The determination of whether a dog or a cat becomes infected is based solely on the mosquito's choice of its next meal. For many months, there may be little or no apparent change in the cat. Strangely, some cats with heartworms have a history of vomiting with no signs of heart failure. Another presentation of cats with heartworms is sudden death. The pumping action of the blood sends some of the worms or blood clots out of the heart and into the lungs.
Cats can tolerate a few adult heartworms in the heart for several months.
However, as the number of worms increases or the worms increase in size, the cat begins to show the clinical signs that we just discussed. 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.