Heartworms are parasites that are able to live inside the arteries of the lung and chambers of a pet's heart and feed on surrounding blood. In severe cases heartworms can grow up to 30cm long and 2cm thick in populations of over 200. The offspring of a heartworm are more commonly referred to as Microfilariae, which can be found in the blood of an infected pet. - See more at: http://www.vetwest.com.au/pet-library/heartworm-disease#sthash.qaOSbj5P.dpuf