Increased prevalence of insecticide resistance in Anopheles coluzzii populations in the city of Yaoundé, Cameroon and influence on pyrethroid-only treated bed net efficacy

In Cameroon, pyrethroid-only long-lasting insecticidal nets (LLINs) are still largely used for malaria

control. The present study assessed the efficacy of such LLINs against a multiple-resistant population of the major

malaria vector, Anopheles coluzzii, in the city of Yaoundé via a cone bioassay and release-recapture experimental

hut trial. Susceptibility of field mosquitoes in Yaoundé to pyrethroids, DDT, carbamates and organophosphate

insecticides was investigated using World Health Organization (WHO) bioassay tube tests. Mechanisms of insecticide

resistance were characterised molecularly. Efficacy of unwashed PermaNet 2.0 was evaluated against untreated

control nets using a resistant colonised strain of An. coluzzii. Mortality, exophily and blood feeding inhibition were

estimated. Field collected An. coluzzii displayed high resistance with mortality rates of 3.5% for propoxur (0.1%),

4.16% for DDT (4%), 26.9% for permethrin (0.75%), 50.8% for deltamethrin (0.05%), and 80% for bendiocarb

(0.1%). High frequency of the 1014F west-Africa kdr allele was recorded in addition to the overexpression of several

detoxification genes, such as Cyp6P3, Cyp6M2, Cyp9K1, Cyp6P4 Cyp6Z1 and GSTe2. A low mortality rate (23.2%)

and high blood feeding inhibition rate (65%) were observed when resistant An. coluzzii were exposed to unwashed

PermaNet 2.0 net compared to control untreated net (p < 0.001). Furthermore, low personal protection (52.4%)

was observed with the resistant strain, indicating reduction of efficacy. The study highlights the loss of efficacy of

pyrethroid-only nets against mosquitoes exhibiting high insecticide resistance and suggests a switch to new generation

bed nets to improve control of malaria vector populations in Yaoundé.