Yes, heartworms are parasites that can set up camp in your pet's heart and lungs. They can multiply rapidly, numbering into the hundreds, and each worm can measure 12 inches.
Some symptoms of heartworms are coughing, lethargy, fatigue, weight loss and difficulty in breathing, but some animals show no symptoms.
Please have your pet checked for heartworms regularly. If untreated, heartworm infestation will kill your pet. However, an annual blood test from your veterinarian can diagnose heartworms, and treatment can begin.
The heartworm condition is VERY preventable! A chewable pill that can be given daily or monthly is available by prescription at your vet's office and will keep heartworms away. -- Heloise
Response letter from the American Heartworm Society.