Selamectin — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	Topical (spot-on)	6 mg/kg (minimum) applied to the skin at the base of the neck	Once monthly	Duration: Year-round for heartworm prevention; for flea control, continue for at least 3 months Notes: For heartworm prevention, administer monthly. For sarcoptic mange, may be given monthly for 2-3 months. For ear mites, a single dose is often effective, but may be repeated in 30 days.
Cat	Topical (spot-on)	6 mg/kg (minimum) applied to the skin at the base of the neck	Once monthly	Duration: Year-round for heartworm prevention; for flea control, continue for at least 3 months Notes: For ear mites, a single dose is often effective, but may be repeated in 30 days. For intestinal parasites, may be given monthly.
Rabbit	Topical (spot-on)	6-12 mg/kg (off-label)	Once monthly	Duration: For ear mites, may require 2-3 doses at 2-week intervals Notes: Use with caution; safety in rabbits not fully established. Consult a veterinarian.
Ferret	Topical (spot-on)	6 mg/kg (off-label)	Once monthly	Duration: For ear mites, may require 2 doses Notes: Use with caution; safety in ferrets not fully established.

Clinical Indications & Species Uses

General Indications

Prevention of heartworm disease
Treatment and control of flea infestations
Treatment of ear mites
Treatment of sarcoptic mange
Control of certain intestinal nematodes

Dog (Canine)

Prevention of heartworm disease (Dirofilaria immitis)
Treatment and control of flea infestations (Ctenocephalides felis)
Treatment of ear mites (Otodectes cynotis)
Treatment of sarcoptic mange (Sarcoptes scabiei)
Control of roundworms (Toxocara canis) and hookworms (Ancylostoma tubaeforme)

Cat (Feline)

Prevention of heartworm disease (Dirofilaria immitis)
Treatment and control of flea infestations (Ctenocephalides felis)
Treatment of ear mites (Otodectes cynotis)
Treatment of roundworms (Toxocara cati) and hookworms (Ancylostoma tubaeforme)
Treatment of intestinal hookworms (Ancylostoma braziliense)

Rabbit & Small Mammals

Treatment of ear mites (Psoroptes cuniculi) (off-label use)
Treatment of fur mites (Cheyletiella spp.) (off-label use)

Exotic & Other Species

Treatment of ear mites in ferrets (off-label use)
Treatment of sarcoptic mange in foxes (off-label use)

Pharmacology & Mechanism of Action

Drug Class: Macrocyclic lactone (avermectin/milbemycin class) | Pharmacological Group: Antiparasitic (endectocide)

Mechanism of Action: Selamectin is a semisynthetic macrocyclic lactone that acts by binding to glutamate-gated chloride channels in nerve and muscle cells of parasites, causing hyperpolarization and paralysis of the parasite. It also enhances the release of gamma-aminobutyric acid (GABA) at the neuromuscular junction, further inhibiting neuronal transmission. This leads to the death of susceptible parasites. Selamectin is active against both nematodes and arthropods, including fleas, heartworm larvae, ear mites, and certain intestinal parasites.

Pharmacodynamics: Selamectin exhibits broad-spectrum antiparasitic activity. It is effective against adult fleas (Ctenocephalides felis), flea larvae, heartworm (Dirofilaria immitis) larvae, ear mites (Otodectes cynotis), sarcoptic mange mites (Sarcoptes scabiei), and certain intestinal nematodes (e.g., roundworms and hookworms). It also has activity against ticks (partially) and some lice. The drug is rapidly absorbed and distributed, with a long duration of action, providing monthly protection.

⚡ Pharmacokinetics Summary

Absorption: After topical application, selamectin is absorbed through the skin and reaches systemic circulation. In dogs and cats, peak plasma concentrations occur within 24-72 hours after application. The drug is also absorbed systemically after oral administration, but topical is the approved route.

Distribution: Selamectin is widely distributed throughout the body, including the skin, subcutaneous tissues, and internal organs. It crosses the blood-brain barrier to a limited extent due to P-glycoprotein efflux, which reduces central nervous system toxicity in mammals. It is distributed to the gastrointestinal tract, which contributes to its efficacy against intestinal parasites.

Metabolism: Selamectin is metabolized in the liver, primarily by oxidation and hydrolysis. The major metabolite is N-demethylated selamectin, which retains some antiparasitic activity. Metabolism is species-dependent, with dogs and cats metabolizing it at different rates.

Excretion: Selamectin and its metabolites are excreted primarily in the feces, with a smaller amount excreted in urine. In lactating animals, it is excreted in milk, but levels are low and not considered clinically significant for nursing offspring.

Half-Life: The elimination half-life is approximately 7-14 days in dogs and cats, allowing for monthly dosing. In other species, half-life may vary.

Bioavailability: After topical application, systemic bioavailability is approximately 50-70% in dogs and cats, with the remainder remaining in the skin and hair. Oral bioavailability is higher, but topical is the approved route.

Protein Binding: Selamectin is highly protein-bound (approximately 99%) in plasma.

Available Formulations & Strengths

Topical solution (spot-on) 15 mg, 30 mg, 45 mg, 60 mg, 120 mg, 240 mg (for dogs and cats based on weight) (Topical)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to selamectin or any component of the formulation
Do not use in puppies or kittens less than 6 weeks of age
Do not use in dogs or cats that are sick, debilitated, or underweight (for heartworm prevention, test for existing heartworm infection first)
Do not use in breeding animals unless specifically indicated (safety not fully established)

Warnings & Clinical Precautions:

For topical use only; avoid oral ingestion by pets or humans
Do not apply to broken or irritated skin
Keep away from eyes and mouth of the animal
Wash hands after application
Do not use in animals with known hypersensitivity to macrocyclic lactones
Use with caution in animals with a history of seizures or neurological disorders
In heartworm-positive animals, treatment may cause adverse reactions due to dying microfilariae; monitor closely
For dogs and cats, test for existing heartworm infection before starting prevention
In collies and other breeds with MDR1 gene mutation, use with caution; although selamectin has a wide safety margin, monitor for neurological signs

Adverse Effects & Reactions

Common:

Transient hair loss at application site
Mild skin irritation or redness
Pruritus (itching)
Gastrointestinal upset (vomiting, diarrhea) if ingested

Serious / Severe:

Neurological signs (tremors, ataxia, seizures) in sensitive animals (especially with MDR1 mutation)
Anaphylactic reactions (rare)
Depression or lethargy (rare)

Rare:

Allergic dermatitis
Localized edema
Anorexia

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other macrocyclic lactones (e.g., ivermectin, milbemycin)	Additive toxicity may occur; avoid concurrent use.	High
Drugs that inhibit P-glycoprotein (e.g., ketoconazole, itraconazole, cyclosporine)	May increase selamectin concentrations and risk of toxicity.	Moderate
Central nervous system depressants (e.g., barbiturates, benzodiazepines)	May enhance CNS depression.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Salivation
Vomiting
Diarrhea
Tremors
Ataxia
Mydriasis
Depression
Seizures (in severe cases)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. There is no specific antidote. If ingestion occurs, induce vomiting (if within a few hours) and administer activated charcoal. Monitor for neurological signs and provide supportive care (e.g., IV fluids, anticonvulsants if seizures occur). In cases of topical overdose, wash the application site with mild soap and water. For severe toxicity, consider hospitalization and intensive care.

Food Animal Withdrawal Times

Not approved for use in food-producing animals. Do not use in animals intended for human consumption.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature, 20°C to 25°C (68°F to 77°F), with excursions permitted between 15°C and 30°C (59°F and 86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep in original container, tightly closed, and protect from light. Do not freeze. Keep out of reach of children and pets.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved by the FDA for use in dogs and cats (Revolution®).

Extra-Label (Off-Label) Use: In the US, selamectin is approved for use in dogs and cats. Extra-label use in other species (e.g., rabbits, ferrets) is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) with a valid veterinarian-client-patient relationship. It is not approved for food animals.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Selamectin is a broad-spectrum antiparasitic agent commonly used for monthly heartworm prevention and flea control in dogs and cats. It is also effective against ear mites and sarcoptic mange. It is generally safe and well-tolerated, but caution is advised in animals with MDR1 gene mutations (e.g., Collies) and in very young animals. For heartworm prevention, it is crucial to test for existing heartworm infection before starting treatment. Selamectin is not effective against adult heartworms, ticks (only partial efficacy), or tapeworms. It is available as a topical spot-on formulation, which should be applied to the skin, not the hair, at the base of the neck. The drug is not approved for food animals, and withdrawal times are not established. Always follow label instructions and consult a veterinarian for off-label use.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)