Detection of Anopheles stephensi Mosquitoes by Molecular Surveillance, Kenya

Abstract

The Anopheles stephensi mosquito is an invasive malaria vector recently reported in Djibouti, Ethiopia, Sudan, Somalia, Nigeria, and Ghana. The World Health Organization has called on countries in Africa to increase surveillance efforts to detect and report this vector and institute appropriate and effective control mechanisms. In Kenya, the Division of National Malaria Program conducted entomological surveillance in counties at risk for An. stephensi mosquito invasion. In addition, the Kenya Medical Research Institute conducted molecular surveillance of all sampled Anopheles mosquitoes from other studies to identify An. stephensi mosquitoes. We report the detection and confirmation of An. stephensi mosquitoes in Marsabit and Turkana Counties by using endpoint PCR and morphological and sequence identification. We demonstrate the urgent need for intensified entomological surveillance in all areas at risk for An. stephensi mosquito invasion, to clarify its occurrence and distribution and develop tailored approaches to prevent further spread.

Keywords: Anopheles stephensi; Kenya; Marsabit; Turkana; malaria; molecular methods; mosquitoes; parasites; vector-borne infections.

Figure 1

Locations of mosquito collection during surveillance conducted by Division of National Malaria Program and partners, Kenya, December 2022. Gray filled circles indicate sites where Anopheles stephensi mosquitoes were present; black filled circles indicate sites where only other vectors (An. gambiae and An. funestus mosquitoes) were collected.

Figure 2

Habitats where Anopheles stephensi mosquitoes were collected, Kenya. A) Animal watering pan in Marsabit County; B) disposed containers containing standing water in Marsabit County; C) old tire in Marsabit County; D) seasonal river pan in Turkana County. Persons pictured gave consent for their photographs to be published in this article.

Figure 3

Pictures of Anopheles stephensi mosquito collected in Kenya as seen under a microscope. The dual banding on the palps characteristic of An. stephensi mosquitoes is circled in red in the closer image at right. Other distinguishing features are not clear in this image.

Figure 4

Phylogenetic tree representing the relationship of Anopheles stephensi mosquito isolates from Kenya (2023KEN0001, 2023KEN0002, and 2023KEN0003) and reference Anopheles spp. isolates using the cytochrome c oxidase subunit 1 region. GenBank accession numbers are provided for reference sequences; accession numbers for Kenya sequences are provided in Table 3. Scale bar indicates 5% nucleotide sequence divergence. Values on the branches represent the percentage of 1,000 bootstrap replicates; bootstrap values >70% are shown in the tree.

Figure 5

Phylogeny of cytochrome c oxidase subunit 1 sequences of Anopheles stephensi mosquito isolates from Kenya (2023KEN0001, 2023KEN0002, and 2023KEN0003) and reference An. stephensi mosquito isolates retrieved from GenBank. GenBank accession numbers are provided for reference sequences; accession numbers for Kenya sequences are provided in Table 3. Scale bar indicates 1% nucleotide sequence divergence. Values on the branches represent the percentage of 1,000 bootstrap replicates; bootstrap values >70% are shown in the tree.

Figure 6

Phylogeny of internal transcribed spacer 2 sequences of An. stephensi isolates from Kenya (OQ275146.1, OQ275144.1, OQ275145.1, OQ878216, OQ87821, and OQ878218) in comparison to An. stephensi isolates from other parts of the world. GenBank accession numbers are provided for reference sequences; accession numbers for Kenya sequences are provided in Table 3. Scale bar indicates 20% nucleotide sequence divergence. Values on the branches represent the percentage of 1,000 bootstrap replicates; bootstrap values >70% are shown in the tree.