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. Sharon White says:

    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.

  2. Angela says:

    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!

  3. Isabella says:

    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.

    • Thank you for your supportive story. I’m saying a prayer for your baby to have an easy and successful treatment. Remember, your first line of defense against embolism is restriction of activity. Click here to read more. Thanks for reading, Dr. Randolph.

    • Mary Lanese says:

      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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