Insights into malaria transmission among Anopheles funestus mosquitoes, Kenya

Abstract

Background: Most malaria vectors belong to species complexes. Sibling species often exhibit divergent behaviors dictating the measures that can be deployed effectively in their control. Despite the importance of the Anopheles funestus complex in malaria transmission in sub-Saharan Africa, sibling species have rarely been identified in the past and their vectoring potential remains understudied.

Methods: We analyzed 1149 wild-caught An. funestus (senso lato) specimens from 21 sites in Kenya, covering the major malaria endemic areas including western, central and coastal areas. Indoor and outdoor collection tools were used targeting host-seeking and resting mosquitoes. The identity of sibling species, infection with malaria Plasmodium parasites, and the host blood meal sources of engorged specimens were analyzed using PCR-based and sequencing methods.

Results: The most abundant sibling species collected in all study sites were Anopheles funestus (59.8%) and Anopheles rivulorum (32.4%) among the 1062 successfully amplified specimens of the An. funestus complex. Proportionally, An. funestus dominated in indoor collections whilst An. rivulorum dominated in outdoor collections. Other species identified were Anopheles leesoni (4.6%), Anopheles parensis (2.4%), Anopheles vaneedeni (0.1%) and for the first time in Kenya, Anopheles longipalpis C (0.7%). Anopheles funestus had an overall Plasmodium infection rate of 9.7% (62/636), predominantly Plasmodium falciparum (59), with two infected with Plasmodium ovale and one with Plasmodium malariae. There was no difference in the infection rate between indoor and outdoor collections. Out of 344 An. rivulorum, only one carried P. falciparum. We also detected P. falciparum infection in two non-blood-fed An. longipalpis C (2/7) which is the first record for this species in Kenya. The mean human blood indices for An. funestus and An. rivulorum were 68% (93/136) and 64% (45/70), respectively, with feeding tendencies on a broad host range including humans and domestic animals such as cow, goat, sheep, chicken and pig.

Conclusions: Our findings underscore the importance of active surveillance through application of molecular approaches to unravel novel parasite-vector associations possibly contributed by cryptic species with important implications for effective malaria control and elimination.

Keywords: Anopheles funestus group; Anopheles longipalpis C; Entomological surveillance; Kenya; Malaria parasite transmission; Molecular approaches.

Fig. 1

Map of Kenya showing the sampling locations

Fig. 2

Relative frequency of species in the Anopheles funestus group, in the different malaria-endemic areas, Kenya. The numbers in parentheses indicate the total sample size of collected mosquitoes per site

Fig. 3

Phylogenetic trees for Plasmodium positive mosquito specimens in the An. funestus group inferred by applying maximum likelihood analysis in MEGA v.6 based on a ITS (267–763 bp), using Kimura-2 model and b cox1 barcode region (615–658 bp) using GTR+G model. Bootstrap support values from 1000 replications are indicated above nodes. Anopheles dthali was included for outgroup purposes. The branch length scale represents substitutions per site. Furthermore, selected sequences of Plasmodium-positive specimens were deposited in GenBank [accession numbers: An. funestus, MH299885-MH299890 (cox1), MH298707-MH298752 (ITS2); An. rivulorum, MH547425 (cox1); An. longipalpis, C MH547426-MH547427 (cox1), MH536653-MH536654 (ITS2)]

Fig. 4

Host blood meal sources for the sibling species in the Anopheles funestus group, Kenya. The numbers in parentheses indicates total sample size for each species analyzed