Invermectin Overdose in Dogs

Charlotte Means, D.V.M., Veterinary Toxicologist, NAPCC

An Apple A Day...Can Kill You

Martin, an eight-year-old Labrador retriever, loved to “help” his owner, Jody, feed the horses. Anything that fell from their mouths he considered dog treats. One day Jody spread ivermectin, a paste wormer for horses, on apple slices. When one horse dropped half an apple slice, Martin caught it before it hit the ground and ate it. Jody was unconcerned; many people she knew used horse wormers for heartworm prevention in their dogs. And after all, Martin weighed 110 pounds! About three hours later, her chores finished, Jody summoned Martin. When he didn’t appear, she started looking. She found him in a stall. Martin got up, but he staggered like he was drunk and showed no interest in his surroundings.

Jody rushed Martin to the office of Robert Bergner, D.V.M. Bergner noted that Martin was ataxic (staggering) and extremely depressed. He also found that Martin was tachycardic (had rapid heartbeat) and panting. Because Bergner, a small animal practitioner, was unfamiliar with the specific brands of equine wormers, he contacted the ASPCA Animal Poison Control Center (APCC).

From APCC veterinarians, Bergner learned that this particular wormer contained 1.87 percent ivermectin. Ivermectin is also found in a monthly heartworm product approved for dogs, though the dose for a dog is considerably different. Veterinarians prescribe ivermectin off-label to treat some forms of mange, but it requires careful dosing.

Martin’s clinical signs were those expected in ivermectin overdose. His veterinarian would continue to observe him for additional signs, including depression, severe weakness and coma. Sometimes the heart can beat extremely slowly or with an abnormal rhythm. In very high doses, muscle tremors and seizures (convulsions) occur. Jody was warned that Martin might experience transient blindness, and that it is not unusual for clinical signs to worsen over several hours.

Special Sensitivities

Bergner recalled that some breeds of dogs are especially sensitive to ivermectin but didn’t think Labs were one of them. The APCC veterinarian confirmed that they weren’t, but added that any dog who ingested enough ivermectin could develop a toxicosis. “White feet, don’t treat,” he said. “Use that rhyme to remember which breeds are especially sensitive [to ivermectin.]” Breeds with white feet—collies, shelties, border collies, Australian shepherds, Old English sheepdogs—may develop clinical signs even at therapeutic doses. Jack Russell terriers also tend to have a higher incidence of toxicity. Bergner was relieved to learn that the prescription heartworm medication he uses is safe for ivermectin-sensitive dogs because only a tiny dose is required to prevent heartworms.

Meanwhile, Bergner’s technician reported that Martin was ready to receive the first dose of activated charcoal. Because ivermectin persists in the body for several days and keeps returning to the gut before being excreted in feces, giving multiple doses of activated charcoal can speed recovery by actually decreasing the dose of drug that the dog receives.

Martin also received intravenous fluids and was monitored closely for several days. Fortunately, he continued to eat and never became comatose. Many dogs require a stomach tube to provide nutrition. Dogs who are unable to stand or walk must be turned frequently to prevent pressure sores.

When Martin walked without staggering after only four days, Jody was ecstatic; some dogs require several weeks to recover. Today Jody is busy making sure that all of her horse-owning friends realize that equine wormers can be hazardous to dogs. Martin, however, is confused. He has no idea why Jody leaves him at home now when she worms the horses!

Dr. Means is a veterinary toxicologist at the ASPCA Animal Poison Control Center in Urbana, IL.

© 2001 ASPCA
ASPCA Animal Watch - Summer 2001

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