Heartworm Disease in Dogs: Symptoms, Prevention, Testing and Cost

How dogs get heartworm

When a mosquito bites an infected animal (a dog, coyote or fox), it picks up baby worms. In 10 to 14 days they develop inside the mosquito into infective larvae, and the next time it bites a dog, it passes them on. It takes about 6 months for the larvae to grow into adult worms, which can live 5 to 7 years in a dog. A dog can carry several hundred worms, which damage the heart, lungs and arteries over time.

Dogs don't catch heartworm directly from each other: a mosquito is always needed. That's also why indoor dogs aren't safe, since mosquitoes get inside.

Symptoms

In the early stages, many dogs show no signs at all, which is why yearly testing matters. As the disease progresses:

A mild, persistent cough.
Reluctance to exercise and tiring after moderate activity.
Decreased appetite and weight loss.
In advanced cases: a swollen belly from fluid buildup and heart failure.

Go to an emergency vet right away if a dog with known or suspected heartworm suddenly has labored breathing, pale gums or dark, bloody urine, or collapses: large numbers of worms can block blood flow (caval syndrome), and only emergency surgery can save them.

Prevention

Start puppies on a preventive as early as the product label allows, and no later than 8 weeks of age.
Give it every month of the year, not just in mosquito season: missed doses leave gaps.
Options include monthly chewables, monthly topicals and injections lasting 6 or 12 months. Many monthly products also treat intestinal worms, and some cover fleas and ticks too. We compare them in how often to deworm a dog.
Preventives require a prescription, and your vet will test your dog before starting one if they're 7 months or older.

Flea, tick and heartworm prevention together cost about $295-$360 a year for an adult dog, according to Rover's 2026 cost survey.

Testing

The American Heartworm Society recommends:

Puppies: start prevention by 8 weeks; test at about 7 months old, again 6 months later, and yearly after that.
Adult dogs: a heartworm test every 12 months, even if they're on year-round prevention, to make sure it's working.

A heartworm test costs about $71 on average in the US (Synchrony/CareCredit 2025-2026 cost study), and it's usually done at the yearly checkup.

Treatment if a dog tests positive

Treatment is possible, but it's expensive, takes months and carries risks:

Your vet will confirm the diagnosis and stage the disease with more tests (bloodwork, X-rays).
The FDA-approved drug, melarsomine, is given as injections in the hospital: typically one injection, a month of rest, then two injections 24 hours apart.
Your dog needs strict rest (crate rest, leash walks only) during treatment and for weeks afterward, because dying worms can block blood vessels in the lungs.
About 9 months after treatment, a test confirms that all worms are gone. Then year-round prevention for life.

Treatment for a medium-size dog averages $1,200-$1,800, according to CareCredit, and severe cases cost more. Prevention for a whole year usually costs far less than that.

What about cats?

Cats can get heartworm too, usually with just a few worms, but there's no approved treatment for heartworm in cats. That's why preventives are also recommended for cats, including indoor cats. Ask your vet.

Frequently asked questions

Do dogs need heartworm prevention in winter?

Yes. The American Heartworm Society recommends year-round prevention everywhere in the US: mosquito seasons vary from year to year, some mosquitoes survive indoors, and a monthly routine is easier to keep.

Can I buy heartworm medicine without a vet?

In the US, heartworm preventives require a veterinarian's prescription, and a negative test is needed before starting a dog older than 7 months on them, because giving some preventives to an infected dog can cause a dangerous reaction.

Is heartworm contagious to people?

No, people are not a normal host. Heartworm is spread only through mosquito bites from infected animals.

Sources

This article is for general information and does not replace diagnosis or treatment by a veterinarian. If you are worried about your pet's health, call your vet clinic.