Question 1

What drugs are dangerous for Collies?

Accepted Answer

Collies and other herding breeds with the MDR1 gene mutation can have severe adverse reactions to several common medications. Ivermectin, used in many heartworm preventatives and deworming products, is the most well-known danger at higher doses. Loperamide, the active ingredient in Imodium, can cause neurological toxicity. Several chemotherapy drugs including vincristine, vinblastine, and doxorubicin require dose adjustments. The sedative acepromazine can cause prolonged and exaggerated sedation. Some antiparasitic drugs like milbemycin and moxidectin may require adjusted dosing. Butorphanol, a common pain medication, and the antibiotic erythromycin can also cause problems. Always inform your veterinarian that your dog is a herding breed and request an MDR1 test before any new medication is prescribed.

Question 2

What is the MDR1 gene mutation?

Accepted Answer

The MDR1 gene, also called ABCB1, provides instructions for producing a protein called P-glycoprotein that acts as a pump in the blood-brain barrier. This pump normally prevents certain drugs from entering the brain in toxic concentrations. Dogs with the MDR1 mutation have a defective pump that allows drugs to accumulate in the brain, causing neurological toxicity that can range from tremors and disorientation to seizures, coma, and death. The mutation is inherited as an autosomal incomplete dominant trait, meaning dogs with one copy of the mutation have intermediate sensitivity while dogs with two copies are most severely affected. Approximately 70 percent of Collies carry at least one copy of the mutation, making testing essential for the breed.

Question 3

Which dog breeds are affected by the MDR1 mutation?

Accepted Answer

The MDR1 mutation is most prevalent in herding breeds and their crosses. Rough and Smooth Collies have the highest frequency at approximately 70 percent affected. Australian Shepherds are affected at about 50 percent, while Shetland Sheepdogs show rates around 15 percent. Other affected breeds include Old English Sheepdogs, English Shepherds, German Shepherds, Border Collies, McNab Shepherds, Silken Windhounds, Long-haired Whippets, and mixed breeds with herding breed ancestry. The mutation traces back to a single ancestor in the herding breed lineage. Because mixed breed dogs can carry the mutation without showing obvious herding breed traits, any dog with unknown heritage should be tested before receiving potentially affected medications.

Question 4

How do you test for the MDR1 mutation?

Accepted Answer

MDR1 testing is a simple DNA test that can be performed using a cheek swab collected at home or a blood sample taken by your veterinarian. The test identifies whether your dog has zero, one, or two copies of the mutation. Results are reported as normal/normal meaning no mutation, normal/mutant meaning one copy with intermediate sensitivity, or mutant/mutant meaning two copies with full sensitivity. The test is available through the Washington State University Veterinary Clinical Pharmacology Laboratory, which pioneered MDR1 research, as well as commercial genetic testing companies. Testing costs between $60 and $90 for a standalone test or is included in many comprehensive breed genetic panels. Results are typically available within two to three weeks and are valid for the dog's entire life.

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