MDR1 Test Kits Now Available at Grand Avenue Veterinary Center

November 17, 2013

Here at Grand Avenue Veterinary Center, we will now be keeping a supply of cheek swab test kits for the MDR1 gene mutation through Washington State University on hand for your use. There is no charge to you for picking up a kit at the clinic; by keeping the kits on hand we hope to make testing your dog faster and more convenient for you. Payment is made directly to WSU when you send the kit to them.

What is MDR1? MDR1, or the multi-drug resistance gene, is responsible for making a drug transport pump in the body called P-glycoprotein. This pump is responsible for limiting the absorption and promoting the excretion of certain types of drugs, especially in the brain. A mutation of the MDR1 gene can lead to absorption of abnormally high amounts of certain drugs, which in some cases increases the risk of side effects. It can also lead to a delayed excretion of certain drugs from the body, meaning that their effects can last longer than normal. The MDR1 mutation has not been reported or identified in cats at this time.

Herding breeds are most commonly affected by the MDR1 mutation, but it has been reported in many breeds. Fortunately, the inheritance of the MDR1 gene is very simple and well understood, and a genetic test has been developed by Washington State University. If you think back to your high school genetics lessons, you probably remember that there are two copies of every gene in the body. MDR1 results are therefore reported as:

1. Normal/normal: These dogs have two normal copies of the gene and are completely unaffected.
2. Normal/mutant: These dogs have one normal and one abnormal copy of the gene. These dogs are unaffected themselves but could pass on an abnormal copy of the gene if they were bred.
3. Mutant/mutant: These dogs have two abnormal copies of the gene. They are affected by the mutation and would pass on their abnormal genes if bred.

What is the advantage of knowing your dog’s MDR1 status? There is a known list of drugs of concern for dogs affected by the MDR1 mutation; these drugs either need to be avoided or the dose reduced to be used safely to avoid excessive side effects, toxicity, and prolonged drug action. It is not necessary to test every dog for MDR1, but it is a good idea to test known or suspected herding breeds and their mixes. If you choose to test your dog for MDR1, simply call the clinic to reserve a test kit. There will be instructions in the kit about how to obtain a sample, and a video demonstration is available at:http://www.vetmed.wsu.edu/depts-VCPL/test.aspx (at the bottom of the page).

A special note about heartworm preventative: There has been some confusion over the safety of Ivermectin, the active ingredient in Heartguard and some other heartworm preventatives, for dogs affected by the MDR1 mutation. At the doses used for heartworm preventative, ALL available heartworm preventatives including Ivermectin ARE safe for these dogs. The confusion is caused because Ivermectin is sometimes used at very high doses to treat certain types of skin mites, and those high doses are NOT safe for these dogs. (For more information about drugs of concern, see: http://www.vetmed.wsu.edu/depts-VCPL/drugs.aspx.)

As always, if you have any questions please feel free to call the clinic and speak to one of our veterinarians.