The presence of clinical signs related to heartworms constitutes heartworm disease. Since the cat is not a normal host for heartworms, some variations do exist. These are the basis for important differences in diagnosing and treating feline heartworm disease. However, cats will usually have normal shaped hearts with pulmonary arteries that are blunted, presumably related to partial obstructions due to the presence of adult worms. Dogs usually have a persistent rise in eosinophils, a normal white blood cell which can be associated with parasitic infection.
Cats have an eosinophil increase that occurs briefly within a few months of infection.
By the time clinical signs are present, most cats have a normal eosinophil count. Because of some of the differences just mentioned, the typical approach to diagnosing heartworms in dogs is not always successful in cats. Since the cat has few, if any, microfilariae, the test to detect microfilariae in the blood is often negative. An elevated eosinophil count is helpful in dogs but will probably be normal in cats. Even the clinical signs seen in dogs are not very reliable in cats, especially when you consider that vomiting in cats may be related to many other disorders other than heartworms.