You are likely aware that there is a shortage of Interceptor and Sentinel right now. I have had several comments on my post about the Novartis shortages from people who are upset because they have a breed of dog who is sensitive to ivermectin and they feel that Interceptor and Sentinel are the only drugs that are safe to use for heartworm prevention for their dogs. I wanted to write a post to explain why this is not true.
The main concern is that some breeds of dog can have a mutation in a gene called MDR1. Dogs who have this mutation can be extremely sensitive to ivermectin which is the main ingredient in Heartgard. (What you may not know is that these dogs are also sensitive to the ingredient milbemycin which is the main ingredient in Interceptor and Sentinel. – More info on this later in the article.)
Dog breeds that are believed to be more likely to be carrying this gene include:
If a dog with the MDR1 gene mutation receives a high enough dose of ivermectin (or milbemycin) they can have serious neurological side effects including tremors, seizures, coma and even death. For this reason, if a vet feels that it is necessary to treat an animal with a significant dose of ivermectin (such as for treatment for sarcoptic mange) they will often recommend testing the dog for the MDR1 gene, especially if the dog is a herding breed.
The key to remember here is the dosage. Dogs who are sensitive to ivermectin can be negatively affected if they receive a dose of 50-100 micograms per kilogram. The amount of ivermectin that is in a dose of Heartgard is 6-12 micograms per kilogram and is not at all toxic to dogs even if they are carrying the MDR1 gene mutation.
For milbemycin, dogs with the MDR1 gene mutation can see neurological side effects at a dose of about 90 micograms per kilogram. Interceptor and sentinel usually delivers about 7-10 micograms per kg of milbemycin to the dog.
Both drugs are very safe to give to any dog as long as we give the recommended dosage.
It is possible that this belief has stemmed from good marketing. When Interceptor first hit the market, the packaging proclaimed, “Safe for ivermectin sensitive collies.” This implied that products like Heartgard that did contain small levels of ivermectin were not safe. But, in reality, both products contained small enough doses to be considered extremely safe even in dogs with the defective gene.
Yes, I do. I have no problem with a collie breed or any other breed with a potential MDR1 gene mutation receiving Heartgard. When it is available, I do prescribe more Interceptor than Heartgard, simply because Interceptor will also be effective against whipworms. But, as far as heartworm prevention goes, both products are very good.
As always, the goal of my website is to try and clear up areas of confusion that exist on the internet. Has this article helped, or do you have more questions? Leave a comment below and hopefully I can help clear up the subject even more.