How do dogs get heartworm?

From a single mosquito bite. An infected mosquito deposits larvae that mature into worms living in the heart, lungs, and nearby blood vessels. It is not contagious between dogs directly, the mosquito is the carrier, which is why heartworm exists anywhere mosquitoes do, meaning all fifty US states.

How much does heartworm prevention cost versus treatment?

Prevention is roughly $70 to $200 a year, about $10 to $20 a month. Treatment of an infected dog commonly runs around $1,000 to $1,800 and can exceed $3,000 once diagnostics and hospitalization are included. Prevention costs a fraction of the cure and avoids the risk.

Is heartworm treatment dangerous for dogs?

It is arduous. The core drug, melarsomine, is arsenic-based and given by deep injection, and the dog must be kept on strict exercise restriction for months because dying worms can cause life-threatening blood clots in the lungs. It works, but it is hard on the dog and the owner.

Do indoor dogs need heartworm prevention?

Yes. Mosquitoes get indoors, and heartworm has been found in all fifty states, so even mostly-indoor dogs are at risk. The American Heartworm Society recommends year-round prevention for all dogs, paired with annual testing.

Can cats get heartworm?

Yes, and it is a special problem. There is no approved drug to kill adult heartworms in cats, so prevention is the only protection. Even one or two worms can cause serious lung disease, sometimes mistaken for asthma, and can be fatal.

Heartworm in Dogs: $14-a-Month Prevention vs a $1,000-Plus Cure

Here is the short version: heartworm is one of the rare health threats where the right choice is genuinely obvious. The worms are spread by mosquitoes, they grow up to a foot long inside the heart and lungs, and they are now found in every US state. Preventing them costs a few dollars a month. Treating them costs many times that, takes months, and is hard on the dog. This guide lays out how it spreads, where it is rising, what treatment really involves, and why cats are a separate, harder case.

What heartworm actually is

Heartworm disease is caused by a parasite, Dirofilaria immitis, spread by mosquito bites.¹ When an infected mosquito bites a dog, it deposits tiny larvae that migrate and mature over months into adult worms living in the heart, the lungs, and the blood vessels between them.¹ Over time they damage those organs and can cause heart failure and death. Because the mosquito does the spreading, a dog does not have to meet another infected dog to be at risk, it just has to be bitten.

It is everywhere now, and spreading

Heartworm has been diagnosed in all fifty states, and the American Heartworm Society estimates that at least 1.1 million US dogs are infected.² Its latest incidence map, built from over a million tests, showed the disease intensifying and reaching areas that used to see little of it. For the first time, Texas topped the nation, and the parasite turned up in unexpected cooler and drier regions from parts of the West to New England.³ The practical message is that "we don't have heartworm here" is no longer a safe assumption anywhere.

all 50 states

now report heartworm, with an estimated 1.1 million-plus infected US dogs and rising spread into new regions

American Heartworm Society; AVMA, 2026

Prevention: cheap, safe, monthly

Prevention is simple and effective. A class of drugs called macrocyclic lactones, given as a monthly chew or topical, or as a longer-acting injection, kills the immature worms before they can grow up.¹ The American Heartworm Society recommends year-round prevention for every dog, paired with an annual test, summed up in its "Think 12" message: twelve months of prevention, tested every twelve months.¹ The cost is roughly $70 to $200 a year, depending on the product and your dog's size.

Treatment: long, risky, and expensive

If a dog is already infected, the contrast is stark. Treatment centers on melarsomine, an arsenic-based drug given by deep injection to kill the adult worms, usually alongside an antibiotic (doxycycline) and a preventive, over a protocol that spans months.⁵ The hardest part for families is the strict exercise restriction: the dog must be kept quiet, often crated, for months, because as the worms die they can break loose and cause life-threatening clots in the lungs.¹ It works, but it asks a lot of the dog and the household.

The bill reflects that. Treatment commonly runs around $1,000 to $1,800, and can climb past $3,000 once you add diagnostics, chest imaging, and hospitalization, with the most severe cases higher still.⁵ Set against $70 to $200 a year for prevention, the math is hard to argue with, which is exactly the point the American Heartworm Society makes when it asks owners to "weigh the costs."⁴

~$1,000–$1,800+

typical cost to treat a heartworm-positive dog, often over $3,000 with diagnostics and hospitalization, versus $70–$200/year to prevent it

PetMD (veterinarian-reviewed); American Heartworm Society

Cats are a different, harder problem

Heartworm in cats deserves its own warning, because the rules change. There is no approved drug to kill adult heartworms in cats, so the treatment that exists for dogs is not an option, which makes prevention the only real protection.⁶ Cats also tend to carry just one or two worms, yet even that small burden can trigger serious lung inflammation, sometimes mistaken for asthma, and can cause sudden death.⁶ If you have cats, heartworm prevention belongs on the conversation list with your vet too.

What to do this week

If your dog is not on year-round heartworm prevention, ask your vet to start it, after the recommended test.
Do not skip the cold-weather months. "Mosquito season" is unreliable, and the Heartworm Society recommends twelve months of coverage.
Set a monthly phone reminder, or ask about a long-acting injectable, so a missed dose never opens a window.
If you have cats, especially indoor-outdoor ones, ask about feline heartworm prevention, since for cats there is no cure to fall back on.

Most of what this site covers involves weighing uncertain risks. Heartworm is not one of those. The disease is widespread and getting more so, the treatment is genuinely grueling, and the prevention is cheap, safe, and effective. If you do one thing after reading this, make sure every dog and cat in your home is covered, every month, all year.