PET TIPS: The how and why of heartworms


f there’s such a thing as heartworm season, we’re in it.

Since heartworms are transmitted by mosquitoes, the recommendation used to be that pets didn’t need protection during the winter months, especially in northern climates. Zero mosquitoes equals zero risk, right?

Even the package inserts for these products used to indicate that they were intended for seasonal use.

But that’s no longer the case, especially when we have weird warm spells that can lure bugs out of their hiding places in the so-called dead of winter.

Randomly stopping and starting your dog’s preventive medication can leave your pet without protection. But the situation is actually much more dangerous than that. Here’s why:

Before starting on heartworm preventative, your pet is supposed to be tested to make sure he doesn’t already have heartworms. That’s because if a dog has microfilaria (immature heartworms) circulating in his bloodstream, giving the preventive chemical can suddenly kill all those worms, which causes the dog to go into anaphylactic shock.

If your dog is screened before starting prevention and he tests positive for heartworms, he’ll be given a treatment that works differently from the preventive medication.

What you don’t want is to get stuck in a Catch-22 like I did. My dog missed a couple doses of Heartgard over the winter. Then I started noticing  mosquitoes inside our house, and I thought, “Uh oh, I better give her the Heartgard so she doesn’t get infected.” Then I realized it was possible she’d already been bitten, and I couldn’t restart the prevention until I could get her to the vet to be tested.

Thankfully, she got the “all clear,” so she’s back on her monthly dosage.

Now, you may be wondering, “Why bother?” Heartworm prevention is very expensive, and perhaps you think it’s not really necessary. Statistically, what are the odds that your dog will get bitten by an infected mosquito?

Let’s look at that question from a slightly different angle. We live in Georgia, and the Southern states have the highest rates of heartworm infection. Plus, it only takes one mosquito bite for an animal to become infected.

And, most importantly, have you ever seen a heart removed from a dog who died of the disease? It looks like it’s completely filled with spaghetti. Imagine how it must feel for an animal to have those long strands choking off the flow of blood to its heart. Don’t let this happen to your pet!


Debbie Gilbert has been handling dogs for 50 years and won numerous AKC obedience competitions with her late Sheltie, Sunny. She lives with her current Sheltie, Daisy. E-mail your Pet Tips questions to

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