Malaria Infection Increases Attractiveness of Humans to Mosquitoes

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

Plasmodium falciparum Produce Lower Infection Intensities in Local versus Foreign Anopheles gambiae Populations

Both Plasmodium falciparum and Anopheles gambiae show great diversity in Africa, in their own genetic makeup and population dynamics. The genetics of the individual mosquito and parasite are known to play a role in determining the outcome of infection in the vector, but whether differences in infection phenotype vary between populations remains to be investigated. Here we established two A. gambiae s.s. M molecular form colonies from Cameroon and Burkina Faso, representing a local and a foreign population for each of the geographical sites. Experimental infections of both colonies were conducted in Cameroon and Burkina Faso using local wild P. falciparum, giving a sympatric and allopatric vector-parasite combination in each site. Infection phenotype was determined in terms of oocyst prevalence and intensity for at least nine infections for each vector-parasite combination. Sympatric infections were found to produce 25% fewer oocysts per midgut than allopatric infections, while prevalence was not affected by local/foreign interactions. The reduction in oocyst numbers in sympatric couples may be the result of evolutionary processes where the mosquito populations have locally adapted to their parasite populations. Future research on vector-parasite interactions must take into account the geographic scale of adaptation revealed here by conducting experiments in natural sympatric populations to give epidemiologically meaningful results

Natural plant diet impacts phenotypic expression of pyrethroid resistance in Anopheles mosquitoes

Success in reducing malaria transmission through vector control is threatened by insecticide resistance in mosquitoes. Although the proximal molecular mechanisms and genetic determinants involved are well documented, little is known about the influence of the environment on mosquito resistance to insecticides. The aim of this study was to assess the effect of plant sugar feeding on the response of Anopheles gambiae sensu lato to insecticides. Adults were fed with one of four treatments, namely a 5% glucose control solution, nectariferous flowers of Barleria lupulina, of Cascabela thevetia and a combination of both B. lupulina+C. thevetia. WHO tube tests were performed with 0.05% and 0.5% deltamethrin, and knockdown rate (KD) and the 24 h mosquito mortality were measured. Plant diet significantly influenced mosquito KD rate at both concentrations of deltamethrin. Following exposure to 0.05% deltamethrin, the B. lupulina diet induced a 2.5 fold-increase in mosquito mortality compared to 5% glucose. Species molecular identification confirmed the predominance of An. gambiae (60% of the samples) over An. coluzzii and An. arabiensis in our study area. The kdr mutation L1014F displayed an allelic frequency of 0.75 and was positively associated with increased phenotypic resistance to deltamethrin. Plant diet, particularly B. lupulina, increased the susceptibility of mosquitoes to insecticides. The finding that B. lupulina-fed control individuals (i.e. not exposed to deltamethrin) also displayed increased 24 h mortality suggests that plant-mediated effects may be driven by a direct effect of plant diet on mosquito survival rather than indirect effects through interference with insecticide-resistance mechanisms. Thus, some plant species may weaken mosquitoes, making them less vigorous and more vulnerable to the insecticide. There is a need for further investigation, using a wider range of plant species and insecticides, in combination with other relevant environmental factors, to better understand the expression and evolution of insecticide resistance

Graphical Representation of Results

<div><p>(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.</p> <p>(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.</p></div

Experimental Set-Up

<p>(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: [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0030298#pbio-0030298-b12" target="_blank">12</a>].</p

Differences in infection prevalence and intensity between sympatric and allopatric vector parasite couples.

<p>*Note that results between PfCM and PfBF cannot be directly compared as mosquitoes were fed on hosts with different infectivities.</p

A Bland-Altman plot showing the differences in oocyst intensity between sympatrically and allopatrically infected mosquitoes.

<p>Arithmetic means are plotted for PfCM infections as blue circles and PfBF infections as red triangles. When points fall below 0 on the y axis, the mean oocysts in the sympatric infection was lower than for the corresponding allopatric infection.</p

Infection summaries.

<p>Ordered according to arithmetic mean oocysts/midgut in sympatric combinations, denoted by a *. # arithmetic mean of infected mosquitoes only in brackets. Tf: trophozoites, Gams: gametocytes.</p