Dear Down Under,
The concern of which you speak is due to a mutation (called ABCB1 or MDR1) that sometimes occurs in herding dog breeds (Australian Shepherds, Collies, Shetland Sheepdogs, German Shepherds, etc.) and mixes of these breeds. This mutation interferes with the formation of a protein called P-glycoprotein (PGP) which prevents certain drugs from crossing into the brain and assists in the breakdown of some medications. Dogs lacking PGPs are at risk of these drugs crossing into the brain and/or reaching toxic levels. Several drugs used in heartworm prevention can be toxic if given at certain doses to affected dogs.
Ivermectin is found in heartworm prevention drugs such as Heartgard, Heartgard Plus, Tri-Heart Plus, and Iverhart Max. Symptoms of toxicity from ivermectin include dilated pupils, blindness, excessive drooling, vomiting, seizures, muscle tremors, unsteady gait, and the dulling of mental function. Levels of other preventative drugs, such as milbemycin (Sentinel), selamectin (Revolution), and moxidectin (Advantage) can also be affected by this mutation.
That said, the doses used in heartworm preventatives, such as those mentioned above, are very low and can be used safely even in dogs with this mutation. The labeled doses are well under the toxic levels for these dogs. So rest easy, you can give your dog a heartworm preventative. Keep in mind, however, if you have livestock that is given high dose ivermectin, make sure your dog does not eat the drug or livestock manure as he may be exposed to high levels of the drug. Also be aware that some pesticides contain drugs similar to ivermectin, such as abamectin (e.g., ant and roach baits). Doses of ivermectin used for treating mange or ear mites can be dangerous for dogs with the mutation and are especially dangerous when combined with spinosad (seen in some flea medications like Comfortis or Trifexis), even for normal dogs. If you’d like to know for sure if your dog carries this gene, there are genetic tests available. You and your vet can determine which heartworm preventive is right for your pet.