Horizontal Transmission of the Symbiont Microsporidia MB in Anopheles arabiensis

Abstract

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