Heartworms In Dogs – The Basics of the Disease and Prevention

Each year, when you take your dog to the vet for his annual wellness visit, your veterinarian probably recommends heartworm prevention. Hopefully, you follow your vet’s recommendation and get heartworm prevention for your dog, but how much do you really know about the disease that you are preventing?

What is heartworm disease?

Heartworm disease is a serious, potentially-fatal condition in dogs. It can also affect cats (although less frequently than dogs), ferrets, coyotes, foxes, and sea lions… and, in rare cases, even humans.

Heartworm disease is caused by the parasite Dirofilaria immitis, also known as the heartworm or dog heartworm. This parasite is spread by mosquitoes.

When an infected mosquito bites a dog, it injects microscopic heartworm larvae into the dog’s bloodstream. These larvae travel through the dog’s blood and tissues, maturing to their adult stage and eventually lodging in the heart and pulmonary arteries (the arteries traveling from the heart to the lungs). If both male and female mature heartworms are present, they begin to reproduce, releasing new pre-larval heartworms (microfilaria) into the bloodstream. When a mosquito feeds on an infected dog, it ingests these microfilariae and is now infected with heartworms. This infected mosquito capable of passing heartworms to a new dog, completing the heartworm life cycle.

Effects Of Heartworms

Mature heartworms are 5-12 inches in length and can live for up to 7 years within the heart.  As you might expect, these heartworms can have significant damaging effects on the heart and lungs. Most dogs show few signs of illness during the early stages of the disease, but their symptoms worsen over time.

The most common signs of heartworm disease include a chronic or persistent cough, reluctance to exercise, and weight loss. The cough and exercise intolerance are related to the effects of the growing heartworms on the heart and lungs. Weight loss is caused by a decrease in appetite, as the dog begins to feel unwell due to damage within the heart and lungs.

As the disease progresses, the heart may struggle to effectively push blood past the growing mass of heartworms. This leads to signs of heart failure. Dogs in heart failure experience increased difficulty breathing (due to fluid accumulation in the lungs) and a swollen belly (due to fluid accumulation in the abdomen).

Some heartworm-infected dogs experience a life-threatening condition known as caval syndrome. In caval syndrome, the heartworms cause a sudden blockage of blood flow within the heart. These dogs experience sudden collapse, labored breathing, and dark coffee-colored urine. This condition is fatal without immediate surgical treatment to remove the heartworm blockage.

Is my dog at risk of heartworm disease?

Heartworms are found in all 50 states; therefore, all dogs in the United States are at risk. Heartworm disease is most common in the Southeastern U.S. and Mississippi Delta region, but even communities outside of this region can see high numbers of heartworm cases depending on other local factors (heavy mosquito populations, large unprotected dog populations, etc.).

Dogs that spend most of their time indoors are still at risk of heartworm disease. It only takes a single mosquito bite for a dog to become infected; mosquitoes can enter your home through an open door, a hole in a window screen, etc. Additionally, even a very brief trip outside can allow your dog to be bit by a mosquito.

Owner is giving his dog a heartworm preventative.How to Protect a Dog Against Heartworms 

Heartworm disease is preventable. In all but the coldest areas of the United States, all dogs should remain on year-round heartworm prevention for protection against heartworm disease. Even in the winter months, mosquitos can be found in isolated areas. Therefore, year-round prevention is essential to fully protect against heartworms.

Heartworm preventions come in a variety of forms, including monthly oral medications, monthly topical treatments, and even a six-month injection. In some cases, heartworm prevention may come combined with flea prevention or other parasite preventions.

Talk to your veterinarian to determine the best heartworm prevention option for your dog.

Can heartworm disease be treated?

Heartworm disease can be treated. However, there are several important considerations regarding the treatment heartworm disease.

  1. Heartworm treatment is expensive, typically costing thousands of dollars.
  2. Heartworm treatment is not a “quick fix.” Treating heartworms takes several months and dogs must remain confined throughout treatment.
  3. Heartworm treatment is not risk-free. While treating heartworms is far safer than leaving them untreated, there is a risk of complications during treatment
  4. Heartworm treatment does not reverse all of the damage caused by heartworms. The inflammation and scarring within the heart and the pulmonary vessels can be reduced by eliminating heartworm infection, but cannot be fully reversed.
  5. Heartworm treatment does not prevent re-infection. Once treated, dogs are immediately at risk of re-infection if heartworm prevention is not started and maintained.

Given these factors, taking measures to actively prevent heartworm disease is strongly preferred over waiting for an infection to arise and relying on treatment.


Heartworms pose a significant health risk for all dogs in the United States. In order to prevent damage to the heart and lungs, as well as avoid the need for costly, risky heartworm treatment, all dogs should receive consistent year-round heartworm prevention, obtained from your veterinarian.

One comment

  1. Heart worms are very expensive to treat. 15 years ago my Golden Retriever had Heart Worms and it cost me about $1200 for her to go through the treatments which included being in the hospital. She recovered but was never the same. She had no stamina and after a very short run of 200 feet she would be winded and would have to stop and rest. Probably cut her lifespan by 3 years. Not worth the chance not to treat.


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