Genotypic variation in parasite avoidance behavior and other mechanistic, non-linear components of transmission

Traditional epidemiological models assume that transmission increases proportionally to the density of parasites. However, empirical data frequently contradict this assumption. General yet mechanistic models can explain why transmission depends non-linearly on parasite density and thereby identify potential defensive strategies of hosts. For example, hosts could decrease their exposure rates at higher parasite densities (via behavioral avoidance) or decrease their per-parasite susceptibility when encountering more parasites (e.g., via stronger immune responses). To illustrate, we fit mechanistic transmission models to 19 genotypes of Daphnia dentifera hosts over gradients of the trophically-acquired parasite, Metschnikowia bicuspidata. Exposure rate (foraging, F) frequently decreased with parasite density (Z), and per-parasite susceptibility (U) frequently decreased with parasite encounters (FxZ). Consequently, infection rates (FxUxZ) often peaked at intermediate parasite densities. Mor...