How is permethrin used for tick prevention in dogs?
Permethrin as tick prevention
Many tick prevention methods for dogs employ permethrin as the primary active ingredient. Frontline Shield, K9 Advantix II, and Vectra3D are some of the most popular brands that utilize this popular tick deterrent. In this topical application, permethrin is cutaneously distributed for dogs predominantly via spot-on treatments and collars. Permethrin has been long used for tick prevention on dogs and is considered safe. It is even considered safe enough for humans to wear gear treated in permethrin once the chemical dries. Given this compound's popularity, many folks ask us “What is permethrin and how does it work?”
What is permethrin?
Two-dimensional chemical structure of permethrin
Permethrin is a synthetic form of pyrethrins—chemicals with pesticidal qualities that occur naturally in some types of chrysanthemum flowers. Because it lacks an ά-cyano group, it is classified as a Type-I pyrethroid. Permethrin is stable under the effects of light, air, acidity, alkalinity, and moisture. It is also non-volatile and poorly water-soluble. Thanks to the enzyme glucuronyl transferase, permethrin is rapidly metabolized in dogs (and many other mammals). Permethrin is highly lipophilic, meaning that the compound is attracted to lipid-containing tissues where it is quickly metabolized via enzymatic hydrolysis, oxidation, and other conjugation processes. Certain animals, particularly cats, lack glucuronyl transferase activity which conjugates permethrin metabolites. As a result, cats cannot metabolize permethrin and severe illness can results if cats are exposed to toxic doses of permethrin. DO NOT EVER USE DOG-SPECIFIC TICK PREVENTION ON CATS.
How permethrin acts on arthropods
An arthropod like a tick comes into contact with permethrin either by ingestion or directly through the outer cuticle. Permethrin is distributed along the arthropod’s nervous system and acts to slow down the activation and inactivation process of the sodium channel gates, which significantly prolongs the sodium ion influx. This interference with the voltage-gated sodium channels of neurons causes continuous nerve discharges resulting in restlessness, paralysis, tremors, respiratory failure, and death of the arthropod.
Permethrin treatments
Permethrin treatments are applied directly to the skin, usually between the shoulder blades or on the back by parting the fur. Permethrin dosage is determined based on body weight of the dog and needs reapplication on a regular (typically monthly) basis. Less than 2% of the permethrin is absorbed transdermally through canine skin. The permethrin treatments spread cutaneously on the animal's skin surface via local tissue-dependent migration from the point of administration through the hair coat and long the stratum corneum; in other words, the chemical is intended to distribute itself across the dog’s body by traveling along or just below the surface of the skin rather than being absorbed deeper into the dog’s body. The goal is for the formulation to eventually cover the entire animal. However, it has been found that not all parts of the dog’s surface are always covered evenly at uniform concentrations. Studies have found that distant body parts, such as parts of the legs and part of the belly, may not be completely covered by permethrin migration. Additionally, time and activity can impact the potency of permethrin treatments. Permethrin often presents peak efficacy between 7-14 days after application. So pet guardians should be aware of the time it takes to completely protect a dog with permethrin, and of its waning efficacy after 21 days of application. Bathing, shampooing, and swimming can also reduce or shorten the efficacy of permethrin treatments.
To mitigate the risk of tick bites during that “ramp-up” period in the first week or so after aplication, tick preventatives often include additional compounds such as imidacloprid or fipronil. Both compounds confer treatment that acts within 48 hours of application to provide more consistent coverage for the month.
No tick preventative product currently available can completely prevent tick bites and the transmission of tick-borne diseases. However, permethrin is a great tool to consider in your arsenal of trick bite prevention. Staying on a prescribed and consistent application of permethrin can drastically reduce tick feeding duration and bite occurrence, thus decreasing the risk of tick-borne disease transmission. Always talk to your veterinarian before starting your pets on any preventatives and NEVER GIVE YOUR CATS PERMETHRIN PRODUCTS!
Additional reading
Pfister K, Armstrong R. Systemically and cutaneously distributed ectoparasiticides: a review of the efficacy against ticks and fleas on dogs. Parasit Vectors. 2016 Aug 8;9(1):436. doi: 10.1186/s13071-016-1719-7. PMID: 27502490; PMCID: PMC4977707.
