Heartworm Explained

Oncology and chemotherapy for pets
Nov 30, 2023

The vets talk about it, dog park gossip is fearful of it… but what is “it” really? Dr Elle Burton-Bradley, Veterinarian at The Village Vet is here to explain all about Heartworm and what you need to know to ensure your pet’s health prevention plan is always up to date.

What is Heartworm?

Heartworm (Dirofilaria immitis) is a parasitic roundworm that is spread by infected mosquitos. Worm larvae are injected by the mosquito which migrate to your pet’s organs over several months, eventually residing in the heart and lungs as adult worms. Adult worms can grow up to 30cm in length. These worms reproduce and release their offspring (microfilaria) into the bloodstream which can then be picked up by mosquitos to continue the spread. Heartworm can infect both dogs and cats.



Due to the nature and dispersal of mosquitos, the true prevalence of heartworm in Australia is poorly understood. 

Heartworm Disease

The worms reside and cause damage to the heart and blood vessels of the lungs. Over time, the worm burden increases and can result in serious complications, including heart failure and death.

Signs of a Heartworm Infection?

During the early stages of a heartworm infection, our pets are often asymptomatic (showing no signs of illness). The first clinical signs are usually a cough, particularly during or after exercise, and exercise intolerance. Due to the lifecycle of the worms, clinical signs typically occur 6-9 months after the initial infection.

As the infection progresses, so too does the severity of the disease, causing weight loss, coughing with or without blood, fainting or collapse, enlarged abdomen, and congestive heart failure.

There are four stages of clinical signs:

  1. No or mild signs with an occasional cough.
  2. Mild signs with an occasional cough, lethargy, tiredness after moderate activity.
  3. More severe signs, unwell, persistent cough, lethargy, difficulty breathing, tiredness after mild activity, changes may be observed on x-rays.
  4. Heart failure due to masses of worms blocking the blood from pumping through the heart (caval syndrome). This is life-threatening.

Heartworm disease in cats is slightly different from dogs. Cats typically only get respiratory signs due to damage of the blood vessels in the lungs. 

Heartworm Prevention

Heartworm can effectively be prevented with a monthly tablet (e.g. Milbemax, Simparica Trio, Nexgard Spectra) or an annual injection (Proheart). It is important that your prevention doesn’t lapse as heartworm infections can be silent for several months. If your dog has missed a treatment please contact your veterinarian to organise a blood test.

How is Heartworm Diagnosed?

The signs of heartworm infection are not unique to this parasitic worm, so diagnostic testing is required. 

To detect early, or asymptomatic infections, a simple blood test is performed. Due to the lifecycle of heartworm, two blood tests several months apart may be required, especially if your pet is overdue for heartworm preventatives. 

If your pet is showing signs of coughing or difficulty breathing, your veterinarian will likely recommend x-rays (radiographs) and a heart ultrasound (echocardiogram). This helps to evaluate the heart and lungs and may give an indication of heartworm infection and establish the severity of the disease. 

What if my Pet Tests Positive to Heartworm?

1. Restrict exercise

Heartworm disease effects the body’s ability to pump blood around the body.

2. Stabilisation

If your pet has clinical signs of disease your veterinarian may need to treat concurrent disease alongside heartworm treatment (e.g. congestive heart failure).

3. Treatment

Treatment varies depending on the stage and severity of your pet’s disease, your pet may require 1-3 stages of treatment.

  • Early stages of the disease have a good prognosis.
  • Lateral stages of the disease may have a poorer prognosis or higher risk of complications.
  • In severe cases, adult heartworms can block the chambers of the heart, requiring surgical removal.


If your pet is showing any signs of illness, bring them to see The Village Vet as soon as possible. Early intervention with any disease is critical to a positive outcome.

Book online at The Village Vet or call us directly at the Pymble Clinic on 9499 4010 or Killara Hospital on 8350 5678.


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