Heartworms in Dogs: Everything Pet Parents Need to Know to Keep Their Furry Friend Safe

Heartworm disease is a potentially life-threatening condition that affects dogs.

The disease is caused by a parasitic worm called Dirofilaria immitis, which is transmitted to dogs through the bite of an infected mosquito.

When a dog is bitten by an infected mosquito, the worms enter its body. They can grow to be several inches long and live in the dog’s heart and blood vessels, causing inflammation and damage to the heart and lungs. 

According to veterinarian Wendy Mandese from the University of Florida College of Veterinary Medicine, a substantial number of worms can lead to irreparable damage to a dog’s blood vessels and heart. That, in turn, can eventually cause pulmonary hypertension or heart failure. 

If left untreated, the condition could also result in death.

In this article, we will explore the causes, symptoms, and treatments of heartworm disease in dogs, as well as ways to prevent your pet from contracting the condition.


A persistent cough is the most common symptom of heartworm disease. As the worms grow inside your dog’s body, they can cause irritation and inflammation in the airways, leading to a cough that won’t go away.

Another symptom to look out for is shortness of breath. The worms can also cause blockages in the dog’s lungs, making it harder for them to breathe. This can cause them to pant or gasp for air, especially after physical activity.

Dogs with heartworm disease may also become more tired and less active. They may have less energy and be less interested in playing or going for walks.

Another symptom is a loss of appetite. As the worms grow and cause damage to the dog’s organs, they may lose interest in food and drink. You may also notice your dog losing weight as a result of all the above symptoms.

Types and Stages

There are two main types of heartworm disease in dogs: acute and chronic.

Acute heartworm disease is the early stage of infection when the worms are still small and have not yet caused significant damage to the dog’s heart and lungs. Dogs in the acute stage may not show any symptoms, or they may have mild symptoms such as a mild cough.

Chronic heartworm disease is the later stage of infection when the worms have grown and caused significant damage to the dog’s heart and lungs. Dogs in the chronic stage may have severe symptoms such as persistent cough, difficulty breathing, and fatigue. They may experience reduced appetite and weight loss.

The disease typically progresses through four stages or classes, which are:

  1. Class 1: In this stage, there are no clinical signs of heartworm disease, and the dog is just starting to be infected.
  2. Class 2: In this stage, mild symptoms may appear, such as occasional coughing or mild exercise intolerance.
  3. Class 3: This stage of infection is more severe, and symptoms such as persistent coughing, difficulty breathing, and fatigue will be more prominent. The dog may also have a decreased appetite and weight loss.
  4. Class 4: This is the most severe stage of heartworm disease, where the dog may have severe symptoms, including heart failure and cardiac arrest.

You may refer to this graphic to better understand the heartworm lifecycle in dogs.


One of the first steps in diagnosing heartworm disease is a physical examination. Your veterinarian will check for symptoms such as difficulty in breathing,  persistent cough, fatigue, etc. They may also check your dog’s heart and lungs to check for any abnormalities.

Another important step in diagnosing heartworm disease is a blood test. This test can detect the presence of heartworm antigens (proteins from the worm) in the dog’s blood. 

Imaging tests such as x-rays or ultrasound scans can also be used to diagnose heartworm disease. These tests can show the presence of worms in the dog’s heart and lungs, as well as any damage that may have been caused by the worms.

A test for the presence of microfilariae is also used to diagnose heartworm disease. Microfilariae are the immature form of the heartworm and can be found in the dog’s blood. Nearly 80-90% of dogs with heartworm disease have microfilariae in their bloodstream.

This test can be done by using a blood sample and looking for the microfilariae under a microscope.


As the saying goes, ‘prevention is better than cure.’ This is especially true in the case of a serious and potentially fatal condition like heartworm disease. 

There are several ways to prevent heartworm infection in dogs. These include:

  1. Use preventative medication: One of the most effective ways to prevent heartworm infection is to give your dog a preventative medication like Heartgard Plus on a regular basis. 

These medications contain ingredients that kill the immature stages of the heartworm, preventing them from developing into adult worms.

Heartgard Plus Chewables are the most commonly used preventatives for this purpose. They contain ivermectin and pyrantel, which attack and kill heartworm larvae before they develop further.

Moreover, these chewables can be administered to dogs as young as six weeks and are made of real beef, making them a treat that your furry friend will relish. 

  1. Reduce your dog’s exposure to mosquitoes: Mosquitoes are the main carrier of heartworm, so reducing your dog’s exposure to them can help to prevent infection. 

This can include using mosquito repellent, keeping your dog indoors during peak mosquito hours, and keeping standing water away from your home.


  1. Regular check-ups and tests: Regular check-ups and heartworm tests with your veterinarian can help to catch the disease early before it becomes serious.


The treatment for heartworm disease in dogs typically involves a combination of medications and close monitoring by a veterinarian. 

The most commonly used medications are called adulticides, which are designed to kill adult heartworms. These medications are usually given as injections and may need to be repeated several times over the course of several months.

In addition to adulticides, dogs with heartworm disease may also be given medications to help control inflammation and prevent complications. These medications may include corticosteroids and antibiotics.

If the disease is caught early and the dog is in good health, the treatment has a high success rate. But if the disease is advanced, or if the dog is in poor health, treatment may not be successful. In these cases, the best option may be to provide supportive care to help the dog feel as comfortable as possible.

This may include medications to control pain and inflammation, as well as other treatments to help support the dog’s overall health.


In conclusion, heartworm disease is a serious condition that requires prompt diagnosis and treatment to prevent severe damage to the dog’s health.

It is crucial for dog owners to understand the importance of preventative measures, regular testing, and early treatment to protect their beloved companions from this debilitating disease.

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