Signs of Heartworms in Dogs

A Heartworm is a dangerous parasite caused by foot-long worms invading a dog's body and living in the heart, lungs, and connected blood vessels. Left untreated, it can cause lung damage, cardiovascular disease and be harmful to other organs in the body. Mosquitos cause heartworms. Prompt treatment is the key to treating heartworm disease in dogs. The best way to treat worms is through preventive medications. Depending on your physical location, these prevention measures need to stay in place part or all year long. 

What is Heartworm Disease?

Also known as Dirofilaria immitis, a blood-borne parasite that's transmitted by mosquitoes. Unfortunately, there's no way to tell if a mosquito is infected. So, the best treatment is heartworm prevention. The infection begins with a filarial worm larva. Then, over 10 to 14 days, the larva matures. The complete maturation process takes six to seven months to transform the larvae into heartworms. 

The final stage involves the mating cycle between males and females, which produces offspring, increasing the number of larvae that mature in the next two to six months if left untreated. Heartworms grow between four and six inches in length, and the females about 10 to 12 inches. They have a  five- to seven-year lifespan living in your pet's heart. The average number of worms living inside a dog is 15, but that can vary from 1 to 250.

Identifying Dog Heartworm Signs and Symptoms

In the beginning stages, right after the infection, there usually aren't any signs or symptoms. Some dogs might experience a mild cough. Once it progresses, your pup might show mild exercise intolerance, and the cough will become more frequent.

During stage three, the symptoms are more profound. You might notice your dog is having a hard time exercising, you can hear them breathing, and their pulse will weaken. Also, impaired blood flow by the worms can cause fainting. 

Additional signs include reduction in appetite, swollen belly from heart failure, and noticeable weight loss. The final stage is life-threatening and leads to permanent organ damage. Dogs can experience cardiovascular collapse with advanced, untreated heartworms. Symptoms consist of labored breathing, dark, coffee-colored urine, and pale gums. 

Diagnosing Heartworms

The earlier your dog receives a diagnosis, the better. For many canines, a blood test for the presence of antigens can detect the disease, even if there aren't any larva. Following the blood test, your dog's veterinarian will conduct additional tests to confirm the presence of heartworms.

Typical screenings include,

  • Radiograph to look for abnormalities in the heart and pulmonary arteries
  • Ultrasound to check for unusual organ shapes and visible worms
  • Echocardiography to visualize worms in the heart chambers

Treating Heartworm

After confirming heartworms, treatment is necessary. The veterinarian will need to restrict your dog's exercise and other activities to reduce the rate of heart and lung damage. With active dogs, this might be difficult. You may need to confine them to a crate for the duration of the treatment.

The only FDA-approved treatment to kill dog heartworms is melarsomine dihydrochloride, a deep intramuscular injection in the lower lumbar region. A second medication is used to remove larva from the bloodstream. Additional treatments can include antibiotics and steroids. Finally, your veterinarian will require heartworm preventives to reduce the chances of re-infection. 

Screening for Heartworms

Even if your dog's never missed a dose of heartworm preventative medicine, they should still be tested regularly. The general rule for screenings begins as a puppy with blood testing for antigens at six months. Keep in mind that it can take up to six months for a positive result to show. For the best results, another test is completed six months after. If your dog's been consistently receiving preventative medicine, no further tests are necessary. If your pup's missed a dose of heartworm medicine, they'll require testing. The same goes for giving them the medication late. If your dog's older than seven and hasn't been on any heartworm preventatives, regular testing should start. 

Heartworm Prevention

You should know dog heartworm signs and symptoms and always be hypervigilant to reduce the chances of infestation. But the best way to prevent the disease is with heartworm medicine.

Most vets recommend year-round preventive measures to reduce exposure. Late and skipped doses can put your pet at risk. To remember these important dates, set up reminders on your smartphone, and put a post-it note where you'll never forget it.

You can also use FDA-approved mosquito repellents to protect your dog from heartworm disease. A bonus of these products is they often offer effective protection against other parasites, such as roundworms, fleas, hookworms, and tapeworms. 


Heartworm disease is a serious condition that requires immediate treatment. Infections can occur in all U.S. states, and untreated cases can result in death. If you notice any signs of the disease, contact your vet for a same-day appointment or have your dog seen as soon as possible.

The best way to protect your dog from heartworms is through prevention. 

How do dogs get heartworm?

Dogs get heartworm from mosquitoes infected with Dirofilaria immitis. Not all mosquitoes have these microfilariae. If left untreated, the larvae grow to adulthood and infest the heart, lungs, and blood vessels between them.

What causes heartworm in dogs?

Heartworm in dogs occurs when the Dirofilaria immitis mature over a two- to six-month period into fully grown worms. Heartworm disease has four stages that start with few signs and slowly progresses. Treatment is essential to restoring your dog's health.

Are heartworms contagious?

No. Transmission can only occur through the mosquito. Instead, the area's high spread rates are determined and how long the mosquito season lasts—many parts of the U.S. experience year-round transmission. Cases of heartworm disease occur countrywide. Although heartworm medicine is highly effective, your dog can still get infected. The breakthrough transmission is why it's vital to know dog heartworm signs and symptoms. You can avoid infestation by using preventative measures. Additionally, periodic pet-safe mosquito fogging in your yard can reduce the chances of heartworms.

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Written by Leo Roux

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