By Hannah Reid, GoPetr staff writer · Updated 3 August 2026
General information only. This article is not a substitute for advice from a registered veterinarian. Every medical point below links to an authoritative source, but your vet knows your dog — consult a licensed vet for diagnosis or treatment specific to your pet.
Spring is nearly here, and with the first warm, still evenings the mosquitoes come back. For a lot of Australian dog owners that’s the cue to finally think about heartworm — and the timing is a trap. Heartworm (Dirofilaria immitis) is a mosquito-borne parasite that can quietly damage a dog’s heart and lungs for months before you notice anything wrong, and by then it’s a serious, expensive problem. Here’s what the disease actually is, why Australian vets treat prevention as a year-round job rather than a summer one, and how the options stack up.
Do dogs need heartworm prevention all year in Australia? Yes. Because mosquito activity is hard to predict across most of the country and even a single missed window can let an infection take hold, the standard advice from Australian vets is continuous, year-round prevention rather than seasonal dosing.
How dogs catch heartworm
Heartworm spreads through mosquitoes, not directly from dog to dog. RSPCA explains that an infected mosquito injects larvae into your dog’s bloodstream with a single bite. From there it’s a slow build: the Australian Veterinary Association says it takes roughly six months for those larvae to mature into adult worms that live in the heart and the blood vessels around the lungs. Early on there are usually no signs at all. As the worm burden grows, the AVA lists exercise intolerance, lethargy and a dry, persistent cough, progressing in severe cases to rapid, shallow breathing, a swollen abdomen and even sudden death after exercise. RSPCA notes the worms can ultimately cause heart and lung failure.
Why ‘summer only’ is the wrong mindset
Heartworm was long framed as a tropical and subtropical issue — Queensland, northern NSW, the Top End and northern WA. But peer-reviewed surveillance has documented the parasite in dogs across a much wider slice of the country, including cooler southern regions. Pair that spread with a simple reality — you can’t reliably predict when mosquitoes will be active in your postcode from one year to the next — and the seasonal approach falls apart. The AVA warns that even one missed dose can put a dog at risk, which is why year-round cover is the norm here. The same logic underpins year-round flea and tick protection, and the higher-risk northern areas overlap with paralysis tick country.
Prevention options compared
There’s no single ‘best’ product — the best one is the one you’ll actually keep up with. The yearly injection is often given at the same visit as your dog’s annual vaccination, which suits owners who struggle to remember monthly doses. Note that the injection and heartworm-only tablets cover heartworm alone, so you’ll still need separate flea and intestinal-worm protection.
| Option | What it covers | Approx. cost (AUD) | Best suited to |
|---|---|---|---|
| Monthly heartworm-only tablet (e.g. Valuheart, Heartgard) | Heartworm only | ~$35–$80 / year | Owners who already use separate flea, tick and worm products |
| Monthly all-in-one chew (e.g. NexGard Spectra, Simparica Trio, Interceptor Spectrum) | Heartworm + intestinal worms + fleas (some add ticks) | ~$180–$320 / year | People who want one product for most parasites |
| Annual injection (ProHeart SR-12) | Heartworm only, 12 months | Vet-administered, weight-based | Owners who forget monthly doses; given at the yearly check-up |
One caution: even the best product only works if it’s given on time. Pet Circle notes that a large share of dogs diagnosed with heartworm were technically ‘on’ monthly prevention when doses were missed, spat out or given late — consistency matters as much as the product you pick.
Test before you (re)start — the step people skip
If your dog has been on continuous prevention since puppyhood, you’re set. But if your dog has never been protected, or cover has lapsed for more than a few months, don’t just restart on your own. Greencross Vets recommends proper diagnosis first — usually a quick blood test — because starting some preventatives in a dog that already carries heartworm can trigger a dangerous reaction. A test now is cheap insurance.
Quick self-check — talk to your vet if any of these apply:
- Your dog has never been on heartworm prevention.
- You’ve missed more than one or two monthly doses.
- You’re switching from monthly dosing to the annual injection.
- You’ve recently adopted a dog with an unknown history.
- You’ve moved to or travelled through a higher-risk northern region.
What treatment looks like (and why prevention wins)
Treatment exists, but it isn’t simple. Greencross notes the registered adulticide is Immiticide (melarsomine), given by injection, and your dog must be strictly rested throughout — the more confined and calm they are, the lower the risk of a dead worm blocking a lung vessel (thromboembolism). It’s costly, too: Pet Circle puts treatment for even a mildly affected dog at roughly $1,200–$2,000, and more if the disease is advanced. Prevention, by comparison, costs a few dollars a week at most.
Frequently asked questions
Does my dog really need heartworm prevention in winter?
In most of Australia, yes. Mosquito seasons are unpredictable and, as the AVA points out, one missed window is enough for an infection to start, so continuous cover is the safer default.
Is heartworm the same as the worms in my dog’s poo?
No. Intestinal worms live in the gut; heartworm lives in the heart and lungs and only spreads through mosquitoes. If you’re due for a gut-worm treatment as well, see our intestinal worming guide.
Can cats get heartworm?
Cats can be infected, though it’s less common and behaves differently in them than in dogs. If you have cats, ask your vet which feline prevention option suits your situation.
Key takeaways
- Heartworm is mosquito-borne and can seriously damage the heart and lungs before any symptoms appear.
- Australian vets recommend year-round prevention, not summer-only dosing.
- Options run from monthly tablets and all-in-one chews to a yearly injection — choose what you’ll actually maintain.
- Get a blood test first if your dog is new to prevention or has lapsed.
- Prevention costs a fraction of treatment.
Sources
- Australian Veterinary Association (Vet Voice) — Heartworm disease in dogs and cats
- RSPCA Queensland — Heartworm and prevention
- Greencross Vets — Heartworm disease and treatment
- Pet Circle (vet-reviewed) — Heartworm treatment and prevention
- Parasites & Vectors — Mosquito-borne Dirofilaria immitis in dogs from Australia
About the author: Hannah Reid writes about pet health and ownership for GoPetr. She is not a veterinarian; each article is researched against veterinary and animal-welfare sources, and questions about your own dog should always go to your vet.
