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7 Symptoms of Heartworm in Dogs

Heartworm symptoms in dogs can take months to appear. Here are some of the common warning signs.

symptoms of heartworm in dogs
Heartworm symptoms may take more than 6 months to appear. Photo: Josh

Heartworm disease in dogs is a silent killer that is often overlooked until it’s too late.

This preventable condition starts with a mosquito bite and ends with either a long, expensive treatment or death.

Symptoms may not appear until the worm reaches maturity (usually after around 6 months).

7 Signs of Heartworm in Dogs

1. A soft, dry cough

The parasites make their way to the lungs and start multiplying in the lungs and surrounding veins.

Coughing may be most noticeable after exercise and may end with fainting. Even light exercise can cause fainting.

2. Inactivity or lethargy

Your pet suddenly seems tired more often, doesn’t want to go outside, or avoids all physical activity.

Dogs with heartworm infection are weakened and find it difficult to be active even when performing small tasks.

3. Weight loss or anorexia

Even minor physical activities, such as eating, can become difficult and exhausting chores.

4. Rapid or difficult breathing

Along with coughing, breathing problems occur when the worms inhabit the lungs and surrounding veins.

Fluid can also build around the blood vessels in the lungs, making it difficult for the lungs to oxygenate the blood.

5. Bulging chest

The ribs may seem to protrude, and the chest has a bulging appearance as a result of adult heartworm infection.

This symptom can result from weight loss and anorexia caused by heartworms. This can also be caused by fluid buildup in response to the parasite’s presence.

6. Allergic reaction

Although allergic reaction is more common in cats, it is possible for dogs to show symptoms similar to an allergic reaction or asthmatic symptoms in response to the heartworms or their offspring.

7. Collapse

Large numbers of heartworms invade the heart and cause blockage of blood flow (known as caval or vena cava syndrome).

Collapse is usually accompanied by shock and red blood cell destruction. Death can follow within days.

Bottle with heartworm, extracted during surgery. By: mvatrabu
Bottle with heartworm, extracted during surgery. Photo: mvatrabu

Other heartworm symptoms are possible, too:

Seizures, lameness and blindness occur when the parasites get lost and end up in places other than the heart or lungs. They can end up in the brain or eyes, although this is rare.

Some of the symptoms above can also be signs of other conditions, which makes it more difficult to detect heartworm infection.

There are other tools used by vets to detect the condition more accurately.

In the video below, Dr. Courtney Campbell, DVM, explains even more about the signs of heartworm in dogs:

YouTube player


Blood tests to detect the various stages of infection can include viewing a blood sample under a microscope and checking for antigens of adult heartworms (a protein the heartworms produce and also the most commonly used test, according to VCA Animal Hospitals).

Other blood tests can check for abnormalities in complete blood counts and evaluate the level of function of the internal organs:

  • X-rays may detect inflammation, enlargement, or swelling of the heart, lungs and the large artery leading to the lungs from the heart (pulmonary artery).
  • Abnormal heart rhythms and enlarged heart chambers can be detected by an electrocardiogram.
  • An ultrasound of the heart (echocardiography) can help determine the health of the heart for treatment and offer a visual of any existing heartworms.

These tests are also used to determine the health status of the internal organs. This is required to evaluate whether the dog is healthy enough for treatment.


Treatment begins by eliminating the adult worms and takes about 1 month to complete.

The dead adult worms are then absorbed by the body.

After this stage, the next treatment gets rid of the younger parasites and offspring. In extreme cases, surgical removal of heartworms from the internal organs may be necessary.

Does My Dog Need a Heartworm Test?

Credit: American Heartworm Society
Protect your pet all year long, and test every 12 months. Graphic: American Heartworm Society

10 Fun and Not-So-Fun Facts About Heartworm

Here are some quick facts about this awful parasite:

1. In endemic areas, such as the South, almost 1 out of every 2 dogs will get heartworm if they are not on prevention medicine.

The risk goes up the warmer the climate and the higher the prevalence of mosquitoes.

2. Heartworm is present in all 50 states.

This map of U.S. cases shows that no state is immune to heartworms:

Heartworm incidence in the United States, 2016. Graphic: American Heartworm Society

3. Heartworm is a real threat if your dog is not on preventive.

The term “heartworm” doesn’t mean a lot to some folks.

These people tell vets that they don’t believe in heartworms. Like fake news, they think the disease is a fake or minor disease made up by vets to make money.

This is not only insulting but misinformed. Vets don’t like to treat heartworm — they want to prevent it. Period.

4. Heartworm preventives may not work like you think they do.

Heartworm prevention medication works to kill all stages of an early infection from the previous month.

Dogs in warm and temperate climates are at risk all year round. But in New England, for example, most mosquitoes usually leave with the Halloween candy, and many people stop giving heartworm preventive too early.

Say you live in a colder climate and you stop giving medication on Oct. 1, and your dog gets bitten by a Halloween mosquito. Your dog will develop heartworm. Had you kept the dog on preventive through the winter months, this wouldn’t be a problem.

With temperatures trending upward nowadays, a few tough mosquitoes hang around for Thanksgiving and Christmas anyway, so most dogs should be kept on heartworm prevention year round — even in New England.

Quick takeaways:

  • Give your dog a monthly preventive every month and on time to get the most effective protection against heartworm.
  • Try to keep your dog or cat inside in the early morning and at dusk, and away from standing water (mosquito breeding grounds).
  • Fix torn window screens, close doors and beware of lighting that can invite mosquitoes into your home. Do everything you can to keep mosquitoes out of the house.

Click here to see the life cycle of heartworm once a dog or cat has been infected by a mosquito.

Parasite prevention is still an important part of your pet’s health regimen. Photo: SkitterPhoto

5. Most heartworm preventives also give your dog extra protection against other parasites.

We used to see so many cases of roundworms and hookworms in dog poop. Now, these pesky diarrhea-causing parasite eggs in fecal samples are rare, thanks to heartworm medication.

6. The earlier the infection, the better the prognosis.

This is true even though a young dog can have a serious infection if the environment has been ripe for heartworms to multiply like crazy. This is why your vet will want to “stage” a heartworm infection with bloodwork and X-rays.

7. The slow-kill method has drawbacks.

Information is rampant in the dog world that if you just keep your heartworm-positive dog on preventive, the infection will go away. This is the “slow-kill” method. Beware of this information.

It can take up to 2 years for the adult heartworms to die off in your dog’s body.

During that time, the worms are still circulating and causing damage to your dog’s heart and pulmonary function. This could cause unnecessary damage and debilitation.

8. Resistance is developing.

Heartworm infection and tick-borne diseases are on the rise across the country. Parasites are getting more sinister. It’s getting harder to control and prevent these infections.

Using heartworm preventive as treatment has been a life-saver for many stray dogs in the South who have been helped out by rescue groups. But this has caused resistant strains of heartworm. This stinks!

In the Mississippi Delta, where heartworms are in paradise and many heartworm-positive dogs are being treated by this slow-kill method for financial reasons, there are new strains of heartworm arising that are resistant to the preventive drugs we have now. When parasites get crafty, they learn to survive.

Luckily, heartworm resistance is still very rare:

  • The vast majority of dogs who take a monthly heartworm preventive like Heartgard or Interceptor will not develop a heartworm infection.
  • If you also protect your dog from getting bitten by the mosquito in the first place, your dog is doubly protected. A product such as Advantage Multi offers this double protection.

But, unfortunately, more drug resistance is likely to occur. Usually, when we begin to document a handful of resistant cases, there are more cases on the horizon.

9. Treatment is scary.

There is risk, but we know much more about the treatment and its risks now that protocols have been greatly improved:

  • We have your dog on protective medications before, during and after the treatments.
  • We insist on complete rest during the treatment period
  • And we often use a protocol that breaks up the injections into 3 separate doses.

If you follow directions, the risks are minimal.

10. Treatment is definitely not cheap, but there are ways to save money.

OK, it can be expensive, but the treatment protocol does give you time to budget, if that is possible.

Here’s our advice if the cost of heartworm treatment is a difficult issue for you, or if you think your estimate is exorbitant:

  • Do some price-comparing at different hospitals. There may be more reasonable pricing around.
  • Keep in mind that treatment is spread over several months, so you won’t get a huge whammy all at once.

How much does dog heartworm treatment cost?

From a recent surgery published in Clinician’s Brief, the heartworm staging ran $150–$500, and the medication given in a series of injections was $200–$700.

So, the total cost of heartworm treatment is in the range of $350–$1,200.

Final Thoughts

This condition is preventable.

Monthly heartworm prevention is not a waste of money — and it’s not “unnecessary.”

Treatment is out there and can restore the vast majority of heartworm-positive dogs to a happy and healthy life.

If your pet is not taking a heartworm preventive, please talk with your vet about getting started. And if you notice some of the above signs of heartworm in dogs, see your vet right away to get a correct diagnosis.


vet-cross60pThis pet health content was written by a team of writers including Dr. Debora Lichtenberg, VMD. It was reviewed for accuracy by Dr. Pippa Elliott, BVMS, MRCVS, and was last reviewed and updated Feb. 19, 2019.

If you have questions or concerns, call your vet, who is best equipped to ensure the health and well-being of your pet. This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis or treatment. See additional information.