Signs of Heartworm Disease

Heartworm disease is sneaky. In the early stages, many dogs show no symptoms at all, which is exactly why annual testing matters so much. As the infection progresses, signs become more noticeable.

Early stages (mild infection):

• Occasional, mild cough

• Slightly reduced energy or exercise tolerance

• Otherwise appears healthy

Moderate infection:

• Persistent cough

• Tiring easily after mild activity

• Reluctance to exercise

• Appearing "off" even at rest

Severe infection:

• Labored or rapid breathing

• Significant weight loss

• Swollen or distended abdomen (fluid buildup)

• Persistent cough, sometimes with blood

• Fainting or collapsing after activity

• Pale or bluish gums

Caval Syndrome (life-threatening emergency): In the most severe cases, such a large mass of worms accumulates that it physically blocks blood from returning to the heart. Signs include sudden collapse, dark or bloody urine, extreme weakness, and cardiovascular shock. This requires emergency surgical worm removal, there is no other option. Without it, death typically occurs within 24–72 hours.

If your dog shows any signs of breathing difficulty, sudden collapse, or pale gums, go to an emergency vet immediately.