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5 facts about heartworms you didn’t know

Heartworm DogsAbout once a month, I get the panicked, guilt-ridden phonecall.

“I just realized Fluffy didn’t get her heartworm pill last month! What should I do? Do I need to test her today?!?!”

First, you are not a terrible person – it happens to the best of us! As soon as you realize it, give the pill (or apply the liquid, depending on the form).

We’ll get through this together.

Here’s the five facts about heartworms most pet owners don’t know.

1-Heartworm preventatives work a little backwards.

We all know heartworms are transmitted by mosquitoes. They suck blood (and worm larvae) from a dog who has heartworms, fly around, then bite your dog and inject the worm larvae into him. These immature worm babies find their way to the bloodstream, where they head for the heart. In the process, they go through another stage of maturation before turning into adult worms. The worm larvae that are initially injected by the mosquito are susceptible to heartworm preventatives, but once they mature into the next stage, those are not. Heartworm preventatives don’t touch them, not matter how many you give. There is some controversy right now as to how long it takes from time of infection to maturation, as vets in the south (with lots of mosquitoes and heartworms) are suspecting it is happening sooner and faster than originally thought. Current consensus is 40-50 days. No worm reads the manual, so there may be individual variation in there as well.

When we give our preventative, we are cleaning house, killing anything that was injected in the past month. It does nothing to help in the approaching month!

2 – There is a (small) grace period with heartworm preventatives.

Because it takes 40-50 days for the larvae to mature past the point of no return, we could in theory give preventative every 40-50 days right? Yes, you could. It’s recommended for once a month use because, with people’s hectic lives, it stands the best chance of actually happening that way. And, if you do give it a week late, you’re still in the clear! If you give it every 40 days, and then miss a dose, you’re more likely to have complications. Shoot for once a month.

3 – The heartworm test is a little sexist…and has a delay.

It takes a total of 6 months from the time of the mosquito bite to the maturation of adult worms. The modern heartworm tests (sometimes called antigen tests) are designed to detect a protein secreted only by female worms. Not that we don’t want to know about male worms, they just don’t secrete anything we’ve discovered yet that we can test for. If your dog only has a couple heartworms, and they’re both dudes, your test will be negative. You also will not get reproduction and worsening of the disease, which is good news! (Cats, due to their small size, can often be infected with only a single worm. That gives this test a 50/50 shot of picking it up!

Bottom line – if you miss a couple months of heartworm preventative, wait 6 months, then test.

4 – Heartworms do not actually live in the heart.

They live in the pulmonary artery. That is the super important blood vessel that connects the heart to the lungs. In order for any blood to get oxygen to continue its travels throughout the body, it has to go through this artery. It being clogged with worms is bad news. Thing is, “pulmonary artery worm” is kind of a mouthful, so we all say “heartworm.”And, with heartworms being 8-11 inches long, they will extend into the heart chambers.

5 – Size DOES matter.

Heartworm preventatives are dosed based on weight of the dog. If your dog weighs 60 lbs, you need to give the 51-100 lb dose, not the smaller dose. Many preventatives’ largest dose goes to 100 lbs, and many dogs weigh over 100lbs. What to do? You have to give two pills. If your dog weighs 115 lbs, you need to give a big dog size and a yorkie size pill at the same time. (Translation, give the 51-100lbs size along with the 5-25lb size so the total weight covered is 125 lbs).

It is safe (and recommended) to round up when dosing preventative. Never round down. You can give a yorkie the lab size heartworm pills every month and it will be fine, just cost you more. Don’t give the lab the yorkie size. True story – I’ve had patients who weighed 107 lbs, were given the heartworm pill for dogs up to 100lbs, and they tested positive for heartworm.

Web-DVM guest blogger Dr. Karen Louis is a practicing small animal veterinarian.  See more of her articles at her blog at

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