Bacterial infection in mosquitoes renders them immune to malaria parasites

Scientists funded by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, have established an inheritable bacterial infection in malaria-transmitting Anopheles mosquitoes that renders them immune to malaria parasites.

Specifically, the scientists infected the mosquitoes with Wolbachia, a bacterium common among insects that previously has been shown to prevent malaria-inducing Plasmodium parasites from developing in Anopheles mosquitoes. Before now, researchers had been unable to create mosquitoes with a stable Wolbachia infection that passed consistently from mother to offspring.

In this study, led by Zhiyong Xi, Ph.D., at Michigan State University, the researchers focused on Anopheles stephensi mosquitoes, the primary malaria carrier in the Middle East and South Asia. The scientists injected Wolbachia into male and female embryos of A. stephensi and, once they matured, mated the adult females with uninfected male mosquitoes. A stable Wolbachia infection was maintained for 34 generations of mosquitoes, at which time the study ended. The researchers also introduced Wolbachia infection into uninfected adult mosquitoes in a series of experiments in which infected female mosquitoes comprised 5 percent, 10 percent or 20 percent of the mosquito population. In all three experiments, 100 percent of the mosquitoes were infected within eight generations, supporting the potential of Wolbachia-infected mosquitoes as a malaria control strategy. Similar approaches have been used successfully to control dengue, another mosquito-borne disease, in certain settings.

In their examination of how Wolbachia affects Plasmodium parasites, the researchers found that the bacterium kills the parasites both in the mosquito midgut, where the parasites mature, and in the salivary glands, from which the parasites are transmitted to humans via mosquito bites. The scientists hypothesize that Wolbachia infection causes the formation of unstable compounds known as reactive oxygen species (ROS), which inhibit the development of the parasites. Future studies might examine whether Plasmodium can become resistant to ROS and explore ways to integrate Wolbachia-infected mosquitoes with existing malaria control strategies, the researchers write.