Heartworm in Dogs & Cats: Prevention, Testing & Treatment in Australia

If you give your dog a monthly heartworm preventative, you probably assume they’re protected. Most owners do.

One of the largest studies of its kind, tracking 228,185 dogs across 52 Australian veterinary practices, found otherwise. Dogs on monthly prevention were, on average, covered for only 7.2 months of the year. The medication wasn’t the problem. Missed and delayed doses were.

This doesn’t mean monthly prevention doesn’t work. For many households, it’s an excellent option. It means heartworm protection depends on consistency, and understanding how the disease spreads, how it’s tested for, and which prevention option actually fits your routine can be the difference between a protected pet and a dangerous gap.

This guide covers how heartworm affects dogs and cats in Australia, the real risk here in Sydney, how vets test for it, what happens if a dog tests positive, the prevention options available, and exactly what to do if you’re not sure your pet is still covered.

Is heartworm really a risk for my dog or cat in Sydney?

Many people assume heartworm is only a problem in tropical parts of Australia. It’s a common misconception, and not a safe assumption to make in Sydney.

Heartworm is found throughout mainland Australia, including New South Wales. The Australian Heartworm Advisory Panel recommends year-round prevention for every dog in the country, regardless of where they live.

One reason Sydney isn’t exempt is its urban wildlife. An older study of foxes around the Sydney region found that roughly 9% carried adult heartworms. Foxes move through parks, waterways and residential areas, and when a mosquito bites an infected fox, it picks up microscopic heartworm larvae, then passes them on the next time it bites a dog or cat.

Your pet never needs to meet another infected animal, or even see a fox, to become infected.

Sydney’s mild, coastal climate also keeps mosquitoes active for most of the year. There’s no clearly defined “heartworm season” here, which is exactly why continuous, year-round protection, not seasonal dosing, is the safest approach for both dogs and cats.

Why missed doses matter

Heartworm prevention doesn’t work like a shield that blocks mosquitoes from biting. It works by clearing out any larvae picked up since the last dose. If there’s a gap, whether a late purchase, a forgotten tablet, or a busy month, any larvae transmitted during that gap keep developing. By the time the next dose arrives, they may already be past the stage the medication can remove.

This is exactly what the Australian compliance study found: dogs on monthly prevention averaged just 7.2 months of covered protection a year, and only 27–37% of them achieved a full year of coverage in any given 12-month period, compared with 93–97% of dogs on the annual injection. A separate, smaller Australian study of dogs that had already tested positive for heartworm found that many owners reported their dog had been on “rigorous” monthly prevention. That’s a reminder that the products themselves are highly effective. The difficulty is almost always maintaining them without a gap, not the medication failing.

For most owners, remembering a tablet every four weeks, every month, for years, is genuinely difficult. That’s a scheduling problem, not a discipline problem, and it’s exactly why the prevention options later in this guide matter.

Signs of heartworm disease in dogs and cats

One of the most unsettling things about heartworm is that most infected pets look completely healthy until real damage has already been done.

In dogs

Adult worms live in the pulmonary arteries, gradually causing inflammation, scarring and rising pressure in the vessels supplying the lungs, which places increasing strain on the heart. Signs may include:

• Persistent dry cough
• Exercise intolerance, or tiring more easily on walks
• Rapid or laboured breathing
• Lethargy
• Reduced appetite and weight loss
• Fluid accumulation in the abdomen, from right-sided heart failure
• Collapse or sudden death in severe cases

By the time these signs appear, the disease is often well advanced. The absence of symptoms doesn’t mean the absence of infection, which is why routine testing (covered below) matters even for dogs that seem perfectly well.

A 2024 US study following outcomes for more than four million veterinary patients found that dogs successfully treated for heartworm carried roughly 3.6 times the risk of right-sided heart failure and 2.8 times the risk of cardiomyopathy later in life, compared with dogs that were never infected. Preventing infection in the first place is far better than treating it after lasting cardiovascular damage has occurred.

In cats

Cats aren’t the natural host for D. immitis. Most larvae never reach adulthood, and an infected cat typically carries only one to three adult worms, rather than the dozens a dog might harbour. That smaller number doesn’t make the disease mild.

Many infected cats develop Heartworm-Associated Respiratory Disease (HARD), an inflammatory reaction that occurs when immature worms reach the lungs. Affected cats may cough, wheeze, breathe rapidly, vomit, or become lethargic: signs that closely resemble feline asthma or chronic bronchitis, which is part of why the disease is so often missed.

The death of even a single adult worm can trigger a sudden, severe inflammatory reaction in a cat’s lungs, occasionally leading to acute respiratory distress or sudden death with little warning.

Being an indoor cat doesn’t remove the risk. One retrospective study found that 27% of cats diagnosed with heartworm were housed exclusively indoors. Mosquitoes get inside through open doors, windows and flyscreens as readily as anywhere else.

Testing for heartworm: what it involves and what it costs in Australia

A heartworm test is a small blood sample, with results usually available quickly, either in-clinic or from a referral laboratory.

For dogs, the standard test looks for heartworm antigen, a protein produced by adult female worms. Because it takes around six months for larvae to mature into worms capable of producing that protein, testing too soon after a suspected exposure can return a false negative. This is also why dogs are usually tested before starting or restarting prevention after a gap: giving a preventative to an already-infected dog without knowing its status carries real risk, so your vet may recommend a test first.

Testing cats is less straightforward. Cats typically carry far fewer worms, which makes the antigen test used in dogs less reliable in cats, so vets often combine it with an antibody test to improve accuracy. Even then, no test is perfect: some studies suggest 15–25% of cats with a genuine adult heartworm infection can still return an antibody-negative result. It’s one of the reasons prevention carries even more weight for cats than testing does.

Testing and prevention are, by a wide margin, the cheaper and far less stressful side of that comparison.

Treatment: why dogs and cats are not the same story

If a dog tests positive, treatment is possible, though it’s involved and it isn’t quick. Your vet will typically stage the disease first, using chest X-rays, bloodwork, an ECG and sometimes a heart ultrasound, to understand how much damage has already occurred and plan treatment accordingly.

Treatment usually involves a series of melarsomine injections, which kill the adult worms in stages rather than all at once, a gradual approach that reduces the risk of a dangerous blockage as the dead worms break down. Strict rest matters just as much as the medication: physical exertion during treatment and recovery significantly increases the risk of complications, so a dog’s activity is deliberately restricted, sometimes for several weeks, while their body clears the worms safely.

Cats are a different story entirely. There is currently no approved drug to eliminate adult heartworms in a cat. Management is supportive: corticosteroids, bronchodilators and oxygen therapy can help a cat cope with the inflammation, but none of it removes the infection itself. For cats, prevention isn’t just the safer option. It’s the only one.

Heartworm prevention options in Australia

When protection is maintained consistently, heartworm infection is almost entirely preventable. In Australia, there are two main approaches for dogs, and one for cats.

Monthly heartworm prevention

Monthly prevention comes as oral chews, tablets, or spot-on treatments, often combined with flea, tick and intestinal worm coverage in one product. Common examples registered in Australia include Simparica Trio, NexGard Spectra, Advocate and Revolution, with the right choice depending on your pet’s species and what else needs covering.

Given every month, on time, these products are highly effective. As covered above, the real-world challenge isn’t the medication. It’s remembering it consistently, year after year. A recurring calendar reminder, an auto-delivery subscription, or dosing on a fixed day each month can all help close that gap.

The annual ProHeart SR-12 injection

ProHeart SR-12 is an extended-release injection, given by your vet once every 12 months, that removes the need for monthly reminders altogether.

In the Australian compliance study referenced above, dogs on the annual injection maintained single-year compliance of 93–97%, against 27–37% for monthly products, largely because the injection happens during a scheduled vet visit rather than relying on an owner remembering it at home.

When given according to the registered label, ProHeart SR-12 also carries a 90-day “reach-back” claim, meaning it can clear larvae picked up in the three months before the injection, which is useful if an annual appointment runs a little late. Your vet will advise whether a test is needed first, based on your dog’s prevention history.

One thing to keep in mind: ProHeart SR-12 protects against heartworm only. If your dog also needs flea, tick or intestinal worm protection, that’s a separate product.

Which option is better?

There isn’t a single “best” option, only the one your pet will actually receive consistently.

Monthly combination products suit owners who already have a reliable routine and want flea, tick and worming coverage in the same dose. The annual injection tends to suit owners who:

• Would rather not rely on monthly reminders
• Have missed doses before
• Want a single scheduled appointment to cover the whole year
• Already use a separate flea or tick product

Your vet can help build a plan around your pet’s age, lifestyle, travel, and your household routine.

Not sure which option suits your dog or cat? Book a consult with one of our vets at Bayside Wolli Creek, Rhodes or Waterloo and we’ll talk you through what fits your routine.

Frequently asked questions

Do cats need heartworm prevention too?

Yes. Cats develop heartworm differently to dogs, but because there’s no approved treatment to remove adult worms once a cat is infected, prevention is the only real protection available. Several monthly spot-on products registered for cats in Australia cover heartworm alongside fleas and other common parasites.

What should I do if I’ve missed a dose?

It depends on how many doses were missed. If it’s just one, and every other dose was on time, resuming your normal schedule is usually fine. If you’re not sure how many were missed, or it’s been several months, the safer step is to book a heartworm test before restarting prevention. Starting a preventative in an already-infected pet without knowing their status isn’t recommended.

How often should my pet be tested?

Most vets recommend an annual heartworm test for dogs, even those on consistent prevention, since no preventative is completely failproof, and a missed or vomited dose can happen without you noticing. Cats are typically tested if there’s a clinical reason to suspect infection, since routine testing is less reliable in cats than in dogs.

Can indoor cats really get heartworm?

Yes. Mosquitoes get inside through open doors, windows and flyscreens, and it only takes one bite. As covered above, 27% of cats diagnosed with heartworm in one study were housed exclusively indoors.