Q: Dr. Habermann, We recently adopted a pet from a rescue group and he has been diagnosed with heartworm disease. They recommended, and we are currently doing, the “slow kill” method of treatment for this parasite. Is this a good idea? 


A: ABSOLUTELY NOT. This topic is becoming a problem.I have three patients adopted from rescues in the last two months that had a diagnosis of heartworms. Frustrating to me, all three, when adopted, were being treated with the “slow kill” method recommended by the staff veterinarian of the rescue.

Let's start with The American Heartworm Society (AHS), the go-to organization for all things heartworm. This organization does not recommend or advocate for this method of treatment. The “slow kill” method of treatment places the patient on an antibiotic (Doxycycline), a medication known to kill a bacterium that helps the heartworms survive. By killing this bacterium, the parasites aren't as strong and are more prone to dying. The other component is to have the dog take a monthly heartworm preventative. This is used to kill any microfilaria (baby heartworms), prevent future heartworm infestation, and slowly (hopefully) kill the adult worms present in the heart and lungs.

At face value, this sounds good, right? But, in practice, when you analyze the science behind it, not so much. First, this process can take many months and, more often than not, years. That means the worms are crawling around heart and lungs doing their damage over the course of this time. Many dogs that succumb to heartworm disease die from damage the worms do to the lining of the vessels and the heart chambers in which they live.

Second, dogs that have heartworm disease should not have any exertional activity whatsoever. Increased heart rate and pumping of blood can cause movement and breaking apart of the worms, leading to a worm thrombus. For the time it takes for these worms to, perhaps, die (and remember, that is many months to years), the pets must be exercise-restricted.

Third, this method is feared to lead to “heartworm preventative resistance”—not unlike antibiotic resistance you hear so much about. This treatment approach can lead to selecting for worms not “killed” by the commonly used monthly preventatives.

So where are the benefits? Far and away, the primary justification is: “It's cheaper.” When I practice medicine and take care of my patients, “best medicine” is the approach, not cheapest medicine. The other argument (not valid in my opinion) is safety. Yes, conventional heartworm treatment (a thorough workup prior with bloodwork, X-rays and urinalysis) with immiticide injections has some risk. However, with 30 years of practice and hundreds of treatments under my belt, I've seen only two bad outcomes. One dog was old as dirt and dying from his heartworm disease, leaving no other choice but to treat, and the other dog was not properly exercise-restricted after treatment and, while chasing a UPS truck, fell over dead from a worm embolism.

As you can tell from the tone of this article, I'm passionate about this “slow kill” method not being used as a treatment unless there's absolutely, positively, no other options. Prior to writing this article, I consulted with two board-certified cardiologists and the most current American Heartworm Association standards to confirm this treatment protocol is not recommended for the vast majority of animals that have heartworm disease.


Direct questions about this topic or any other you would like Dr. Habermann to address in future articles to foothillsvet@windstream.net or 706-216-1356.