Dw. Kelly et Ce. Thompson, Epidemiology and optimal foraging: modelling the ideal free distribution of insect vectors, PARASITOL, 120, 2000, pp. 319-327
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.