Heartworm Prevention for Dogs
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Heartworm is spread by mosquito bites and prevented with a monthly medication that clears recent larvae before they mature into adult worms. The American Heartworm Society advises year-round prevention for all dogs, plus a yearly test — because treating an established infection is costly, risky and far harder than preventing it.
Heartworm disease is serious and potentially fatal, but almost entirely preventable with a simple monthly routine. This guide explains how the disease spreads, why prevention has to run year-round, what testing looks like, how prevention compares to treatment on cost, and what to do if a dose slips.
How heartworm spreads
Heartworm has one route into a dog: the bite of an infected mosquito. When a mosquito bites an infected animal it picks up microscopic larvae, then deposits them into the next dog it bites. Over roughly six to seven months those larvae migrate and mature into adult worms — which can grow to a foot long — living in the heart and the arteries of the lungs. There they damage blood vessels and, over time, cause coughing, exercise intolerance, heart failure and death if untreated.
Because it travels by mosquito, heartworm is not passed directly between dogs, and no amount of keeping your dog away from other dogs prevents it. Any dog a mosquito can reach — indoors or out — is at risk.
Why prevention must be year-round
The American Heartworm Society recommends year-round prevention for every dog. There are a few reasons this matters more than it might seem:
- Preventives clear larvae, not adults. Monthly products work retroactively — they kill the larvae a dog picked up over the previous month. They do not kill adult worms, so once an infection is established, prevention alone cannot fix it.
- The 30-day window is unforgiving. Dosing must stay on a monthly rhythm; a late or skipped dose can let larvae survive long enough to mature past the point a preventive can catch them.
- Mosquitoes aren't strictly seasonal. Warm spells, indoor mosquitoes and shifting climates make exposure hard to predict, so continuous protection is the safe default.
Prevention vs. treatment: the cost
The financial case for prevention is stark, and the health case is stronger still — treatment is hard on a dog and requires strict cage rest for weeks. These are ballpark United States figures, not quotes.
| Item | Typical cost (USD) | Notes |
|---|---|---|
| Monthly preventive | ~$11–$25 / month | Roughly $130–$300 per year |
| Annual heartworm test | ~$55–$130 | Usually a quick blood antigen test |
| Prevention, per year (all-in) | ~$185–$430 | Preventive plus the yearly test |
| Treating an infected dog | ~$1,200–$1,800 | Higher for larger dogs or complications; low-cost programs may be less |
Put simply, a year of prevention typically costs a small fraction of a single course of treatment — and treatment carries real risk to the dog, which prevention avoids entirely.
Testing: when and how often
Prevention and testing go together. Even a well-protected dog is tested to catch any gap early, and preventives should generally be started only after confirming a dog is heartworm-negative.
- Annual antigen test for most dogs, usually a simple in-clinic blood test.
- Puppies can begin prevention young (often by 8 weeks), but their first test is at around 7 months, because the infection cannot be detected before the worms mature.
- After any lapse in prevention, your vet will test promptly, then retest about six months later, since newly acquired worms take months to show up.
What a missed dose means
A single late dose isn't a reason to panic, but it isn't nothing either. The concern is that during the gap your dog may have picked up larvae that a resumed dose can no longer kill once they've matured. The general approach:
- Give the missed dose as soon as you notice and return to your monthly schedule.
- Contact your vet and follow their advice.
- Expect to test now and retest around six months later, because an infection contracted during the gap won't be detectable immediately — heartworms must be roughly seven months old before a test can find them.
The monthly dose that can't be late
Tavi keeps your dog's heartworm preventive on a strict monthly rhythm and reminds you before the 30-day window closes — plus a nudge for the yearly test. Free for your first pet.
Choosing a preventive
Heartworm preventives come as monthly chews, monthly topicals and a longer-acting injection given at the vet. Many products also cover intestinal worms, fleas or ticks in one dose, which can simplify your routine. Your vet will recommend a product based on your dog's weight, health and what else you want it to cover. It pairs naturally with your dog's deworming schedule and flea and tick prevention.
Frequently asked questions
How do dogs get heartworm?
Heartworm is spread only by mosquito bites. An infected mosquito injects immature heartworm larvae into a dog's skin; over about six to seven months they mature into adult worms that live in the heart and lung arteries. It is not passed directly from dog to dog. Any dog bitten by an infected mosquito is at risk.
Why is year-round heartworm prevention recommended?
The American Heartworm Society recommends year-round prevention for all dogs. Mosquitoes can be active outside the warm months, monthly preventives work by clearing recent larvae rather than adult worms, and giving them every 30 days without gaps keeps that protection continuous. Year-round dosing also builds a reliable habit and avoids risky lapses.
How often should a dog be tested for heartworm?
Most dogs should have an annual heartworm test, usually a blood antigen test, even when on prevention. Puppies can start prevention young but need their first test at around seven months, because the infection cannot be detected earlier. After any lapse in prevention, your vet will retest sooner and again six months later.
How much does heartworm prevention cost versus treatment?
In the United States, monthly prevention commonly costs about 11 to 25 dollars, roughly 130 to 300 dollars a year, plus an annual test around 55 to 130 dollars. Treating an infected dog often costs about 1,200 to 1,800 dollars and is hard on the dog. Prevention is far cheaper and safer.
What happens if I miss a dose of heartworm prevention?
Give the dose as soon as you remember and get back on the monthly schedule, then contact your vet. Because a gap can allow an infection that testing cannot yet detect, vets generally advise testing now and retesting about six months later. Do not simply skip ahead without your vet's guidance.
Do indoor dogs need heartworm prevention?
Yes. Mosquitoes get indoors easily, and heartworm has been diagnosed in all types of dogs, including indoor ones. Because a single infected mosquito bite is enough and treatment is costly and risky, most veterinarians recommend year-round prevention for indoor and outdoor dogs alike.
Tavi's guides are informational and not a substitute for professional veterinary advice, diagnosis or treatment. Costs, products and testing intervals vary by region, clinic and your dog's health — always follow your own veterinarian's recommendations, and when in doubt, contact your vet. Sources include the American Heartworm Society, the American Veterinary Medical Association (AVMA), PetMD, the American Kennel Club (AKC) and the Merck Veterinary Manual.