Talking About Heartworm: We know preventives are important, but do our clients know as well?

Talking About Heartworm: We know preventives are important, but do our clients know as well? 

 Andrew R. Moorhead, DVM, MS, Ph.D., Dip. ACVM 

Heartworm infection can be life threatening. As with many parasites, an ounce of prevention is worth a pound of cure. Due to the pathologic changes that occur due to heartworm infection, as well as the associated cost of treatment, prevention of heartworm is a necessity, not a luxury. Convincing clients of this necessity in the age of the internet, coupled with reports of lack of efficacy, has created challenges that were not present when preventatives first came on the market. It truly has become a brave new world. 

As a note, the majority of the information referenced in these proceedings can be located at The American Heartworm Society

The life cycle

Before we can discuss the challenges associated with heartworm prevention, we must first discuss how preventives function in relation to the life cycle. 

The most logical place to start is with the adult worms, which live in the pulmonary arteries and heart. Adults can live up to 5-7 years in dogs. During this time, adults will mate and produce microfilariae, which circulate in the blood. The microfilariae are then ingested by a mosquito intermediate host. The mosquito is ABSOLUTELY REQUIRED for the heartworm to develop. Once inside the mosquito, the microfilariae migrate within the mosquito for an average period of approximately 10-14 days, during which time, the microfilariae become first-stage larvae. After this, they molt twice to become an infectious third-stage larvae (L3), which at this point is in the mouthparts of the mosquito. The L3s are approximately one mm in length. Upon landing on a host, the mosquito will then take a bloodmeal. During feeding, the larvae will “bust out” of the fleshy mouthparts. When the mosquito removes its stylet, the larvae then enters the host through the hole made by the stylet. This is in contrast to the commonly-held belief that the larvae are “injected” into the host. 

Once inside the definitive host, the L3s follow a complicated migration pathway. The L3 remains in the vicinity of the sight of entrance for approximately 3-4 days. Also during this time, L3 undergoes a molt to the fourth-stage larvae (L4). This is normally completed by 4 days post-infection, but could be complete until day 12 post-infection. This is the most important part of the life cycle with regards to prevention. Heartworm preventives, as a rule, kill the migrating stages of heartworm acquired during the previous month. They do not kill “forward.” That is, the the majority of monthly administered products do not work by killing infective L3s acquired after administration of the preventive with. For 6- and 12- month injectable moxidectin, obviously the drug is continually on board, so it acts on the L3/L4s at some point after they enter the host. 

It is generally agreed that the molt to the last stage, known as the juvenile adult, occurs by day 58. At day 70, the worms first arrive at their final location, the pulmonary artery. Most worms have reached this location by day 120. By day 180, worms are sexually mature and have begun to produce microfilariae, thus completing the life cycle. 

 A brief history of the evolution of heartworm preventives 

(Disclaimer: Mention or lack thereof of a particular product does not constitute bias on the author’s part. At this point, it is almost impossible to mention every domestic small animal parasiticide on the market.) 

Initially, heartworm preventives were administered daily and contained diethylcarbamazine (DEC). The most well-known of these was Filaribits®, which many of us still remember (although, there are more and more blank stares when I mention this drug to veterinary students). In the mid-1980’s, preventives underwent a monumental change that has shifted the industry forever. This is when the first ivermectin-based monthly preventive was launched. A monthly preventive can kill migrating L3s and L4s, thus giving it a distinct advantage over a daily dose of DEC, which is effective at a distinct point in the life cycle. With monthly prevention, owners had less “pills” to remember, and veterinarians no longer had to stock boxes and boxes of Filaribits® bottles in their practice bathrooms and offices. The industry has not looked back, since the advent of the monthly preventive. 

Macrocyclic lactone drugs contain all currently labeled heartworm preventives. These drugs are administered monthly with one exception discussed below. 

Ivermectin is in the macrocyclic lactone class of drugs.Ivermectin is normally combined with pyrantel in such products as Heartgard Plus® (Boehringer-Ingelheim), Tri-heart Plus® (Merck Animal Health), and Iverhart Plus® (Virbac). This combination of ivermectin and pyrantel was the start of another important shift in the industry: the combination product. Now with ivermectin and pyrantel, one could “prevent” heartworm, as well as certain intestinal parasites, specifically hookworm and roundworm. 

Next on the market was Interceptor® (Novartis Animal Health), which contained milbemycin oxime. Milbemycin oxime had a different spectrum of coverage compared to the ivermectin/pyrantel-based drugs in that it was also labeled for whipworms. Interceptor would later be combined with the flea preventive lufenuron to create Sentinel®. 

Next to market was Revolution® (Zoetis), which contains selamectin. Revolution® was unique at the time because it was a topical versus oral heartworm preventive. While anecdotally, most clients prefer to give their dog a pill, some do prefer administering drugs topically. Also, Revolution® provided cat owners a much easier route to administer a heartworm/intestinal preventive to their pet. 

Another topical, Advantage Multi® (Bayer Animal Health) would come to market, thereafter. Advantage Multi® contains moxidectin and the flea preventive, imidacloprid, which is in the flea preventive Advantage®. Advantage Multi®, like Interceptor®/Sentinel®, will treat and prevent whipworm, and like Revolution® is topical. A few years after that, Trifexis® (Elanco) hit the market. This combination of milbemycin oxime and spinosad (aka Comfortis®), a flea adulticide, took the market by storm with ever increasing sales. The market has again changed with the advent of the oral insecticidal/acaricidal isoxazolines. These products (Bravecto®, Credilio®, Nexgard®, Simparica®, please note that these were listed in alphabetical order) seemed to have become the product of choice for flea and tick control. Earlier this year, Simparica Trio® was launched which combines moxidectin, pyrantel, and sarolaner. This was the first to market product that covered some gastrointestinal parasites, fleas, ticks and heartworms. There will likely be more “all-in-one” products to follow. 

But wait, you forgot one… 

ProHeart® 6 and 12 are injectable moxidectin products and offers the non-compliant client a means to protect their pet from heartworm for 6 and 12 months, respectively. These products are also labeled for the “treatment of existing larval and adult hookworm (Ancylostoma caninum and Uncinaria stenocephala) infections.” These products require relatively less inventory space on your shelf when compared to its monthly competitors. 

A brave new world… 

There are many perceived problems surrounding parasite prevention. I cannot begin to detail every one. However, I have listed the most important and pressing issues. 

  1. Internet Pharmacies: This debate could and has dominated hour-long roundtable discussions. As cost has become more of an issue in the eyes of the client, they have turned to these “cheaper” sources. Another problem, product diversion, allows these pharmacies to thrive. The “grey” market, which buys products from veterinarians and then sells them to these pharmacies, is one of the central culprit. 
  2. “I read on the internet…”: Clients increasingly turn to the internet in order to obtain veterinary advice. While some sites are reputable, many are not. Unfortunately, clients cannot always tell the difference. This could lead to use of internet pharmacies and unproven herbal remedies. We need to guide the clients to reputable sites. 
  3. Cost: With more limited income, clients have to make tough choices between preventives and other necessities. 
  4. Lack of efficacy cases: Reports of suspected lack of efficacies, which cannot be attributed to non-compliance or a failure to administer preventives correctly, has raised doubts among some clients and veterinarians about these products. 
  5. Not communicating the value of prevention to the client: As veterinarians, we need to send a consistent message of compliance starting with the front desk, then the technician, and finally the veterinarian. We need to emphasize not only the importance of prevention, but the importance of the veterinarian’s role in tailoring a parasite prevention program to the individual client. The internet will never be able to accomplish this task. 

Improving Adherence 

We can combat the lack of adherence of clients about heartworm prevention by emphasizing the real need for prevention and the consequences to the pet when we do not. 

Giving the heartworm talk 

The bottom line is that clients cannot comply if they do not know they should. When we talk to clients we should ideally do the following: 

  1. Give consistent, clear, and concise recommendations 
  2. Focus on what is best for the pet 
  3. Highlight why owners need to adhere to your recommendation. 

When highlighting the need for prevention, one should emphasize 3 to 5 main points. Any more than that and the client may “tune out.” Such points may include: 

  1. Stress that heartworm disease can be deadly, 
  2. Heartworm is carried by mosquitoes, 
  3. Heartworm is preventable, but it is expensive to treat. 


The dental industry has led the way with automated reminders via email, text, etc. Some ways you can leverage this to improve adherence are as follows: 

  1. Send text reminders at the beginning of the month to remind clients about administering preventives. 
  2. Set up appointments for heartworm testing before the client leaves. 
  3. Post links about heartworm and other parasites on your social media platforms. 
  4. Have a board out in front of your clinic that lets clients know how many heartworm cases that have been diagnosed at your clinic. This will help emphasize that the risk of heartworm is real. 


While there are many challenges ahead of the profession with regards to parasite prevention, I believe that we can address these issues by emphasizing the value of these products to the client, and the benefits to the health of their pets.