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What Heartworm Preventative Is "Safe" for Collies and Aussies?

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Owners of certain herding breeds like Australian shepherds or collies may wonder whether there is a heartworm preventive that is safe for collies and Aussies. This is an important question considering that several herding dog breeds can be highly sensitive to ivermectin because of a mutation in the MDR1 gene (also known as ABCB1 gene). Failure to acknowledge the risks, may result in serious neurological side effects such as seizures, coma, and even death. While there is no specific antidote for ivermectin poisoning and treatment is mostly supportive, the good news is that there are several heartworm preventives that are considered "safe" for collies and Aussies, but it's important to follow the vet's guidelines and remember that no drug can be considered totally safe.

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About the Sensitivity 

Over 100 years ago, a new discovery was made: If a blue dye was injected into the bloodstream of an animal, the whole tissues of the body would turn blue expect for the brain. Why was that? Scientists at that time theorized that the brain was protected by a special barrier that was called the "blood-brain-barrier." Nowadays, we know much more about this structure.

The blood brain barrier is a semi-permeable structure that protects the brain from being accessed by foreign substances in the bloodstream.

In dogs with the mutation of the MDR1 gene as seen in some collies, Australian shepherds, but also Shetland sheepdogs, Old English sheepdogs, German shepherds and associated mixed breeds, the blood-brain barrier does not prevent ivermectin from accessing their central nervous system as it should.

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When given high doses of ivermectin, these affected dogs therefore end up accumulating high concentrations in their brain tissue causing serious and potentially deadly neurological problems. This suggests that these dogs lack a fully functional blood-brain barrier, explains Dr. Katrina L. Mealey, a board-certified veterinarian specializing in internal medicine.

Adverse Drug Reactions

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Dogs with the mutation of the MDR1 gene are genetically predisposed to develop adverse reactions to several different drugs.

These drugs include ivermectin and milbemycin which are the active ingredients in most heartworm medications, but also several other drugs such as the anti-diarrhea drug loperamide (Imodium), the antiparasitic drugs selamectin, milbemycin, and moxidectin, the tranquillizer acepromazine, butorphanol, a pain reliever and pre-anesthetic agent and some chemotherapy drugs such as vincristine, vinblastine, paclitaxeland doxorubicin.

It's important to be aware of the dangers these drugs pose to a dog with the mutation of the MDR1 gene as some of these may be available over the counter.

This is one reason why it's always important consulting with a vet before administering any medications to dogs. Being over the counter, doesn't necessarily mean safe, and even more so in dogs breeds prone to drug sensitivities!

Did you know? Dogs with the MDR1 mutation are also at risk if they happen to eat the feces of farm animals that were treated with ivermectin, warns veterinarian Dr. Karen Becker.

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A Matter of Dosage

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If you own a breed that can be predisposed to having a mutation of the MDR1 gene, you may be wondering if there are alternative heartworm drugs that can be used. Turns out, ivermectin (Heartguard) or milbemycin (Interceptor) given at their recommended dosages are safe for dogs with the MDR1 gene mutation.

It's a common misconception that Heartgard, given at the appropriate dosage is toxic to shelties, points out Critical Care Vet, a veterinarian specializing in Emergency and Critical Care. The truth is that the problem with ivermectin and certain breeds is dosage. 

For instance, dogs without any drug sensitivities, can take doses of ivermectin as high as 400 µg/kg safely; whereas, in a dog with the MDR1 mutation, a dose as low as 50 to 100 µg/kg is toxic. Dogs with the MDR1 mutation are therefore safe in getting a monthly dosage of an ivermectin product like heart guard considering that it only contains 6 µg/kg, explains veterinarian Dr Rebecca.

Owners of collies with the double mutant gene may be overly concerned and may think to use ivermectin-free products such as Revolution which contains selamectin or Advantage Multi which contains moxidectin, but, according to Washington State University, these drugs have also been documented to cause problems to dogs with the mutation. However, these drugs are also considered safe when given for heartworm prevention and at the manufacturer’s recommended dose.

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Did you know? Dogs with sarcoptic mange are treated with invermectin given at a dosage as high as 300 µg/kg, while dogs affected by demodectic mange are treated with dosages as high as 400 to 600 µg/kg. Before treating any dog that can potentially have the MDR1 gene mutation with such dosages it's obviously highly recommended testing for the MDR1 gene mutation.

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"In mutant/mutant & mutant/normal dogs, all US FDA-approved heartworm preventives (ie, ivermectin, milbemycin, moxidectin, selamectin) are considered safe for use. See specific trademark products for label indications. "~Katrina Mealey

A Helpful Test

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If your dog belongs to a breed that is predisposed to drug sensitivities, you may find it helpful knowing that, nowadays, there is a new test that can test your dog for multidrug sensitivity. This means that, once you get the testing results, you and your vet may determine whether you will need to take particular precautions when it comes to certain drugs or not. Consider that not all collies or Aussies, or other predisposed breeds, have the MDR1 mutation.

If you are therefore concerned about the possibility that your dog has the MDR1 mutation, for peace of mind, you may find it reassuring getting the test done. For those interested, Washington State University's College of Veterinary Medicine offers MDR1 genotyping for dogs. The test is not invasive and simply involves getting a cheek swab and sending it off to the lab.

So what heartworm preventive is safe for collies and Aussies? The truth is that there is no drug that is totally safe, and this is true even for dogs without the MDR1 mutation. If you have any concerns about the best heartworm medication or any other medications for your dog, consult with your vet. He or she is the best person to discuss any questions or concerns you may have. Heartworm medication is only available by prescription so you will have to go through your vet in any case.

Did you know? Dog owners are often to blame for causing ivermectin toxicity in their dogs with the MDR1 mutation. This tends to happen when they give a too concentrated strength of ivermectin made for for horses or cattle. Always contact your vet for dosage questions and advice.

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  • DVM360, On the Forefront: A new tool that detects ivermectin and other drug sensitivities in dogs
  • Healthy Pets, Use Extreme Caution with Ivermectin
  • Mealey KL. Canine ABCB1 and macrocyclic lactones: heartworm prevention and pharmacogenetics. Vet Parasitol 2008;158:215-222.
  • March 2016 Plumb’s Therapeutics Brief, How Should I Treat Dogs & Cats with MDR1 Mutation? Katrina Mealey, DVM, PhD, DACVIM, DACVCP Washington State University

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