Effect of wall type, delayed mortality and mosquito age on the residual efficacy of a clothianidin-based indoor residual spray formulation (SumiShield™ 50WG) in southern Mozambique

Abstract

Background: Indoor residual spraying (IRS) is one of the main malaria vector control strategies in Mozambique alongside the distribution of insecticide treated nets. As part of the national insecticide resistance management strategy, Mozambique introduced SumiShield TM 50WG, a third generation IRS product, in 2018. Its residual efficacy was assessed in southern Mozambique during the 2018-2019 malaria season. Methods: Two different wall surfaces, cement and mud-plastered surfaces, daily mosquito mortality up to 120 hours post-exposure, and older mosquitoes (13-26d old) were included in standard WHO (World Health Organization) cone bioassay tests. Lethal times (LT) 90, LT50 and LT10 were estimated using Bayesian models. Results: Mortality 24h post exposure was consistently below 80%, the current WHO threshold value for effective IRS, in both young and old mosquitoes, regardless of wall surface type. Considering delayed mortality, residual efficacies (mosquito mortality equal or greater than 80%) ranged from 1 to ≥12 months, with the duration depending on mortality time post exposure, wall type and mosquito age. Looking at mortality 72h after exposure, residual efficacy was between 6 and 9 months, depending on wall type and mosquito age. Mortality of older mosquitoes was significantly higher on mud-surfaces compared to cemented-surfaces 24h post exposure, but this difference was not significant for the delayed mortalities. The LT 50 and LT 10 (i.e. 90% of the mosquitoes survive exposure to the insecticides) values were consistently higher for older mosquitoes using the 24h post-exposure observations and ranged from 0.2 to 5.7 months and 0.2 to 7.2 months for LT50 and LT10, respectively. Conclusions: The present study highlights the need for assessing mosquito mortality beyond the currently recommended 24h post exposure. Failure to do so may lead to underestimation of the residual efficacy of IRS products, as delayed mortality will lead to a further reduction in mosquito vector populations and potentially negatively impact disease transmission. Monitoring residual efficacy on relevant wall surfaces, including old mosquitoes that are ultimately responsible for malaria transmission, and assessing delayed mortalities are critical to provide accurate and actionable data to guide vector control programmes.

The Residual Efficacy of SumiShield™ 50WG and K-Othrine® WG250 IRS Formulations Applied to Different Building Materials Against Anopheles and Aedes Mosquitoes

“…Subsequently, in this current study 72-hour mortality was used to determine the products efficacy. If 24-hour mortality had been selected the residual efficacy over time would be reduced, as has been observed previously [39]. When products with a novel MoA are tested, it is vital that preliminary studies consider a wider range of outcomes than rapid knock down or kill.…”

Comparison of cone bioassay estimates at two laboratories with different Anopheles mosquitoes for quality assurance of pyrethroid insecticide-treated nets

“…manufacturing processes [47][48][49], and possibly poor shipping or storage conditions [50]; as well as bioassay methods including sample preparation e.g. using a net sample straight from the fridge, mosquito age [51,52] and fitness [53], test procedures [40], temperature [54][55][56] and inter-operator variability [31].…”

Cone Bioassays Provide Reproducible Bioefficacy Estimates with Different Anopheline Mosquitoes and Can Be Used for Quality Assurance of Pyrethroid Insecticide Treated Nets

“…manufacturing processes [42][43][44], poor shipping [45] or storage conditions [46]; as well as bioassay methods including sample preparation e.g. using a net sample straight from the fridge (Skovmand, personal communication), mosquito age [47,48] and tness [49]), test procedures [36], temperature [50][51][52] and inter-operator variability [12].…”