Heartworms begin their lifecycle as microscopic larvae that are transmitted to hosts through the bite of a mosquito. The worms travel through the bloodstream and lodge in the heart and lungs, where they can grow up to a foot long, clogging the heart and the blood vessels in the lungs. Although there may be no sign of heartworm disease in the early stages, animals with heartworms will eventually show symptoms such as coughing, low tolerance for exercise, and generally poor condition. Both cats and dogs can acquire heartworms. Cats may experience symptoms like vomiting, fluid in the abdomen, or seizures. Without treatment, animals gradually become sicker, and the disease can even lead to death.