Use of a portable field-adapted liquid chromatographic system (C-Vue machine) to estimate the quantity of deltamethrin on insecticide-treated nets paired with WHO cone bioassays to determine ITN bioefficacy as part of three-year durability monitoring in Mali

Abstract

Background Monitoring insecticide levels and physical integrity over time is essential for assessing the durability of insecticide-treated nets (ITNs), which largely depends on the net handling habits of users. This study aimed to determine the insecticide content and effectiveness of ITNs (Yorkool and PermaNet 2.0) at 6, 12, 24 and 36 months after mass distribution in Mali. Methods At 12, 24 and 36 months postdistribution from 2018 to 2020, 30 nets were randomly collected from households in the districts of Kenieba and Kita in the southern part of Mali, together with information about ITN use and washing practices. The insecticidal effectiveness of the ITNs was assessed with the World Health Organization (WHO) cone test using a laboratory-reared susceptible colony of Anopheles coluzzii. The residual insecticide content was measured by a nondestructive sampling technique with a portable field-adapted liquid chromatographic system (C-Vue®) validated by running samples in parallel with standardized WHO HPLC methods. Results At each time point, nets were washed an average of three times over the previous 6 months, most commonly using detergent or bleach. For Yorkool nets, the average deltamethrin concentration was 55 mg/m² at 6 months and gradually decreased to 14 mg/m² at 36 months. The values for the PermaNet 2.0 nets were 45 mg/m² at 6 months and 6 mg/m² at 36 months. Until the 24-month evaluation, the proportion of nets with minimal effectiveness (at least 75% Knock Down "K.D." or 50% 24 h mortality) was greater than 80% for both net types and sites. According to the WHO criteria, the proportion of net who met the WHO optimal effectiveness criteria (at least 95% K.D. or 80% mortality) was less than 80%. The WHO standardized cone test and C-Vue evaluation demonstrated that net type effectiveness and insecticide content were consistently lower than expected at 3 years. Conclusion The nets did not meet the WHO criteria of optimal effectiveness for the efficacy of ITNs in this 3-year survey, and users washed nets with local soap containing detergent or bleach. The C-Vue portable chromatographic device was used successfully for the first time in Mali to measure the insecticide concentration of ITNs. These results were consistent with those of cone bioassays.