I forgot a dose of heartworm prevention, why retest?

Q: I was a few weeks late giving my dog’s heartworm preventative, and now my vet says I should test him again in six months. I only missed ONE dose.  Is she trying to rip me off?

A: Nope, she’s trying to protect your dog’s long-term health.

Heartworm preventatives keep your dog from getting sick from heartworms, but they don’t keep infected mosquitoes from biting him and leaving tiny larval heartworms on his skin. From there, the worms migrate through the body, growing bigger and bigger, until they reach the heart and lungs as foot-long adults about six months later.

When you give a dose of the medication, it kills larval worms younger than about six weeks old–before they get into your dog’s heart and lungs and cause heartworm disease. Once the worms are 45 days old, though, preventatives can’t kill them. Now they’re free to make their way into the heart and cause big trouble.

Why the six month wait? Well, heartworm tests only detect worms more than about six months old. We’ve got to wait long enough for the worms to be detected by testing, but we don’t want to wait any longer than necessary. The sooner you know if your dog was infected, the sooner treatment can begin–and the more likely he is to respond to treatment.

So, yes, retesting is critical. In the meantime, read about what to do if you forget a dose of heartworm prevention.