A study conducted by researchers at the University of North Carolina at Chapel Hill and the University of Rhode Island evaluated the effectiveness of long-lasting permethrin-impregnated clothing for reducing bites from blacklegged ticks (Ixodes scapularis) among outdoor workers. Their findings showed as much as 65 percent fewer tick-bite encounters among study participants wearing the treated clothing compared to those wearing non-treated clothing. (Photo by Armed Forces Pest Management Board, via Flickr, CC BY-NC-ND 2.0)

By John P. Roche, Ph.D.

Tick-borne diseases such as Lyme disease are a serious health problem in the United States. About 30,000 cases of Lyme disease are reported to the U.S. Centers for Disease Control and Prevention each year, but the CDC states that the real number of cases in the U.S. might be as high as 300,000. Thus, strategies to reduce exposure to tick bites are an important public health goal.

One promising strategy is wearing clothing impregnated with the insecticide permethrin. Permethrin repels ticks, causing them to drop off clothing and, with longer exposure, makes them unable to bite. In previous work, Steve Meshnick, M.D., Ph.D., of the University of North Carolina at Chapel Hill and colleagues found that permethrin-treated clothing was 80 percent effective in reducing bites from the lone star tick (Amblyomma americanum) in the first year of use. But how effective is permethrin-treated clothing in protecting against blacklegged ticks (Ixodes scapularis), the primary vector of Lyme disease, and are there ways to improve its efficacy? These questions were investigated recently by Meshnick and University of Rhode Island collaborator Thomas Mather, Ph.D., and their teams, and their findings are presented in a new report published this month in the Journal of Medical Entomology.

The study is the first random control field trial to test the effectiveness of long-lasting permethrin treatment on clothing against bites by the blacklegged tick. In addition to vectoring Lyme disease, blacklegged ticks are vectors of anaplasmosis, babesiosis, hard tick relapsing fever, and Powassan virus.

The research team studied 132 subjects who worked outdoors in a variety of professions in Southern Massachusetts or Rhode Island. Eighty-two of the subjects completed one or more years of the study and were used in the analysis. The subjects were divided into a treatment group that wore long-lasting, factory-impregnated garments and a control group that did not. Subjects were divided into the groups randomly, and neither participants nor study investigators knew which group individual subjects were in during the course of the trial. Previous studies found that the amount of permethrin absorbed by individuals wearing long-lasting permethrin-impregnated garments was far below EPA and European safety guidelines.

A study conducted by researchers at the University of North Carolina at Chapel Hill and the University of Rhode Island evaluated the effectiveness of long-lasting permethrin-impregnated clothing for reducing bites from blacklegged ticks (Ixodes scapularis) among outdoor workers. Their findings showed as much as 65 percent fewer tick-bite encounters among study participants wearing the treated clothing compared to those wearing non-treated clothing. (Photo courtesy of Steve Meshnick, M.D., Ph.D.)

Subjects in the study recorded incidences of contact with ticks and incidences of bites from ticks. In the first year, an incidence rate of 0.13 bites per 100 outdoor work hours was recorded among subjects in the treatment group. An incidence rate of 0.37 bites per 100 outdoor work hours was recorded among subjects in the control group. In the second year, an incidence rate of 0.25 bites per 100 outdoor work hours was recorded among subjects in the treatment group, and an incidence rate of 0.50 bites per 100 outdoor work hours was recorded among subjects in the control group. Therefore, the use of long-lasting permethrin-impregnated clothing significantly reduced tick bites, by 65 percent in the first year and by 50 percent in the second year.

TickEncounter.org, operated by the University of Rhode Island, provides educational materials such as this poster to inform the public about ways to reduce incidence of tick bites. (Image provided by Thomas Mather, Ph.D.)

Effectiveness of the clothing declined in the second year in this study as well as in the previous study conducted in North Carolina by Meshnick and colleagues. This is a result of the permethrin gradually being washed out during repeated launderings. Study coauthor Mather says, “These results suggest that annual retreatment will provide the most effective tick protection.”

Treatment with permethrin can be done at home, but having clothing treated professionally provides longer-lasting protection. “The InsectShield-treated clothing used in this study is permethrin-impregnated in a factory using special treatment equipment, including ovens,” Meshnick says. “This special method can’t be replicated outside of the factory.” Both clothes originally purchased as permethrin-impregnated products and regular clothes not previously treated can be sent in for factory treatment with permethrin.

The long-lasting permethrin-treated clothing was more effective in the first year than in the second year, but it’s first-year efficacy was still only a 65 percent reduction in tick bites. Speculating why the clothing wasn’t 100 percent effective, coauthor Mather says, “We aren’t able to know whether the ticks that subjects reported biting them were acquired when they were wearing the treated clothing or were acquired when they were not wearing the treated clothing. Ticks can be acquired by outdoor workers while on the job but also when they’re not on the clock and not in their permethrin-impregnated clothing.” And, as Meshnick points out, “Some ticks might first land on exposed skin and miss the clothing entirely.”

None the less, this study provides solid evidence of the effectiveness of permethrin-treated clothing as a tool for reducing tick bites. It has broad practical significance, and not only for outdoor workers. Mather says, “Our study suggests that tick-repellent clothing will be helpful for protecting people both at work and at play. The safety findings of the study, showing that people wearing the long-lasting permethrin clothing consistently had no higher levels of permethrin metabolites in their urine than folks that were not wearing treated clothing, suggests that people are not at any greater risk for exposure to permethrin by wearing permethrin-treated clothing.”

More information about obtaining permethrin-treated clothing or having your own clothes treated with long-lasting permethrin can be found from commercial suppliers, including Insect Shield of Greensboro, North Carolina.

Also see previous articles on Entomology Today:

• “Tick-Repellent Clothing: How Laundry Suds Affect Your Permethrin-Treated Duds,” August 30, 2018
• “New CDC Tick Study Adds to Promise of Permethrin-Treated Clothing,” May 24, 2018

John P. Roche, Ph.D., is an author, biologist, and educator dedicated to making rigorous science clear and accessible. Director of Science View Productions and Adjunct Professor at the College of the Holy Cross, Dr. Roche has published over 200 articles and has written and taught extensively about science. For more information, visit https://authorjohnproche.com. Email: scienceviewinfo@zoho.com.