Heartworm in Dogs: Prevention, Testing & Treatment (USA 2026)
Heartworm is one of the few dog diseases that's almost entirely preventable for a few dollars a month — yet treating it once the worms are established can cost thousands and put months of your dog's life on hold. The gap between those two numbers is the whole story. This guide walks through how mosquitoes spread heartworm, why your vet tests before handing you a preventive, and what treatment actually looks like if a dog tests positive, following American Heartworm Society (AHS) and CAPC guidance.
Heartworm spreads only through mosquito bites, never dog to dog, and worms mature in the heart and lungs over about six to seven months, often with no early signs. Vets test before starting a preventive because giving one to an already-infected dog can cause a dangerous reaction; the American Heartworm Society advises "Think 12" — test every 12 months and give a preventive year-round, since each monthly dose works retroactively on the prior month's larvae. Prevention costs roughly USD 6–18 a month, while treating an established infection commonly runs USD 1,000–3,000+ with melarsomine injections, doxycycline, and weeks of strict rest. Leaving it untreated risks progressive, often irreversible heart and lung damage. Confirm the right product and schedule with your veterinarian.
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
| Prevention option | How it's given | Good to know |
|---|---|---|
| Monthly chewable | A flavored tablet or chew, once a month | Many combos also cover fleas and/or intestinal worms |
| Monthly topical | Liquid applied to the skin, once a month | Useful for dogs that spit out or won't eat pills |
| 6-month injectable | Moxidectin shot given at the clinic | Removes the 'did I miss a dose?' problem for half a year |
| 12-month injectable | Longer-acting shot given at the clinic | One visit covers a full year; vet-administered only |
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.
Frequently asked questions
Can my dog catch heartworm from another dog?
No. Heartworm needs a mosquito as an intermediate host — a mosquito bites an infected animal, the larvae develop inside it for about two weeks, and then it passes them to your dog through a bite. It never spreads directly between dogs, which is why mosquito exposure, not contact with other dogs, is the risk that matters.
Why does my vet test before giving heartworm prevention?
Preventives kill larvae, not adult worms or circulating microfilariae. If a dog is already silently infected, starting a preventive can trigger a dangerous reaction as larvae die. Because antigen tests usually can't detect infection until about six to seven months after the bite, a dog over roughly seven months old that hasn't been on prevention should be tested first.
Does my dog need heartworm prevention in winter?
Yes — the American Heartworm Society recommends year-round prevention. Each monthly dose works retroactively, clearing the larvae from the previous month rather than protecting ahead, so a winter gap lets any larvae picked up on a mild day keep developing. Mosquitoes can be active indoors and on warm days, and heartworm has been found in all 50 states.
How much does heartworm prevention cost vs. treatment?
Monthly prevention typically runs about USD 6–18 per dose depending on your dog's weight and the product. Treating an established infection commonly costs USD 1,000–3,000 or more, including a pre-treatment workup (USD 200–500), melarsomine injections and hospital care (USD 500–1,500), plus weeks of restricted activity. Prevention is dramatically cheaper.
What happens if heartworm goes untreated?
Untreated heartworm is progressive. As adult worms accumulate in the heart and lungs, dogs develop exercise intolerance, coughing, and often irreversible cardiac and pulmonary damage, along with the risk of a fatal clot (pulmonary thromboembolism). The standard treatment — doxycycline plus melarsomine injections with strict rest — is far safer done early, so prompt veterinary care matters.
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