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New study confirms efficacy of permethrin-treated clothing against tick bites
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CDC researchers have conducted a series of experiments testing the effects of permethrin-treated clothing against tick species that are known to transmit infection to humans. They found that just 1 minute of exposure reduced some ticks’ ability to bite, highlighting the promise of permethrin-treated clothing in preventing tick-borne diseases.
“Of special note in our study is the very strong impact of permethrin-treated clothing on nymphal [Ixodes scapularis] ticks, which are considered the primary vectors in the United States of seven human pathogens, including the causative agents of Lyme disease, anaplasmosis and babesiosis,” Lars Eisen, PhD, research entomologist at CDC’s National Center for Emerging and Zoonotic Infectious Diseases, and colleagues wrote in the Journal of Medical Entomology.
Permethrin toxic against three tick species
According to the researchers, numerous studies published over the past 4 decades have demonstrated that permethrin-treated clothing reduces the motility of tick vectors in the U.S., including the blacklegged tick (I. scapularis), the lone star tick (Amblyomma americanum) and the American dog tick (Dermacentor variabilis). However, two studies published last year suggest that the impact of permethrin may be weaker for field-collected vs. laboratory-reared ticks. Therefore, the researchers used bioassays to further investigate the irritancy and toxicity of permethrin in both field-collected and laboratory-reared ticks, all of which were primary human-biting disease vectors (I. scapularis nymphs and females; A. americanum nymphs and females; and D. variabilis females). The permethrin-treated clothing items included T-shirts, long-sleeved shirts, pants and socks. All items were manufactured by Insect Shield and in “pristine condition,” meaning they had not been washed or worn.
Results from the experiments showed that both laboratory-reared and field-collected ticks lost their ability to move after contact with permethrin-treated clothing and “posed no more than minimal risk of biting.” Permethrin-treated clothing had the strongest impact on I. scapularis nymphs. After 1 minute of exposure, none of the I. scapularis nymphs had normal movement within 1 hour. Meanwhile, 98% of A. americanum females, 82% of D. variabilis females, 38% of I. scapularis females, and 14% of A americanum nymphs had normal movement. According to the researchers, it required 2 minutes of permethrin exposure for A. americanum nymphs to lack movement and 5 minutes of exposure for all female ticks to lack movement.
“All tested tick species and life stages experienced irritation — the ‘hot-foot’ effect — after coming into contact with permethrin-treated clothing,” Eisen said in a press release. “This caused the ticks to drop off from a vertically oriented treated textile designed to mimic a pant leg or the arm of a shirt. We also found that sustained contact with permethrin-treated clothing — up to 5 minutes — resulted in loss of normal movement for all examined tick species and life stages, leaving them unable to bite.”
The effects of a 5-minute session of permethrin exposure lasted for more than 24 hours among all I. scapularis nymphs, 90% of A americanum nymphs, and up 86% of all female ticks, according to the researchers.
Despite the promising results, Eisen and colleagues noted that important questions remain, including whether permethrin-treated clothing loses efficacy after repeated wash and wear.
“Ultimately, we’d like to be able to provide more specific guidance about the use of permethrin-treated clothing, including what types of clothing provide the best protection,” Eisen said in the release. “Additional research in this area can improve public health recommendations.”
Tick vectors increase in US
Recently, the CDC released data showing the number of illnesses caused by the bite of infected ticks, mosquitoes and fleas has more than tripled in the U.S. from 2004 to 2016. In addition, preliminary findings from a new surveillance effort known as Project Vector Shield, led by Indiana University’s Environmental Reliance Institute and the Prepared for Environmental Change Grand Challenge, indicate that the prevalence of Lyme disease vectors is expanding in the Midwest. The project regularly monitors ticks and mosquitoes at 20 sites in Southern Indiana to assess the threat of disease transmission in the region.
During the project’s first tick collection in April, Keith Clay, PhD, project leader and distinguished professor of biology at Indiana University, and colleagues identified several I. scapularis ticks in southern Indiana, even though they are not commonly found in this region, according to a press release.
“We have not sorted and identified all of the ticks from all sites in the first census yet. But of the first 10 sites analyzed, deer ticks were found in all 10 sites, and made up 10% of the total number of ticks collected,” Clay told Infectious Disease News.
Clay attributed climate change and human behavior, such as international travel, to the emergence of tick vectors into new regions.
“The only way to quickly detect new species or diseases entering an area is regular, long-term data collection, which is rare,” he said in a press release. “Our goal is to analyze ticks and mosquitoes for species that previously did not occur — or were less common — in Indiana so people are able to take the appropriate precautions.” – by Stephanie Viguers
References:
Frequently asked questions about Insect Shield. https://www.insectshield.com/pages/faqs. Accessed September 15, 2021.
Indiana University. IU bug-borne disease monitoring project finds deer ticks on the rise in Indiana. https://news.iu.edu/stories/2018/05/iub/releases/24-bug-borne-disease-monitoring-project-finds-deer-ticks-on-the-rise-in-indiana.html. Accessed May 29, 2018.
Prose R, et al. J Med Entomol. 2018;doi:10.1093/jme/tjy062.
Rosenberg R, et al. MMWR Morb Mortal Wkly Rep. 2018;doi:10.15585/mmwr.mm6717e1_w.
Disclosures: Eisen and colleagues report no relevant financial disclosures. Infectious Disease News was unable to confirm Clay’s relevant financial disclosures at the time of publication.