Efficacy of Long-lasting Insecticidal Nets With Declining Physical and Chemical Integrity on Aedes aegypti (Diptera: Culicidae)

Abstract

Abstract Fitting long-lasting insecticidal nets (LLIN) as screens on doors/windows has a significant impact on indoor-adult Aedes aegypti (L.), with entomological reductions measured in a previous study being significant for up to 2 yr post-installation, even in the presence of pyrethroid-resistant Aedes populations. To better understand the mode of LLIN protection, bioassays were performed to evaluate the effects of field deployment (0, 6, and 12 mo) and damage type (none, central, lateral, and multiple) on LLIN efficacy. Contact bioassays confirmed that LLIN residual activity (median knockdown time, in minutes, or MKDT) decreased significantly over time: 6.95 (95% confidence interval [CI]: 5.32–8.58) to 9.24 (95% CI: 8.69–9.79) MKDT at 0- and 12-mo age, respectively, using a pyrethroid-susceptible Aedes strain. Tunnel tests (exposing human forearm for 40 min as attractant) showed that deployment time affected negatively Aedes passage inhibition from 54.9% (95% CI: 43.5–66.2) at 0 mo to 35.7% (95% CI: 16.3–55.1) at 12 mo and blood-feeding inhibition from 65.2% (95% CI: 54.2–76.2) to 48.9% (95% CI: 26.4–71.3), respectively; both the passage/blood-feeding inhibition increased by a factor of 1.8–2.9 on LLINs with multiple and central damages compared with nets with lateral damage. Mosquito mortality was 74.6% (95% CI: 65.3–83.9) at 0 mo, 72.3% (95% CI: 64.1–80.5) at 6 mo, and 59% (95% CI: 46.7–71.3) at 12 mo. Despite the LLIN physical integrity could be compromised over time, we demonstrate that the remaining chemical effect after field conditions would still contribute to killing/repelling mosquitoes.