Slow-Kill And Fast-Kill Heartworm Treatment

A bold divide exists among veterinarians on the topic of how to rid dogs’ bodies of heartworms. Few lack passion for this subject.

Immiticide is the “gold standard” for heartworm treatment in dogs.

“Slow kill” or “fast kill?”

Traditional “fast kill” heartworm treatment which, by the way, is not particularly fast, involves the use of Immiticide. (And, the newer medication, Diroban.)

Two regimens are described in the drug manufacturer’s package insert accompanying each box of Immiticide.

The first is aimed at dogs with relatively light loads of heartworms. The heartworm burden can be approximated by combining results from a semiquantitative heartworm test, pulmonary and cardiac ultrasound, chest X-ray and physical examination. This regimen starts with an initial injection of Immiticide followed by another 24 hours later. If the patient is not already on heartworm preventive, one is begun and a heartworm test is performed six months later to determine whether the treatment was successful. Thirty to 45 days of activity restriction is strictly enforced after the first injection is administered.

For those canids infected with heavier burdens of heartworms a 3-injection series of Immiticide injections is performed. A first injection is given, at which time activity restriction is begun. Approximately 30 days later another Immiticide injection is administered, followed by a third 24 hours later. (The American Heartworm Society recommends the 3-injection series for all dogs with heartworms.) A heartworm test demonstrates success or failure six months later.

Heartworm disease is easily and inexpensively prevented. No dog or cat need suffer from this infestation.

The failure rate is between 2 and 5%.

Advocates of the “slow kill” approach cite the occasional pulmonary embolism and the infrequent incidence of fatal complications as reasons to avoid Immiticide use.

However, slow kill is not without its own drawbacks.

How does it work? Essentially it is the process of allowing adult heartworms to live to their life expectancy and die of old age, while the dog is administered a monthly heartworm preventive to keep him from getting even more heartworms.

Before the treatment can begin, his microfilaria status must be determined. (Click here to read about microfilaria, the offspring of adult heartworms.) This step is common to both slow-kill and Immiticide treatments. Most heartworm preventives are also very effective killers of microfilaria. When microfilaria die suddenly in large numbers a substantial percentage of dogs will experience anaphylaxis, a violent allergic reaction that can be fatal. If your heartworm-positive dog also has microfilaria, the treatment of those must occur under strict supervision of your veterinarian. If anaphylaxis occurs, emergency medications can be administered immediately in hopes of stopping the reaction.

If there are no microfilaria, heartworm preventive can be given right away and is continued monthly. Testing is usually performed semiannually so that we can know as soon as possible when the patient reverts to a heartworm-negative status.

Old, female heartworms are expected to die first, usually in 6-12 months.

Strong young male heartworms may live as long as five years. However, male heartworms will not trigger a positive heartworm test because heartworm tests detect molecules of the female heartworm’s uterus.

My professional opinion is that all dogs should have their heartworms treated with Immiticide. I came to that conclusion because it is also the opinion of the American Heartworm Society. No less distinguished expert than
Ray Dillon, D.V.M., Diplomate of the American College of Veterinary Internal Medicine (subspecialty in cardiology)is a member and known worldwide as a leading heartworm expert.

Dr. Ray Dillon’s stance is clear: No slow kill. No way. No how.

Studies Dr. Dillon has participated in show that the longer heartworms live in a dog’s body, the greater the damage occurring to the heart, lungs and pulmonary arteries. No one, not even advocates of slow kill heartworm treatment, argues against that.

What some proponents of slow kill say, however, is that they don’t see that as a clinical syndrome in their heartworm treatment patients.

To me, that is equivalent to saying, “What you don’t see can’t hurt you.”

I believe the experts who say that these pets are susceptible to respiratory tract diseases of all kinds including Infectious Tracheobronchitis and Canine Influenza. In addition, no long-term studies have been performed evaluating patient longevity and rate of complications later in life. Furthermore, those studies will never be performed. It would require allowing a pet to contract heartworms, subject it to slow-kill treatment, then plan on a necropsy when its life is over.

Is slow-kill heartworm treatment better than no heartworm treatment at all? Absolutely!

Is slow-kill an acceptable first step while a dog owner saves his money for Immiticide treatment? Absolutely!

Is slow-kill the best heartworm treatment available to today’s dogs infested with heartworms? Not in my opinion.

See you next week, Dr. Randolph.



  1. Hello,
    We’ve adopted Bailey from Texas (of course). She was given Proheart6 injection dec 2021 and another July 2022. She tested positive Sept 2022 and again Jan 2023 but both times with no microfilaria.
    Currently taking Heartgard monthly tablets. I notice the occassional weazing which makes me want to do the radiology to determine the extent of infection. She is now in Ontario Canada and has had 4 months of cold weather. She’s 2.5 yrs old shepherd mix and a very sweet girl. What are the chances of complications if opting for the Melasormine injections? Thank you

    • I copied this statement from the American Heartworm Society’s Web page: “‘Heartworm disease is a complex disease, and there are no shortcuts to appropriate treatment,’ the AHS leader emphasizes, noting that the AHS protocol was designed to kill adult worm infections with minimal complications while stopping the progression of disease. ‘Skipping any one of these steps can affect both the safety and efficacy of heartworm treatment.'” As you see in our article, Dr. Dillon says, “No slow kill.” ASSUMING your local veterinarian agrees, and the pre-treatment laboratory testing and chest radiograph don’t preclude it, you could proceed to treatment. Click on “Heartworm Category” on the right side of any page to read more on heartworm disease. Thanks for reading, Dr. Randolph.

  2. Thanks, lots of good info here.
    I rescued a 2 year old setter rottie mix from a shelter 5 months ago. He was/is HW positive and the shelter did slow kill (Doxi and advantage multi) bc that is their method of choice. I have continued the advantage multi topical and am considering injections. However, I find it odd that the worm burden is never considered. For example, if the worm burden is low (say a dozen adult worms) wouldn’t the slow kill be a good option? If high worm burden, I feel injections are probably a necessary (somewhat risky and expensive) procedure. It concerns me to hear blanket statements like “The only approved treatment for adult heartworms is immiticide.” I feel all factors including dogs age/health and worm burden should be considered. Please advise.
    Oh, also, I have been letting Lobo run at the dog park and chase squirrels while on slow kill. Is this really dangerous and I can’t let him run?? Please advise, and thank u for writing. Great info here!

    • 1. Heartworm disease researchers emphasize that there is ongoing damage as long as worms are present. THAT’S why slow kill (also called no-kill, the heartworms are simply dying of old age, eventually) is not recommended. 2. Worm burden can be difficult to assess. Some tests are called “semiquantitative,” meaning they give us a general idea of heavy or light burden. Pre-treatment chest radiography tells us about how much response the body has made to the adult heartworms and the damage therefrom. 3. The presence of worms can be evaluated by ultrasound. The bottom line is that ALL patients need activity restriction AT LEAST after adulticide therapy has begun. The American Heartworm Society recommends activity restriction AS SOON AS THE DIAGNOSIS HAS BEEN MADE. Therefore, AHS would suggest NOT allowing your dog to have heart-rate-increasing activity anywhere, including your dog park. We’ll say a prayer that your dog has proper treatment and an excellent outcome. Thank you for reading, Dr. Randolph.

      • Thanks for the great info. No more running for Lobo 🙁 I am thinking I need to get injections. In your personal opinion, Is there ever a situation where the worm burden is so low and dogs health so good that slow kill is the better option? I just want to do what’s best for Lobo and give him the best quality of life. I adopted him 4 months ago and he doesn’t ever cough and shows no symptoms. The shelter said he was a “low positive for HW. Do injections typically add multiple years to the dogs life compared to slow kill? I know these are hard questions to answer and there is not a clear-cut definitive answer. Just looking for general knowledge and your opinion. Thanks for all the advice and help you give us on here for our four-legged friends.

        • In your professional opinion, Is there ever a situation where the worm burden is so low and dogs health so good that slow kill is the better option? No.

          Do injections typically add multiple years to the dogs life compared to slow kill? Longer life and better life.

          Thanks for all the advice and help you give us on here for our four-legged friends. You are most welcome and let us know how Lobo does. Dr. Randolph.

          • Ok, thanks. It sounds like injections is definitely the best option any time the dog is healthy enough to qualify. I guess the shelter I got him from probably does slow-kill simply for economic reasons. Thanks again and I will let you know how it goes for Lobo.

  3. I adopted a Doberman and my veterinarian thinks he is around 1 1/2 years old. He was positive for heartworms and was started on doxycycline 200mg bid x 21 days. He was given Heartgard after finishing the doxy. It has been 5 weeks since treatment was started. I asked the vet tech (who has worked with my vet for years) how much longer I need to keep him from activity. She said it was fine for him to do whatever he wants. I was not able to talk with my veterinarian but everything I’ve read is 3 months of inactivity. Advice please, thanks.

    • This is a direct quote from the American Heartworm Society: “From the first injection until six to eight weeks following the last injection, it will be absolutely essential to keep your dog quiet. That means strictly restricting all exercise and activity that would elevate your dog’s heart rate or increase his blood pressure.” Did your dog have injectable treatment with melarsomine, or oral medication only? Dr. Randolph.

  4. My Duke is a full blooded chocolate lab. I believe him to to be 12 years old. He was surrendered to a shelter and I adopted him at approximately 2 years old. He was diagnosed with heart worms in January, he did not have signs of the larvae. The vet told me the 2 options but suggested the slow kill due to his age & health. He has now been on slow kill for 6 months. We have also done rounds of lasix & prednisone when he starts hacking. However his breathing I feel may be worse. Could you please tell me your opinion? Duke is 110 lbs. it’s hot & humid here so he is not able to go out like he used too or he simply can’t breathe. I don’t want him to suffer but I don’t want to give up on him, he has so much love for me and the look in his eyes is asking me for help & to fix it!

    • Ha! No reason to give up on Duke! Keep doing what you’ve been doing, limit his time in the heat and humidity and restrict walks to the cool of the morning. Medicate him when he’s coughing or having trouble breathing. Follow his doctor’s advice, he has all of the information to make informed decisions for Duke. Enjoy the time he has left. Sadly, those of us who love big dogs know that they don’t live as long as most small-breed dogs. Thanks for reading, Dr. Randolph.

    • The same medication is aimed at both genders of heartworm, it’s just that the young male heartworms are the hardest to kill. I have had a few patients who had a single injection in some other state before moving here, and they were consistently negative on the occult heartworm test thereafter. BUT, the American Heartworm Society recommends the three-injection series for ALL dogs, and the success rate when following their recommendations is very high. Thanks for reading, Dr. Randolph.

  5. Hello my dog jasper was diagnosed with heart worms today and we decided to do the heart worm prevention medicine while saving the money for the shots. It slipped my mind while I was at the vet since I was so upset he was positive. But my question is even with the heart worm prevention (slow kill way) how long does he need to rest after taking it? Thank you

    • The American Heartworm Society recommends activity restriction from the moment of diagnosis to reduce the risk of pulmonary embolism. Good for you saving your money for adulticide treatment. Many experts, including Dr. Dillon, call it “No Kill” instead of “Slow Kill.” Thanks for reading, Dr. Randolph.

  6. Dr. Randolph I have a 3 1/2-year-old Boston terrier that had been in a bad home and miss treated. It’s been about six months since I relived this family member of Beau and due to the Covid crisis to get a regular veterinarian appointment was difficult, emergency visit only. Yesterday I was able to get into my regular vet to find out that he is heartworm positive. He has received his first injection to get rid of the ones that are in the blood he will come back in a month to get his main treatment injection then to return in 30 days for his two other injections following the treatment guidelines that you discussed in a fast kill method. This dog is very unique and I know everyone says that, he is one of a kind his caring and display of affection, his emotions and especially his intelligence is unreal, even my vet said he is one in 1 million and it’s taking a special interest in him and his treatment. Yesterday after his heartworm preventative he was in a coma toast state not wanting to drink any water and just in a daze. This morning the vet had contacted me to check on him I explained how he had been acting since leaving his office and he seemed to think it was more to do with everything that he had to go trough in his office and him never really being to a vet but he wanted me to bring him in today. Once there they gave him an injection of ketalog. I believe that was to treat for infection and also side effect being it cause thirsting beheavor. Now my question is two parts one, he is super active, him and I play 4 to 5 hours a day with his toy, he has a lot of energy, just him having his toy in a crate or even in just a room would that be OK And with the symptoms that he had from the first preventative injection even though he is 90% better today and becoming his regular self plus consuming water now should I be concerned when the actual real treatment starts I have read where you have said the treatment that he received yesterday can be the most harmful to an animal I am just concerned if the medicine that he gets 30 days from now is stronger than the prevention medication received yesterday then what to expect. Due to how he did is the risk enough for him to do the slow kill method. I prefer not to but if the fast kill treatment is a higher risk in his situation then should he go the slow route. Lastly his treatment yesterday was the test rabies vaccine a steroid and nail trim. A very traumatic experience to say the least. Thank you for your post they have been very helpful.

    • If I understand correctly, your Boston had an injection to kill microfilaria yesterday, and your pet’s doctor is also using that injection as his first dose of heartworm preventive. If that’s correct, know that it’s not unusual for reactions to occur when microfilaria die. I’m assuming your veterinarian gave him a corticosteroid injection to stop the reaction. If my understanding is correct so far, know that the process is totally different from the adulticidal injections. Dr. Dillon says that instead of “slow kill” it should be called “no kill,” as the process kills no adult heartworms, it only allows them to live their usual life expectancy, causing damage to the patient’s heart and lungs for the entire time. With your veterinarian’s close attention, proceed with adulticide injections when the time is right, follow his recommendations for activity restriction, and be sure to give his heartworm preventive every month on time for the rest of his life. Thanks for reading

  7. We agreed to foster a heartworm positive, pregnant dog for our local Humane Society. Rescued is our favorite breed, but we’ve never had a positive pup before. She gave birth to 13 puppies in November. Twelve beautiful babies survived and were returned to the Humane Society when they were eight weeks old. We fell in love with the momma and are continuing to foster her and plan to adopt her once she is healthy. She started a 30 day Doxy regimine on December 29 and was spayed on Jan 21. She is scheduled for heartworm treatment on February 4 & 5. We are terrified of the responsibility entrusted to us for her after treatment. She is rambunctious even on a four foot leash. She jumps up on the furniture inside the house: yes, we are those people, our pups sleep where they please! I’ve read several of your heart worm articles and appreciate the valuable information. How worried should we be with a young, energetic dog? We’ll do our best of course, but completely still, that seems almost impossible with this girl.

    • Did you notice that we have a heartworm category? You can access it by scrolling down any page where all of the categories are listed in alphabetical order. There are 32 articles in the heartworm category and one of them discusses the need for activity restriction. A little jumping on the couch doesn’t affect cardiac output, an increase of which is required for thromboembolism. If worse comes to worse, you can always simply keep a leash on her. It’s “only” three months, and could be lifesaving. Thank you for doing this for what you’ve described to be an incredibly sweet little dog.

  8. Thank you for this great information. I am considering adopting a heartworm positive great dane mix from local kill shelter. I have had several rescue dogs before, but never one heartworm positive. I was trying to find out as much as I could to make sure I could commit to his treatment. Thank you!

  9. Thank you so much for this article. We live in the Caribbean and rescued a dog with heartworm. We have had her for 2 years and treated her with the slow kill to no avail. We are bringing her in today to get her first fast-kill shot and it’s scary. Our last dog was also a rescued heartworm positive dog. Unfortunately she was pretty advanced when we found her. We gave her a wonderful life and treated her with the slow-kill method, but she died after only a year with us from a severe pulmonary embolism. I can’t recommend enough for people to practice preventative methods for heartworm. No animal should have to suffer like that. Thank you again for your very helpul article. It made us more positive we are doing the right thing for her.

    • We rescued a 3 year old Great Dane from Mississippi three months ago. Heartworm positive, of course. We’ve done the doxy, and she had her first shot of Diroban on Friday. She’s doing really well, and we keep looking at NewYears day as the start of her “new year” full of running around like crazy. I think it’d be harder if she were a herding breed or a terrier, but she likes her rest, so we’re hopeful. Fingers crossed for your pup!

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