Impact of Water Quality on Laboratory Evaluation of Insecticide-Treated Mosquito Nets

Abstract:
Laboratory testing of long-lasting insecticide treated nets (LN’s) includes assessment of bio-efficacy and determination of insecticide retention after washings. The standard WHO washing method recommends the use of deionizded water with a hardness of 0 ppm (water without Ca++ and Mg++). In the context of Bayer’s LN development we conducted laboratory tests to evaluate the impact of washings with water of varying hardness (0 - 500 ppm) on the bio-efficacy and the active ingredient retention of two Bayer Deltamethrin binder-coated LN’s: K-O TAB® 1-2-3 for hand treatment of nets in the field, and the industrially treated DAWA Plus net. The results demonstrate that nets washed with water containing any level of Ca++ and Mg++ ions showed considerably higher knock down and mortality compared to nets washed with deionized water (0 ppm). The chemical analysis surprisingly showed that this is not a function of a stronger wash loss of Deltamethrin after washing in deionized water, but in contrast suggests that water without Ca++ and Mg++ ions may have a detrimental impact on the chemical-physical conditions of the coating technology. The results of our studies imply that the laboratory bio-efficacy evaluation of K-O TAB ® 1-2-3 or DAWA Plus nets after washing with deionized water may not reflect the full potential activity of bed nets treated by the coating technologies