Abstract:
Background: The development and spread of resistance among local vectors to the
major classes of insecticides used in Long-Lasting Insecticidal Nets (LLINs) and Indoor
Residual Spraying (IRS) poses a major challenge to malaria vector control programs
worldwide. The main methods of evaluating insecticide resistance in malaria vectors are
the WHO tube bioassay and CDC bottle assays, with their weakness being determination
of resistance at a fixed dose for variable populations. The CDC bottle assay using
different insecticide dosages has proved applicable in ascertaining the intensity of
resistance.
Methods: We determined the status and intensity of permethrin resistance and
investigated the efficacy of commonly used LLINs (PermaNet® 2.0, PermaNet® 3.0 and
Olyset®) against 3-5 day-old adult female Anopheles mosquitoes from four subcounties; Teso, Bondo, Rachuonyo and Nyando in western Kenya. Knockdown was
assessed to 4 doses of permethrin; 1× (21.5 μg/ml), 2× (43 μg/ml), 5× (107.5 μg/ml) and
10× (215 μg/ml) using CDC bottle assays.
Results: Mortality for 0.75% permethrin ranged from 23.5% to 96.1% in the WHO tube
assay. Intensity of permethrin resistance was highest in Barkanyango Bondo, with 84%
knockdown at the 30 min diagnostic time when exposed to the 10× dose. When exposed
to the LLINs, mortality ranged between- 0-39% for Olyset®, 12-88% for PermaNet® 2.0
and 26-89% for PermaNet® 3.0. The efficacy of nets was reduced in Bondo and Teso.
Results from this study show that there was confirmed resistance in all the sites;
however, intensity assays were able to differentiate Bondo and Teso as the sites with the
highest levels of resistance, which coincidentally were the two sub-counties with reduced
net efficacy.
Conclusions: There was a reduced efficacy of nets in areas with high resistance
portraying that at certain intensities of resistance, vector control using LLINs may be
compromised. It is necessary to incorporate intensity assays in order to determine the
extent of threat that resistance poses to malaria control.