Introduction

Selamectin is a novel macrocyclic lactone of the avermectin class. Since its 1999 market introduction, selamectin’s wide margin of safety, broad spectrum of coverage, and convenient administration and packaging have resulted in the wide use of this product in both small animal (e.g. dog, cat) and companion exotic practice. Selamectin has a spectrum of activity encompassing a range of external and internal parasites of dogs and cats, including fleas (Ctenocephalides spp.) and larval stages of the canine heartworm (Dirofilaria immitis).

Mechanism of action

As an avermectin, selamectin activates glutamate-gated chloride ion channels at muscle synapses by binding to gamma-aminobutyric acid (GABA) receptors, disrupting neuromuscular transmission in nematodes and arthropods. In invertebrates, these avermectin binding sites are found in peripheral tissues, while in vertebrates, they are confined to the central nervous system. Therefore, selamectin’s wide safety margin in companion animals is attributed to the inability of this compound to penetrate the blood-brain barrier. Selamectin is considered safe to use even in avermectin sensitive collies[1].

For B. malayi, more recent data have indicated that the GluCls exist in the excretory/secretory pore of microfilaria and the reproductive tissue of the adult worm. The drug causes rapid clearance of microfilariae and long-term sterilization of the female worm by inhibiting the secretion of immunomodulatory molecules, which leads to suppression of the Mf population for several months.

Selamectin and ivermectin

Selamectin is considered safe for use in both large and small animals and pregnant and lactating cats and dogs because it does not cross the blood-brain barrier in mammals and achieves large volume distribution, tissue penetration, and accumulation in sebaceous glands, thereby acting as a drug reservoir. Although selamectin and ivermectin are both macrocyclic lactones, selamectin has been modified to improve its safety profile. Topical selamectin is easier to administer than oral or parenteral ivermectin, and its toxicity has been reported to be lower.

Labeled use in dogs and cats

Fleas. Revolution? and Stronghold? are both labelled for the treatment and prevention of fleas (Ctenocephalides spp.), the treatment of ear mites (Otodectes cynotis), and heartworm prevention (Dirofilaria immitis) in dogs and cats. When topically applied at 4- 8 mg/kg, the active antiparasitic agent remains effective against fleas for up to 28 days in dogs and cats. As per label claims, selamectin (Stronghold?) disrupts the flea life cycle at several stages of development, including being adulticidal on the host animal and preventing the hatching of eggs and killing larvae in the environment by contact with debris from treated animals. This is an important consideration when integrating topical flea control into a comprehensive strategy that also addresses prevention of reinfestation and environmental decontamination[2].

Heartworm. While selamectin is effective against immature L3 stage D. immitis, it has no activity against adult heartworms. Although no adverse effects were noted in heartworm infected dogs or cats, a heartworm test is recommended before treatment. Selamectin is an effective heartworm preventative at 2 mg/kg orally or 4-8 mg/kg topically for up to 30 days in dogs. Efficacy against immature D. immitis does wane 2 months after exposure, which suggests patients should be tested if there is more than a 60 day gap in treatment. Selamectin may be used in heartworm-positive cats to prevent subsequent infections, which may apply to preventative care in small exotic carnivores, such as ferrets.

Ticks. Selamectin kills ticks, but does not repel nor maintain residual activity against these arthropods, therefore recommendations for prevention increase to 14 day treatment intervals due to the higher risk of re-infestation. Revolution? has label claims against the American dog tick (Dermacentor variabilis). At an oral dose of 6 mg/kg, selamectin was effective in killing both the brown dog tick (Rhipicephalus sanguineus) and the American dog tick.

Mites and nematodes. Selamectin is labeled for treatment of adult roundworm (Toxocara canis), sarcoptic mange (Sarcoptes scabiei), and biting lice (Trichodectes canis) in dogs and of biting lice (Felicola subrostratus), adult roundworm (Toxocara cati) and hookworm (Ancylostoma tubaeforme) infections in cats. It is 100% effective against hookworms and other roundworms (e.g. Ancylostoma caninum, Uncinaria stenocephala, Toxascaris leonine) in dogs at doses as low as 2 mg/kg when administered orally every 7 days. However, this treatment method precludes the inclusion of these parasites on label claims.

Reference

[1] Patsharaporn T Sarasombath. “First study of topical selamectin efficacy for treating cats naturally infected with Brugia malayi and Brugia pahangi under field conditions.” Parasitology Research 118 4 (2019): 1289–1297.

[2] Jacobs, D E. “Selamectin - a novel endectocide for dogs and cats.” Veterinary parasitology 91 3–4 (2000): 161–2.