WHAT IS HEARTWORM DISEASE? Canine heartworm disease develops when a dog is bitten by a mosquito carrying the parasite. As a mosquito feeds, infection occurs and the parasite begins their migration into the dog’s bloodstream, where they grow into adult worms. Adult heartworms can grow 10 to 12 inches in length! They make their home in the heart and vessels of the lungs, often causing lung disease and heart failure. Although easy to prevent, heartworm disease continues to be a