Afoxolaner: An Effective Insecticide and Acaricide for Flea and Tick Infestations in Dogs

General Description

Afoxolaner is an isoxazoline class insecticide and acaricide that acts on ligand-gated chloride ion channels, primarily by inhibiting the gamma-aminobutyric acid gated channels. It is used for the treatment of flea and tick infestations in dogs, with a recommended dosage based on the dog's weight range. Afoxolaner is nearly completely absorbed when orally administered to dogs and has a plasma half-life of around two weeks. The drug is metabolized into hydrophilic compounds and primarily excreted through bile, with some elimination through urine. Afoxolaner's unique mode of action results in the efficient killing of fleas and ticks upon contact or ingestion by treated dogs. Certain precautions should be taken when using Afoxolaner, including cautious use in pregnant dogs, puppies younger than 8 weeks, or dogs weighing less than 2 kg.

Figure 1. Afoxolaner

Pharmacological Effects

Afoxolaner is an isoxazoline class insecticide and acaricide that acts on ligand-gated chloride ion channels, particularly by inhibiting the gamma-aminobutyric acid (GABA) gated channels. This action blocks the transmission of chloride ions from the presynaptic membrane to the postsynaptic membrane, leading to increased neuronal activity and overexcitation followed by death in insects. When administered orally at a dose of 2.5 mg/kg body weight in dogs, Afoxolaner is nearly completely absorbed, with an absolute bioavailability of 74%. The time to reach peak concentration (Tmax) is around 2-4 hours, with a peak concentration (Cmax) of 1655 (±332) ng/ml. The volume of distribution in tissues is approximately 2.6±0.6 l/kg, plasma clearance is 5.0±1.2 ml/hr/kg, and the plasma half-life is around 2 weeks. However, there may be variations in different breeds of dogs, such as in Border Collies where a dose of 25 mg/kg body weight can result in a half-life of approximately 47.7 days. Afoxolaner is metabolized in dogs into more hydrophilic compounds, and both the metabolites and the parent form are primarily excreted through bile, with no enterohepatic circulation. Some portion of the drug is also eliminated through urine. ¹

Indications

Afoxolaner is indicated for the treatment of flea and tick infestations in dogs. As an isoxazoline class insecticide and acaricide, Afoxolaner effectively targets and eliminates fleas and ticks on dogs, providing relief from these common parasitic infestations. Fleas and ticks not only cause discomfort for dogs but can also transmit diseases, making effective treatment crucial for canine health and well-being. The unique mode of action of Afoxolaner, through its interference with ligand-gated chloride ion channels and inhibition of the gamma-aminobutyric acid (GABA) gated channels in insects and acarids, results in the rapid and efficient killing of these parasites upon contact or ingestion by treated dogs. This makes Afoxolaner an important therapeutic option for managing and preventing flea and tick infestations, helping to improve the overall health and quality of life for dogs. ²

Dosage

Afoxolaner is an oral medication used for the treatment of flea and tick infestations in dogs. The dosage of Afoxolaner varies depending on the weight range of the dog. For dogs weighing between 2 kg and 4 kg, the recommended dosage is 11.3 mg chewable tablets. Dogs weighing between 4.1 kg and 10 kg should be given 28.3 mg chewable tablets, while those weighing between 10.1 kg and 25 kg require 68 mg chewable tablets. For dogs weighing between 25.1 kg and 50 kg, 136 mg chewable tablets are recommended. Dogs weighing over 50 kg can be given a combination of chewable tablets to achieve a dose within the range of 2.7 to 6.9 mg/kg body weight. When administered as directed, Afoxolaner has shown no reported adverse effects in dogs. However, it is important to note that in Border Collies, administration of Afoxolaner at five times the recommended dose (25 mg/kg body weight) has been associated with diarrhea and vomiting. Certain precautions should be taken when using Afoxolaner, including cautious use in pregnant dogs, puppies younger than 8 weeks, or dogs weighing less than 2 kg. Additionally, Afoxolaner requires contact with fleas and ticks to be effective, so there is still a risk of disease transmission via parasites. If the dog refuses to consume the chewable tablet directly, it can be mixed with their food. After removing the required number of chewable tablets, any remaining tablets should be returned to the original packaging, and hands should be washed after use. Expired products should not be used, and Afoxolaner should be stored out of reach of children. ³

Reference

1. Letendre L, Huang R, Kvaternick V, Harriman J, Drag M, Soll M. The intravenous and oral pharmacokinetics of afoxolaner used as a monthly chewable antiparasitic for dogs. Vet Parasitol. 2014;201(3-4):190-197.

2. Mendoza-Roldan JA, Napoli E, Perles L, et al. Afoxolaner (NexGard®) in pet snakes for the treatment and control of Ophionyssus natricis (Mesostigmata: Macronyssidae). Parasit Vectors. 2023;16(1):6.

3. Rania Gollakner, BS, DVM, MPH. Afoxolaner. VCA animal hospitals.