Heartworm is a nasty, potentially fatal disease caused by parasitic worms living in an animal’s heart and blood vessels. It’s a bad deal. Fortunately, Banfield Pet Hospital’s 2016 State of Pet Health Report found a 33.1 percent decline in the number of confirmed heartworm cases in their hospitals between 2011 and 2016. Woo! This trend has a lot to do with proactive prevention on the part of pet owners. Here are three simple things you can do to protect your dog from this horrible disease.