Heartworm disease may cause a combination of medical problems within the same dog including dysfunction of the lungs, heart, liver and kidneys. The worms are found in the right side of the heart and in the major vessels that bring blood to and from the right chambers, where they cause inflammation and interfere with blood flow. This primarily causes pulmonary thromboembolisms (clots in the lungs) and congestive heart failure. It can also lead to liver or kidney failure. Death can be caused by one or a combination of these problems.