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  • Writer's pictureJessie Miller


Updated: Jul 8, 2021

Sometimes when a litter does not go as planned extra precautions are made! This litter did not go as planned but here they are! All puppies will be tested for the MDR1 defect so we will be awaiting results patiently/ not patiently lol! Every breeder does their best to make sure their dogs are genetically healthy. Both parents are genetically healthy, however they both carry a copy of the MDR1 gene so together they do not make the best combination for the litter. Here are some facts about MDR1 affected dogs:

Approximately three of every four Collies in the United States have the mutant MDR1 gene. The only way to know if an individual dog has the mutant MDR1 gene is to have the dog tested. Having the MDR1 mutation is a problem if you don’t know it is there.  If a dog with even one copy of the mutation receives too much of certain drugs it can make the dog very sick or even kill it.  But dogs have survived for millennia without these drugs and for at least a century – before the development of many of our modern drugs – some dogs have survived with the MDR1 mutation with no ill effect.  In wild animals which don’t go to the vet this mutation would have null effect.

The MDR1 gene codes for the production of a protein known as p-glycoprotein. In animals with two mutated copies of the MDR1 gene, however, the p-glycoprotein that is produced is abnormal. Without a functional p-glycoprotein system, drugs can reach higher levels within the cell and therefore these dogs are more sensitive to certain drugs.

For some drugs the veterinarian can now know ahead of time that there could be a bad reaction and she can select another drug.  No dog will go untreated because of the MDR1 mutation; it is a matter of personalizing the treatment to the individual dog.

Ivermectin and other related drugs (milbemycin, selamectin) are commonly used in the prevention and treatment of parasites. They are common components of most canine heartworm preventives. At low doses, such as those used in heartworm prevention, ivermectin is safe even for dogs with MDR1 mutation. At high doses, however, such as those used in treating parasites such as demodectic mange, ivermectin can cause neurologic effects in dogs with the MDR1 mutation.

Loperamide (Imodium®) is another drug with potential neurologic effects in dogs with the MDR1 mutation. This drug does not affect the brain in most dogs, due to p-glycoprotein’s ability to pump the drug out of the brain. Dogs with the MDR1 mutation, however, can experience neurologic effects from loperamide.

The MDR1 mutation isn’t desirable but so long as testing of dogs in the affected breeds is standard and owners take a few basic precautions  there need never be a dog that is lost to a drug reaction to those particular drugs because the veterinarian didn’t know there was a risk.

Hope this is helpful!

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