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Ivermectin Toxicity in Dogs

By: Dr. Bari Spielman

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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 animals and animals 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 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 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.

  • Dilated pupils
  • Depression
  • Excessive drooling
  • Vomiting
  • Tremors
  • Disorientation
  • Weakness
  • Recumbency (inability to rise)
  • Blindness
  • Nonresponsiveness
  • Slow heart rate
  • Anorexia (lack of appetite)
  • Stupor
  • Slow respiratory rate
  • Coma or death
  • Seizure

    Diagnosis

    Routine baseline diagnostics to include a complete blood count, biochemical profile and urinalysis are generally within normal limits. Abnormalities in blood gas analysis may be seen in association with respiratory depression, which is slower and more shallow breathing.

    A temporary return to consciousness or alertness after the administration of physostigmine (a medication) supports, but does not confirm, a diagnosis of ivermectin toxicity.

    Ivermectin sensitivity testing (the presence MDR-1 mutant gene) is available at Washington State University College of Veterinary Medicine. They use a cheek brush sample for analysis. For more information, call their lab at 509-335-3745 or see their website at http://www.vetmed.wsu.edu/depts-VCPL/. Test kits can be ordered online directly through the university.

    Treatment

    Treatment is largely supportive and symptomatic, as there are no specific reversal agents available to treat ivermectin toxicity. However, if an accidental exposure was recent (with 4 to 6 hours), induction of vomiting may be recommended, gastric lavage and activated charcoal to minimize drug absorption.

    General care may include:

  • Intravenous fluid and supplemental electrolyte therapy.

  • Nutritional support via intravenous feeding or through a feeding tube.

  • Appropriate bedding, frequent turning, physical therapy and lubricants for the eyes for the recumbent patients. Heat support or fans may be used to support high or low body temperatures. Urinary catheters may be placed for patients that are recumbent.

  • Mechanical ventilation (respirator) for the patients who have respiratory depression.

  • Treatment of seizures using medications such as diazepam or propofol.

    Prognosis

    Prognosis depends on the severity of the clinical signs and your pets individual response to treatment. Some pets 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 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:

  • Ivermectin
  • Selamectin
  • Milbemycin
  • Moxidectin
  • Loperamide
  • Acepromazine
  • Butorphanol
  • Vincristine
  • Vinblastine
  • Doxorubicin


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