A Comparison of Surface and Total Deltamethrin Levels of Insecticide-Treated Nets and Estimation of the Effective Insecticidal Lifetime

The ability to anticipate the useful lifetime of an insecticide-treated mosquito net (ITN) would provide a proactive approach for planning net distribution programs. Therefore, we used an exponential decay model of deltamethrin depletion to predict the effective insecticidal lifetime of PermaNet® 2.0 nets used in the Lao PDR. Residual deltamethrin was measured using two nondestructive analytical field methods; X-ray fluorescence (total levels) and a colorimetric field test (surface levels) at 12 and 24 months postdistribution. The model assumes that the 12-month depletion rate can be used to predict future levels. The median total and surface deltamethrin levels for the Lao nets at 12 months were 31.2 and 0.0743 mg/m2, respectively. By defining a failed net as having total deltamethrin levels of less than 15 mg/m2 or a surface level less than 0.0028 mg/m2, it was predicted that 50% of the group of nets will fail at about 27 months after distribution. Insecticide-treated bednets (ITNs) are recognized as important tools for reducing malaria transmission in malaria-endemic regions.1-3 The effectiveness of ITNs at reducing malaria transmission relies on their ability to act as chemical as well as physical barriers. Over time, the accumulation of holes, rips, and tears as well as depletion of insecticide potency, reduces the efficacy of ITNs. Although the accumulation of holes results in decreased personal protection, the presence of remaining insecticide still has the potential to reduce malaria.4 It has been suggested by mathematical models that 94% of transmission can be prevented if 80% of the population continues to use these nets.4 Thus, monitoring insecticide levels along with physical integrity are important in recognizing when an ITN is no longer effective. Although damage to the nets can be visually ascertained, monitoring insecticidal potency is often assessed by using mosquito bioassays or chemical techniques. Mosquito bioassays, such as the WHO Cone Test, are the "Gold Standard" for assessing ITNs. Although mosquito bioassays are important elements in evaluating net efficacy, it is difficult to compare net performance across geographical regions where mosquito behavior and insecticide resistance are quite variable. Therefore, this report focuses on measuring surface and total residual insecticide levels by chemical means as a practical way to monitor and predict net longevity. The chemical techniques used to measure insecticide levels usually result in the partial destruction of an ITN. Spectroscopic methods such as X-ray fluorescence (XRF)5,6 and surface level measurements such as the colorimetric field test for cyanopyrethroids (CFT)7 provide alternative insecticide analysis techniques that are nondestructive to the net, thus allowing the same net to be monitored for insecticide levels over time. The XRF method measures the total (TL) amount of insecticide per area and the CFT measures available insecticide on the net surface (SL) via an abrasion technique using filter paper. Deltamethrin adhered to the filter paper is measured using a colorimetric cyanopyrethroid analysis method.7 The deltamethrin molecule contains both cyano and bromine groups, thus allowing it to be detected by the CFT and XRF, respectively.5,7 In this report, we describe the use of both XRF and CFT methods were on the same net after 12 and 24 months of use. The objectives of this report are to apply an exponential decay model for predicting the effective longevity of ITNs based on TL and SL deltamethrin measured after 12 months of use. The model is based on an assumption that by 12 months, factors contributing to insecticidal loss, such as washing and storage habits have become routinely established, thereby resulting in a depletion rate constant, from which future levels can be predicted.