Suppression of Plasmodium falciparum infection by Microsporidia MB in wild Anopheles arabiensis populations

Symbiont-based approaches offer a promising strategy for reducing malaria transmission by suppressing Plasmodium development in Anopheles mosquitoes. Microsporidia MB, a naturally occurring microsporidian symbiont of Anopheles arabiensis, inhibits Plasmodium falciparum development in the laboratory, but its efficacy under natural transmission conditions is unknown. Here we monitored Microsporidia MB in wild An. arabiensis over 30 months in western Kenya and identified distinct low- and high-intensity infection phenotypes consistent with horizontal and vertical transmission, respectively. Overall, Microsporidia MB was associated with approximately 50% lower odds of P. falciparum infection. This association strengthened with symbiont intensity: high-intensity infections were associated with approximately 60% lower odds of Plasmodium infection compared with approximately 34% for low-intensity infections, identifying symbiont intensity as a key determinant of parasite-blocking efficacy. These findings provide evidence that Microsporidia MB suppresses P. falciparum infection in wild Anopheles populations, providing a key component of the field validation needed to advance Microsporidia MB as a candidate for malaria-control applications.