Adult heartworms can reach 12 inches long and live five to seven years in dogs. Heavy infestations damage the heart, lungs, and blood vessels, causing coughing, exercise intolerance, and potentially fatal heart failure. Cats are atypical hosts and may show respiratory signs or sudden death with even one or two worms. Year-round monthly preventive medication is the cornerstone of protection and is far safer and cheaper than treatment. Diagnosis in dogs uses a simple blood antigen test. Treatment involves months of activity restriction and injectable medication to kill adult worms. There is no approved treatment for cats, making prevention even more critical.