Symptoms of Heartworms in Dogs and Cats

Heartworm disease symptoms

Pets with heartworm disease often cough. They find it difficult to exert themselves and don't enjoy exercise. Most pets lose weight, and some develop episodes of fainting. Cats with heartworm disease develop asthma-like symptoms and their hearts enlarge. Unfortunately heartworm disease in cats is often fatal once signs of the disease occur. Left untreated, dogs experience signs associated with lung disease and heart failure.


  • Coughing
  • Weight loss
  • Edema
  • Exercise Intolerance
  • Fainting


  • Difficulty breathing
  • Coughing/gagging
  • Vomiting
  • Weight loss
  • Lethargy
How is heartworm disease diagnosed?

Heartworm disease is diagnosed with blood tests and X-rays. Some pets have blood tests that may also show a high level of eosinophils, the type of white blood cell that is elevated in the presence of allergies or parasites.

X-rays diagnose heartworm disease by revealing an unusually large heart that is expanded on the right side—it is enlarged because the chamber has adapted to holding too much blood. The pulmonary vessels and lungs are also unusual: enlarged and tortuous because the heartworms reside in these areas. In some pets, x-rays show an allergic reaction throughout the lungs that is caused by the heartworms.

Why does my pet need a blood test for heartworm medication?

Heartworm medication requires an annual test and a prescription because this medication has the potential to harm—even kill—a pet that is already infected with heartworms.