According to the American Heartworm Society, coughing, difficult breathing, fatigue, vomiting and loss of appetite are common symptoms of heartworms in cats. Cats can die suddenly after being infected with heartworms even in early stages of the infection.
Heartworms initially infest the lungs and bronchial tubes causing cats to exhibit signs similar to asthma or bronchitis. After a period of four to six months, the worms and the larvae move into the heart and cause chronic heart trouble, which is often worsened by running or other exercise. Because cats are not ideal hosts for the parasites, the worms die quickly once inside the body. Inflammation in the heart, lungs and arteries occurs both before and after the heartworms die. Inflammation and bloody sputum can occur as early as four months after the cat contracts the parasites.
Serious heartworm infestations can cause congestive heart failure and damage blood vessels, according to WebMD. Liver and kidney failure are also possible when heartworms are left untreated. Vomiting and changes in eating patterns, including refusal to eat, occur during all stages of the infection.
Cats can also experience sudden death after becoming infected. Heartworms are often larger than the cat's arteries, which can result in significant damage to the heart within six months. Fainting, blindness and fluid in the chest cavity are less common symptoms found in cats with heartworms. Typically, cats experience a range of symptoms that include respiratory problems, anorexia and digestive issues that last for the duration of the infection.