Heartworm disease

04 Apr 2019

Found in numerous locations across Australia, heartworm is an often-deadly parasitic disease of dogs spread by mosquitos. Larvae are picked up by mosquitos after biting an infected dog, before being transmitted to a healthy dog via another mosquito bite. After six months the larvae have grown into adult worms that live in the heart and vessels around the lungs.

Left untreated, heartworms can reproduce and grow, causing blockages of the major vessels and serious disease. Fortunately, there are effective means of preventing this disease.


Fortunately, there are many effective heartworm preventatives on the market. If heartworm is endemic in your area (your veterinarian will be able to tell you), you need to ensure your dog is on heartworm prevention medication year-round, as even one missed dose can put them at risk of heartworm disease.

The two main types of preventatives; monthly preventatives that come in a spot on or tablet form (often combined with other types of worming and flea medications) and a yearly injection.

Your veterinarian can help you with the best preventative plan for your pet. Many pet owners manage the risk of heartworm disease with the yearly injection, as this removes the risk of forgetting a dose of preventative medication. Your veterinarian can administer this injection as part of your pet’s regular health check.

Symptoms of heartworm disease

When dogs are infected with larvae after being bitten by an infected mosquito, they don’t usually show any clinical signs of the disease. However, once adult worms have grown and reproduced, disease associated with blockage is often seen.

Symptoms of clinical heartworm disease include:

  • Low exercise threshold (exercise intolerance)
  • Lethargy and weakness
  • A dry and persistent cough
  • Weight loss

In more severe cases, when the heart and lungs have been extensively damaged by the worm burden, symptoms include:

  • Rapid and shallow breathing
  • Swollen abdomen
  • Sudden collapse
  • Blueish tinge to gums
  • Sudden death (particularly after exercise)

Diagnosis and treatment of heartworm disease

Diagnosing heartworm disease in dogs involves a couple of steps. After six months from the first infected mosquito bite, adult heartworms can be detected on a rapid bedside test from a drop of blood. Your veterinarian will look at your dog’s blood under the microscope too, looking for worm larvae. They might also send a sample off to a diagnostic lab.

Once confirmed on a blood test, your veterinarian can stage the infection by taking chest X-rays and conducting further blood tests to check the degree of damage caused by the infection. Treatment can be risky and prolonged, so prevention is definitely the best cure in the case of heartworm disease.

Heartworm disease in cats

Heartworm can infect cats as well as dogs. However, the disease is very different in cats. Most cats that are bitten by infected mosquitos only have a short, limited infection, as their immune system kills the larvae before they can grow into adult worms.

This also means that heartworm disease in cats is more difficult to diagnose in cats. Although the infection is usually self-limiting in cats, it can cause an allergic reaction and immune response that damages the lungs.

Symptoms of feline heartworm disease can often mimic feline asthma, another respiratory condition of cats. If you’re in an endemic heartworm region and your cat is showing signs of feline asthma, your veterinarian might consider testing for heartworm, even though it’s a rare condition in cats.

Fortunately, there are monthly spot-on treatments for cats which protect them against heartworm disease, and as with dogs, prevention is always better than cure.

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