Signs and Symptoms of Heartworm in Dogs
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Heartworm symptoms often appear only after damage is done. Here's what to watch for — and why prevention is everything.
Early stages are silent
In the early stages, dogs often show no symptoms at all — which is exactly why heartworm is so dangerous. By the time signs appear, worms may already be damaging the heart and lungs.
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Symptoms to watch for
As disease progresses, look for a persistent mild cough, reluctance to exercise, fatigue after activity, decreased appetite, and weight loss. Advanced cases can cause a swollen belly and heart failure.
Why testing matters
Because early disease is invisible, annual heartworm testing is essential — even for dogs on prevention. Testing catches infection early and confirms it's safe to continue preventives.
Prevention beats treatment
Treatment is expensive, lengthy, and hard on the dog. A monthly preventive like [Heartgard Plus](/products/heartgard-plus-chewables) avoids all of it. Claim our [free pet savings card](/card) to save on prevention.
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Frequently asked
- What are the first signs of heartworm in dogs?
- Early infection is often symptom-free. The first noticeable signs are usually a mild persistent cough and tiring easily during exercise.
- How is heartworm diagnosed?
- A simple blood test detects heartworm infection. Annual testing is recommended even for dogs on prevention, to catch any infection early.