MDR1 - What the heck is that? And what does it mean for my dog?

MDR1 - What the heck is that?  And what does it mean for my dog?

I had never heard of MDR1 until my friend Sarah brought it to my attention.  Now I can't believe that I had never heard of it before!This is a guest blog post about MDR1 by Sarah Duncan.

Meet Hattie, she is an 18 month-old, sweet, sensitive and loving mini-Aussie mix. I love her with my whole heart. She is always ready for a snuggle and a pet. Ball crazy like only an Aussie-parent will understand (yes, even you Lab parents have got it a tiny bit better!).

At Hattie’s recent check-in with Dr. Weeks at Woodhaven Vet, she mentioned I should have Hattie tested for the MDR1 gene. I vaguely remember this from when my older Aussie-mix, Hugo, was a puppy. But, just barely.Dr. Weeks and I had been chatting about how much more on-the-go Hattie is. Hugo would rather be at home, herding the goats and guarding the front porch from our barn kitties. Hattie, however, comes with us camping, to the barn to see the horses, and just about every other time the car leaves the drive way. In short, Hattie is the co-pilot to all my wanderlusting adventures.HattieAnyways, this is when Dr. Weeks brought up that the all-in-one flea and tick treatment we were using may not be the best one on the market anymore. But, that before we switch anything up, we should get her tested for the MDR1 gene. I asked all questions that came to mind, and if you have ever met me you know that came be a ton of questions, mercifully Dr. Weeks had all the answers for me. We really do love Dr. Weeks!"If you have a herding breed dog, or even think you may have a herding mix, I think MDR1 testing is an awesome test that can really save lives. The samples are easy to collect and the test is fairly inexpensive. The knowledge we can gain regarding which drugs to avoid or which drugs to prescribe at a lower dose is important. In Hattie's case, if she is negative for the gene, great! If she is positive, now I know that as things happen throughout her life, I can be better prepared to deal with them." - Dr. Karen Weeks, DVM.

What is MDR1? And what does it stand for? Herding dogs are predisposed to carry a gene that makes them sensitive to certain drugs. They call this the MDR1 gene (Multi-Drug Resistance 1). These drugs include Ivermectin, a drug commonly used to treat worms in horses and dogs.    Imodium, another commonly used drug, used to treat diarrhea and Butorphanol, used as a pre-anesthetic agent during medical procedures.
What does it mean if she has it? She can have a severe reaction to certain medications that can be life threatening. Basically, the drug goes into the dogs brain and the brain fails to pump it out. This causes severe neurological problems up to and including death.
Why would we test her/ what benefit would it be to know? You need to know so that you are able to communicate to your dog’s health care providers anytime they are administering flea, tick, heartworm treatments, anesthesia, or tranquilizers. For me personally, because I have horses and Hattie is around horses, its important to know because she can pick up the Ivermectin if she eats the manure from recently treated horses. (Try as I might to dissuade her, I am not sure she has completely grown out of her manure eating “stage”.)
Is it common? About 1 in 3 herding dogs have it. (Me: Really?! That seems like a pretty high percentage!)
Is there much risk in the procedure? Not at all! It’s a super easy-peasy DNA test.
How much does it cost? Dr. Weeks didn’t know for sure, but said she would get back to me. Turns out its $60 plus any costs associated with the blood draw (see below for more info on that).
How long will it take for results? Again, she wasn’t sure right off the top of her head, but said she would get me the info ASAP.

Dr. Weeks spent the time with me to discuss all this and then sent me an email that same afternoon with the link to the WSU website.Hi Sarah,

It was so good to see you today!  I looked up the information for the herding breed genetic mutation.  You can order the kit yourself and send in a cheek swab OR we can draw blood to send in (there would be some fees associated with the blood draw and submission).  There does not seem to be any benefit to one over the other.  The fees for the test are on the website.
Here is the website:
Let me know what you think!
Karen Weeks, DVM

The website shows the costs associated with the testing and has links to more information on MDR1. The website also has the link to order the DNA sample kit and a video on how to take the sample at home.I ultimately decided I wanted to do a blood draw and have Dr. Weeks send in the DNA sample. Dr. Weeks was really nice and advised me of the costs to 2-day the sample (not in-expensive). She also advised me that the DYI sample kit was really just as good of an option. So, just to be clear, you certainly don’t have to have to have your vet do the blood draw, but I went with it for convienance. Plus I really wanted a reason to have Julie take more photos of Hattie… seriously, if you are here, reading this, you must know she is amazing! But I digress.Here is where I found more info on MDR1: and

Which breeds are affected?
  • Australian Shepherds (50%)
  • Border Collies (<5%)
  • Collies (Rough and Smooth) (70%)
  • English Shepherds (15%)
  • German Shepherd Dogs (10%)
  • Long-Haired Whippets (65%)
  • McNabbs (30%)
  • Miniature American (Australian) Shepherds (50%)
  • Mix-breeds (5%)
  • Old English Sheepdogs (5%)
  • Shetland Sheepdogs (15%)
  • Silken Windhounds (30%)

Drugs that have been documented to cause problems in dogs with the MDR1 mutation include:Ivermectin (antiparasitic agent) - While the dose of ivermectin used to prevent heartworm infection is SAFE in dogs with the mutation (6 micrograms per kilogram), higher doses, such as those used for treating mange (300-600 micrograms per kilogram) will cause neurological toxicity in dogs that are homozygous for the MDR1 mutation (MDR1 mutant/mutant) and can cause toxicity in dogs that are heterozygous for the mutation (MDR1 mutant/normal).Selamectin, milbemycin, and moxidectin (antaparasitic agents) - Similar to ivermectin, these drugs are safe in dogs with the mutation if used for heartworm prevention at the manufacturer’s recommended dose. Higher doses (generally 10-20 times higher than the heartworm prevention dose) have been documented to cause neurological toxicity in dogs with the MDR1 mutation.Loperamide (ImodiumTM; antidiarrheal agent) - At doses used to treat diarrhea, this drug will cause neurological toxicity in dogs with the MDR1 mutation. This drug should be avoided in all dogs with the MDR1 mutation.Acepromazine (tranquilizer and pre-anesthetic agent) - Based on collaborative research, the VCPL has determined that dose reductions are required for dogs MDR1 mutant/mutant and MDR1 mutant/normal.Butorphanol (analgesic and pre-anesthetic agent) - Dose reduction required for dogs MDR1 mutant/mutant and MDR1 mutant/normal.Chemotherapy Agents (Vincristine, Vinblastine, Doxorubicin, Paclitaxel) - Based on collaborative research, the VCPL has determined that dose reductions are required for dogs MDR1 mutant/mutant and MDR1 mutant/normal in order to avoid SEVERE toxicity.

Hattie’s appointment went really well and I will let you know the outcome in the next 1-2 weeks (tests are done in batches and if you get in on Monday’s batch the results are sent back that same week).

Sarah is currently writing a follow-up piece on Hattie's vet visit to be tested and the final outcome of Hattie's test.  She will also have some information on what you as a pet-parent can do if your pet tests positive. We all have our fingers and paws crossed Hattie's test will come back negative.

Sarah will be writing a follow up piece about having Hattie tested.Thanks Sarah!