Intensity of pyrethroid resistance in An. gambiaes.l.before and after a mass distribution of insecticide treated nets in Kinshasa and from 11 provinces of the Democratic Republic of Congo

BackgroundBetween 2011 and 2018, an estimated 134.8 million pyrethroid-treatedlong-lasting insecticidal nets (LLINs) were distributed nationwide in the Democratic Republic of Congo (DRC) for malaria control. Pyrethroid resistance has developed in DRC in recent years, but the intensity ofresistance and impact on LLIN efficacy was not known. Therefore, the intensity of resistance of Anopheles gambiae s.l. to permethrin and deltamethrin was monitored before and after a mass distribution of LLINs in Kinshasa in December 2016, and in 7sites across the country in 2017 and 11 sites in 2018.Materials and MethodsIn Kinshasa, CDC bottle bioassays using 1, 2, 5, and 10 times the diagnostic dose of permethrin and deltamethrin were conducted using An. gambiae s.l. collected as larvae and reared to adults. Bioassays were conducted in four sites in Kinshasa province six months before a mass distribution of deltamethrin-treated ITNs and then two, six, and ten months after the distribution. One site in neighboring Kongo Central province was used as a control (no mass campaign of ITN distribution during the study). Nationwide intensity assays were conducted in six sites in 2017 using CDC bottle bioassays and in 11 sites in 2018 using WHO intensity assays. A sub-sample of An. gambiae s.l. were tested by PCR to determine species composition and frequency of kdr-1014F and 1014S alleles. ResultsIn June 2016, before LLIN distribution, permethrin resistance intensity was high in Kinshasa; the mean mortality rate was 43% at the 5× concentration and 73% at the 10× concentration. Bioassays at 3 time points after LLIN distribution showed considerable variation by site and time and there was no consistent evidence for an increase in pyrethroid resistance intensity compared to the neighboring control site. Tests of An. gambiaes.l. in 7 sites across the country in 2017 and 11 sites in 2018 showed all populations were resistant to the diagnostic doses of 3 pyrethroids.In 2018, the intensity of resistance varied by site, but was generally moderate for all three pyrethroids, with survivors at ×5 the diagnostic dose. An. gambiae s.s. was the most common species identified across 11 sites in DRC. However in Kinshasa, An. gambiae s.s.(91%) and An. coluzzii (8%) were sympatric.ConclusionsModerate or high intensity pyrethroid resistance was detected nationwide in DRC and is a serious threat to sustained malaria control with pyrethroid LLINs. Next generation nets (PBO nets or bi-treated nets) should be considered for mass distribution.