Topical permethrin may increase blacklegged tick (Ixodes scapularis) repellency but is associated with cutaneous irritation in horses

Authors: Poh, Karen; COLE, ZOEY - Pennsylvania State University; SMARSH, DANIELLE - Pennsylvania State University; SPRINGER, HAYLEY - Pennsylvania State University; KELLY, KATHLEEN - Pennsylvania State University; KENNY, LAURA - Pennsylvania State University; MACHTINGER, ERIKA - Pennsylvania State University

Submitted to: American Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/19/2023
Publication Date: 2/20/2023
Citation: Poh, K.C., Cole, Z.T., Smarsh, D.N., Springer, H.R., Kelly, K., Kenny, L.B., Machtinger, E.T. 2023. Topical permethrin may increase blacklegged tick (Ixodes scapularis) repellency but is associated with cutaneous irritation in horses. American Journal of Veterinary Research. 84(4). https://doi.org/10.2460/ajvr.22.10.0176.
DOI: https://doi.org/10.2460/ajvr.22.10.0176

Interpretive Summary: Ticks are common ectoparasites of horses and can transmit pathogens that cause debilitating diseases in equine populations. With no available vaccine for Lyme disease or anaplasmosis for horses, tick bite prevention via chemical sprays is recommended. Permethrin is a common active ingredient in chemical sprays used to protect horses from flies and ticks. Even though permethrin is a common active ingredient in many registered insecticides that are used on horses, the safety and efficacy of permethrin use on horses against ticks has not been fully evaluated. Therefore, this study evaluated the safety of repeated exposures to permethrin concentrations (0% control, 1.5%, 5%, and 10%) on the necks and faces of horses and assessed the efficacy and longevity of permethrin as an equine tick repellent. The treatments were repeatedly applied to horses four times a day, for up to 10 days. An 8 mm biopsy was taken from the treatment areas to evaluate dermal responses. Treatments were then applied to the face of the horses four times a day, for up to five days. Tick bioassays tested the repellency response of five female blacklegged ticks (Ixodes scapularis) to repellent treatments applied to each leg of the horse. While higher concentrations of permethrin repelled more ticks and lasted longer, these same concentrations also produced greater skin damage after repeated exposures, according to histologicala results. This underscores the need for additional tick control methods for horses.

Technical Abstract: To address tick-borne diseases in horses, the goals of this study are to evaluate the safety of repeated exposures to permethrin concentrations (0% control, 1.5%, 5%, and 10%) on the necks and faces of horses and to assess the efficacy and longevity of permethrin as an equine tick repellent. Each treatment was applied to the neck of each horse (0.01 m2) four times a day, for up to 10 days. An 8 mm biopsy was taken to evaluate post-exposure dermal responses. Any treatments that were not withdrawn were applied to a quadrant of the horse's face four times a day, for up to five days. For tick bioassays, a treatment was applied to one leg of a horse and five female blacklegged ticks (Ixodes scapularis) were evaluated as “repelled” or “not repelled” by the treatment. The tick bioassays were repeated up to five days, but treatment application took place only on the first day of the experiment. Histological results of biopsies from the neck indicated that more repeated exposures or higher concentrations resulted in more dermal damage. Tick bioassays showed that 5% and 10% permethrin had the greatest efficacy and longevity as a tick repellent, but the differences in tick repellency across all treatments were not significant overall. While higher concentrations of permethrin can repel more ticks with longer-lasting residual repellent effects, higher concentrations also produced greater skin damage after repeated exposures. These opposing findings emphasize the need for better tick prevention and control methods that balance safety and efficacy for the equine community.