Everything You Need To Know About Heartworms In Cats & Dogs Every pet owner must know that heartworm disease is a serious disease that can be fatal, but, the good news is that it is entirely preventable. The worms are known as 'heartworms' as they live in the heart, lungs and associated blood vessels of your infected pet. The disease is caused by a parasitic worm 'Dirofilaria Immitis', which is spread through the bite of an infected mosquito.