Heartworm disease may cause a combination of medical
problems in the same dog including dysfunction of the lungs, heart, liver and
kidneys. The disease may have an acute onset but usually begins with barely
detectable signs resulting from a chronic infection and a combination of
physiologic changes. Dogs with a low number of adult worms in the body that are
not exercised strenuously may never have apparent signs of heartworm disease.
However, in most dogs, the heart and lungs are the major organs affected by
heartworms with varying degrees of clinical signs.