Microsporidia: a promising vector control tool for residual malaria transmission

Abstract

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) have resulted in a major decrease in malaria transmission. However, it has become apparent that malaria can be effectively transmitted despite high coverage of LLINs/IRS. Residual transmission can occur due to Plasmodium-carrying Anopheles mosquitoes that are insecticide resistant and have feeding and resting behavior that reduces their chance of encountering the currently deployed indoor malaria control tools. Residual malaria transmission is likely to be the most significant hurdle to achieving the goal of malaria eradication and research and development towards new tools and strategies that can control residual malaria transmission is therefore critical. One of the most promising strategies involves biological agents that are part of the mosquito microbiome and influence the ability of Anopheles to transmit Plasmodium. These differ from biological agents previously used for vector control in that their primary effect is on vectoral capacity rather than the longevity and fitness of Anopheles (which may or may not be affected). An example of this type of biological agent is Microsporidia MB, which was identified in field collected Anopheles arabiensis and caused complete inhibition of Plasmodium falciparum transmission without effecting the longevity and fitness of the host. Microsporidia MB belongs to a unique group of rapidly adapting and evolving intracellular parasites and symbionts called microsporidia. In this review we discuss the general biology of microsporidians and the inherent characteristics that make some of them particularly suitable for malaria control. We then discuss the research priorities for developing a transmission blocking strategy for the currently leading microsporidian candidate Microsporidia MB for malaria control.

Draft genome of Microsporidia sp. MB—a malaria-blocking microsporidian symbiont of the Anopheles arabiensis

“…Phylogenomic analysis using OrthoFinder v2.5.4 ( 25 ) showed Microsporidia sp. MB is closely related to Vittaforma corneae within the Enterocytozoonidae group, consistent with previous taxonomic classification based on SSU rRNA ( 4 , 8 , 10 ), and contained the highest proportion of core BUSCO genes ( Fig. 1 ).…”

“…Moreover, it exhibits positive effects on the fitness of its host, contributing to its spread in host populations ( 8 , 9 ). Its unique characteristics and life cycle adaptations make it an intriguing subject for research in mosquito-borne disease control ( 10 ). We aimed to sequence and assemble the genome of this important symbiont isolated from Anopheles arabiensis mosquitoes in Kenya.…”

Host-specific effects of a generalist parasite of mosquitoes

“…4a ), showing an ability to clear the infection independently of sex (χ 2 = 1.28, df = 1, p = 0.258). Spore density (on mosquitoes harbouring spores) increased with the age of the mosquitoes (χ 2 = 19.68, df = 1, p < 0.001) up to a plateau of about 10 4 to 10 5 spores per mosquito. While spore density was similar for males and females (χ 2 = 1.07, df = 1, p = 0.300), sex influenced the increase with age (interaction age * sex: χ 2 = 6.07, df = 1, p = 0.014, Fig.…”

“…Microsporidians are obligate parasites 1 of many animals, particularly insects and fish 2 , including humans 3 . Almost half of the species infecting insects use dipterans as hosts 4 and more than 150 species have been found in 14 genera of mosquitoes [5][6][7][8][9][10][11][12] . While their life cycles vary from simple cycles in a single host to complex cycles involving several species 11 , they are all transmitted to larval mosquitoes and kill some larvae, letting others survive to cause, in adults, lower fertility, shorter lifespan and loss of vigour 13,14 .…”

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

“…In addition, Microsporidia MB is both vertically and sexually transmitted (11). In conjunction with the Plasmodium blocking, the high efficiency of vertical and sexual transmission of Microsporidia MB , which could facilitate the spread and maintenance of Microsporidia MB in Anopheles mosquito populations, has led to the suggestion that this symbiont could be deployed as a new tool for malaria control (12).…”