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Miniature American Shepherd running in a field.

Dogs of some herding breeds and some mix-breed dogs can have a genetic mutation that makes them dangerously oversensitive to ivermectin, the active ingredient in some commonly used heartworm prevention medicines for dogs.

Given at the proper doses and under the supervision of a veterinarian, ivermectin is safe for most dogs and is very effective in treating and preventing a number of parasites. However, a dog with the mutation who ingests the drug can have a severe, life-threatening reaction called ivermectin toxicity.

This sensitivity is because of a mutation in what is known as the MDR1 gene. In dogs who have the mutation, the drug crosses the blood-brain barrier and causes neurological damage, which can be lethal.

Dogs can ingest ivermectin not only in the form of heartworm preventative, but also if they eat the manure of livestock that were treated with the drug for parasite control. For this reason, owners of vulnerable breeds should be extra vigilant when their dogs are around horses, sheep, or other livestock.

Owners of herding breeds or other vulnerable dogs should be careful that their dogs do not eat the manure of sheep or other livestock that may have been treated with ivermectin.

Dogs with the mutation are hypersensitive to other medications as well, including loperamide (Imodium), acepromazine, and some chemotherapy drugs.

Symptoms of Ivermectin Toxicity

The signs of ivermectin toxicity can be acute or mild. Acute symptoms can occur within 4 to 12 hours of ingestion, while milder symptoms may become apparent over 2 or 3 days. Symptoms can include:

  • Lethargy or depression
  • Disorientation
  • Drooling
  • Loss of appetite
  • Slow heartbeat
  • Dilation of pupils
  • Trembling or seizures
  • Inability to stand
  • Difficulty breathing
  • Sudden blindness

Which Breeds Can Be Sensitive to Ivermectin?

The following types of dogs have been found to be prone to the mutation:

It’s important to understand that not all individual dogs in the breeds listed above have the mutant gene.

Test for Ivermectin Sensitivity

Fortunately, a simple genetic test is now available that indicates whether or not a dog has the mutation. Veterinarians recommend that owners of herding-breed dogs have this simple test performed on their pets. The test involves a small brush that is quickly swiped in the dog’s mouth, with the sample then sent to a testing lab at Washington State University.

If you are concerned about your dog being vulnerable to ivermectin or other drugs or have questions about the use of heartworm preventative, be sure to speak with your vet.

Related article: How to Socialize Your Dog From Home
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