College of Veterinary Medicine

Veterinary Clinical Pharmacology Lab 

Frequently Asked Questions

Can Collie crosses or other herding breed crosses carry the mutant MDR1 gene and have an adverse reaction to a normal dose of drugs?

Yes, it is less likely in a mixed breed, but still possible. We have found the mutant gene in a Saint Bernard mix that had an adverse drug reaction. The veterinarian did note that each eye was a different color, like some Australian Shepherds.

How old must a dog be before it can be tested?

A puppy can be tested as soon as it is weaned from its mother. Why do we recommend waiting until the puppy is weaned? Since we sample the inside of the dog's mouth, and milk can contain a few cells from the mother, it is possible that the puppy's sample could contain enough of the dam's DNA to generate a false result.

Can mixed breed dogs have the MDR mutation?

YES! We have found the MDR1 mutation in many mixed breed dogs - even dogs that don't look like herding breed dogs. Mixed breed dogs should be tested for the mutation before receiving anti-mange doses of ivermectin.

I have a mixed breed dog. Will the MDR1 test indicate what breeds my dog is?

The MDR1 test will determine if your dog has the MDR1 mutation, which predisposes dogs to many adverse drug reactions. It is true that the MDR1 mutation has been identified only in herding breed dogs, so if your dog tests positive for the MDR1 mutation, then it is highly likely that your dog has an ancestor that is one of the herding breeds.

Why are affected dogs called "mutants"?

A gene is a section of DNA that is responsible for producing a particular protein in the body. When a dog (or other organism) has a DNA sequence that produces a defective protein, that animal is said to have a mutation in that gene. Individual animals that have a mutation in a particular gene are considered to have the mutant form of the gene. Unfortunately, the word "mutant" in lay language often carries a negative connotation.

What heartworm prevention products can I use if my dog has the MDR1 mutation?

Fortunately, the dose of ivermectin, selamectin, milbemycin and moxidectin in the commercial heartworm preparations are low enough to be used safely even in dogs with the MDR1 mutation. It is only when the drugs are used at high doses, such as those used to treat mange (50 times higher dose than the heartworm prevention dose), that dogs with the mutation will develop neurological toxicity. Attempting to use large animal formulations of these drugs is likely to cause neurological toxicity because it is difficult to accurately measure

My dog has tested positive for the MDR1 mutation. Now that Interceptor is no longer available, what heartworm preventive product should I use for my dog?

Any of the monthly heartworm preventives (administered at the label dose) can be used in dogs with the MDR1 mutation. We recommend avoiding the 6-month or 12-month injectable products and we prefer the single-ingredient products to the multiple ingredient products.

Can individuals outside the United States order a test kit?

Yes, we can run samples from any country except Australia , New Zealand or European countries (including Britain). For those countries, please see the links below. If you have questions please contact us at

For individuals in Australia and New Zealand, the exclusive licensee for MDR1 genotyping is The Gribbles Group, and they may be contacted at

Dr Richard McCoy, PhD
Head Scientist, Molecular Diagnostics
Gribbles Veterinary Pathology
1868 Dandenong Road
ClaytonVIC 3168

Phone: +61 3 9538 2241
Fax: +61 3 9538 6778

The Veterinary Clinical Pharmacology Laboratory will test dogs from all European countries except: the United Kingdom, Germany, France, Switzerland, Netherlands and Austria. For individuals in the United Kingdom, Germany, France, Switzerland, Netherlands and Austria, the exclusive licensee for MDR1 genotyping is Genetic Counseling Services, and they may be contacted as listed below.
0252 532 284
Genetic Counseling Services
Weeresteinstraat 45
2182 GR Hillegom
The Netherlands

Last Edited: Dec 19, 2014 10:00 AM   

Veterinary Clinical Pharmacology Laboratory PO Box 609 , Washington State University, Pullman WA 99163-0609, 509-335-3745, Contact Us Safety Links