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UNITED STATES DEPARTMENT OF LABOR

Protective effectiveness of long-lasting permethrin impregnated clothing against tick bites in an endemic Lyme disease setting: A randomized control trial among outdoor workers (Mitchell, 2020)

Review Guidelines

Causal Evidence Guidelines

Absence of conflict of interest

Citation

Mitchell, C., Dyer, M., Lin, F. C., Bowman, N., Mather, T., & Meshnick, S. (2020). Protective effectiveness of long-lasting permethrin impregnated clothing against tick bites in an endemic Lyme disease setting: A randomized control trial among outdoor workers. Journal of Medical Entomology, 57(5), 1532-1538. https://doi.org/10.1093/jme/tjaa061

Highlights

  • The study's objective was to examine the impact of Long-Lasting Permethrin-Impregnated (LLPI) clothing on health and safety outcomes. 
  • This study was a randomized controlled trial assessing the efficacy of LLPI clothing in reducing tick bites. Using questionnaires and online logs, the authors conducted statistical tests to compare differences in outcomes between treatment and control group members. 
  • The study found that LLPI clothing significantly reduced tick bites in both years. 
  • This study receives a high evidence rating for year 1 outcomes. This means we are confident that the estimated effects are attributable to the LLPI clothing, and not to other factors. However, the study receives a low evidence rating for the year two outcomes. This means we are not confident that the estimated effects are attributable to the LLPI clothing; other factors are likely to have contributed.  

Features of the Study

This randomized controlled trial assessed the efficacy of LLPI clothing in reducing tick bites among outdoor workers in Rhode Island and Massachusetts. The study followed two cohorts of participants (cohort 1: 2016-2017 and cohort 2: 2017-2018) for two tick seasons (March through December). To be eligible for the study, participants were required to be at least 18 years old, work outside for at least an average of 10 hours each week during the peak of tick season, and provide written informed consent. People were excluded from the study if they were pregnant or planning to become pregnant during the study, were unable to speak English, or knew they were allergic or sensitive to insecticides. Of the 132 eligible participants, 65 were randomly assigned to the treatment group and 67 were randomly assigned to the control group.  

At the start of each season participants were told to put their study ID on their work clothing and ship them to be treated. The treatment group participants were sent back clothing that was treated with permethrin and the control group participants were sent back clothing that was washed and dried. All participants were instructed to follow their normal tick bite prevention methods and to wash their clothing as usual. Participants were also required to complete weekly online tick bite logs. The authors removed participants from the study if they did not send in the clothing or did not maintain the tick bite logs. The final sample included 82 outdoor workers (40 treatment and 42 control). The sample was mostly male (76%), with a median age of 40. Data sources included weekly online logs, a baseline questionnaire, and questionnaires conducted after each tick season. The authors used statistical tests to compare tick bites among treatment and control group members. 

Findings

Health and Safety 

  • The study found that LLPI clothing had a 65% protective effectiveness against tick bites in the first year. The treatment group had significantly fewer tick bites than the control group (24 vs. 78). 
  • The study found that LLPI clothing had a 50% protective effectiveness against tick bites in year 2. The treatment group had significantly fewer tick bites than the control group (36 vs. 88). 

Considerations for Interpreting the Findings

The study had high attrition in the second year and the authors did not sufficiently control for baseline group differences in race/ethnicity as required by the protocol. Therefore, the year 2 outcomes are not eligible for a high or moderate evidence rating. Additionally, there was not 100% compliance with wearing the clothing submitted for treatment although participants were asked to wear the clothing they sent in for treatment.  

Causal Evidence Rating

The quality of causal evidence for year one outcomes is high because it was based on a well-implemented randomized controlled trial with low attrition. This means we are confident that the estimated effects are attributable to LLPI clothing and not to other factors. However, the quality of evidence for year two outcomes is low because it was based on a randomized controlled trial with high attrition and the authors did not control for potential baseline differences between the treatment and control groups. This means we are not confident that estimated effects on these outcomes are attributable to LLPI clothing; other factors are likely to have contributed. 

Reviewed by CLEAR

May 2024