Five heartworm facts you didn’t know

About 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.

Here's what heartworms look like when dissected.

Here’s what heartworms look like when dissected.

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! Here’s my article on heartworm in cats.)

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.

Finally, heartworm preventatives kill a lot more than heartworm larvae! Here’s my article on the various intestinal worms they can treat.


Posted in General health, Heartworm, Parasites.


  1. Hi. My dog gets diarrhea with the heartworm meds. What is the best way to help his intestines tolerate them? Is there any way he can be infected in the coldest months (central NY) when there haven’t been any mosquitoes around for several months—if he misses one dose of his HW med?

    Thanks for any advice

    • I have this happen sometimes – not fun. Ask your vet about Revolution! It’s a liquid that goes on their back and prevents fleas, ear mites, and heartworms! I use it for my patients that don’t want to eat the heartworm pill, it upsets their tummy, or they are undergoing a food trial. It’s prescription only, but your vet can order it for you if they do not routinely carry it. And no, infection is not very likely when it’s freezing out and there are no mosquitoes. I still recommend not missing a dose, but life happens sometimes!

  2. This article was super helpful. My St. Bernard puppy outgrew his six month supply of heartworm chewables for dogs up to 100 lbs. I could not find any chewables for dogs over 100 lbs. which he is now at 10 months old. I am so glad I came across your article. Now I know I can give him two chewables, one up to 100 lbs and one up to 25 lbs. Thanks so much!

  3. My 6lb chihuahua ate his heartworm tab and vomited three hours later. Is he still protected? This is the second time it’s made him sick. He always gets it with some chicken or beef because he hates it. I even tried giving a day later. It’s tri-heart. Thanks for any help!

  4. My puppy weighs 55 pounds. I have 2 doses leftover of the 26 to 50 pound Heartgard. Would it be okay to give her those 2 doses before I start giving her the 51 to 100 size?

    Thank you.

  5. Hi, I adopted a 3 1/2 yo Dachshund mix from SPCA. He had all his shots and was fixed. Bought Flea Prevention and Heart-worm meds. Unfortunately he was a biter and very dominant/aggressive. He bit my son and snapped at me. As much as I hated to turn him back in I did. He hated cats and every noise imaginable (constant barking). A month later we adopted a 3 month old Chi puppy that is 4 lbs. We had her fixed last week and the rest of her shots and she weighs 7 lbs now (4 mths). I have 2 questions.
    # 1 – I have (5 mths worth) heart worm meds (Sentinel) that were Charlie’s (11-20 lbs). Can I cut the pills in half or something to give to her (Ruby).
    # 2 – Ruby is potty training…..She does pretty good with going on the pads but she likes to shred them now of course. She always poops inside…..She has only went once since we got her outside. I have taken her feces and throw it out to the area I want her to use. I don’t know what else to do. She does not mess in her crate when she is in there. Please help!
    Any suggestions?

  6. My German Shepherd outgrew her puppy size heartguard pills but I still have so many left. She is currently 70 lbs, could I give her 2-3 pills of heartguard for the up to 25 lbs dogs?

    • It appears so. The article shows that by using addition on the max size of the heartgard pill box, you can obtain the correct dose for your dog.

  7. So, I usually give my 65ld dog a (60-120lb) trifexis pill hidden in peanut butter. Well, he wised up halfway through this dose, he ate the first half and won’t eat the second. Through trying all the different tactics the second half of the pill is ruined. I have neck drops, should I put a full dose on him to make sure he is covered, or try half? From everything I read above, I shouldn’t just let him have a half…. thanks for the help

  8. In South america, vets only recommend it every 3 months, and i have yet to come across a dog who’s had heart worm! We play by the 3 month rule here and it works perfectly.
    Pharma in North America is much more $ driven than here. And i’ll leave the rest to common sense 😉

  9. Thanks, when should I give heartguard plus not certain they go their doses two days ago of heartguard plus – one is 9 lb. poodle and one is 15 lb. poodles.

  10. I have a 43lb lab mix, and a 64lb pitbull mix. Can I give the 43lb dog the larger dose(51-100lb pill) of heartworm medicine?

  11. So I have a suspicion that my dog has a very small number of worms — possibly only a single, adult worm. He tested negative at the shelter we rescued him from, positive on the antigen test, but negative on the confirmation, looking for larvae (we had to test him multiple times before it was confirmed). He also got to the shelter in January, and got “de-worming” treatments until we got him in the summer. We’ve done the first step of the process, and are almost ready to start the injections — but I wanted to know if this is an unusual case that might need special precautions. He must have had the worm for a long time before we got him, but to test negative for larvae? Does that mean his case/treatment may be more complex, or require anything else in addition to the treatment? I want what’s best for him, and I’m worried.

  12. I have 2 yorkies that weigh under 7 lbs. The heartworm dose that I give them is for dogs 1-25 lbs in weight. Is it OK to give my dogs half of the recommended dose, since they are under 1/3 of the total maximum weight (25 lbs)? I do not want to give my dogs unnecessary quantitities of drugs if I don’t have to. Thanks

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