Why it's dangerous
Ivermectin acts as an anthelmintic that can cross the blood-brain barrier in sensitive breeds or at high doses, leading to central nervous system depression and severe neurological impairment.
Toxic compound: Avermectins
How vets treat it
No treatment plan written for cats was found from Merck or the poison lines. In the published series of 20 cats accidentally overdosed by injection, vets gave an IV lipid (fat) emulsion. A first dose followed by a 30-minute drip appeared to reduce signs better than a single dose, and the cats that became ill received more until their signs cleared. Thin cats had more severe signs.
Outlook
No survival figures for cats were found. Pet Poison Helpline warns that without aggressive treatment, ivermectin poisoning can be deadly. In the series of 20 overdosed cats, the cats that fell ill were treated until their signs had fully cleared.
Where pets are exposed
- Horse dewormer pastes, such as Zimectrin. They contain large amounts of ivermectin, and many poisonings happen when a dog chews up a tube.
- Ivermectin products for cattle, pigs and other livestock, such as drenches, injections and medicated feed. Ivermectin is also sold under names such as Ivomec and Bimectin.
- Manure from horses, sheep or other animals that were recently dewormed. Dogs have been poisoned by eating it.
- Monthly heartworm preventives for dogs and cats, such as Heartgard and Iverhart. At the labeled dose they are considered safe, but eating a larger number of tablets may be a concern.
- Higher doses used off-label for mange and other skin parasites, and doses given by mistake, such as an overdose injected into a cat.
Prevention
- Never give your dog or cat a dewormer made for horses, cattle or other livestock. These products contain large amounts of ivermectin.
- Give heartworm preventives only as your vet prescribes. At the labeled dose, ivermectin heartworm preventives such as Heartgard are considered safe even for dogs with the MDR1 mutation.
- If your dog is a collie, Australian shepherd, Shetland sheepdog, long-haired whippet or another herding breed or cross, ask your vet about an MDR1 test before any other ivermectin treatment. A genetic test is the only way to know, and reactions can happen in other breeds too.
- Keep horse dewormer tubes in a closed cabinet, not in an open tack trunk where a dog can chew them, and keep dogs away from manure of recently dewormed animals.
- Store heartworm preventives where pets can't reach them. Eating a larger number of tablets at once may be a concern.
Good to know
Certain breeds with the MDR1 (ABCB1) gene mutation are hypersensitive. Poisoning often occurs from horse dewormer ingestion or eating feces of recently dewormed horses.
Common questions
Is ivermectin safe for dogs?
At the doses in monthly heartworm preventives, yes. Washington State University says the FDA found Heartgard safe at label doses even for dogs with the MDR1 mutation. At higher doses, ivermectin can poison any dog, and MDR1 dogs react at much lower doses than other dogs. Never give your dog ivermectin made for horses or livestock, and call your vet or a pet poison line if your dog swallows more than its prescribed dose.
My dog ate horse dewormer, what do I do?
Call your vet or an animal poison line now, even if your dog seems fine. Horse dewormer paste contains large amounts of ivermectin: Pet Poison Helpline says an average 20-pound dog would start to show signs after swallowing about a fifth of a tube. Don't make your dog vomit unless a vet tells you to. Keep the tube, and tell the vet your dog's weight and breed.
Which dog breeds are sensitive to ivermectin?
Mostly herding breeds. Washington State University estimates the MDR1 mutation is carried by about 70% of collies, 50% of Australian shepherds and long-haired whippets, 30% of silken windhounds and McNabs, and 15% of Shetland sheepdogs and English shepherds. It is also found in German shepherds, Old English sheepdogs, Border collies and mixed breeds. A genetic test is the only way to know. If your dog is one of these breeds or a cross and swallowed ivermectin, call your vet now.
Is ivermectin toxic to cats?
It can be. Around 4% of cats carry an MDR1 mutation, according to Washington State University, and overdoses poison cats too: in one published series, 20 cats were accidentally injected with 4 mg/kg, and some became severely ill despite early treatment. Pet Poison Helpline says 1 to 2 extra heartworm tablets are unlikely to be a concern in animals that aren't sensitive. Never give a cat a product made for dogs, horses or livestock, and call your vet if your cat gets too much.
How long does ivermectin poisoning last in dogs?
First signs can appear within 4 to 12 hours, and slower signs can take 2 to 3 days. Severe cases can last weeks: in a study of Collies, most got worse until about day 6 and took 3 weeks to recover, and some needed a ventilator. Call your vet as soon as you know your dog swallowed ivermectin, without waiting for signs.
Sources
- Merck Veterinary Manual — Antiparasitic Drugs for Integumentary Disease in Animals
- ASPCA Animal Poison Control Center — Ivermectin toxicosis in dogs: a retrospective study (Journal of the American Animal Hospital Association, 2009)
- Pet Poison Helpline — Ivermectin
- Pet Poison Helpline — Beware of These Nasty Canine Toxins in Your Tack Trunk
- Pet Poison Helpline — Ivermectin Seizures in Dogs
- Washington State University, Washington Animal Disease Diagnostic Laboratory — MDR1 in Dogs
- Journal of Veterinary Internal Medicine — Ivermectin toxicity in 17 collies (2002)
- Journal of Veterinary Emergency and Critical Care — Intravenous lipid emulsion therapy in 20 cats accidentally overdosed with ivermectin (2015)
- Pet Poison Helpline
This page is informational and is not veterinary advice. If your pet has been exposed to something risky, contact your vet or an animal poison control line right away.
