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Signs and Symptoms of Heartworm in Dogs

Heartworm disease is a dangerous and sometimes fatal disease that can affect dogs, cats, ferrets, and other mammals. It can also affect humans in rare instances. Cats are not as susceptible to heartworms as dogs; they are an atypical host. So most heartworms do not survive in adult cats. However, even immature worms can cause real damage to a cat. The treatment dogs receive for heartworm disease does not apply to cats. Prevention is the only way to protect your cat.


Heartworm in Dogs

Dirofilaria immitis is the worm that causes heartworm disease. They spread through mosquito bites, and can cause organ damage, severe lung disease, heart failure, and even death. Dogs are the natural hosts of heartworms. Heartworms can mate and produce offspring inside their host, allowing them to grow and multiply.


Symptoms of Heartworm

A worm burden is the number of worms living inside a pet. The symptoms of heartworm disease may not be easily visible in dogs that don’t have a high worm burden. The average worm burden or average number of worms living inside dogs is 15 worms.

The severity of symptoms depends on the worm burden, immune response, infection duration, and activity level of your dog. These vary in the different stages of the disease.


  • Stage One

In this stage, the most noticeable symptom is a cough. You will hear your dog coughing from time to time. It will mostly cough after exercising. You may think it is kennel cough. Either way, you should contact your vet in Greenwood Village so they can help identify the cause of their cough.


  • Stage Two

In addition to the cough, your dog will become tired after moderate activity. Your dog’s stamina changes, making your dog more lethargic. This is a result of the worms getting into their heart and blocking normal bloodflow.


  • Stage Three

The symptoms become more severe in this stage. Your dog may appear sickly, have trouble breathing, and may show signs of heart failure. They could also have a loss of appetite that results in weight loss.

As the worms mature, they migrate into the lungs and the heart.  This can cause your dog to have extreme lethargy, severe coughing (often with blood), and extreme lethargy.


  • Stage Four

A large mass of worms can block the blood flowing back to your pet’s heart. If you do not treat heartworm disease, it can progress and damage your dog’s heart. It will also damage the lungs, liver, and kidneys and can cause heart failure.



It is often recommended to test your dog for heartworms even if they appear healthy because testing can identify heartworms even when symptoms are minimal or not present. This is typically done in conjunction with their annual examination.

If you do not test your dog before starting preventive measures, it could be dangerous for your pet if microfilariae are present. The preventive may cause the microfilariae to die and their death may trigger shock to your pet’s system.

For more information on the signs and symptoms of heartworm in dogs, call Greenwood Village Veterinary Clinic in Greenwood Village, Colorado at (303) 771-6304 today.

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