Heartworm is one of the most serious yet most preventable diseases your dog can face. Spread by a single mosquito bite, it develops silently over months before causing lasting damage to the heart and lungs. The good news is that a simple monthly preventive makes it almost entirely avoidable. Here's what every dog owner should understand about the symptoms, causes, diagnosis, treatment and prevention of heartworm.
What is heartworm disease?
Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. Adult worms — which can grow up to a foot long — live in the heart, lungs and associated blood vessels of an infected dog. Over time they cause inflammation, damage the vessels, and force the heart to work harder, eventually leading to heart failure and organ damage if left untreated. It's a progressive disease, which is why catching or preventing it early makes such a difference.
How dogs get heartworm (causes)
The lifecycle always runs through a mosquito. When a mosquito bites an infected animal, it picks up microscopic baby worms called microfilariae. Inside the mosquito these mature into infective larvae, and when that mosquito bites your dog, the larvae are deposited onto the skin and enter through the bite wound. Over about six months, they travel through the body and mature into adult worms in the heart and lungs. Because a mosquito is always required, heartworm can't pass directly between dogs — but anywhere mosquitoes live, dogs are at risk.
Symptoms and signs
Early heartworm is often completely silent, which is what makes it so dangerous. As worms accumulate and disease progresses, signs appear and worsen:
- A mild, persistent cough
- Tiredness or reluctance to exercise, and fatigue after moderate activity
- Reduced appetite and weight loss
- Difficulty breathing
- A swollen belly from fluid buildup in advanced cases
- In severe cases, collapse — a life-threatening emergency
Because symptoms only appear once damage is underway, annual testing is essential even for dogs that seem perfectly healthy.
Diagnosis
Vets diagnose heartworm primarily with a simple blood test that detects proteins from adult female worms (an antigen test), often paired with a test for microfilariae. If the result is positive or the dog shows symptoms, further tests such as X-rays, ultrasound and bloodwork help stage the disease and check how much damage has occurred. Testing is recommended every year, and before starting any new preventive, because giving preventives to an already-infected dog can be dangerous.
Treatment and management
Treating an established infection is a serious undertaking. It typically involves:
- Stabilisation of the dog and staging of the disease first.
- A course of medication to kill the worms, usually given as a series of deep injections over several weeks.
- Strict exercise restriction for weeks to months — this is critical, because as worms die, fragments can block blood vessels and cause life-threatening complications if the dog is active.
- Follow-up testing to confirm the infection has cleared.
The treatment is costly, lengthy and stressful for both dog and owner, which is exactly why prevention is so strongly recommended.
Year-round preventives make heartworm almost entirely avoidable. Pair vet-prescribed prevention with supportive wellness and parasite-support products to keep your dog protected.
Shop health & wellness →Prevention
Prevention is simple, safe and inexpensive compared with treatment. Vets recommend year-round preventive medication — available as monthly chewables, topicals or a long-acting injection — which kills larvae before they can mature. Combine this with annual heartworm testing so any breakthrough is caught early. Reducing mosquito exposure by limiting standing water around your home and keeping dogs indoors at peak mosquito times adds an extra layer of protection. Browse general health and wellness supplies to round out your dog's care routine.
When to see a vet
See your vet promptly if your dog develops a lingering cough, tires easily, loses weight, or seems short of breath — especially if heartworm prevention has lapsed. Any collapse, severe breathing difficulty, or a suddenly swollen belly is an emergency. Even without symptoms, book an annual heartworm test and talk to your vet about the right preventive for your dog and region.
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Download the MyFurtopia AppFrequently asked questions
How does a dog get heartworm?
Dogs get heartworm through the bite of an infected mosquito. The mosquito deposits tiny larvae onto the skin, which enter through the bite wound and, over several months, mature into worms that live in the heart and lungs. Heartworm cannot spread directly from dog to dog; a mosquito is always the go-between, which is why prevention focuses on year-round preventive medication.
What are the first signs of heartworm in dogs?
Early heartworm often shows no symptoms at all. As it progresses, the first signs are usually a mild persistent cough, tiredness after moderate activity, and reduced appetite. Later stages can bring weight loss, a swollen belly from fluid, and difficulty breathing. Because early disease is silent, annual testing is the only reliable way to catch it before symptoms appear.
Is heartworm in dogs curable?
Heartworm is treatable in most dogs, but the treatment is long, expensive and carries real risk, involving a series of injections and strict rest for months to prevent dangerous complications as the worms die. It's far safer and cheaper to prevent heartworm with year-round preventive medication than to treat an established infection, so prevention is always the better path.
This guide is educational and not a substitute for professional veterinary care. If you're worried about your dog, contact your vet.