Photo of Heartworm Disease in Dogs Heartworms. It is the diagnosis that every pet owner dreads hearing during a routine checkup. Hearts drop at the thought of what might happen to a beloved best friend. But what exactly are heartworms, and is the disease as serious as it sounds? According to Humane Society of North Texas veterinarian Dr. Cynthia Jones, heartworm disease is the resulting damage caused by parasitic worms, which can lead to severe lung damage, heart failure, and various organ...