Malaria infection increases attractiveness of humans to mosquitoes

Abstract

Do malaria parasites enhance the attractiveness of humans to the parasite's vector? As such manipulation would have important implications for the epidemiology of the disease, the question has been debated for many years. To investigate the issue in a semi-natural situation, we assayed the attractiveness of 12 groups of three western Kenyan children to the main African malaria vector, the mosquito Anopheles gambiae. In each group, one child was uninfected, one was naturally infected with the asexual (non-infective) stage of Plasmodium falciparum, and one harboured the parasite's gametocytes (the stage transmissible to mosquitoes). The children harbouring gametocytes attracted about twice as many mosquitoes as the two other classes of children. In a second assay of the same children, when the parasites had been cleared with anti-malarial treatment, the attractiveness was similar between the three classes of children. In particular, the children who had previously harboured gametocytes, but had now cleared the parasite, were not more attractive than other children. This ruled out the possibility of a bias due to differential intrinsic attractiveness of the children to mosquitoes and strongly suggests that gametocytes increase the attractiveness of the children.

Figure 1. Experimental Set-Up

(A)–(C) Top (A), cross-sectional (B), and three-dimensional (C) views of the olfactometer used. The fan (a) draws air (∼130 l/min per tent) from the three tents (b) to the outside environment via PVC pipes (c), trap chambers (d), and a central chamber (e). Each trap chamber contains a collecting cage (f) into which an exit trap opens (g). The fan pipe and release cup (h) are fitted to the top and bottom of the central chamber, respectively. Diagrams are not shown to scale; all dimensions are in centimetres. Source: [12].

Figure 2. Graphical Representation of Results

(A) Number of mosquitoes attracted to each class of children. Points show means of 12 groups; vertical lines show standard errors of the means. Solid diamonds show data of children before treatment; open diamonds denote children after treatment. (B) Proportion of the responsive mosquitoes (i.e., the ones that were attracted to any of the children within a group) attracted to the children who harboured gametocytes (before treatment; dark bars) and to the children who had cleared their gametocytes (after treatment; light bars). The horizontal line shows the proportion expected if the mosquitoes showed no preference.