Understanding And Preventing Heartworm Disease

Heartworm disease is a serious disease that can lead to severe lung damage, heart failure, other organ failure, or death in our beloved pets. Heartworm disease is caused by a parasitic worm called Diofilaria immitis, which are spread through the bite of a mosquito. With mosquito season in full swing, learn about heartworm, the symptoms and preventative treatments; to ensure your pets live a healthy, long and active life.

Is Heartworm Disease Dangerous?

So you might be wondering how does heartworm start and is my pet in danger? The spread of heartworm begins when an infected dog is bitten by a mosquito, which then results in that mosquito being infected. When that infected mosquito bites another dog, the mosquito spreads the infection. In the newly infected dog, it will take between six and seven months for the infective larvae to mature to adult heartworms. The adult heartworms mate and the females release their offspring into the host’s bloodstream, completing the lifecycle.  

Inside a dog, a heartworm’s lifespan is five to seven years. The number of worms living inside an infected pet is called the ‘worm burden’. The average worm burden is 15 worms for dogs, but can range from 1 to 250 worms. Heartworm disease is not contagious, meaning that a dog can’t catch the disease from being near an infected dog. Heartworm disease is only spread through the bite of a mosquito, which makes it difficult to monitor without an effective preventative treatment plan. 


The severity of heartworm disease is directly related to how many worms are living inside the host (the worm burden). There are four classes, or stages, of heartworm disease. The higher the class, the worse the disease and the more obvious the symptoms. 
  • Class One: No symptoms or mild symptoms such as an occasional cough.
  • Class Two: Mild to moderate symptoms such as an occasional cough and tiredness after moderate activity.
  • Class Three: General loss of body condition, a persistent cough, and tiredness after mild activity. Trouble breathing and signs of heart failure are common. For class 2 and 3 heartworm disease, heart and lung changes are usually seen on chest x-rays.
  • Class Four: Also called ‘caval syndrome’. The severity of the worm burden will physically block blood flowing back to the heart, due to the large mass of worms. Caval syndrome is life-threatening and quick surgical removal of the heartworms is the only treatment option. The surgery is highly risky, and even with surgery, most pets with caval syndrome die.

Prevention is much much better than cure! Dogs older than six to seven months of age should be tested for heartworms before starting heartworm prevention. It is very important to understand that preventive treatments will not kill adult heartworms. If a heartworm-positive dog is not tested before starting a preventive, the dog will remain infected with adult heartworms until it gets ill enough to show symptoms. Also, giving heartworm preventive to a dog that has an adult heartworm infection may be harmful or deadly. Annual testing of all dogs on heartworm prevention is recommended. Talk to your veterinarian about the best time for your dog’s annual heartworm test. 

Once your pet is cleared of heartworm disease, there are a range of options to provide your pet with year round protection from this deadly disease. 

VetShopAustralia’s Dr. Mark recommends, Revolution for Dogs or Advocate for Dogs if you prefer topical treatments and would like to treat against fleas at the same time, or Sentinel Spectrum or Heartgard Plus, which are administered orally as a tasty meaty chew.

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