Anopheles stephensi is an invasive malaria vector that is endemic to south Asia and the Arabian Peninsula. It was recently reported in the Horn of Africa countries including Djibouti (2012), Ethiopia, Sudan (2019), Somalia (2019) and most recently Nigeria (2020). This mosquito is a competent vector for both Plasmodium falciparum and P. vivax. It is characterized by a high degree of behavioral plasticity and the ability to reproduce in various types of breeding sites including containers and therefore has the potential to propagate malaria transmission in rapidly urbanizing settings with poor drainage and disposal of waste containers. The World Health Organization (WHO) has called on all countries to scale up surveillance efforts to detect and report invasion by this vector and institute appropriate and effective control mechanisms. In Kenya, the Division for National Malaria Program (DNMP) and its partners have been conducting entomological surveillance in all coastal and northern counties that are suspected to be at risk of An. stephensi invasion as well as in all counties at risk of malaria. These efforts were supported by molecular surveillance of all unidentified Anopheles mosquitoes from other studies conducted by the Kenya Medical Research Institute (KEMRI) to try and identify An. stephensi. In this article, we report the first detection of An. stephensi in two sub counties of Marsabit County, Kenya in December 2022. We used Polymerase Chain Reaction (PCR) as the primary method of identification and confirmed results using morphological keys and sequencing of the ITS2 region. With the detection of this vector in Kenya, there is an urgent need for intensified surveillance to determine its occurrence and distribution and develop tailored approaches towards control to prevent further spread.

Background: The discovery of the Anopheles symbiont, Microsporidia MB in An. arabiensis and the subsequent demonstration that it blocks malaria transmission, undergoes vertical and horizontal transmission suggests that it is a promising candidate for developing a symbiont-based malaria transmission-blocking strategy. We investigated the prevalence and characteristics of Microsporidia MB in An. gambiae s.s., another primary malaria vector species in Kenya. Methods: Field collected females were confirmed Microsporidia MB positive after oviposition. The egg counting of Microsporidia MB infected and non-infected individuals used to infer the effects of Microsporidia MBon fecundity. The duration to pupation, the adult sex ratio and survival were quantified to determine if Microsporidia MBinfection has similar characteristics in An. gambiae and An. arabiensis hosts. The intensity of Microsporidia MB infections in tissues such as midgut, gonads and carcasses was determined by qPCR. To investigate horizontal transmission, virgin individuals of opposite sexes that were either Microsporidia MB infected or non-infected were placed in the standard cages for 48 hours and allowed to mate. After this period, transmission was confirmed by assessment of Microsporidia MB infection status by qPCR. Results: We observed that Microsporidia MB is naturally found at low prevalence in Anopheles gambiae s.s. collected in Western Kenya. Microsporidia MB shortened development time from larvae to pupae, but other fitness parameters such as fecundity, sex ratio, and adult survival did not differ between Microsporidia MB infected and non-infected host. Microsporidia MB intensities were high in the male gonadal tissue. Transmission experiments indicate that Microsporidia MB utilizes both maternal and horizontal transmission in An. gambiae s.s. Conclusions: The finding that Microsporidia MB naturally infects An. gambiae s.s., undergoes maternal and horizontal transmission and is avirulent signifies that many of the characteristics of Microsporidia MB infection observed in An. arabiensis hold true for Anopheles gambiae s.s. This is an early indication that Microsporidia MB could equally be developed as a tool for malaria transmission blocking across different Anopheles species.

Anopheles stephensi is an invasive malaria vector recently reported in Djibouti (2012), Ethiopia, Sudan (2019), Somalia (2019) and most recently Nigeria (2020). The World Health Organization (WHO) has called on all countries to scale up surveillance efforts to detect and report invasion by this vector and institute appropriate and effective control mechanisms. In Kenya, the Division for National Malaria Program (DNMP) conduct entomological surveillance in counties at risk of An. stephensi invasion. These efforts were supported by molecular surveillance of all sampled Anopheles mosquitoes from other studies conducted by the Kenya Medical Research Institute (KEMRI) to identify An. stephensi. In this article, we report the first detection of An. stephensi in Marsabit and Turkana Counties using PCR and confirmed by sequencing and morphological identification. This detection shows the urgent need for intensified surveillance to determine its occurrence and distribution and develop tailored approaches towards control to prevent further spread.