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Heartworm Symptoms - The Four Stages of Heartworm in Dogs

Does Your Dog Have Heartworm? Track the 4 Stages of Heartworm Symptoms

By April 25, 2012 | See Comments

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    PetCareRx Staff Veterinarian


Dog with heartworm symptoms

Heartworm symptoms can be subtle and tricky to detect. Learn more about heartworm symptoms here.


While potentially fatal if untreated, heartworm symptoms can be initially quite mild. Early in the disease’s progression, there may not be any symptoms present at all, which is part of what makes heartworm such a difficult disease to diagnose. It is because of these reasons that monthly treatment with a preventative, like Heartgard for dogs, Iverhart Max, or Trifexis is so important.

As with cancer, heartworm disease is divided up into four stages. The mild cough of stage one or two progresses into a persistent cough by stages three and four. Without treatment, the symptoms become increasingly disruptive and severe, and the impact on a dog’s lungs and heart is negative. Your dog’s stage of heartworm disease – as well as other considerations like age and health – will play a major role in determining your veterinarian’s approach to treatment.

The primary observable heartworm symptoms are a cough that just won’t seem to go away, difficulties breathing, being tired after exercise, and at a certain point, a reluctance to exercise at all. Below, see how symptoms match to each of the progressively more entrenched stages of the disease.

Stage 1

During the first stage of heartworm disease, dogs generally will not have any symptoms at all, although you may notice a slight cough. This is a very mild stage, and dogs will appear happy and healthy. Even with a physical, this disease is difficult to detect, and blood tests may come back with a negative result for heartworm disease.

Stage 2

Stage two of heartworm disease is accompanied by moderate symptoms such as a lingering cough or fatigue after exercise. The symptoms may now be advanced enough for you to detect them. During this phase, the impact of the heartworms on a dog may show in tests.

Stage 3

By stage three, the disease is having a real impact on your dog’s health, and the heartworm symptoms are correspondingly more severe and noticeable. Dogs will continue to cough, will experience fatigue after exercise, may be reluctant to exercise, and can have trouble breathing. During this stage, dogs may also cough up blood. By stage three, the disease is quite evident on x-rays.

Stage 4

Dogs in stage four of the disease have very visible heartworm symptoms, which are accompanied by long-term implications for the dog’s health. As in other stages, dogs will be reluctant to exercise, tired after exercising, and will exhibit a cough. Dogs will probably experience trouble breathing as well. Testing may reveal the impact of the disease in the form of abnormal lung sounds within the dog’s lungs, an enlarged liver, and heart noises. Untreated, this stage of heartworm disease can lead to death.

At each stage of the disease, treatment methods become increasingly more severe and invasive. Be mindful of your dog’s typical behavior, so that potential symptoms like a reluctance to exercise or extended lethargy don't get overlooked. Taking heartworm medication has a good success rate at keeping larval infections from maturing into adult heartworms, especially when augmented by annual heartworm tests to detect signs of the disease.

How PetPlus Can Help

If your dog is taking heartworm medication, or anything else, PetPlus can help make buying their medications much more affordable. With discounts on brand name heartworm medications like Heartgard, Tri-Heart, and Iverhart Max as high as 47% off, PetPlus can save pet parents a considerable amount of money.

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by,your veterinarian with respect to your pet. It has, however, been verified by a licensed veterinarian for accuracy.


Heartworms are not spread to other dogs. They get them from mosquitoes. Google it or ask your vet. Heartworms are only spread through mosquito bites, and it cannot bite a dog and transfer it due to the amount of time it incubates the parasite.


If your dog was diagnosed 5 years ago, and you have been giving monthly heartworm preventative since then, your dog probably no longer has heartworms. You didn't let 'nature take its course', you treated your dog with the "slow kill" method. The HW preventative kills all of the eggs and does not allow any more to be produced and it also greatly shortens the adults life span. Generally about 2 years of treatment has completely removed all of the worms, although the damage is still there. Not treating them at all would be considered letting nature take its course.


I Understand the situation you're in with the age of your furbaby, and especially the severity of the drugs to treat heartworm. However this isn't helping to control the spread of heartworm to healthy dogs!


I have just had two dogs tested and it has come back positive, I am waiting for the vet to contact me to see what is to be done. I recently lost a podenco to heartworm which is why the other two have been tested. They are from Spain.


Liz, my dog also has heartworm disease and treating heartworm at certain ages is very painful in of itself for dogs that are older. The medications used make dogs very sick, just like in cancer treatments..not to mention thousands of dollars that many people cannot afford. If your dog is already far into age, it's actually more humane to not treat them with those meds so they can at least live the rest of their lives out in peace. I still give him heartworm guard to prevent new ones...and lots of love. So, when you say stuff like that, you need to understand the scope of things.


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my dog is exsibiting sighns of heartworms I don't know for sure If that Is what the problem Is or not but he Is having trouble breathing exspecially after exercise but then again It Is summer also It looks like he is loosing a little weight I don't know what to think so I am taking him to the vet but I don't want them charging me way over what I really should pay!!!

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