Evidence of a multiple insecticide resistance in the malaria vector Anopheles funestus in South West Nigeria

Abstract

Background: Knowing the extent and spread of insecticide resistance in malaria vectors is vital to successfully manage insecticide resistance in Africa. This information in the main malaria vector, Anopheles funestus sensu stricto, is completely lacking in the most populous country in Africa, Nigeria. This study reports the insecticide susceptibility status and the molecular basis of resistance of An. funestus as well as its involvement in malaria transmission in Akaka-Remo, a farm settlement village in southwest Nigeria.

Results: Plasmodium infection analysis using TaqMan protocol coupled with a nested PCR revealed an infection rate of 8% in An. funestus s.s. from Akaka-Remo. WHO susceptibility tests showed this species has developed multiple resistance to insecticides in the study area. Anopheles funestus s.s. population in Akaka-Remo is highly resistant to organochlorines: dieldrin (8%) and DDT (10%). Resistance was also observed against pyrethroids: permethrin (68%) and deltamethrin (87%), and the carbamate bendiocarb (84%). Mortality rate with DDT slightly increased (from 10 to 30%, n = 45) after PBO pre-exposure indicating that cytochrome P450s play little role in DDT resistance while high mortalities were recorded after PBO pre-exposure with permethrin (from 68 to 100%, n = 70) and dieldrin (from 8 to 100%, n = 48) suggesting the implication of P450s in the observed permethrin and dieldrin resistance. High frequencies of resistant allele, 119F in F₀ (77%) and F₁ (80% in resistant and 72% in susceptible) populations with an odd ratio of 1.56 (P = 0.1859) show that L119F-GSTe2 mutation is almost fixed in the population. Genotyping of the A296S-RDL mutation in both F₀ and F₁ samples shows an association with dieldrin resistance with an odd ratio of 81 (P < 0.0001) (allelic frequency (R) = 76% for F₀; for F₁, 90 and 10% were observed in resistant and susceptible populations, respectively) as this mutation is not yet fixed in the population.

Conclusion: The study reports multiple insecticide resistance in An. funestus from Akaka Remo. It is, therefore, necessary to pay more attention to this major malaria vector for effective malaria control in Nigeria.

Keywords: Anopheles funestus; Insecticide resistance; Malaria control; Nigeria; Resistance mechanisms.

Fig. 1

Seasonal density of An. funestus per room at Akaka-Remo. m/r mosquitoes per room

Fig. 2

Insecticide resistance profiles of An. funestus s.s. from Akaka-Remo. Error bars represent standard error of the mean

Fig. 3

Insecticide resistance profiles of An. funestus after exposure to PBO. Error bars represent standard error of the mean; Control 1 Mosquitoes that were neither exposed to PBO nor insecticides; Control 2 Mosquitoes that were exposed to only PBO

Fig. 4

Screening of L119F-GSTe2 mutation (a) shows a high presence of RR and RS individuals and a low presence of SS in F₀ females An. funestus from Akaka-remo. b F₁ An. funestus s.s. from Akaka-Remo showing high presence of RR and RS and a relatively low presence of SS individuals in both the resistant (alive) and susceptible (dead) individuals post DDT exposure

Fig. 5

Genotype distribution of A296S-RDL mutation (a) showing a significant presence of RR and RS individuals in the F₀ population of An. funestus from Akaka Remo (b) F₁ An. funestus s.s. from Akaka-Remo post dieldrin exposure showing high presence of RR individuals in resistant (alive) and the absence of RR and low presence of RS in susceptible (dead) individuals