In a study at Western Connecticut State University, subjects performed outdoor field work while wearing clothing impregnated with the pesticide permethrin, which can deter many types of arthropods, including human-biting ticks. The clothes were worn and washed 16 times over an eight-week period and then tested to determine what effect the usage and laundering had on the repellency of the permethrin in the clothing. (Photo credit: Peggy Stewart, Western Connecticut State University)

By Neeta Connally, Ph.D.

In recent years, clothing that has been factory-impregnated with the pesticide permethrin has become readily available for hikers, gardeners, outdoor enthusiasts, and even children, as a retail product for deterring many types of arthropods, including human-biting ticks. A recent study by researchers at the U.S. Centers for Disease Control and Prevention (CDC) demonstrated that factory-treated permethrin clothing was quite effective at repelling and incapacitating several species of ticks, including the primary Lyme disease vector, Ixodes scapularis (the blacklegged tick).

Neeta Connally, Ph.D.

In that study, however, the clothing tested had not been worn or laundered. In fact, despite a growing body of evidence that factory-treated permethrin-treated clothing is effective for repelling ticks, studies showing what happens when ticks are exposed to treated clothing that has been worn, washed, and dried, have been scarce in the literature. So, my colleagues and I at Western Connecticut State University, Colorado State University, and the CDC set out to investigate this further.

In our study, published Monday in the Journal of Medical Entomology, four human subjects conducted outdoor activities while wearing factory-treated permethrin-impregnated clothing. Each subject wore a long-sleeved cotton shirt, cotton work pants, cotton/synthetic-blend socks, and canvas high-top sneakers, all factory-impregnated with permethrin by the Insect Shield company in Greensboro, North Carolina. The subjects wore the treated clothing for two days per week for eight weeks, and all clothing items except shoes were laundered after each day of wear (16 times total). After eight weeks, we conducted laboratory bioassays to assess the contact irritancy (i.e., repellent effect) and toxicity of the clothing, and snips of the clothing were sent to the Colorado State University Proteomics and Metabolomics Facility to assess permethrin concentration.

The lab bioassays followed a model we developed a couple years ago, one of which involves placing ticks on a segment of the tested fabric, held at a 45-degree angle, and observing the ticks’ behavior, such as the “hot-foot” effect, in which they actively try to remove themselves from the permethrin-treated fabric.  A second bioassay was also used, in which ticks were forced to contact the treated fabric on a horizontal surface for a predetermined time period, and later observed at two time points for signs of normal movement.  We compared tick bioassay results and permethrin concentration of laundered and worn clothing to pristine treated clothing (not worn or washed), washed but unworn clothing, and untreated fabric.

Here is what we found:

• There was a large reduction in permethrin concentration, ranging from 50-90 percent, in clothing items laundered 16 times over the eight-week period.
• Treated shoes that were worn but not laundered retained permethrin concentration comparable to that of the pristine treated shoes.
• Washed/dried and worn clothing was less repellent and less toxic to ticks overall when compared to pristine treated clothing.
• Nonetheless, washed/dried and worn treated clothing items still outperformed untreated control fabric: between 31-67 percent of ticks displayed normal movement one hour after contact with washed and worn treated fabric compared with 90-100 percent displaying normal movement after contact with untreated control fabric.

We sought to evaluate the impact of laundering and wearing permethrin-treated clothing on its contact irritancy and toxicity to blacklegged ticks. What we found was that washed and worn clothes are less effective at knocking down ticks than pristine treated clothes, but washed and worn permethrin-treated clothes were still more effective than untreated clothes.

If we want to understand whether wearing retail tick-repellent clothing is truly protective against tick-borne diseases, then prospective human-intervention studies are needed. For now, the results of our study suggest that wearing factory-impregnated permethrin-treated clothing, including clothing that has been worn and laundered several times, has the potential to offer some protection against tick bites in a real-world scenario.

Neeta Connally, Ph.D., is a medical entomologist and associate professor of biology at Western Connecticut State University in Danbury, Connecticut, where she oversees the WCSU Tickborne Disease Prevention Laboratory. Email: connallyn@wcsu.edu