Exposure of Anopheles mosquitoes to trypanosomes reduces reproductive fitness and enhances susceptibility to Plasmodium

During a blood meal, female Anopheles mosquitoes are potentially exposed to diverse microbes in addition to the malaria parasite, Plasmodium. Human and animal African trypanosomiases are frequently co-endemic with malaria in Africa. It is not known whether exposure of Anopheles to trypanosomes influences their fitness or ability to transmit Plasmodium. Using cell and molecular biology approaches, we found that Trypanosoma brucei brucei parasites survive for at least 48h after infectious blood meal in the midgut of the major malaria vector, Anopheles coluzzii before being cleared. This transient survival of trypanosomes in the midgut is correlated with a dysbiosis, an alteration in the abundance of the enteric bacterial flora in Anopheles coluzzii. Using a developmental biology approach, we found that the presence of live trypanosomes in mosquito midguts also reduces their reproductive fitness, as it impairs the viability of laid eggs by affecting their hatching. Furthermore, we found that Anopheles exposure to trypanosomes enhances their vector competence for Plasmodium, as it increases their infection prevalence. A transcriptomic analysis revealed that expression of only two Anopheles immune genes are modulated during trypanosome exposure and that the increased susceptibility to Plasmodium was microbiome-dependent, while the reproductive fitness cost was dependent only on the presence of live trypanosomes but was microbiome independent. Taken together, these results demonstrate multiple effects upon Anopheles vector competence for Plasmodium caused by eukaryotic microbes interacting with the host and its microbiome, which may in turn have implications for malaria control strategies in co-endemic areas.

In nature, females Anopheles mosquitoes that transmit the malaria parasites Plasmodium, take successive blood meals to maximize their offspring. During these blood meals, mosquitoes are exposed to a variety of microbes present in the host blood in addition to Plasmodium, the obligate parasite that causes malaria. The Trypanosoma parasites, causing trypanosomiases, are sympatric with the malaria parasites in numerous African regions, therefore, a single female mosquito could be in contact with both pathogens concurrently or through successive blood meals. In this work, we showed that exposure of females Anopheles mosquitoes to Trypanosoma enhanced their susceptibility to malaria parasites, reduced their reproductive fitness and modulated their bacterial gut flora. While the effect of trypanosomes ingestion on Plasmodium infection is microbiome dependent, the phenotype on the reproductive fitness is microbiome independent. These results highlight the need for considering the effect of eukaryotic microbes on Anopheles biology for malaria control strategies.