Epidemiology and optimal foraging: modelling the ideal free distribution of insect vectors

Existing models of the basic case reproduction number (R-0) for vector-born e diseases assume (i) that the distribution of vectors over the susceptible host species is homogenous and (ii) that the biting preference for the sus ceptible host species rather than other potential hosts is a constant. Empi rical evidence contradicts both assumptions, with important consequences fo r disease transmission. In this paper we develop an Ideal Free Distribution (IFD) model of host choice by blood-sucking insects, predicated on the arg ument that vectors must have evolved to choose the least defensive hosts in order to maximize their feeding success. From a re-analysis of existing da ta, we demonstrate that the interference constant, in, of the IFD can vary between host species. As a result, the predicted distribution of insects ov er hosts has 2 desirable and intuitively plausible behaviours: that it is h eterogeneous both within and between host species; and that the intensity o f heterogeneity varies with host and vector density. When the IFD model is incorporated into R-0, the relationship with the vector:host ratio becomes non-linear. If correct, the IFD could add considerable realism to models wh ich seek to predict the effect of these ecological parameters on disease tr ansmission as they vary naturally (e.g. through seasonality in vector densi ty or host population movement) or as a consequence of artificial manipulat ion (e.g. zooprophylaxis, vector control). It raises the possibility of tar geting transmission hot spots with greater accuracy and concomitant reducti on in control effort. The robustness of the model to simplifying assumption s is discussed.