Heartworm disease is a life threatening parasitic disease caused by a long, thin worm that lives in the blood vessels and heart of infected cats and dogs. The disease is spread by mosquitoes that bite a dog or cat with the Heartworm infection and then passes it on to another dog or cat. The microscopic worms in the infected dog or cat will grow into a parasite exceeding a foot in length. The disease impacts the lungs, arteries of the lungs, and the heart. Symptoms include tiring, coughing, weight loss, and heart failure.