MDR1 Info

Some breeds of dogs are more sensitive to certain drugs compared to other breeds. For example, Collies, Border Collies, German Shepherds, Shelties, Australian Shepherds and other breeds are often more sensitive to the antiparasitic drug, ivermectin.

Why are some breeds more sensitive to the effects of drugs than other breeds? Which drugs have been reported to cause problems? At Washington State University's College of Veterinary Medicine you can get your dog tested for drug sensitivity and keep up with the latest research.

It is well known that Collies and related breeds can have adverse reactions to drugs such as ivermectin, loperamide (ImodiumĀ®), and others. It was previously unknown why some individual dogs were sensitive and others were not. Advances in molecular biology at the Veterinary Clinical Pharmacology Laboratory at Washington State University's College of Veterinary Medicine have led to the discovery of the cause of multi-drug sensitivity in affected dogs. The problem is due to a mutation in the multi-drug resistance gene (MDR1). This gene encodes a protein, P-glycoprotein, that is responsible for pumping many drugs and other toxins out of the brain. Dogs with the mutant gene can not pump some drugs out of the brain as a normal dog would, which may result in abnormal neurologic signs. The result may be an illness requiring an extended hospital stay--or even death.

A test has recently been developed at Washington State University to screen for the presence of the mutant gene*. Instead of avoiding drugs such as ivermectin in known susceptible breeds, veterinarians can now determine if a dog is normal, in which case the drug can be administered or abnormal, in which case an alternative treatment can be given.

Owners and breeders can submit samples for testing. All that is needed for the test is a cheek brush sample that can be obtained by the owner and sent by mail for analysis.
Affected Breeds

Approximately 3 of every 4 Collies in the United States have the mutant MDR1 gene. The frequency is about the same in France and Australia, so it is likely that most Collies worldwide have the mutation. The MDR1 mutation has also been found in Shetland Sheepdogs (Shelties). Australian Shepherds, Old English Sheepdogs, German Shepherds, Long-haired Whippets, Silken Windhounds, and a variety of mixed breed dogs.
The only way to know if an individual dog has the mutant MDR1 gene is to have the dog tested. As more dogs are tested, more breeds will probably be added to the list of affected breeds.

For more information on the MDR1 Gene Click here:


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Last Updated: May 19, 2016