My Dog Has Been Diagnosed with Heartworm Disease. Now What?

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The American Heartworm Society does not endorse any specific heartworm medication preventative and is not responsible for any cross-linked

No pet parent wants to hear the words “Your dog has heartworms” spoken by their veterinarian. But if this happens, it’s important to note that you and your dog are not alone—in fact, the American Heartworm Society estimates that roughly one million dogs in the U.S. become infected with heartworms every year.

While heartworm preventives are almost 100% effective in preventing heartworm disease, they are only effective if your pup receives his medication on time each month and you never skip a dose. If your dog is not on year-round heartworm prevention—or if you’ve forgotten to give your dog’s preventive for even a month or two—heartworm disease becomes a real possibility.

If your dog is diagnosed with heartworm disease, it’s important to understand how the disease can affect them, what treatment entails, and what kind of home care your dog will need in the coming weeks and months as they recover.

What Is Heartworm Disease in Dogs?

Heartworm disease is a potentially life-threatening condition in dogs, caused by the parasitic infestation of long, thread-like worms within the heart, lungs, and blood vessels, transmitted through mosquito bites. Heartworm disease in dogs is a serious medical issue that requires time and money for veterinary medications and treatment.

Heartworm infection, while serious, varies in severity. Dogs who were only recently infected and have just a few adult heartworms may not show any outward signs of heartworm disease.

Roughly one million dogs in the U.S. become infected with heartworms every year.

Inside your pup, however, much more is happening. Within just a couple of months after a dog is infected, before it is even possible for a heartworm test to yield a positive result, damage to arteries and organs begins.

About six months after the initial infection, heartworms may have grown up to 12 inches in length and can cause significant damage to the heart, lungs, and pulmonary arteries. Dogs can also become infected with more heartworms if they are bitten by more infected mosquitoes.

How Veterinarians Diagnose Heartworm Disease in Dogs

A heartworm diagnosis starts with a blood test that is typically performed during a routine checkup or when a veterinarian has reason to suspect a dog has become infected with heartworms.

The American Heartworm Society recommends two types of routine heartworm tests, both requiring a small blood sample:

  • Antigen test: detects the presence of adult female worms in a dog’s blood.

  • Microfilaria test: detects the presence of microscopic heartworm larvae, called microfilariae.

Microfilariae are produced when adult heartworms already living in your dog mate and reproduce. While these heartworm “babies” will not mature to become adult worms inside your dog, they can be spread to other animals if a mosquito bites your dog.

When a mosquito bites a dog with microfilariae, the microfilariae are transferred to the mosquito, where they incubate for a short period and become larvae. When the mosquito then bites another animal, the heartworm infection is spread.

If your dog tests positive on one or both of these tests, your veterinarian will likely want to run additional tests to confirm the diagnosis and evaluate your dog’s condition. These tests may include one or more of the following:

  • A second antigen test. This is conducted to verify the presence of heartworms and confirm the initial test results.

  • Chest radiographs. While these are not performed in every heartworm case, they allow the veterinarian to assess the amount of damage from heartworms to a dog’s heart, lungs, and arteries, as well as the potential for complications to occur during treatment.

  • Echocardiogram. If your dog is showing clear signs of heartworm disease, such as coughing, difficulty breathing, loss of appetite, and weight loss, your veterinarian may want to perform a more advanced evaluation of your dog’s cardiovascular system.

How Will My Dog Be Treated for Heartworm Disease?

The goal of heartworm treatment is to stop heartworm damage and eliminate the heartworms inside your dog. Heartworm treatment must be managed carefully to minimize complications.

As worms die within your dog, dead and decomposing worms can create life-threatening blockages and inflammation in your dog’s blood vessels. The longer your dog has been infected and the greater the number of worms, the greater the likelihood of complications.

If your dog undergoes heartworm treatment, it is imperative to follow your veterinarian’s instructions. The American Heartworm Society recommends a multi-step approach involving various medications over several months.

Initially, doxycycline, an antibiotic, weakens the heartworms. Next, melarsomine will be administered by your veterinarian; this is used to kill the adult worms. Additionally, anti-inflammatory medications may be prescribed in certain cases. Heartworm preventive medications are administered to guard against further infection.

Some of these medications can be administered at home by you, while others require injection at the veterinarian’s office. Your veterinarian will determine the best treatment regimen based on the severity of your dog’s heartworm disease.

Surgery for Heartworm Disease in Dogs

One severe complication of heartworm disease is a condition called caval syndrome, when large numbers of heartworms obstruct blood flow through the heart and interfere with heart function. Dogs with caval syndrome become weak and lethargic, short of breath, and may have a swollen abdomen. Without prompt treatment, the condition is almost always fatal.

The treatment for caval syndrome is surgical removal of the adult worms, followed by medications to stabilize the dog and to kill any remaining adult worms. The worms will die and then are eliminated over time by the dog’s immune system.

What To Expect During Treatment of Heartworm Disease in Dogs

As a pet parent, you play a critical role in your dog’s recovery from heartworm infection—which is the role of keeping your dog calm and quiet. The risk of complications increases significantly if a dog’s heart rate is elevated.

For that reason, running, jumping, chasing, and any form of exercise or excitement should be strictly avoided from the time of diagnosis until at least a month after treatment has been completed. While your dog will still need to take short outdoor bathroom breaks, they should be kept on a short leash during these breaks and otherwise kept quiet and rested.

The goal of heartworm treatment is to stop heartworm damage and eliminate the heartworms inside your dog. Heartworm treatment must be managed carefully to minimize complications.

This doesn’t mean you can’t give your dog love and affection during heartworm treatment. In fact, your dog will be relying on you more than ever to keep them company and find quiet ways to keep them from becoming bored and restless.

Veterinarians may also prescribe calming medications or supplements when needed. You and your veterinarian should discuss your dog’s energy level and any difficulties you anticipate in keeping your dog calm and quiet during treatment.

How to Prevent Heartworm Disease in Your Dog

Dogs who have previously had heartworms do not develop immunity from their infection. It is crucial to start your pup on a preventative heartworm medication immediately upon their diagnosis. Even during treatment, infected dogs can acquire more and more worms if bitten by additional infected mosquitoes.

Once on heartworm prevention, your dog must consistently receive their medication throughout the year. Whether you prefer a monthly chewable tablet, a spot-on medication, or a once- or twice-a-year injection administered by your veterinarian, there are many safe, effective, and affordable options.

Your veterinarian will work with you to determine the best heartworm preventative based on your dog’s needs and lifestyle.

The good news is that with proper treatment, most dogs will recover from heartworm disease and go on to live a full, healthy life. By educating yourself about heartworm disease and its management, you can help ensure that your dog successfully recovers and remains protected from heartworm disease in the future.



Andy Moorhead DVM, American Heartworm Society


Andy Moorhead DVM, American Heartworm Society


Dr. Andy Moorhead is a small animal parasitologist and Associate Professor at the University of Georgia, College of Veterinary Medicine....

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