Signs Your Dog has Heartworm and What to Do About It

By now we’ve all been made aware of the risks related to heartworm. Recently, we realized that some information on heartworm was biased or incomplete. So, we decided to take a deeper dive into exactly what heartworm is, after one of our own dogs was diagnosed and we were left with some unanswered questions ourselves. Luckily, we have a close network of incredible veterinarians here and throughout the U.S. that helped to create an individualized plan. We wanted to share some of what we learned through our experience as well as touch on why we see an increasing prevalence of heartworm in the Northeast US and what we can do about it.Heartworm is a mosquito-borne illness caused by Dirofilaria immitis. The Dirofilaria immitis or “heartworm” is a parasite which is described as foot-long worms. These worms reside in the heart, lungs and blood vessels, most commonly in our beloved dogs. But why do they choose to mature and replicate in our furry friends? Well, dogs are considered a “natural host,” which makes growing and living quite easy.1 A dog’s body is the perfect climate for the heartworm to live. Although heartworm is most common in dogs, it can also be found in cats—but it is quite rare. Contrary to popular belief, cats are not natural hosts for these worms, and the worms often don’t mature as they do in dogs.1 Cats could have heartworm, but only have 1-3 worms in their system. This definitely makes detection difficult.

Heartworm: How the infection happens

When a mosquito bites a heartworm-infected animal, the mosquito ingests larvae, or immature worms. After some maturation within the mosquito, the mosquito is able to deposit the larvae into the skin of another animal. The larvae then make their way into the subcutaneous tissue (or fatty/connective tissue layer) which is deeper within the skin. The subcutaneous tissue has a bunch of small blood vessels that lead to larger blood vessels. The larvae travel through the blood vessels and eventually make their way to the vessels within the lungs. Here, at about 6 months after the mosquito bite, they are able to reproduce microscopic larvae, called “microfilaria.” 1 The microfilaria, since so small, are able to travel through the heart and through the blood stream. The microfilariae are always present in the blood once adult worms are mature enough to reproduce. They will continue to be present as long as the worms are still reproducing.5 The immature worms that reside in the smaller blood vessels cause inflammation and thickening of the blood vessel walls. As these worms grow, they are unable to fit through the smaller vessels and are forced into the larger vessels, which are the main arteries in the dog’s lungs. This is where complications can arise, and where dogs can start showing symptoms.

Signs: What do I look for?

Symptoms and signs of heartworm in dogs can vary in severity. In the early stages of heartworm infection, dogs may show minimal to no symptoms at all. As the infection progresses, the symptoms typically become more severe and non-specific. Dogs with preexisting health conditions or dogs who are heavily infected with worms are more likely to show symptoms.  Most commonly, dogs will develop a cough, reluctancy to exercise, loss of appetite, and increasing fatigue. As the disease continues to advance, dogs can develop fluid overload secondary to heart failure.5

How is heartworm infection diagnosed?

 Due to microfilaria taking approximately 6 months to manifest, heartworm testing is typically done at an annual exam by your vet. Unfortunately, this means that usually our rescues aren’t tested if they are under 1 year, and even if they are, they may not test positive until at least 6-8 months of age. This is unfortunate because dogs are often rescued from southern U.S.—where mosquitos and heartworm are more prevalent. Rescue dogs are occasionally given heartworm preventative medication, but this doesn’t mean they are guaranteed heartworm free.3 That said, it is our responsibility, as dog parents, to ensure our pets are tested appropriately.


Heartworm testing is done by blood sampling. The first test performed is typically testing for antigens. Antigens are proteins that are released into the dog’s bloodstream by female heartworms. These commercial tests are very specific, but the accuracy of this test is based on how many female worms are present within the dog’s body. The rapid, in-clinic antigen testing that is performed can detect “46%-76.2% of patients infected with a single female worm, and 84%-100% of patients with 3 or more female worms.”2 This means that the more worms present, the more accurate the antigen test is. However, it is recommended that both antigen and microfilariae testing are performed to confirm the diagnosis. The microfilariae can also be detected via blood sampling.

But why exercise restriction?

Personally, when I first heard the term ‘exercise restriction’ I was crushed. Immediately, I began to think about our usual long hikes and lengthy play dates were going to be a thing of the past. More importantly, I began thinking about how we were possibly going to burn all that pent-up energy. As I explored treatment options, I learned this doesn’t have to be the case. Instead, we chose a treatment option that allows for monitored exercise—so maybe we’ll just skip the weighted pack on our walks.


There are multiple treatment options available for heartworm infection, and it is important to weigh these options with your vet. The protocol is decided based on the severity of the disease, whether the dog has a pre-existing condition, and cost for pet parents. Heartworm treatment can be expensive and both mentally and physically taxing on you and your dog. It is recommended by the American Heartworm Society (AHS) to begin restricting exercise once the diagnosis is confirmed, regardless of which protocol is followed. However, not all veterinarians may recommend exercise restriction—this is based on the treatment option, your pet’s individual situation and health status. It is important to consult with your vet about what they think is best.


When exercise restriction is recommended, it is because it may increase the rate at which the heartworms damage the heart and lung tissue. The AHS makes a blanket strict exercise restriction recommendation that means no short walks, full crate rest during treatment, and leashed potty breaks. That said, AHS also states, “the more severe the symptoms, the less activity your dog should have.”1 In other words, a heartworm diagnosis doesn’t necessarily mean your dog must live in a crate for the duration of treatment.

Heartworm Treatment: The traditional ways

Upon a heartworm diagnosis, it is important to confirm your dog is in tip-top shape before beginning treatment. Usually, initial bloodwork will be performed which includes assessing the dog’s immune system, electrolytes, and liver function. Basically, baseline lab values are checked to ensure the dog can handle whatever treatment prescribed.


There are two types of traditional treatments—a series of injections or the use of a topical medication. The injection, although costly, is highly recommended as it has a 95% success rate, according to the American Heartworm Society. The medication, called Melarsomine, “is an arsenic-containing drug that is FDA-approved to kill adult heartworms in dogs.”4 This medication is injected into the deep tissue of a dog’s gluteal muscles, near their tail, on either side of their spine. The injection is often painful and can require pain medication and occasionally an overnight stay at the vet. The course is typically an injection, followed by 30 days of rest, another injection, then 24 hours later, the last injection. After the last injection, there is 4-8 weeks of continued exercise restriction, prior to being retested to see if the pup is still heartworm positive. Alongside the injections, an antibiotic (doxycycline) and a steroid (prednisone) are administered. Sounds exhausting on the body, right?


The other treatment option is known as “the slow kill” method. This is frequently the treatment of choice for shelters due to its price point. This option is less expensive than the injections—as the injections require frequent visits to the vet for check-ups. The slow-kill method uses a heartworm preventative medication over many months. This method also involves the use of doxycycline. This option is not highly recommended as it does not kill all life stages, it only prevents maturation of microfilariae.


Therefore, the most recommended traditional route, according to AHS, is the use of Melarsomine, in conjunction with an antibiotic and steroid, as prescribed by your vet. However, a potentially fatal complication of the use of the injections is the chance for blood vessel blockages. Said differently, as the treatment progresses the worms die and break up, they can block some of the various blood vessels and cause pulmonary emboli (blood clots in the lung). This is the reason AHS recommends strict exercise restriction and crate rest with this option.

Are ‘natural’ treatments an option?

My first thought as a pet parent— “Do I REALLY need to give our dog all these medications?” This was bothersome to me because, to us, Susie isn’t sick. She has plenty of energy, no coughing, and has a beautiful black coat. We wanted to find a way to kill the heartworms without compromising Susie’s loving (and sometimes annoying) personality. We didn’t want her to be in pain.


Like humans, a healthy diet generally supports a strong immune system and it’s important to consider this when choosing a traditional verses holistic treatment. As far as ‘natural’ treatments go, many have caught a bad name – and rightfully so. There are multiple blogs and opinions stating that one supplement is the magic bullet, but there is no science to support it. Obviously, there is no magic bullet – but there are alternative options to explore. Some include a combination of traditional and alternative treatments, and some may only include alternative options. In discussing the options with our veterinarians, we know that there is not a specific medication or protocol that all alternative practice veterinarians follow. That said, most have their preferences based on personal experience and most assert they can be successful with compliance, patience and supporting the immune system. Some examples include black walnut—said to expel and weaken worms, alongside the heart, circulatory, and immune supplementation.5


When our dog was diagnosed, we reached out to a vet who had plenty of experience treating dogs with heartworm. She had suggested a specific protocol for us to follow, which includes both traditional medications as well as supplementation that supports the immune system and heart. Our protocol also involves frequent check-ups—mostly to listen to heart and lung sounds to ensure the heartworms are not advancing.

Our best advice? Ask questions.

Questions like:

  1. What are all the options?
  2. What experience do you have with each of these options?
  3. Are there side effects I should be aware of?
  4. What health conditions does my pet have that can complicate treatment or outcome?
  5. How can I keep my pet comfortable?
  6. How and can we implement exercise?
  7. Are there ways I can help support my pet’s immune system?

These are just a few, but it is important to consider these, alongside the treatment options, with your family and your vet. Certain treatments may be beneficial for some dogs, but not other dogs. This depends on a workup that is performed—usually including bloodwork, x-rays, and other heart imaging. Regardless, just like with human health, obtaining a second opinion on treatment, and alternative options is never a bad idea! Believe it or not, most vets are happy to refer out for a second opinion – or offer one!

The MOST important question: WHY?

 Always ask why. Whether it be human medicine or animal medicine, it is important to know what (e.g. supplement, medication) you are giving and WHY. No one should take supplements, medicines, or any treatment without asking what they are for. It is also important to understand the side effects associated with the medications or supplements. If you take nothing else from this let it be the lesson: Never be afraid to ask questions.

Susie has started her treatment and is doing just fine! ?

[vc_single_image image=”4994″]*This article is for informational purposes only. It is not meant to provide medical advice or replace the advice of a qualified veterinarian. If you think your pet has heartworm or any medical condition please seek the advice of a qualified veterinarian.

About the Author:

Michelle Yaglowski

Michelle is a Registered Nurse, holding her bachelor’s degree in Nursing with both Emergency and ICU experience. It goes without saying that she has incredible attention to detail, the ability to see past the obvious and a knack for research. Like many in this industry, she had a sick pet which developed her keen interest in animal nutrition, and her experience in human medicine and the ability to think critically serve her well in this space. Her quest for knowledge drives her to dive into topics that may be considered controversial, or that don’t have much research in animal nutrition. This allows her to provide a unique perspective to other pet owners which also encourages them to ask the tough questions and challenge the status quo. When she is not working in the hospital or researching and contributing to the NPP Journal she can be found spending time with her Dog Susie and cats Stout and Archer. If you have a topic or a question you would like an evidence-based research answer to you can email Michelle here.


  1. org. 2020. Heartworm Basics – American Heartworm Society. [online] Available at: <> [Accessed 12 October 2020].
  2. Little S, Saleh M, Wohltjen M, Nagamori Y. Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance. Parasites & Vectors. 2018;11(1):186. doi:1186/s13071-018-2736-5
  3. Managing Heartworm Disease in Shelter Animals | Today’s Veterinary Practice. Accessed October 13, 2020.
  4. Medicine C for V. Keep the Worms Out of Your Pet’s Heart! The Facts about Heartworm Disease. FDA. Published online July 29, 2020. Accessed October 13, 2020.
  5. Treating Heartworm Holistically – Whole Dog Journal. Accessed October 13, 2020.