The near full recovery observed for pyrethroids in
An. funestus after pre-exposure to PBO indicates that resistance is mainly conferred by metabolic resistance particularly by cytochrome P450s [
40]. This is in line with previous reports of the absence of
kdr in this species in Cameroon [
5] and across Africa [
41]. The frequency of the 119F-GSTe2-resistant allele in Mibellon field population (28%) is lower than in the northern part of Cameroon in Gounougou (52%) [
5] or in Ghana (44.2%) [
42] and Benin (56.25%) [
30]. The frequencies in Mibellon are closer to that observed in the eastern part of Africa in Uganda (20.4%) [
21,
32]. GSTe2 has been shown to confer cross-resistance between DDT and pyrethroids [
42]. The partial recovery of DDT susceptibility after PBO assays suggests that GSTe2 is probably playing a role in the resistance in this
An. funestus s.s. population.
The frequency of the 296S-RDL-resistant allele is only 9.7%, which is lower than was observed in the northern region at Gounougou. This low frequency could be as a result of recovery to susceptibility for dieldrin after this insecticide was removed from public and agricultural sectors in Cameroon, as observed in Gounougou where the frequency of this mutation went down from 80% in 2006 to 40% in 2012 and 14.6% in 2015 [
5,
43]. Such reversal of resistance is encouraging for the implementation of resistance management strategies.
The very high resistance levels to pyrethroids in
An. gambiae s.s. (no mortality to permethrin), correlates with the high frequency of the 1014F
kdr allele (63.9%). This is in line with other reports from Africa where high pyrethroid resistance in
An. gambiae s.l. has been associated with nearly fixed
kdr allele in the population, as observed recently in DR Congo [
38], or previously in Côte d’Ivoire [
44]. However, this high level of
kdr is in contrast to frequencies observed in other locations across Cameroon as highlighted by a recent review [
17]. The very low frequency of the 1014S
kdr allele in Mibellon is similar to previous reports across Cameroon showing that this marker, originally present in East Africa, has now migrated to Central and West Africa although still at very low frequencies [
45]. Overall, the fact that 1014F
kdr frequency is not fixed in Mibellon in the presence of such high pyrethroid and DDT resistance suggests that other mechanisms are playing an important role, probably metabolic resistance as shown for other
An. gambiae s.l. populations in Cameroon [
18,
19,
46]. Further investigation of the resistance mechanisms will help elucidate the molecular basis driving resistance in this population. The total absence of the 119S
ace-
1 mutation is in line with the susceptibility of this population to organophosphate and carbamate as this mutation is responsible for organophosphate and carbamate resistance [
47,
48].