Permethrin-Treated Clothing as Protection against the Dengue Vector, Aedes aegypti: Extent and Duration of Protection

Abstract

Introduction: Dengue transmission by the mosquito vector, Aedes aegypti, occurs indoors and outdoors during the day. Personal protection of individuals, particularly when outside, is challenging. Here we assess the efficacy and durability of different types of insecticide-treated clothing on laboratory-reared Ae. aegypti.

Methods: Standardised World Health Organisation Pesticide Evaluation Scheme (WHOPES) cone tests and arm-in-cage assays were used to assess knockdown (KD) and mortality of Ae. aegypti tested against factory-treated fabric, home-dipped fabric and microencapsulated fabric. Based on the testing of these three different treatment types, the most protective was selected for further analysis using arm-in cage assays with the effect of washing, ultra-violet light, and ironing investigated using high pressure liquid chromatography.

Results: Efficacy varied between the microencapsulated and factory dipped fabrics in cone testing. Factory-dipped clothing showed the greatest effect on KD (3 min 38.1%; 1 hour 96.5%) and mortality (97.1%) with no significant difference between this and the factory dipped school uniforms. Factory-dipped clothing was therefore selected for further testing. Factory dipped clothing provided 59% (95% CI = 49.2%- 66.9%) reduction in landing and a 100% reduction in biting in arm-in-cage tests. Washing duration and technique had a significant effect, with insecticidal longevity shown to be greater with machine washing (LW50 = 33.4) compared to simulated hand washing (LW50 = 17.6). Ironing significantly reduced permethrin content after 1 week of simulated use, with a 96.7% decrease after 3 months although UV exposure did not reduce permethrin content within clothing significantly after 3 months simulated use.

Conclusion: Permethrin-treated clothing may be a promising intervention in reducing dengue transmission. However, our findings also suggest that clothing may provide only short-term protection due to the effect of washing and ironing, highlighting the need for improved fabric treatment techniques.

Fig 1. Flow chart of study design and experiments performed.

*Denotes that the treatment technique was not included in all efficacy evaluation tests. FDSU was not included in Arm-in-cage assays and HDC was not included in the cone assays.

Fig 2. WHOPES arm-in-cage biting and landing full coverage protection for three different clothing impregnation methods; factory-dipped clothing (FDC), micro-encapsulated (MC), hand dipped clothing (HDC).

Protection is measured against the control clothing (FCC) for both biting and landing, Whiskers correspond to 95% confidence intervals.

Fig 3. One hour knockdown and 24 hour mortality of factory dipped clothing after using WHO and machine washing techniques after a 3 minute exposure to clothing.

Whiskers correspond to 95% confidence intervals

Fig 4. Permethrin content (mg/cm²) of WHO bottle and machine washed materials at 0, 1, 5, 10, 15, 20, 25 & 30 washes.

Whiskers correspond to 95% confidence intervals.

Fig 5. Permethrin content (mg/cm²) of treated material exposed to UV-light, ironing and washing in combination.

Whiskers correspond to 95% confidence intervals. Clothing was exposed to a combination of UV, Ironing and Washing in combination; 1 day (1 wash, 20min UV, 30 seconds Ironing); 1 week (2 washes, 100 min UV, 1 minute Ironing), 1 month (8 washes, 400 min UV,4 minutes ironing), 3 months (24 washes, 1200 min UV, 12 minutes ironing).

Fig 6. Permethrin content (mg/cm²) of treated material exposed to ironing.

Whiskers correspond to 95% confidence intervals. Clothing was exposed to Ironing for 0 seconds, 30 seconds, 1 minute, 4 minutes and 12 minutes to simulate 0, 1 day, 1 week, 1 month and 3 months exposure.

Fig 7. Permethrin content (mg/cm²) of treated material exposed to UV-light.

Whiskers correspond to 95% confidence intervals. Clothing was exposed to UV-light for 0 seconds, 30 seconds, 1 minute, 4 minutes and 12 minutes to simulate 0, 1 day, 1 week, 1 month and 3 months exposure.