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Heartworm in Dogs & Puppies

By Colleen Williams and medically reviewed by Brittany Kleszynski, DVM
published: May 6, 2018 - updated: January 18, 2023 • 3 min. read
heartworm in puppies

Key Takeaways

  • Heartworms, a type of roundworm are transmitted through mosquito bites, can enter the bloodstream.
  • Monthly heartworm preventative is the key to reducing your dog’s risk of contracting heartworm disease, and much easier than treating the condition later.
  • Treatment consists of a month-long course of an antibiotic and keeping your dog inactive for 6-8 weeks.

Heartworms are blood-borne parasites that cause disease of the cardiovascular system in dogs. Transmission increases as the weather warms up because larvae are spread by mosquitoes. It is important that your dog receives year-round monthly heartworm prevention to reduce the risk for infection. Although small, these parasites can cause severe internal damage. Prevention is much easier and less costly than treatment.

What causes heartworm?

Heartworms, which are a type of roundworm scientifically known as Dirofilaria immitis, is transmitted through mosquito bites.  After a dog is bitten by an infected mosquito, heartworm larvae enter the bloodstream. They mature into adult heartworms within 6-7 months and become lodged into the pulmonary artery of the heart. Female heartworms begin producing microfilariae at this time, which circulate throughout the bloodstream. Adult heartworms can live 3 to 5 years in untreated dogs, and the parasite burden can become very high.

Protect your pet

Symptoms of heartworm disease

During the early stages of infection, your dog may be subclinical, which means he or she may not show any signs. As the disease progresses, clinical signs may include the following:

  • Coughing
  • Exercise intolerance
  • Anemia
  • Hypertension (high blood pressure)
  • Difficulty breathing
  • Tachycardia (rapid heartbeat)
  • Fainting
  • Heart failure

Diagnosis & treatment

heartworm awareness month

Your veterinarian will collect a small blood sample from your dog to run a heartworm antigen test. Results are available within 10 minutes. This test specifically detects the presence of adult heartworms, so it will not show a positive result until 6-7 months after infection. This is why your veterinarian will not perform a heartworm test on a dog that is less than 7 months of age because it will be negative regardless of whether larvae are present within the blood.

If your dog tests positive for adult heartworms, another blood test to check for circulating microfilariae will be done. This test is very simple and takes less than one minute to perform. It is important to know whether microfilariae are present or not so that the treatment protocol can be tailored accordingly. Staging of heartworm disease also includes performing chest radiographs (x-rays) and bloodwork.

How is heartworm disease treated?

Heartworm disease treatment is expensive compared to the cost of monthly prevention. Treatment consists of a month-long course of an antibiotic, which weakens the heartworms and makes them more susceptible to adulticide. If your dog is microfilariae positive, he or she will be given a dose of heartworm prevention to eliminate those in circulation as well as an antihistamine and steroid to reduce the risk for a reaction. The most common protocol to eliminate the adult heartworms consists of two adulticide injections 24 hours apart. In more severely clinical patients, there may be a series of 3 injections that are spread further apart.

While your dog is undergoing heartworm treatment, it is essential to restrict his or her physical activity for the next 6-8 weeks. Without exercise restriction, the dying heartworms can break into small pieces and become lodged in blood vessels, within the lungs, and within the brain. These complications can be fatal.


Monthly heartworm preventative is the key to reducing your dog’s risk of contracting heartworm disease. The medication is based on your dog’s weight, and it must be given the same day each month. If a monthly preventative dose is given even a day late, your dog may be at risk. Monthly preventatives come in both oral and topical formulas, and they are relatively affordable.

Heartworm disease in cats

Unlike dogs, cats do not commonly get heartworm disease. If they do, it is usually a very low worm burden and is often not detected on the yearly heartworm test. However, clinical signs of heartworm disease can be much more severe in cats. There is also no treatment available, so prevention is the best defense.

Remember: test annually

Yearly testing is recommended to ensure your dog stays free of disease even if monthly prevention is given. Early detection of heartworm disease reduces the risk of severe internal damage and complications associated with treatment.

Visit the American Heartworm Society to learn more about heartworm disease.

The content is not intended to be a substitute for professional veterinarian advice, diagnosis, or treatment. Always seek the advice of your veterinarian or other qualified health provider with any questions you may have regarding a medical diagnosis, condition, or treatment options.

Beating heartworm is tough enough – with Healthy Paws, pet parents don’t have to choose between their pet and their wallet. By signing up for pet insurance when pets are young, ongoing treatments will be covered up to 90%. Find out more by getting a free quote.

colleen williams
By Colleen Williams

Over the past decade, Colleen has written about health, wellness, beauty, and even pets for The New York Times, The Cut, Refinery29, xoVain, Healthy Paws Pet Insurance, and Seattle Met Magazine, as well as many beauty brands. She has a BFA in Art History from the University of New Mexico and an AAS in Fashion Design from Parsons School of Design in New York.

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About DVM contributor, Brittany Kleszynski
By Brittany Kleszynski, DVM

Dr. Brittany Kleszynski is a freelance veterinary and medical writer for Healthy Paws who specializes in creating meaningful content that engages readers and speaks directly to the intended audiences. She writes and edits educational articles for pet parents and creates continuing education and online learning modules for healthcare professionals. She has worked in research and small animal practice since graduating veterinary school and is a member of the American Veterinary Medical Association.

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