warning signs your dog has heartworms
Symptoms of heartworm disease may take more than 6 months to appear. By: Josh

Heartworms are silent killers that are often overlooked until it’s too late.

This preventable condition starts with a mosquito bite and ends with either a long, expensive treatment or death. Understanding the condition starts with knowing how it develops and affects your dog.

Heartworm Life Cycle in Dogs

A tiny little mosquito starts this long, painful process. The insect provides the transport for the parasite known as Dirofilaria immitis. This parasite is a small, thread-like roundworm that is transmitted from a host when a mosquito bites it.


The worm enters the mosquito and goes through 2 molts, the last molt moving the parasite to the mosquito’s saliva glands. It’s here where the parasite becomes infectious and waits for the mosquito to bite a new host.

Once the mosquito bites a new host, the parasite enters the body and goes through 2 more molts before becoming an adult heartworm. The adult heartworm usually travels to a chamber in the heart or the lungs and multiplies in size (sometimes as large as 10 times its original size).

The worms will also start producing offspring at this time (around 6 months or later from the original bite) that can be detected in the blood. This means that symptoms may not appear until after 6 months since the original mosquito bite.

In the video below, a small-animal veterinarian discusses the heartworm life cycle in dogs, as well as treatment:

Warning Signs That Your Dog Has Heartworms

Symptoms may not appear until the worm reaches maturity (usually after around 6 months).

1. Coughing

A soft, dry cough can be one of the red flags of heartworms in dogs. The parasites make their way to the lungs and start multiplying in the lungs and surrounding veins. Coughing may be most noticeable after exercise and may end with fainting. Even light exercise can cause fainting.

2. Inactivity or Lethargy

Your pet suddenly seems tired more often, doesn’t want to go outside or avoids all physical activity. Dogs with heartworms are weakened and find it difficult to be active even when performing small tasks.

3. Weight Loss or Anorexia

Even minor physical activities such as eating can become difficult and exhausting chores for a dog with heartworms.

4. Rapid or Difficult Breathing

Along with coughing, breathing problems and abnormalities occur when the heartworms inhabit the lungs and surrounding veins. Fluid can also build around the blood vessels in the lungs, making it difficult for the lungs to oxygenate the blood.

5. Bulging Chest

The ribs may seem to protrude and the chest has a bulging appearance as a result of adult heartworm infection. This symptom can result from weight loss and anorexia caused by heartworms. This can also be caused by fluid buildup in response to the parasite’s presence.

6. Allergic Reaction

Although allergic reaction is more common in cats, it is possible for dogs to show symptoms similar to an allergic reaction or asthmatic symptoms in response to the heartworms or their offspring.

7. Collapse

Large numbers of heartworms invade the heart and cause blockage of blood flow (known as caval or vena cava syndrome). Collapse is usually accompanied by shock and red blood cell destruction. Death can follow within days.

Other symptoms possible with heartworms:

  • 8. Nosebleeds
  • 9. Secondary pneumonia
  • 10. Increased blood pressure
  • 11. Excessive sleeping
  • 12. Seizures
  • 13. Blindness
  • 14. Lameness

Seizures, lameness and blindness occur when the parasites get lost and end up in places other than the heart or lungs. They can end up in the brain or eyes, although this is rare.

Some of these symptoms can also be symptoms of other conditions, and this makes it more difficult to detect heartworms. There are other tools used by vets to detect heartworms more accurately.

Diagnosis and Treatment of Heartworm in Dogs

Blood tests to detect the various stages of heartworm infection can include viewing a blood sample under a microscope and checking for antigens of adult heartworms (a protein the heartworms produce and also the most commonly used test, according to VCA Animal Hospitals).

Other blood tests can check for abnormalities in complete blood counts and evaluate the level of function of the internal organs.

  • Radiographs (X-rays) can also be used to detect inflammation, enlargement or swelling of the heart, lungs and the large artery leading to the lungs from the heart (pulmonary artery).
  • Abnormal heart rhythms and enlarged heart chambers can be detected by an electrocardiogram (EKG or ECD).
  • An ultrasound of the heart (echocardiography) can help determine the health of the heart for treatment and offer a visual of any existing heartworms.

These tests are also used to determine the health status of the internal organs. This is required to evaluate whether the dog is healthy enough for treatment.

Treatment begins by eliminating the adult roundworms and takes about one month to complete. The dead adult worms are then absorbed by the body. After this stage the next treatment gets rid of the younger parasites and offspring. In extreme cases, surgical removal of heartworms from the internal organs may be necessary.

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This condition is preventable. If your pet is not taking any type of heartworm preventive, please talk with your veterinarian about getting started.

Cats can also contract heartworms and have a much lower chance of recovery than dogs because only a few heartworms can cause death. Ferrets and other animals are also susceptible to heartworms, so take precautions now for the health of your pet.

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