Heartworm Disease in Cats

Heartworm disease in cats occurs when parasitic worms transmitted by mosquitoes settle in the blood vessels of the lungs and, less commonly, the heart. Unlike in dogs, where large numbers of adult worms may develop, cats typically harbour only a few worms, and many infections resolve without the worms reaching maturity. The condition behaves differently in cats than in dogs, and even a small worm burden can trigger significant inflammatory reactions in the lungs. Owners often arrive at this topic because their cat has developed respiratory signs—coughing, difficulty breathing, or episodes that resemble asthma—or because heartworm has been mentioned as a possibility during investigations for these symptoms. In some cases, cats show vague signs such as reduced appetite, weight loss, or vomiting, or the condition is discovered incidentally during testing for other reasons. Sudden collapse or death can occur, though this is uncommon. This page explores what signs may be observed, what is happening in the lungs and heart when heartworm is present, how the condition is investigated, and what approaches exist for managing affected cats and reducing future risk.