If your dog or cat becomes infected with heartworms, the worms can cause serious damage to their lungs, heart, and blood vessels. Symptoms may include coughing, difficulty breathing, fatigue, weight loss, and in severe cases, heart failure or death. Treatment for heartworm disease can be complicated, expensive, and does not come without risks. It typically involves a series of injections to kill the adult worms, along with medications to manage symptoms and support the pet's recovery over a period of 9 months. Cats are more susceptible to severe and sometimes fatal reactions during treatment.