For many infectious diseases, contacts between individuals are a prerequisite for successful transmission of the pathogen, and the rate of contacts will determine the rate of transmission. Contacts between animals are influenced by many behavioral factors, several of which (e.g. territoriality, mate choice) are expected to change with population density. Such density-dependent changes in contacts can have important effects on pathogen transmission, invasion and persistence, but very little is known about how density can affect contacts or pathogen transmission, and most disease models assume transmission to be either independent of or linearly dependent on density without assessing the effects of this assumption. To address this empirical as well as conceptual knowledge gap, we combined a field experiment with a mathematical modelling study.
By manipulating rodent population densities in field enclosures, we found that foraging contacts at a baiting station increased with density in a nonlinear, sigmoidal fashion, where contacts initially remained low, but then increased rapidly from a threshold density until reaching a plateau. Using these contact-density data we then simulated how different shapes of the contact-density function would affect the transmission of a directly-transmitted pathogen, and found that although prevalence and incidence patterns were relatively unaffected, pathogen invasion and persistence were highly dependent on the exact shape of the contact-density function. In accordance with previous conceptual studies, these results show that the contact-density function should be chosen with care when modelling disease transmission in fluctuating populations.