critically on their interaction. Low levels of pathogen exposure are prevalent for months prior to epidemics, and epidemics only emerge when host immune defenses decline to a critical minimum. Both exposure and susceptibility are temporally dynamic and show strong among-population variation, and I discuss the extent to which ecological processes may be driving this variation.