Spinosad
Spinosad is a systemic, oral macrocyclic lactone insecticide derived from the naturally occurring soil bacterium *Saccharopolyspora spinosa*. It is primarily indicated for the prevention and treatment of flea (*Ctenocephalides felis*) infestations in dogs. **Key Clinical Advantages:** * **Rapid Onset:** Begins killing fleas within 30 minutes, with complete flea death in 4 hours, effectively breaking the flea life cycle by killing adults before they can lay eggs. * **Waterproof:** Because it is administered systemically (orally), its efficacy is completely unaffected by swimming, bathing, or dermatologic shampoos. **Clinical Pearls:** * While primarily a flea adulticide, pilot studies have shown high efficacy in killing adult brown dog ticks (*R. sanguineus*) within 24 hours at higher off-label doses, though it is not officially labeled for tick control. * *Note on Feline Use:* While this specific monograph notes it is not approved for cats, a feline-specific formulation of Comfortis was subsequently approved by the FDA. Always refer to current product labeling for feline dosing.
Mechanism: Spinosad targets the insect nervous system through a unique mechanism distinct from other parasiticides: * **Primary Pathway:** Acts as an agonist at the **nicotinic acetylcholine D-alpha receptors** → causes motor neuron activation → leads to involuntary muscle contractions and tremors → prolonged exposure results in paralysis and rapid flea death. * **Secondary Pathway:** Opens **GABA-gated chloride channels** in insects, similar to the mechanism of other macrocyclic lactones, further contributing to neuromuscular paralysis. Because mammalian nicotinic receptors are structurally different from insect receptors, spinosad exhibits a high margin of safety in dogs.
Dosing by species
- Flea infestation · 50-75 mg/kg · PO · q28d · Give with or immediately after food.
- Prevention or treatment of fleas infestations · 30-60 mg/kg (minimum dosage of 30 mg/kg) PO once monthly with food. · PO · q30d · 1 month · Must be given with food for optimal absorption.
- Flea infestation · 45-70 mg/kg · PO · q28d · Give with or immediately after food.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Cats (per this specific monograph's label status)
- Dogs with known pork or soy allergies (chewable tablets contain pork proteins and hydrolyzed soy)
- Dogs and cats weighing < 1.2 kg
- Dogs and cats < 14 weeks of age
- Dogs with MDR1 mutation (relative contraindication due to increased risk of adverse effects)
Adverse effects
- Vomiting (most common)
- Depression/lethargy
- Anorexia
- Ataxia
- Diarrhea
- Pruritus
- Trembling
- Hypersalivation
- Seizures (rare)
- Vomiting (occasional)
- Lethargy (rare)
- Diarrhoea (rare)
- Anorexia (rare)
- Ataxia (rare)
Drug interactions
- Ivermectin · Concurrent use with high extra-label doses of ivermectin (e.g., for demodicosis) increases the risk of neurotoxicity. However, spinosad is safe to use with standard low-dose heartworm preventatives.
- Macrocyclic lactones (e.g., avermectins, ivermectin) · Increased risk of severe neurological toxicity. Do not use in combination. · major
- Digoxin · Competition for P-glycoprotein efflux pump, potentially increasing digoxin levels. · moderate
- Doxorubicin · Competition for P-glycoprotein efflux pump, potentially increasing doxorubicin toxicity. · moderate
Monitoring
- Flea control efficacy
- Adverse gastrointestinal or neurologic effects
- Efficacy of flea control
- Gastrointestinal tolerance (vomiting)
- Adverse neurological signs
Overdose
Spinosad has a wide margin of safety. The oral LD50 in mice is >5000 mg/kg. In canine dose tolerance studies (up to 16.7X the normal dose for 10 days), the primary sign was **routine vomiting**. No significant changes in hematology or coagulation were noted, though mild elevations in ALT and phospholipidosis (vacuolation) of lymphoid tissue occurred.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.