Localization and tissue tropism of the symbiont Microsporidia MB in the germ line and somatic tissues of Anopheles arabiensis.

IMPORTANCE: Microsporidia MB is a symbiont with a strong malaria transmission-blocking phenotype in Anopheles arabiensis. It spreads in mosquito populations through mother-to-offspring and sexual transmission. The ability of Microsporidia MB to block Plasmodium transmission, together with its ability to spread within Anopheles populations and its avirulence to the host, makes it a very attractive candidate for developing a key strategy to stop malaria transmissions. Here, we report tissue tropism and localization patterns of Microsporidia MB, which are relevant to its transmission. We find that Microsporidia MB accumulates in Anopheles arabiensis tissues, linked to its sexual and vertical transmission. Its prevalence and intensity in the tissues over the mosquito life cycle suggest adaptation to maximize transmission and avirulence in Anopheles arabiensis. These findings provide the foundation for understanding the factors that may affect Microsporidia MB transmission efficiency. This will contribute to the development of strategies to maximize Microsporidia MB transmission to establish and sustain a high prevalence of the symbiont in Anopheles mosquito populations for malaria transmission blocking.

Horizontal Transmission of the Symbiont Microsporidia MB in Anopheles arabiensis.

The recently discovered Anopheles symbiont, Microsporidia MB, has a strong malaria transmission-blocking phenotype in Anopheles arabiensis, the predominant Anopheles gambiae species complex member in many active transmission areas in eastern Africa. The ability of Microsporidia MB to block Plasmodium transmission together with vertical transmission and avirulence makes it a candidate for the development of a symbiont-based malaria transmission blocking strategy. We investigate the characteristics and efficiencies of Microsporidia MB transmission between An. arabiensis mosquitoes. We show that Microsporidia MB is not transmitted between larvae but is effectively transmitted horizontally between adult mosquitoes. Notably, Microsporidia MB was only found to be transmitted between male and female An. arabiensis, suggesting sexual horizontal transmission. In addition, Microsporidia MB cells were observed infecting the An. arabiensis ejaculatory duct. Female An. arabiensis that acquire Microsporidia MB horizontally are able to transmit the symbiont vertically to their offspring. We also investigate the possibility that Microsporidia MB can infect alternate hosts that live in the same habitats as their An. arabiensis hosts, but find no other non-anopheline hosts. Notably, Microsporidia MB infections were found in another primary malaria African vector, Anopheles funestus s.s. The finding that Microsporidia MB can be transmitted horizontally is relevant for the development of dissemination strategies to control malaria that are based on the targeted release of Microsporidia MB infected Anopheles mosquitoes.

A microsporidian impairs Plasmodium falciparum transmission in Anopheles arabiensis mosquitoes.

A possible malaria control approach involves the dissemination in mosquitoes of inherited symbiotic microbes to block Plasmodium transmission. However, in the Anopheles gambiae complex, the primary African vectors of malaria, there are limited reports of inherited symbionts that impair transmission. We show that a vertically transmitted microsporidian symbiont (Microsporidia MB) in the An. gambiae complex can impair Plasmodium transmission. Microsporidia MB is present at moderate prevalence in geographically dispersed populations of An. arabiensis in Kenya, localized to the mosquito midgut and ovaries, and is not associated with significant reductions in adult host fecundity or survival. Field-collected Microsporidia MB infected An. arabiensis tested negative for P. falciparum gametocytes and, on experimental infection with P. falciparum, sporozoites aren't detected in Microsporidia MB infected mosquitoes. As a microbe that impairs Plasmodium transmission that is non-virulent and vertically transmitted, Microsporidia MB could be investigated as a strategy to limit malaria transmission.

Arboviruses and Blood Meal Sources in Zoophilic Mosquitoes at Human-Wildlife Interfaces in Kenya.

Background: Zoophilic mosquitoes play an important role in the transmission of arboviruses of medical importance at human-wildlife interfaces, yet arbovirus surveillance efforts have been focused mostly on anthropophilic mosquitoes. Understanding the diversity of zoophilic mosquitoes and their associated feeding patterns and arboviruses can inform better vector control strategies. Materials and Methods: We morphologically identified mosquitoes collected from two game reserves in Kenya, the Maasai Mara National Reserve (MMNR) and locations near the Shimba Hills National Reserve (SHNR). Representative mosquitoes were also identified by cytochrome c oxidase subunit 1 (COI) barcode sequencing. In addition, we identified the vertebrate hosts of mosquito blood meals from the contents of each mosquito's abdomen by high-resolution melting (HRM) analysis and sequencing of COI, 16S ribosomal RNA, and cytochrome b gene PCR products. Similarly, mosquito arbovirus infections were identified by HRM analysis and sequencing of Alphavirus- and Flavivirus-specific RT-PCR products. Results: Of 2858 mosquitoes collected, 51 were engorged with blood meals from seven different vertebrate hosts, including humans, birds, domestic, and peridomestic animals and wildlife. Culex was the most abundant mosquito genus, with Culex pipiens being the most abundant species in both study regions. Among MMNR samples, we detected dengue serotype-2 virus (DENV-2) for the first time in Aedes tarsalis and Aedes tricholabis, as well as Sindbis virus in male Cx. pipiens. We also detected DENV-2 in Aedes aegypti sampled from locations near the SHNR. Human and diverse wildlife blood meals were identified, including bushbuck blood in the dengue-infected Ae. tarsalis and both human and hippopotamus blood in a single Eretmapodites chrysogaster mosquito. Conclusions: Our findings highlight the potential risk of sylvatic dengue and Sindbis transmission to humans by zoophilic mosquitoes at human-wildlife interfaces in Africa. Of specific importance, we provide evidence of sylvatic DENV-2 in Ae. tarsalis and Ae. tricholabis, representing potential new dengue vectors.