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Heartworm Disease in Dogs

Heartworm disease in dogs is a serious disease caused by a parasitic worm called Dirofilaria immitis, which lives in the lungs, heart, and arteries of an infected dog.

This worm, which is usually referred to as the heartworm, is spread by mosquitoes. The severity of the disease depends on the number of worms present.

When a mosquito bites an infected dog, it becomes infected with the immature form of the worm (microfilariae). Microfilariae become infective larvae inside the mosquito and when the infected mosquito bites another dog, the transmission occurs.

Heartworms will enter a dog’s bloodstream as tiny, invisible larvae, but can be up to 12 inches long when fully grown.

It can take many years before a dog will show the symptoms of heartworm disease and usually occurs in dogs from two to nine years old.

It is unusual to find heartworm disease in puppies because the microfilariae can take from six to eight months to mature into adult heartworms after infection.

And by the time clinical signs are seen, the disease is usually well advanced.

Dogs playing at the beachSymptoms of Heartworm Disease

  • Mild persistent cough
  • Reluctance to exercise
  • Labored breathing
  • Fatigue after moderate activity
  • Decreased appetite
  • Vomiting
  • Weight loss
  • Heart failure and the appearance of a swollen belly (due to excess of fluid in the abdominal cavity).
  • Sudden blockages of blood flow within the heart (Caval syndrome), characterized by labored breathing, pale gums, and dark bloody or coffee-colored urine.

Some dogs won’t show any symptoms until the infection is in the later stages which is why early testing is important.

There are four stages of heartworm disease

According to fda.gov

Class 1:  No symptoms or mild symptoms (e.g. occasional cough).

Class 2:  Mild to moderate symptoms (e.g. occasional cough and fatigue moderate activity). Heart and lung changes are usually seen on chest x-rays.

Class 3: General loss of body condition, a persistent cough, and fatigue after mild activity.  Heart and lung changes are usually seen on chest x-rays.

Class 4:  A large mass of heartworms prevents blood from flowing back to the heart causing a cardiovascular collapse known as Caval syndrome.

Diagnosis of Heartworm Disease

Heartworm disease is diagnosed by a thorough vet exam, radiographs, ultrasound and/or blood tests.

The treatment will depend on the severity of the disease.

Heartworm Treatment

When diagnosed with heartworm disease, your vet will conduct a thorough exam to determine the best way to treat your dog with the associated potential risks.

The most common treatments are usually injections of drugs that are inserted in the dog’s muscle.  While this cure has a high success rate, your dog will need to be hospitalized.

In some dogs, the injections can be conducted as an outpatient.

Some of the most common treatments for heartworm are the following:


This is the main ingredient of many heartworm preventive products and it is capable of killing microfilariae. Usually, infected dogs are administered a microfilariae-killing drug to prevent them from developing into adults and then an adult-killing drug is administered.


Milbemycin is also found in heartworm preventive products (e.g. Sentinel and Interceptor) and can kill the microfilariae much faster than Ivermectin. This can be disadvantageous because large numbers of microfilariae dying at the same time can cause a circulatory shock.

Melarsomine dihydrochloride (Immiticide® by Merial)

This is the only FDA approved drug to kill adult worms. Treatment consists of two or three deep injections into the back muscles. These injections are very painful so your dog may also need pain relievers.

After treatment, your dog should be put on a preventative medication to make sure that the heartworm disease does not re-occur.

Are Any Dogs At A Higher Risk for Heartworm Disease?

Dogs that live in a humid climate, like Florida, are at a higher risk for heartworm disease.

Heartworms can occur in any dog but since mosquitoes are the carriers and thrive in humid clients, dogs living in those climates have a higher likelihood of contracting the infestation.

Can Pet Insurance Help With Heartworm Disease?

Depending on the severity of the heartworm infection, the cost can range anywhere from $200 to $2,500.

Pet insurance can help offset the cost of the hospital visits and the expensive injections and treatment that your dog might need.

Plans like Healthy Paws can cover up to 90% of the costs of all the medical bills.

There are also wellness plans, like those from Embrace that can give you an annual allowance toward the expense of the monthly heartworm prescriptions that your dog might need.

Tips For Heartworm Prevention

The best way to prevent your dog from getting heartworm disease is to make sure your dog is screened annually, particularly before spring.

If you live in a humid climate, it is always best to give your dog a monthly chewable pill or another heartworm preventative medication.

There are also topical ointments that you can put on your dog’s fur or skin.

As always, your vet will give you the best recommendation for your dog.

If you think your dog might not have heartworm, but maybe another common health issues, our glossary of dog health issues is a great place to start!

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