Selamectin
Selamectin is a broad-spectrum, semi-synthetic avermectin topical parasiticide used extensively in veterinary medicine. **Key Clinical Features:** * **Broad Spectrum:** Effective against a wide range of internal and external parasites, including heartworm larvae, fleas, ear mites, sarcoptic mange, and certain ticks. * **Feline Intestinal Parasites:** In cats, it is also effective against hookworms and roundworms. * **Safety Profile:** Highly safe, even in avermectin-sensitive breeds (e.g., Collies with the MDR1 mutation) when used at the recommended topical label doses. * **Systemic Absorption:** Unlike many topical flea/tick products that remain in the lipid layer of the skin, selamectin is absorbed systemically, which is necessary for its efficacy against heartworm and internal parasites.
Mechanism: Selamectin acts by binding to **glutamate-gated chloride channels** in the nervous system of invertebrates. Binding โ increased permeability of the cell membrane to chloride ions โ hyperpolarization of the nerve or muscle cell โ **flaccid paralysis and death** of the parasite. *Clinical Pearl:* Mammals generally lack glutamate-gated chloride channels, and macrocyclic lactones do not readily cross the intact mammalian blood-brain barrier (mediated by P-glycoprotein), conferring a high margin of safety.
Dosing by species
- Heartworm prevention, flea control, ear mites, hookworms, roundworms ยท 6 mg/kg ยท topical ยท monthly (ear mites, hookworms, roundworms = once, repeat in one month if necessary)
- Notoedric (face/head) mange (Notoedres cati) ยท 6 mg/kg ยท topical ยท once
- Biting lice ยท 6 mg/kg ยท topical ยท once
- Flea treatment/prevention, heartworm prevention, ear mites, biting lice, roundworms, hookworms ยท 6 mg/kg ยท topical ยท monthly ยท Ongoing for prevention ยท Apply to the skin at the base of the neck in front of the shoulder blades.
- Heartworm prevention ยท 18 mg/kg ยท topical ยท every 30 days
- Flea treatment/prevention, heartworm prevention (Off-label / specific regional labels) ยท 15 - 18 mg/kg ยท topical ยท monthly ยท Ongoing for prevention ยท Use the kitten/small cat pipette.
- Ear mites (P. cunuculi) in rabbits ยท 6-18 mg/kg ยท topical ยท once
- Mites and fleas (Off-label) ยท 6 - 18 mg/kg ยท topical ยท monthly ยท As needed ยท Off-label use. Dose varies by species (e.g., rabbits, guinea pigs).
Routes of administration
Contraindications
- Application to broken skin
- Application to wet hair coats
- Use in severely debilitated or underweight animals (relative)
- Puppies and kittens under 6 weeks of age
- Sick, debilitated, or severely underweight animals
- Known hypersensitivity to the active substance or excipients
Adverse effects
- Transient localized alopecia at application site (rare)
- Mild, transient gastrointestinal signs (vomiting, diarrhea) if ingested
- Transient local alopecia at the application site
- Application site irritation or pruritus
- Rarely: vomiting, diarrhea, lethargy, or anorexia
- Rarely: neurological signs (tremors, ataxia) in highly sensitive individuals
Drug interactions
- Other macrocyclic lactones (e.g., ivermectin, milbemycin) ยท Potential for additive toxicity, though selamectin has a high margin of safety. ยท minor
- P-glycoprotein inhibitors (e.g., ketoconazole, cyclosporine) ยท May theoretically increase systemic absorption and CNS penetration, increasing the risk of toxicity. ยท moderate
Monitoring
- Clinical efficacy
- Owner compliance with treatment regimen
- Efficacy of parasite control
- Application site for signs of irritation or alopecia
Overdose
Generally very safe. Overdoses up to 10 times the recommended dose typically do not produce clinical signs in dogs or cats. If massive accidental ingestion occurs, monitor for neurologic signs (ataxia, tremors, mydriasis) and provide supportive care.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.