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Heartworm in Dogs and Cats: Signs, Testing and Prevention

Heartworm is spread by mosquitoes and has been found in all 50 US states. How it harms dogs and cats, how it's tested for, and why year-round prevention is advised.

Updated October 2026 · 7 min read · By the Hushku editorial team · 1 sources

Short answer

Heartworm is a parasitic worm spread by mosquito bites that lives in the heart and lungs. The American Heartworm Society recommends year-round prevention for dogs and cats and a heartworm test for dogs every year. Infected dogs can be treated; there is no approved treatment for cats, so prevention matters even more.

Key facts

Heartworm is spread by mosquitoes and has been found in all 50 US states.
The American Heartworm Society recommends year-round prevention and annual testing for dogs.
There's no approved treatment to kill heartworms in cats.
Indoor pets can be infected because mosquitoes get inside.

Definition

Heartworm disease is caused by Dirofilaria immitis, a worm transmitted by mosquitoes. Adult worms live in the heart, lungs and blood vessels, causing lung disease, heart failure and organ damage.

When an infected mosquito bites a pet, it leaves tiny larvae that migrate through the body and grow into adult worms over about six months. In dogs, adult worms can reach a foot long and live for years, and a dog can carry dozens. Cats are less natural hosts and usually have fewer worms, but even immature worms can cause serious lung disease, sometimes called heartworm-associated respiratory disease (HARD).

Heartworm was once thought of as a disease of the southern US, but the American Heartworm Society reports cases in all 50 states. Indoor pets are not fully protected, because mosquitoes get inside.

Signs and symptoms

Often none in early infection
A mild, persistent cough
Tiring easily on walks
Weight loss and poor appetite
A swollen belly from fluid (late stage)
In cats: coughing, asthma-like attacks, vomiting, or sudden collapse

Causes and risk factors

Bites from infected mosquitoes
Missed or late doses of preventive
Living in or travelling to areas with heartworm (which now includes most of the US)

Treatment

Dogs: a staged course of injections that kill adult worms, with strict exercise restriction for weeks
Supportive care and medicines to reduce complications
Cats: no approved treatment to kill the worms; care focuses on managing signs

Prevention

Year-round heartworm preventive (monthly chewable or topical, or an injection every 6 or 12 months for dogs)
Annual heartworm testing for dogs, even on prevention
Testing before starting prevention in dogs over 7 months old
Mosquito control around the home

Why prevention is given all year

Preventives work by killing larvae picked up in the previous month. Because a missed month can let larvae mature, and mosquito seasons are getting longer, the American Heartworm Society advises giving prevention every month of the year, even where winters are cold.

Why dogs need an annual test

No preventive is 100% effective, and doses get missed or vomited. An annual test catches infection early, when treatment is easier. Starting prevention in an already-infected dog can cause a reaction, which is why dogs over about 7 months are tested first.

Heartworm in cats

Cats can be infected even if they live indoors. Testing is harder in cats, and there's no approved drug to kill the worms, so monthly prevention is the main protection. Ask your vet about combined flea, worm and heartworm products.

Free tools for this

Puppy and kitten vaccination schedule

Sources

American Heartworm Society: Heartworm facts

We check guides against veterinary sources and update them when guidance changes. How we write and review.

Questions.

Do indoor cats need heartworm prevention?

Many vets recommend it, because mosquitoes get indoors and there is no approved treatment for cats once they're infected. Ask your vet about the risk where you live.

Can heartworm spread from dog to dog?

No. It needs a mosquito to pass from one animal to another.

Is heartworm treatment dangerous?

Treatment in dogs is effective but carries risks as the worms die, which is why dogs must be kept very quiet for weeks during treatment. Prevention is far safer and cheaper.