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. Thank you for this article. We adopted a dog from Texas who came to us heartworm positive. There was a lot of discussion about which treatment method we should use, with the Rescue advocating, slow kill, and my vet advocating fast kill. After doing some research on them and conferring with our vet, we chose the fast kill method. Our vet told us that based on current rates, slow actually wasn’t much cheaper than the standard treatment (which is the prime selling point for rescues) but was more likely to result in our dog experiencing more damage from the worms. Our little guy went through the treatments successfully and will be rechecked in January. We’re hoping that things are smooth sailing from there.

  2. Hi,
    I have a foster dog that I can’t make medical decisions on. The rescue says they don’t do fast kill. My foster dog is 5 and is on slow kill doxycycline and hesrtworm meds every other week. I don’t see anything about this method and its effectiveness in your article. Can you give any information about this plan and how long they need to be on doxycycline

    • I apologize for the delay. I thought I’d already responded to your question. Look at Dr. Dillon’s slide on this page; it says “NO SLOW KILL.” He says “slow kill” should be called “no kill,” it simply allows the heartworms to die of old age while cardiopulmonary damage continues. WHY does the shelter not perform adulticide therapy? Will they “allow” you to take YOUR dog to a private practice for proper heartworm treatment? If you want to read more, look at the right side of each page; we have a Heartworm Category, in which I’ve created a collection of all of the heartworm-related articles for you. Read one, click on the BACK button on your browser and continue up or down the list. Thanks for reading, Dr. Randolph.

  3. Dr. Randolph,

    Thank ypu for the informative article. I am unable to find any research (if it exists) on the efficacy of the “slow kill” method with the 2 shot method. Do you have any insight?
    My adopted dog will be getting the 2 shot method from the shelter we adopted from but he came down with distemper shortly after adoption and shots had to be delayed. In the meantime we have kept this “strong positive ” little guy on Advantage Multi. It’s been 6 months and now we will do the shots. I am very hopeful the 2 shot method with Advantage Multi will be enough to kill all of the worms.

    • Two injections are NOT recommended by the American Heartworm Society. They recommend, and we follow, the three-injection protocol ONLY. Maintain Advantage Multi (or switch to ProHeart6 or ProHeart12 plus Simparica, you’ll get equally good heartworm prevention AND substantially better flea and tick control). Be sure ALL of the American Heartworm Society guidelines are followed. Thanks for the update, Dr. Randolph.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.