Heartworm in Dogs: Symptoms, Prevention & Treatment Explained
Veterinary references checked with PetHealth+ · Last reviewed June 2026
How dogs get heartworm
When a mosquito bites an infected animal, it picks up microscopic heartworm larvae. If that mosquito then bites your dog, larvae enter the bloodstream, mature over months, and settle in the heart and nearby blood vessels. They can grow up to 30 cm long and live for years, damaging blood flow and lung tissue.
Heartworm is widespread in warm, humid regions (including parts of the USA, southern Europe, and elsewhere) and is spreading as climates change. Your vet can tell you whether your area is at risk.
Symptoms of heartworm disease
Early infection may show no signs. As worms accumulate, you may notice:
- Soft, persistent cough
- Tiring easily on walks
- Reduced appetite and weight loss
- Reluctance to exercise
- Swollen belly (fluid from heart failure)
- Collapse in severe cases
Because symptoms appear late, many dogs are diagnosed on a routine test before they look ill — which is why regular screening matters.
Prevention vs treatment
Prevention — Monthly tablets, chews, or topical products (prescription-only in most countries) kill larvae before they mature. They are safe, affordable, and vastly preferable to treating disease.
Testing — An annual blood test detects adult worms. Dogs should be tested before starting prevention and regularly thereafter.
Treatment — If a dog has active heartworm, killing adult worms requires a controlled veterinary protocol (often including strict rest), carries risk of complications, and takes months. It is not a simple course of tablets.
Never give preventive medication to a dog with unknown heartworm status without testing first — consult your vet.
Cats and heartworm
Cats can also get heartworm, though they are less typical hosts. There is no approved adulticide treatment for cats in many regions — prevention is the main strategy where the parasite is present.
Related guides: Dog vaccinations · How to care for a dog · Fleas and ticks in pets
Frequently asked questions
Can heartworm be cured in dogs?
Yes, but treatment is costly, lengthy, and carries risks. Prevention is far safer and cheaper than treating established disease.
Do indoor dogs need heartworm prevention?
In many regions, yes — mosquitoes can enter homes. Follow your vet's advice based on local heartworm prevalence.
How often should dogs be tested for heartworm?
Most vets recommend an annual blood test, even for dogs on prevention, to catch any breakthrough infection early.
Is heartworm contagious between dogs?
No — it spreads only through mosquito bites, not direct contact between dogs.