Heartworm in Dogs and Cats

A serious disease that's easy to prevent

There are two major types of worms that affect pets — heartworm and intestinal worms. You can read more about intestinal worms on our intestinal worm page, but here we’ll focus on heartworm, a potentially life-threatening parasite that affects both dogs and cats.

What is heartworm?

Heartworm (Dirofilaria immitis) is a serious parasite spread by mosquitoes — so your pet doesn't need direct contact with other animals to become infected.

Once bitten by an infected mosquito, heartworm larvae travel through your pet’s bloodstream and develop in the heart and lungs, where they can cause severe damage over time. In advanced cases, heartworm can lead to heart failure, respiratory issues, and death if left untreated.

Heartworm is found across most of Australia, including Victoria. (Tasmania and the most arid inland areas are the only regions with low risk.)

How does heartworm spread?

1. A mosquito bites an infected dog and ingests immature heartworm (microfilariae)
2. These larvae develop inside the mosquito over 10–14 days
3. The infected mosquito bites another pet and transfers the larvae
4. The larvae migrate through the pet’s body, eventually settling in the heart and major blood vessels

Over several months, they mature into adult worms, growing up to 30 cm in length. In dogs, the heart and lungs become damaged; in cats, even a small number of worms can be fatal.

Symptoms of heartworm disease:

In early stages, your pet may not show signs. As the disease progresses, symptoms can include:

• Lethargy or reluctance to exercise

• Coughing
• Difficulty breathing
• Weight loss
• Collapse or sudden death (especially in cats)

Prevention is simple and effective

The good news? Heartworm is completely preventable — and prevention is far easier and safer than treating an established infection.

We offer a range of effective heartworm prevention options:

• Monthly chews or tablets
• Monthly spot-on treatments
• An annual heartworm injection for dogs, administered by one of our vets

💉 The annual injection is a popular option for busy owners who want peace of mind all year round.

What if my pet hasn’t had heartworm prevention?

If your pet has missed doses or has never been on heartworm prevention, we recommend a simple blood test before starting any new prevention program. This ensures it's safe to begin and helps us tailor the right plan for your pet’s needs.

📞 Not sure which prevention is right for your pet?
Call us on (03) 5333 5959 or book an appointment to speak with our team.
We're here to help you protect your pet’s health — today and into the future.