Heartworms in dogs: how to diagnose, treat and prevent them

Heartworm disease in dogs

Heartworm or dirofilariosis is a serious disease that can lead to lung and/or heart problems in dogs.

What causes heartworm disease?

Heartworm disease is caused by the heartworm (Dirofilaria immitis). When dogs are bitten by an infected mosquito, they become infected with heartworm larvae. The heartworm larvae get under the dog's skin. In six to seven months, the larvae develop into adult worms in the dog’s body and settle in the pulmonary arteries and heart. Adult worms can then remain in the body for years until they die. These adult worms form larvae which circulate in the blood. A mosquito that sucks up the blood (and therefore the larvae) of an infected animal can pass on the infection again.

Heartworm symptoms

A heartworm infection can go on without symptoms for months and often years.
If the adult worms are in the lungs and heart for a while they can cause unpleasant and dangerous symptoms and even lead to heart failure or serious lung problems. Typical heartworm symptoms:

  • cough
  • fatigue
  • weight loss
  • breathlessness
  • enlargement of the heart
  • damage to the kidneys and liver

Diagnosis of heartworm in dogs

There are three ways to determine whether your dog has heartworms. First, your dog's poop can be examined by your vet with a microscope to see if any larvae are present. Unfortunately, this method only works in about 25 percent of infected dogs. The second way is a blood test. The pet’s blood is tested for antibodies to heartworms. Finally, an ultrasound or X-ray examination can show whether your dog has heartworms.

How to treat heartworms?

Heartworm infection is difficult to treat and the disease carries significant danger for the pet. So prevention is always better than treatment. Treatment to combat the parasites involves many side effects and the death of heartworms can cause thrombosis in the lungs. The treatment consists of a number of medicine courses and takes several months. You should consult your vet who can prescribe appropriate treatment depending on your dog’s condition.

How to prevent heartworm infection in dogs?

It is important to prevent your dog from being bitten by mosquitoes that can transmit heartworms, as treating an already infected dog is very difficult and the disease can then lead to death.

Main ways to prevent heartworm infection:


  • An odorless tick collar that you can put on your dog. This band protects against mosquitoes that can cause heartworms but also against ticks and sand flies.
  • Administering an anti-flea agent with a pipette. Certain medicine can protect your dog against tropical mosquitoes, such as the heartworm mosquito. Ask your vet for the correct product;
  • Another option is to treat your pet with a dewormer every four weeks. This kills the larvae that are in the blood before they grow into dangerous worms.

Tick bands and anti-flea agents are not 100 percent, so it is advisable to deworm your dog regularly in addition to using a tick band or flea pipette.