Ivermectin: Is Your Dog At Risk By Breed?

It never hurts to remind folks that certain breeds are sensitive to certain drugs, and during this time of year when the climate turns warm, it’s especially important to talk about Ivermectin, an effective medication for humans used to treat any number of maladies in humans, including fighting parasites, head lice, scabies, river blindness, and strongyloidiasis. It is, in fact,  on the World Health Organization’s list of the most important medications needed in a basic health care system. 

Ivermectin was discovered in 1975, came into medical use in 1981, and was first used for dogs in 1982.  We shudder to think how many dogs it took before it was realized that certain dog breeds with the MDR1 gene (the P-glycoprotein gene) can be lethally sensitive to ivermectin, the active ingredient in a product like Heartgard. The gene mutation allows the drug to build up in the brain where it can cause neurological reactions, including tremors, disorientation and blindness. Sensitivity to Ivermectin is not a given,  but the owners of certain breeds need to be aware that their dogs are more at risk. These breeds are:

  • Old English Sheepdog
  • English Shepherd
  • Shetland Sheepdog
  • Bearded Collies
  • Australian Shepherd
  • German Shepherd
  • Certain Whippets
  • Silken Windhound
  • Skye Terrier
  • Border Collie
  • Collie
  • Miniature American Shepherd

Washington State University College of Veterinary Medicine first discovered the MDR1 mutation, and in 2010, developed test procedures to identify dogs with the mutation.  Mixed breeds are not immune.  Collie-type mixed breeds are at risk, too.  Learn more about the genetic test here:

There’s one more thing – a hugely important thing – for breed owners to know. Dogs, especially those with the MDR1 mutation, should never be given large animal formulations that contain ivermectin. You might think to yourself, “Who on earth would do that?”

A rancher would. But not to the dog. A rancher, farmer, or sheep owner would reasonably treat their stock with large animal formulations, and how this impacts a dog is best illustrated by what happened to an Australian Shepherd last year.  The dog and his owner attended a sheep herding session, and as is often the case, the dog found the sheep pellets to be tasty morsels. Unbeknownst to anyone other than the owner of the sheep, the sheep had been treated the day before with Ivermectin.  Few of us would think to ask in advance of taking our dogs to go herding when the last time was that the sheep were treated with drugs.

The dog became critically ill within a short time, and it was quite literally touch-and-go for the dog for weeks. To our knowledge, the Aussie did make a complete recovery, but there was never a time early on when death wasn’t a realistic outcome for the dog.

Be aware, ask questions, and run the gene test on your dog if his or her breed warrants it.

Image: “Mick” by Terry d. Chacon

12 thoughts on “Ivermectin: Is Your Dog At Risk By Breed?”

  1. Thank you for this article. I was aware of the MDR1 factor with Aussie Shepherds as am involved with this breed in Australia. However I was not aware of its existence within Skye Terriers which I have been involved with, on a breeding and showing level, since 1985 This was rather a surprise as I have used Heart guard in the past. Luckily none of my dogs showed any ill effects.
    Might just get my current Skye Terrier tested, out of curiosity.

    • Let us know what the test results yield, Lorraine, on your Skye? We were also surprised to see the breed on this list, and came across it only once. Rather than take chances, however, we included it since the web site where we found the breed listed was a veterinary one. Their DVM trumps us, lol.

  2. I have a 3 year old, male, NSDTR who must have eaten a blob of Ivermectin which would have been dropped out of a horse’s mouth after deworming with an Ivermectin product. Within 24 hours he was totally blind, but otherwise seemed fine. Fortunately, his sight completely restored in about 2 days.

    • What a horrific thing to have happened, Glenna, you must have been ecstatic when his sight returned.

  3. Thank you for posting this article. I am sorry, but I was not able to find the author’s name. It is very important to know that the sensitivity to ivermectin is dose dependent. The heartworm prevention dose does not cause disease even in dogs with the MDR1 mutation.

    As you have said we have known about the sensitivity since the 1980’s in collies and collie type breeds, but it is important to know that there are other sensitive breeds. Thank you!

  4. Two other critical things people need to know – First this is not a purebred problem – MDR1 inherits as a dominant fashion – and there are effects from one or two copies. That means that even if it originally came from a purebred in its ancestry, a dog could be mixed for generations and still have a copy of the gene and be affected. The mutation itself predates the isolation of breeds by registry (http://www.pnas.org/content/101/32/11725.short)

    That means any dog who has it can pass it on to offspring and they are affected no matter if they have one or two copies. The level of tolerance for some drugs varies with one or two copies, but not all which brings the second thing that people need to know.

    There are are more drugs than just ivermectin that cause problems for dogs that have copies of the gene for MDR!, including some that are more dangerous that ivermectin.

    For example, there have been a lists of over the counter drugs and doses that make it around the internet that include the antidiarrheal loperamide (Imodium). Whereas there are doses of ivermectin considered safe enough to give to dogs with MDR1 genes, there is no safe dose known for loperamide.

    From Washington State University: “The most serious adverse drug reactions involve several antiparasitic agents (ivermectin, milbemycin and related drugs), the antidiarrheal agent loperamide (Imodium), and several anticancer drugs (vincristine, doxorubicin, others).”

    Here is their page on drugs with adverse reactions: http://vcpl.vetmed.wsu.edu/problem-drugs

    Another source http://www.busteralert.org/vet-fact-sheet/

    • Great information, Karen, thank you for posting it here where more people can see it!

  5. Many years ago we had an Australian Terrier who has bad tremors after getting his heartworm medication. None of our other dogs had bad reactions but we never gave the medication to the boy who did again.

    • Wise decision, J. Presumably, the Aussie never had tremors again?

  6. We don’t give ivermectin. And all our Skyes go with a detailed letter to the vet stating ivermectin is NOT to be given. They are required to sign it and date it and stamp it and have it sent back. And the warning is on every Skyes file. NO ivermectin.
    I’ve heard to many horror stories of Skyes dying from it. Not worth the risk to me. Or the owners. It’s too much unwanted suffering that can easily be avoided with a little caution.

    • Thank you for sharing your input, Shal. We couldn’t agree more with you, it’s simply not worth the risk.

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