Evaluation of Different Methods to Assess the Duration of Effectiveness of Permethrin-Treated Clothing against Aedes albopictus and Aedes aegypti Mosquitoes after Repeated Washing

Abstract

Mosquitoes can be vectors of pathogens that cause many diseases, such as malaria, West Nile encephalitis, dengue and Zika. Mosquito-borne diseases continue to be a global problem, affecting outdoor workers (e.g. forestry workers) and individuals participating in recreational activities, such as camping, hiking, hunting, or gardening. Measures to minimize the impact of arthropod exposure (e.g. mosquito and tick bites) are vital to protect public health. Personal protective measures, such as wearing insecticide-treated clothing, can provide some level of protection from arthropod exposure. Consequently, the purpose of the current laboratory study is to assess the effects of repeated washing on the effectiveness of permethrin-treated clothing against mosquitoes. The specific aims are to: 1) quantify the amount of permethrin in two different fabric types after repeated washing, 2) investigate the knockdown/mortality of permethrin-susceptible and permethrin-resistant populations of Aedes mosquitoes after exposure to permethrin-treated clothing, and 3) examine the extent which mosquito exposure method impacts knockdown/mortality rate after exposure to permethrin-treated clothing. Permethrin-treated and untreated clothing of two fabric types (50% cotton/50% polyester and 100% cotton) were machine-washed and -dried for up to 15 cycles. Fabric swatch samples (n=2) were cut from each garment after 0, 5 and 15 washing/drying cycles. Two species of Aedes mosquitoes (Ae. albopictus and Ae. aegypti) known to be susceptible or resistant to permethrin (previously assessed by Centers for Disease Control and Prevention bottle bioassay) were used for this study. Subsets of mosquitoes were exposed to swatches for 2 min using 2 different exposure methods (Environmental Protection Agency petri dish method and World Health Organization cone method), and then observed for knockdown/mortality at 2 h and 24 h post-exposure. Each fabric swatch was analyzed for permethrin content using gas chromatography. Analysis of variance was used to evaluate differences in permethrin content between treatment groups, while chi-square testing was used to evaluate proportions of mosquito knockdown and mortality against different variables. Results showed that permethrin-treated clothing was effective against the mosquito population that was susceptible to permethrin (Ae. albopictus) but not the resistant population (Ae. aegypti). Permethrin content decreased with increased number of washings. The petri dish and cone methods resulted in no significant differences in knockdown/mortality rate observed for either tested mosquito population. We expect the findings of this study to contribute to current research on permethrin-treated clothing as personal protection for reducing risk to mosquito-borne diseases. Understanding the extent to which fabrics retain permethrin may lead to the development of fabric blends that maximize permethrin retention. Moreover, understanding the efficacy of different mosquito exposure methods may help improve or standardize testing methods.