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Use Extreme Caution with Ivermectin

March 06, 2013

Story at-a-glance

  • Ivermectin is the active ingredient in many popular heartworm preventive drugs like Heartgard®.
  • The dose of ivermectin in heartworm preventives is small (6 µg/kg) compared to the amounts recommended for off-label use of the drug. For example, off-label use for the treatment of sarcoptic mange requires a dose of 300 µg/kg, and for demodectic mange, 400-600 µg/kg.
  • High doses of ivermectin can cause neurologic signs in dogs, including weakness or paralysis of one or more limbs, balance disorders, vision disorders, pain, and seizures.
  • Animals at highest risk are very young dogs, multi-drug sensitive breeds, dogs that may be exposed to ivermectin-containing products for large animals and/or the feces of large animals (horses, cows, pigs) treated with ivermectin.
  • There is no antidote for ivermectin toxicosis, so treatment is primarily supportive and depends on the amount of drug ingested and the susceptibility of the dog.

By Dr. Becker

Ivermectin is an anti-parasite medication given to dogs to prevent heartworm disease and to treat various other parasite-related conditions like demodectic and sarcoptic mange, and ear mites.

As a heartworm preventive, ivermectin is the active ingredient in products including:

  • Ivomec® and Heartgard® by Merial
  • Zimectrin® by Farnam
  • Iverhart® by Virbac
  • Tri-Heart® by Intervet
  • Various generics

Ivermectin works by causing neurological damage to specific parasites, which results in paralysis and death. Currently, the FDA approves use of ivermectin for heartworm prevention and the treatment of ear mites (Acarexx®) only in dogs. All other uses of the drug – for example to treat certain varieties of mange, or to clear heartworm larvae in dogs with active heartworm infection -- are considered off-label.

The dosages of ivermectin in heartworm preventives are significantly lower than doses used for other purposes. Off-label use of ivermectin typically involves much higher dosages than the amount in a heartworm preventive – as much as 50 times higher, in fact. This is important information every dog owner should be aware of.

High Doses of Ivermectin Can Cause Neurologic Disorders in Dogs

When ivermectin is given in high enough doses, it can cross the blood-brain barrier and cause neurologic signs in dogs, including weakness or paralysis of one or more limbs, balance disorders, vision disorders, pain, and seizures.

Ivermectin toxicity can occur whether the drug is given orally, topically (applied to the skin) or parenterally (by IV or injection).

Dogs most at risk are very young animals with immature blood-brain barriers; multi-drug sensitive breeds including the Border Collie, Australian Shepherd, long-haired Whippet, Silken Windhound, Rough- and Smooth-Coated Collies and associated mixed breeds; dogs that may be exposed to ivermectin-containing products for large animals and/or the feces of large animals (horses, cows, pigs) treated with ivermectin.

In multi-drug sensitive breeds, ivermectin toxicity can occur at doses as low as 100 µg/kg. In non-sensitive breeds, doses greater than 2000 µg/kg are typically required to induce toxicosis. As a reference point, the recommended veterinary dose of ivermectin for heartworm prevention in dogs is 6 µg/kg. For treatment of sarcoptic mange the dose is 300 µg/kg, and for demodectic mange it's 400-600 µg/kg.

Symptoms of Ivermectin Toxicity

Symptoms of ivermectin toxicosis include loss of coordination, prolonged abnormal pupil dilation, reduced alertness that can progress to stupor or coma, excessive saliva production, vomiting, blindness, tremors and seizures.

Diagnosing ivermectin toxicity involves taking the patient's history, performing a thorough physical exam, and blood tests to determine the serum ivermectin concentration. Depending on your dog's breed or breed mix, your vet might also test for the ABCB1 gene mutation, which is present in multi-drug sensitive dogs as described above.

Since serum ivermectin test results aren't always immediately available, in the interim, your vet might also try to rule out other causes for your dog's symptoms, including exposure to other potentially toxic substances, diseases affecting the brain, and metabolic disorders.

Diagnosis and Treatment

Unfortunately there are no specific antidotes for ivermectin poisoning, so treatment is primarily supportive in nature.

If the drug was ingested orally in the last one to four hours, your vet will attempt to induce vomiting, administer activated charcoal, and monitor your dog's electrolyte levels. If the drug was taken within 24 to 36 hours, activated charcoal will be administered.

If the ivermectin was applied topically, your dog should be washed with mild dishwashing detergent and water before other treatments are attempted.

Regardless of how the toxicosis occurred, your dog will require IV fluid therapy. Dogs experiencing respiratory depression will be intubated to aid breathing.

IV lipids have been used with some success in treating ivermectin toxicity with relatively few adverse effects.

Supportive care may include keeping the patient's body temperature in the normal range, preferably with circulating warm water or air blankets since many patients are not alert enough to know when to move away from a heat source. Dogs that are semi-conscious or unconscious should have their bodies rotated every four to eight hours.

There are a number of other supportive care measures that may be necessary depending on the animal's condition, including catheterization, intubation to keep the airway clear, seizure control, eye lubrication, and delivering nutrition to a dog who can't or won't eat on his own.

Depending on the level of veterinary care your dog requires, treatment costs can range from around $100 to over $1,000.

The good news is most dogs respond well to treatment for ivermectin toxicosis, and the prognosis is very good if supportive care is given early and aggressively.

Depending on how much ivermectin is ingested and the sensitivity of the patient, treatment can be lengthy, and symptoms can take several weeks to disappear.

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