Heartworm in Dogs: Prevention, Testing & Treatment (USA 2026)

How a dog actually gets heartworm

It always starts with a mosquito — heartworm can't pass directly from dog to dog. A mosquito bites an infected animal, picks up microscopic larvae (microfilariae), and those larvae develop inside the mosquito for about two weeks. When that mosquito bites your dog, it deposits infective larvae into the skin. From there the worms migrate through the body, reaching the heart and lungs and maturing into foot-long adults over roughly six to seven months, at which point they start producing new microfilariae. That long, silent runway is exactly why heartworm is so easy to miss early — many dogs show no signs at all until there's real damage to the heart and lungs.

Heartworm prevention options compared

Why the vet tests before starting prevention

Test first: AHS and CAPC advise testing a dog for heartworm before starting or restarting a preventive. The reason is safety: monthly preventives target larvae, not the adult worms or the microfilariae circulating in an already-infected dog. Giving a preventive to a dog that's silently positive can trigger a serious reaction as microfilariae die off. Antigen tests generally can't detect an infection until about six to seven months after the bite, so a dog older than about seven months that hasn't been on prevention should be tested first.

Then test every year: AHS sums it up as "Think 12" — test every 12 months, and give a preventive 12 months a year. Annual testing matters even for dogs that never miss a dose, because a pill can be vomited, spat out, or poorly absorbed, and no product is 100% foolproof. Vets also typically recheck about six months after a dog starts or switches products, so any gap gets caught while it's still early.

Why prevention has to be year-round

It's tempting to stop preventives in winter, but that's where dogs slip through the cracks. Monthly preventives work retroactively — each dose clears the larvae a dog picked up over the previous month, rather than protecting forward. Skip a dose or stop for the season and those larvae keep developing toward adulthood unchecked. Mosquitoes can be active on any mild day, indoors and out, and heartworm has been diagnosed in all 50 US states. Year-round dosing removes the guesswork entirely, which is why AHS recommends it even in colder climates — the cost of consistency is trivial next to the alternative.

Prevention vs treatment: the cost gap

• Monthly preventive: roughly USD 6–18 per dose depending on the dog's weight and product
• 6- or 12-month injectable: given at the clinic, often comparable per-month
• Pre-treatment workup if positive (bloodwork, chest X-rays): USD 200–500
• Melarsomine injections plus hospital care: USD 500–1,500 by dog size
• Full heartworm treatment, all in: commonly USD 1,000–3,000+

Prevention runs a few dollars a month; treating an established infection routinely lands in the four figures — before you count the strict activity restriction and repeat visits. That lopsided math is the core argument for staying on prevention, and it's why most vets frame heartworm as a budgeting question as much as a medical one.

If a dog tests positive: treatment vs. no treatment

The AHS-recommended protocol pairs a monthly preventive with a 28-day course of doxycycline (which weakens Wolbachia bacteria the worms depend on) and a series of melarsomine injections — the only FDA-approved drug that kills adult heartworms. Studies show adding doxycycline lowers the rate of respiratory complications. The catch is that killing worms is risky in itself: strict crate rest and leash-only bathroom breaks for weeks to months are essential, because a spike in activity can dislodge dying worms and cause a fatal clot. A slower "slow-kill" (moxidectin plus doxycycline) route exists when melarsomine isn't an option, but it clears adults far more slowly and raises resistance concerns, so it's a fallback, not a shortcut. Simply not treating isn't neutral — untreated dogs face progressive, often irreversible heart and lung damage, exercise intolerance, and the risk of clots. Your vet stages the infection first and tailors the plan to it.

Track heartworm prevention with PetCare AI

The single biggest cause of preventable heartworm is a missed or forgotten dose. Log each monthly preventive and your dog's annual test date in PetCare AI's care calendar and get reminders so a gap never opens. Ask the AI vet assistant things like "When should my puppy start heartworm prevention?" to understand the timeline, and use the in-app finder to locate a nearby clinic for testing. Always confirm the specific product, dose, and test schedule with your veterinarian, who will match them to your dog's weight and history.