Heartworm in Dogs: Symptoms, Prevention and Whether Your Dog Needs It

Your vet has recommended heartworm prevention, and you're quietly wondering whether your dog actually needs it.

It's a fair question. Heartworm isn't evenly spread across Australia. A dog in Townsville and a dog in Hobart are not in the same position.

What doesn't vary is the consequence. Heartworm is silent for months, and by the time there's a cough the worms are already in the heart. If your dog has just been diagnosed, skip to the treatment section.

I'll take you through the risk where you live, what prevention actually covers, and why heartworm is a different disease from the intestinal worms your all-wormer handles.

Heartworm causes no signs at all for months, and a soft persistent cough is usually the first thing an owner notices.

The disease is well established before it announces itself. There is no early warning sign to watch for, which is the entire reason prevention and annual testing exist rather than a symptom checklist.

When signs do appear, they usually start as a mild persistent cough, a reluctance to exercise, tiredness after moderate activity, reduced appetite and weight loss. As it progresses, dogs can develop heart failure and a swollen abdomen from fluid. In severe cases, a sudden collapse with laboured breathing, pale gums and dark or coffee-coloured urine signals caval syndrome, which is an emergency.

See a vet promptly if a dog that is not on prevention develops a cough that will not settle, or starts tiring on walks it used to manage easily.

A blood test looks for a protein shed by adult female heartworms. It is quick and it is the standard first step.

Its one limitation is the same six-month gap that governs everything else about this disease. The test detects mature worms, so a dog infected recently can test negative while carrying developing larvae. This is why a single negative test is not proof a dog has never been exposed, and why the timing of a test after a lapse in prevention matters.

If a test is positive, imaging follows. X-rays and ultrasound show how much damage the worms have done and how advanced the disease is, which determines how treatment is approached.

Dogs on prevention are still tested annually. Not because the products fail often, but because a missed or vomited dose leaves a gap nobody notices at the time.

Puppies can start prevention without a test, with testing at around six and twelve months to confirm nothing was missed.

Heartworm prevention is given monthly, and the real decision is whether you want it bundled with flea, tick and worm cover or kept as a separate tablet.

Monthly chews and tablets

The most common option. Some cover heartworm alone, like the Valuheart range. Others fold it into broader cover: HeartGard 30 Plus adds roundworm and hookworm, while NexGard Spectra and Simparica Trio add fleas, ticks and intestinal worms to the same monthly dose. Our all-in-one dog parasite protection range covers those combinations.

Monthly spot-ons

Advocate and similar spot-ons cover heartworm alongside fleas and intestinal worms, applied to the skin rather than given by mouth. They suit dogs that will not reliably take a tablet.

When to start, and why year-round

Puppies start young, and our puppy flea, tick and worm range is filtered to products registered for the smallest and youngest dogs.

For adult dogs, prevention is generally given year-round rather than seasonally. In the north that reflects real year-round mosquito activity. Further south the argument is more practical than biological: a schedule you stop and restart is a schedule that gets forgotten, and the gap it leaves is invisible for six months.

Most all-wormers treat intestinal worms only. The few products that cover both only do it when they are given monthly.

This is the most common misunderstanding about heartworm, and the product naming does not help. "All-wormer" means all intestinal worms.

That said, treatments do genuinely fail sometimes, usually for one of these reasons.

• A pet in the house was missed, or is on a different schedule.
• The dose was wrong for your dog's weight. Estimating rather than checking is common.
• A spot-on was applied to a recently bathed dog, or the dog was bathed too soon afterwards.
• The product was out of date, or stored somewhere hot.
• The house was never treated, so the reservoir is still full.

If none of those apply and you are still finding live fleas after two months of consistent treatment, speak to your vet rather than simply switching brands.

Some intestinal all-wormers contain an active ingredient that would prevent heartworm if it were given every month.

Used on the usual three-monthly worming schedule, it does not, because heartworm prevention only works at monthly intervals. The active ingredient is right and the timing is wrong, and the timing is what counts.

What to check on the pack: the words "prevents heartworm" or "heartworm prevention", and the dosing interval. If it says monthly, it can. If it says every three months, question it.

Our guide to intestinal worms in dogs covers what each wormer class actually treats.

Restart prevention straight away, then talk to your vet about when to test.

Restarting immediately is right in every case. What changes with the length of the gap is the testing plan, not the restart.

If you missed one month

Give the dose as soon as you realise and carry on with the normal schedule. A single missed month is the common case and usually ends there. Mention it at your dog's next visit so the annual test is read with that in mind.

If you missed several months or longer

Restart, then speak to your vet about testing. They will usually want a test around six months after the gap rather than immediately, and there is a reason for the wait.

Why the test has to wait

Two things are happening. The antigen test only detects mature worms, so testing a month after a lapse can return a negative while larvae are still developing, which is false reassurance rather than a clean result. And a dog that has developed a heartworm infection can be carrying microfilariae in its blood, where giving a preventative carries a risk of reaction.

Neither is a reason to delay restarting prevention. Both are reasons to let your vet set the testing date rather than assuming a quick negative means everything is fine.

Rescue and imported dogs

A dog with no known prevention history is the highest-risk category there is, and it is worth treating as such. If you have taken on a rescue, particularly one that came from northern Australia, ask about heartworm testing as part of the initial health check rather than waiting for the next annual visit.

Treating heartworm means killing adult worms inside the heart and lungs, which is why it carries real risk and demands months of rest.

The drug used is melarsomine, given by injection in a veterinary hospital across a staged course. Preventative medication is usually given for around two months beforehand. After the injections, most adult worms die and are cleared over one to three months.

The danger is in the dying. As worms break down, fragments travel into the lung arteries, and that is what causes the serious complications. This is why physical activity must be restricted as soon as the diagnosis is confirmed, because exertion increases the rate at which the worms damage the heart and lungs.

That restriction is the part owners consistently under-anticipate. It is not a quiet week. It is months of no running, no off-lead exercise and no stairs where avoidable, for a dog that has no idea why. For a young active dog, it is the hardest part of the whole process.

Treatment is also considerably more expensive than prevention, and involves repeat hospital visits and follow-up testing.

None of that is a reason to panic if your dog has just been diagnosed. Dogs are treated successfully every year. It is a reason to take prevention seriously if your dog is not yet in that position.

Our heartworm prevention tablets for dogs range covers the monthly options, from heartworm-only tablets to combinations that fold in fleas, ticks and intestinal worms.

To compare the monthly options properly, our guide to the best flea, tick and worm treatments for dogs sets them side by side on coverage and dosing.

If you came here weighing up whether your dog needs prevention, the honest answer is that it depends on where you live, how your dog spends its time, and where you take it. 

That is a conversation with your vet, and it is worth having properly rather than defaulting either way. If you have already decided, the thing that matters most is not which product you pick but whether the dose actually gets given. 

Our parasite protection guide covers how heartworm fits alongside fleas and ticks in one plan.