Actually, it would be more accurate to say that any larval heartworms he contracted in August are definitely now out of his system. And that’s not inconvenient, because Buster and I did a fair bit of camping in mosquito country last month.
Heartgard (it always has, and always will, annoy me to type that word. Dr. Deb, the company still could have trademarked the name if it had spelled guard correctly!) Plus and all monthly oral heartworm preventatives work by killing larval heartworms in the bloodstream before they can become dangerous mature worms. The pill kills any larvae that were contracted in the prior 31 days.
Heartgard (sic) Plus comes in flavored tablets. Buster absolutely loves them. In fact, he loves them so much that on August 1, he broke into the closet where they were stored (“someone” hadn’t closed it fully) and ate four of them â€” after I had already given him one. That made five in one day.
Fortunately, the active ingredients in Heartgard (sic) Plus have good safety margins. Ivermectin, the ingredient that kills heartworm larvae, is present in such low doses that Buster probably could have eaten 12 tablets without ill effects. But Buster is not a Collie or a Sheltie or any type of dog susceptible to the MDR1 mutation â€” these dogs are much more sensitive to that ingredient.
The other active ingredient, pyrantel pamoate, kills intestinal worms. Its safety margin isn’t as good as the non-MDR1 safety margin of ivermectin, but it’s still good.
In short, when I discovered that Buster had eaten our entire stash of heartworm preventative, I did not worry that any harm would come to him. I did, however, have to buy a new box. I will continue to give him one pill each month, on the first day of the month.
Is that what you should do if your dog does what Buster did? Probably. However, although the safety margin of Heartgard (sic) Plus is good, there are variations in reactions among individual dogs. So you should contact your vet if your dog overindulges in his heartworm preventative.