Contrasting Plasmodium infection rates and insecticide susceptibility profiles between the sympatric sibling species Anopheles parensis and Anopheles funestus s.s: a potential challenge for malaria vector control in Uganda

Abstract

Background: Although the An. funestus group conceals one of the major malaria vectors in Africa, little is known about the dynamics of members of this group across the continent. Here, we investigated the species composition, infection rate and susceptibility to insecticides of this species group in Uganda.

Methods: Indoor resting blood-fed Anopheles adult female mosquitoes were collected from 3 districts in Uganda. Mosquitoes morphologically belonging to the An. funestus group were identified to species by PCR. The sporozoite infection rates were determined by TaqMan and a nested PCR. Susceptibility to major insecticides was assessed using WHO bioassays. The potential role of four candidate resistance genes was assessed using qRT-PCR.

Results: An. funestus s.s. and An. parensis, were the only members of the An. funestus group identified. Both species were sympatric in Masindi (North-West), whereas only An. parensis was present in Mityana (Central) and Ntungamo (South-West). The Plasmodium falciparum infection detected in An. parensis (4.2%) by TaqMan could not be confirmed by nested PCR, whereas the 5.3% infection in An. funestus s.s. was confirmed. An. parensis was susceptible to most insecticides, however, a moderate resistance was observed against deltamethrin and DDT. In the sympatric population of Masindi, resistance was observed to pyrethroids (permethrin and deltamethrin) and DDT, but all the resistant mosquitoes belonged to An. funestus s.s. No significant over-expression was observed for the four P450 candidate genes CYP6M7, CYP9K1, CYP6P9 and CYP6AA4 between deltamethrin resistant and control An. parensis. However, when compared with the susceptible FANG An. funestus s.s strain, the CYP9K1 is significantly over-expressed in An. parensis (15-fold change; P < 0.001), suggesting it could play a role in the deltamethrin resistance.

Conclusion: The contrasting infection rates and insecticide susceptibility profiles of both species highlights the importance of accurate species identification for successful vector control programs.

Figure 1

Map of Uganda showing the three study districts (with the red indicator). Major towns in Uganda are also included for further guidance.

Figure 2

Susceptibility levels in F₁ adults from Mityana and Masindi after 1 hr exposure to insecticides. The data shown are mean + SEM (n ≥ 4).

Figure 3

Susceptibility levels of An. parensis from Mityana after 30 and 20 minutes exposure to insecticides. The data shown are mean + SEM (n ≥ 4).

Figure 4

Transcriptional profiling of candidate resistance genes by qRT-PCR in An. parensis from Mityana. Mosquitoes alive after 30 minutes exposure to deltamethrin (Res) were compared to a control sample not exposed to insecticide (Cont) and to a susceptible laboratory strain (Sus-FANG) of An. funestus.