Heartworm Treatment Methods

Is the “Slow-Kill” Method of Heartworm Treatment Safer Than Other Methods?


by Lorie Huston, DVM on August 31, 2013

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Recently, I published a post about a major change in the guidelines for treatment of heartworms. In response to this post, one of my readers asked a very good question, one that many of you are probably wondering about. Her question was this:

“The overall question/comment myself and my dog community friends have is this: We’ve been told for years that we need to test before giving preventative, because the preventative could kill a heartworm positive dog. My understanding was that if you kill off too many heartworms at once, you can clog arteries with worms faster than the system can break them down.

So if we move to this fast kill process, doesn’t that risk go up significantly?”

I’d like to answer her question here.

Which method of heartworm treatment is safest for the infected dog?

Is the “Fast-Kill” Heartworm Treatment New?

In actuality, the “fast-kill” process is not new and has always been the recommended treatment for heartworm infections in all but the most debilitated dogs. This method involves treating the infected dog with a medication known as melarsomine to kill the adult heartworms.

The advantage to the melarsomine method of heartworm treatment is that the heartworm infection is eradicated within a much shorter period of time. Once eradicated, the adult worms are no longer present to continue causing damage. With the so-called “slow-kill” method, the dog can remain infected with adult heartworms living in the heart and blood vessels for two years or longer. During this time, the worms can cause damage to the heart and vessels, and that damage is irreversible.

How Does the “Slow-Kill” Method Differ From Melarsomine Treatment?

The “slow-kill” method involves using heartworm preventive medications to kill the microfilaria (larval heartworms in the blood stream) and prevent further infection. The adult heartworms then die off over time, of natural causes, not because the heartworm preventive medication kills them.

Melarsomine treatment, on the other hand, does kill the adult worms, removing them so that they cannot continue to cause damage and disease.

Isn’t the “Fast-Kill” Method More Dangerous Than the “Slow-Kill” Method?

The reaction that heartworm positive dogs may have to heartworm preventive medications is different than that seen with melarsomine treatment, or the so-called “fast-kill” method.

The reaction to heartworm preventive medication is a result of a massive die-off of microfilaria, the larval form of heartworms. This is essentially a shock-type of reaction. It has nothing to do with die-off of adult heartworms.

The complication we most often see associated with melarsomine treatment is lung damage due to the death of the adult heartworms. These dead worms can cause what is known as pulmonary thromboemolism, which means that the blood vessels that supply the lungs become clogged with dead worms or worm fragments, causing damage to the areas of the lung supplied by those vessels.

It is true that there are risks to using the melarsomine (“fast-kill”) method, just as there are risks in using the “slow-kill” method. When we treat heartworms with melarsomine, we minimize the risk in several different ways. We know that pre-treatment with ivermectin and doxycycline reduces the risk of lung damage when melarsomine is administered. The current recommendation (though still somewhat controversial because of the resistance factor) is administering ivermectin-based heartworm prevention for two months prior to beginning treatment with melarsomine. Doxycycline is also administered in the time prior to melarsomine injections and seems to decrease the risk of complications as well. Other medications (like glucocorticoids) may be used to further decrease the risk of lung damage during the melarsomine treatment process. And exercise restriction is essential during this entire period. The more active the dog, the higher the risk of complications.


I know it’s confusing, especially when the terminology “fast-kill” and “slow-kill” is used. I use them because they’re the terms that many pet owners know and use, but these terms aren’t entirely accurate. When we talk about “slow-kill”, we’re really not talking about using medication that kills the adult worms. The heartworm preventives used in the slow-kill method are macrocyclic lactones such as ivermectin. These drugs kill the larval form of the heartworm (microfilaria) and also prevent any further infections from occurring. But the adults survive until they die of natural causes. Essentially, with this method, the dog isn’t cleared of the infection until the adult heartworms have died of what can be thought of as old age. As noted previously, as long as the adult worms are present, they continue to damage the heart and blood vessels. This damage, in itself, can cause significant disease and even death.

The “fast-kill” method, by comparison, kills the adult heartworms which are responsible for most of the damage and symptoms caused by heartworm disease. More specifically, treatment with melarsomine kills the adult heartworms. However, this method of heartworm treatment also incorporates heartworm prevention to prevent further infection and increase the chance of a successful treatment outcome. Treating with heartworm preventives for two months prior to treatment with melarsomine gives the migrating heartworm larvae time to mature to a stage that the melarsomine can kill.

Cost Factors in the Treatment of Heartworm Disease

It’s probably worth pointing out also that melarsomine treatment is quite expensive. So, in situations where finances are an issue, as they are for so many of us, the “slow-kill” method has the advantage of being much more affordable. That fact has, at least in part, led to the “slow-kill” method becoming so popular, for obvious reasons. Of course, we didn’t realize until recently that this method was also contributing to the development of a drug resistant heartworm population.

I hope that clarifies matters and doesn’t just cause more confusion. If you have other questions, please feel free to ask.

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About Lorie Huston, DVM Lorie Huston is an accomplished veterinarian, an award winning blogger, a talented author and a certified veterinary journalist. She is available for writing assignments, blogging and social media consultation, and SEO strategy.

This article is posted and shared through the courtesy of the Pet Health Care Gazette