Heartworm in Dogs: Signs, Testing and Prevention
Quick answer
Heartworm is a serious, potentially fatal disease caused by worms living in the heart, lungs and blood vessels, spread by mosquito bites. The American Heartworm Society says many dogs show few or no signs early on, so it advises testing every 12 months and giving a preventive 12 months a year ("think 12"). It has been diagnosed in all 50 US states. Most infected dogs can be treated, but treatment is complex.
From the American Heartworm Society. Not veterinary or financial advice. Worried about your pet now?
On this page (9 sections)
What it is and how it spreads
The American Heartworm Society describes heartworm disease as serious and potentially fatal, caused by long worms that live in the heart, lungs and blood vessels. Mosquitoes spread it: a mosquito picks up microscopic young worms from an infected animal and passes larvae to the next animal when it bites. Dogs, cats and ferrets can be infected, and wild animals such as coyotes and foxes can carry it. Source: American Heartworm Society, Heartworm basics.
Signs in dogs
Early on, many dogs show few or no signs. The Society lists:
- A mild, persistent cough
- Reluctance to exercise, and tiredness after moderate activity
- Reduced appetite and weight loss
- In advanced disease, heart failure and a swollen belly
Heavy infections can cause a sudden, life-threatening collapse with laboured breathing and dark urine. That is an emergency: find an emergency vet. Source: American Heartworm Society, Heartworm basics.
Testing
The Society says dogs should be tested every year, usually at a routine visit. Puppies under seven months can start prevention without a test, and are then tested six months after the first visit, six months after that, and yearly from then on. Adult dogs not already on prevention are tested before starting. Yearly testing still matters for dogs on prevention, because preventives are highly effective but not 100 percent. Source: American Heartworm Society, Heartworm basics.
Prevention: "think 12"
The Society's message is "think 12": test every 12 months and give preventive 12 months a year. Heartworm preventives are prescribed by a vet, who will choose the product for your dog. Source: American Heartworm Society, Heartworm basics.
Treatment
Most infected dogs can be successfully treated, the Society says, but treatment is complex: the dog may need stabilising first, and exercise has to be restricted. Prevention is far simpler than treatment. Source: American Heartworm Society, Heartworm basics.
Cats get heartworm too
Signs in cats can be very subtle or very dramatic: coughing, asthma-like attacks, vomiting, and loss of appetite or weight. Sudden collapse can be the first sign. The Society says there is no approved drug treatment for cats, and the drug used for dogs is not safe for them, so prevention matters for indoor and outdoor cats alike. Source: American Heartworm Society, Heartworm basics.
Where it is found
Heartworm has been diagnosed in all 50 US states and in many other parts of the world, and the Society says its range is spreading. If you travel with your dog, ask your vet about heartworm before you go. Source: American Heartworm Society, Heartworm basics.
Questions people ask
What are the first signs of heartworm in dogs?
The American Heartworm Society says many dogs show few or no signs early on. Signs can include a mild persistent cough, reluctance to exercise, tiredness after moderate activity, reduced appetite and weight loss.
How do dogs get heartworm?
From mosquito bites. A mosquito picks up microscopic young worms from an infected animal and passes larvae to the next animal it bites.
How often should a dog be tested for heartworm?
The American Heartworm Society advises testing every 12 months, even for dogs on prevention, and giving preventive 12 months a year.
Can heartworm in dogs be treated?
The American Heartworm Society says most infected dogs can be successfully treated, but treatment is complex and involves restricting exercise. There is no approved drug treatment for cats.
Sources
- American Heartworm Society, Heartworm basics, read 11 November 2026.
Related pages
Lungworm in dogs, flea treatment in winter, kennel cough and everything for dog owners.