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What is MDR1?

MDR1 is  a mutation in the genetic makeup of the dog which causes problems in the dogs' ability to remove certain drugs from the brain to avoid toxicity even at 'normal' doses for the drug. MDR1 affects a great number of herding breeds including the Collie. The mutation in Collies is identified as either normal/normal (no genetic mutation), normal/mutant (carrier for the mutation) or mutant/mutant (double carrier for the mutation).  


Long before this test was available, common knowledge was that you treat every Collie (and every herding breed with the piebald white pattern of a color, white feet, white tail tip) as if the dog would react to some problem drugs. The MDR1 test made it easy in one way but doesn't always correctly asses risk and can give a false sense of security. Dogs who have tested 'normal' have had reactions, and dogs that have tested 'mutant' have not and there is a wide range of susceptibility in between that testing cannot guarantee. For this reason, I ALWAYS treat a Collie like it is at risk. ALWAYS. We have a list of drugs that we know are problems but we also have drugs that are not problems and they are easily available and do not cost any different.

-N/N  Dogs testing as normal/normal are not considered to be at risk but there are exceptions.
-N/M  Dogs testing as Normal/Mutant (carriers) may or may not react to normal doses therefore a carrier should be treated as susceptible despite the presence of only one mutation.
-M/M  Dogs testing as affected being double carriers will be expected to experience problems with certain drugs at normal dosing levels including life threatening reactions.

How Does the MDR1 Mutation Affect My Collie On A Daily Basis?

- The MDR1 status of your dog will come into play whenever you apply flea/tick products or dose for heartworm.
- Certain wormer ingredients should be avoided.
- If your dog has exposure to livestock that may be wormed, dogs can be poisoned from livestock wormers by   eating livestock feces.
- Certain diarrhea medications should be avoided.
- Surgery involving certain anesthesia's, some pain medications, and some cancer medications must be used with caution.

The list of drugs that may have an impact on MDR1 sensitive dogs is constantly changing. Always check this link for the most up to date information:

How Do I Find Out My Dogs MDR1 Status?

Breeding stock is screened however it is important to note that due to the prevalence of MDR1 in the Collie breed (estimated 75% are either M/M or N/M), each Collie is considered affected until proven otherwise.  As such, precautions must be taken when your Collie requires medication or surgery.

Do not assume that your veterinarian will be aware of the MDR1 mutation. Ensure documentation is in the dogs' file even if the MDR1 status is unknown so that the dog is treated as if the dog is susceptible. Talk about the MDR1 status of your dog with your vet before every medical procedure and whenever flea, tick and/or heartworm medication is prescribed.

For information on how to test your dog, see the link at the top of this page for testing by the Washington State University.


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