The short answer
A mild, persistent cough. But here’s the problem: by the time you notice symptoms, the worms have been growing inside your dog’s heart and lungs for months. Heartworm is one of those diseases where waiting for symptoms means waiting too long.
The early signs
The first thing most owners notice is a soft cough that doesn’t go away. It tends to come on after exercise or excitement and can be easy to dismiss as kennel cough or a minor respiratory issue. But unlike a cold, it doesn’t clear up in a week or two.
Shortly after, you might notice your dog tiring more quickly on walks. A dog that used to go the full loop around the neighbourhood without slowing down now wants to stop and rest. Exercise intolerance is one of the more reliable early indicators.
Decreased appetite and mild weight loss can follow. These are subtle and easy to miss, especially in dogs with irregular eating habits.
Why symptoms come late
After a mosquito deposits heartworm larvae into your dog, it takes 6 to 7 months for those larvae to mature into adults living in the heart and pulmonary arteries. During that entire period, there are no visible symptoms. The dog looks and acts completely normal.
Symptoms only appear once the adult worms start affecting blood flow through the heart and lungs. By that point, you’re dealing with an established infection that requires serious treatment.
This is exactly why annual heartworm testing matters more than symptom-watching. A blood test in spring catches infections from the previous summer, well before your dog shows signs. Getting tested before starting seasonal prevention is standard practice.
Advanced symptoms
If heartworm goes undetected and untreated, the signs get worse:
Persistent coughing becomes more frequent. Laboured breathing, even at rest. Lethargy beyond normal tiredness. Swollen belly from fluid buildup as the heart struggles. Pale gums. Collapse after exertion.
In severe cases, a sudden blockage of blood flow called caval syndrome can cause collapse and dark-coloured urine. This is a medical emergency. Without immediate surgical intervention, it’s usually fatal.
Dogs can survive heartworm with treatment, but outcomes are much better when the infection is caught early through routine testing rather than after advanced symptoms develop.
Key takeaways
- A mild persistent cough is usually the first sign, often after exercise.
- Symptoms don’t appear until 6 to 7 months after infection, when adult worms are established.
- Exercise intolerance, decreased appetite, and weight loss follow.
- Annual blood testing catches heartworm before symptoms appear. Don’t rely on symptoms alone.
- Advanced heartworm causes heart failure, fluid buildup, and can be fatal without intervention.
References
- American Heartworm Society. “Heartworms in Dogs.” heartwormsociety.org
- VCA Animal Hospitals. “Heartworm Disease in Dogs.” vcahospitals.com
- Cornell University College of Veterinary Medicine. “Heartworm Disease.” vet.cornell.edu
- PetMD. “Heartworm Disease in Dogs: Symptoms, Causes and Treatment.” petmd.com