Is Ivermectin safe for dogs and cats?

Ivermectin toxicity in dogs is profoundly influenced by ABCB1 (MDR1) gene mutation status. Dogs with the ABCB1-1Δ (MDR1) frameshift mutation — affecting approximately 75% of rough collies, 50% of smooth collies, and significant proportions of Shetland sheepdogs, Australian shepherds, Old English sheepdogs, McNabs, long-haired whippets, Silken Windhounds, and crosses of these breeds — lack functional P-glycoprotein at the blood-brain barrier. In MDR1-mutant dogs, ivermectin crosses freely into the CNS, causing severe neurotoxicity (mydriasis, ataxia, disorientation, tremors, seizures, coma) at standard anti-parasitic doses that are safe for non-mutant dogs. MDR1-wild-type dogs tolerate ivermectin at typical heartworm prevention doses (6 μg/kg monthly) without issue, but toxic signs can occur even in wild-type dogs at the much higher doses used in some livestock formulations if accidentally ingested. Common exposure scenarios for dogs include ingestion of horse feces containing ivermectin residues, licking of large-animal pour-on formulations, or owners unknowingly using ivermectin-containing livestock products on dogs. A DNA test is available to determine MDR1 status. Treatment of toxicosis requires extended supportive care; intravenous lipid emulsion has been used as adjunctive therapy.

Cats are generally more sensitive to ivermectin than most dogs, but the drug is used in cats for certain indications (ear mites, cheyletiellosis) at doses below the toxic threshold. Cats lack the MDR1 mutation issue but have lower P-glycoprotein efflux capacity than dogs; neurotoxicity (mydriasis, salivation, ataxia, tremors, coma) occurs at overdose. Accidental exposure scenarios include owners applying dog-label ivermectin products to cats, cats grooming themselves after application of ivermectin-containing dog products, and ingestion of concentrated livestock formulations. Products labeled for dogs sometimes contain pyrethroids in combination with ivermectin — permethrin co-formulation represents an additional extreme feline toxicity risk independent of ivermectin. ASPCA Animal Poison Control Center lists ivermectin as a significant cat toxin. Treatment is supportive; intravenous lipid emulsion has been reported as potentially beneficial in severe cases.