A possible mechanism for the suppression of Plasmodium berghei development in the mosquito Anopheles gambiae by the microsporidian Vavraia culicis

Abstract

Background: Microsporidian parasites of mosquitoes offer a possible way of controlling malaria, as they impede the development of Plasmodium parasites within the mosquito. The mechanism involved in this interference process is unknown.

Methodology: We evaluated the possibility that larval infection by a microsporidian primes the immune system of adult mosquitoes in a way that enables a more effective anti-Plasmodium response. To do so, we infected 2-day old larvae of the mosquito Anopheles gambiae with one of 4 isolates of the microsporidian Vavraia culicis and reared one group as an uninfected control. Within each treatment, we fed half the adult females on a mix of P. berghei ookinetes and blood and inoculated the other half with a negatively charged CM-25 Sephadex bead to evaluate the mosquitoes' melanisation response.

Conclusions: The microsporidian-infected mosquitoes were less likely to harbour oocysts (58.5% vs. 81.8%), harboured fewer oocysts (8.9 oocysts vs. 20.7 oocysts) if the malaria parasite did develop and melanised the Sephadex bead to a greater degree (73% vs. 35%) than the controls. While the isolates differed in the number of oocysts and in the melanisation response, the stimulation of the immune response was not correlated with either measure of malaria development. Nevertheless, the consistent difference between microsporidian-infected and -uninfected mosquitoes--more effective melanisation and less successful infection by malaria--suggests that microsporidians impede the development of malaria by priming the mosquito's immune system.

Figure 1. Success of infection by P. berghei in control mosquitoes and mosquitoes infected by one of four isolates of V. culicis.

(a) The proportion of mosquitoes that harboured at least one oocyst 10 days after blood feeding. (b) The mean number of oocysts in mosquitoes with at least one oocyst. In both panels, the vertical lines show the standard errors of the estimates, the horizontal, dotted lines show the means of the controls, and the numbers in the bars indicate the number of mosquitoes analyzed. The isolates are numbered in order of increasing melanisation efficacy (see Figure 2).

Figure 2. Efficacy of melanization response in control mosquitoes and mosquitoes infected by one of four isolates of V. culicis.

Each point represents the mean proportion of a Sephadex bead melanized by a mosquito. The vertical lines represent the standard errors of the means and the horizontal, dotted line shows the mean of the controls. Again, the numbers in the bars indicate the number of mosquitoes sampled.

Figure 3. The association between the melanization response and the success of infection by P. berghei.

Each point shows the mean within a block of the control or the microsporidian-infected mosquitoes (where the four isolates were pooled), and the horizontal and vertical lines show the standard errors of the estimates. (a) Association between the melanization response and the proportion of mosquitoes harbouring at least one oocyst 10 days after bloodfeeding. (b) Association between the melanization response and the mean number of oocysts.