Ivermectin Toxicity in Dogs

Overview of Canine Ivermectin Toxicity

Ivermectin toxicity is seen when an overdose of the drug ivermectin is administered or consumed or in pets with increased sensitivity to the drug. Toxicity results in any number or combination of clinical signs ranging from mild to extremely severe, including death.

Ivermectin is a commonly used parasite drug that causes neurologic damage to the parasite, resulting in paralysis and death. Ivermectin has been used to prevent parasite infections, as with heartworm prevention, and to treat infections, as with ear mites.

The most common causes of ivermectin toxicity are due to administration of excessive doses (10-20X the recommended dose) and breed sensitivity. Breed sensitivity to lower doses occurs in some dogs such as in the collie, Australian shepherd Shetland sheepdogs, Old English sheepdogs and English sheepdogs although not every “sensitive” animal in the breed is affected. Toxicity may also result when ivermectin is used in high does in conjunction with the drug Comfortis™ (spinosad).

Toxicity can occur in cats, although it is uncommon.

Very young dogs and dogs with a history of head trauma or other central nervous system diseases that affects the blood-brain barrier appear to have an increased risk for toxicosis.

For dog breeds predisposed to the disease, an autosomal recessive trait (MDR-1) gene has been identified that causes a defect in the p-glycoprotein multidrug transporter to the blood brain barrier. This allows for the ivermectin pass into the brain at low dosages thus causing toxicity. This trait may also cause toxicity from other related drugs and should also be avoided or used in lower doses in susceptible breeds.

Toxicity can occur from a one-time injection or from daily dosing.

What to Watch For

Signs of ivermectin toxicity in dogs can occur acute or subacutely. Acute clinical signs of toxicity often develop within 4 to 12 hours of drug exposure. Subacute signs will often occur with in 48 to 96 hours of exposure.

Diagnosis of Ivermectin Toxicity in Dogs

Treatment of Ivermectin Toxicity in Dogs


Prognosis depends on the severity of the clinical signs and your dogs individual response to treatment. Some dogs may require days to weeks of supportive care.

Home Care and Prevention

Follow your veterinarian’s directions when administering products containing ivermectin. If you have a susceptible dog breed, consider having the Ivermectin Sensitivity Testing complete (see information above).

Use caution if administering ivermectin to susceptible breeds.

In dogs with MDR 1 gene mutation, the following drugs should be avoided or used with caution: