Heartworms are parasitic worms which live in susceptible host bodies (dogs and other canids, cats, ferrets and even sea lions and rarely humans) and are transmitted in a complicated life cycle via mosquito vectors. After infection, these worms can live in the arteries of the lungs and in the heart and cause inflammation and tissue destruction in those locations. 

Symptoms of heartworm disease can range from no clinical signs to mild/moderate coughing, vomiting, exercise intolerance and weight loss to severe respiratory distress and sudden death. Cats tend to be more resistant to heartworms but can also exhibit signs such as vomiting, trouble breathing, weight loss and decreased appetite. 

Heartworm disease is usually detected in dogs via a blood test that checks for the presence of adult female heartworms, which is detectable 6-7 months after the initial infection. Blood testing for heartworm disease in cats checks for previous exposure and the presence of antibodies to the heartworms that the immune system produces. Evaluating the stage and severity of the heartworm disease may include other diagnostics such as a microfilaria test to check for the presence of baby heartworms, chest X-rays and possibly a cardiac ultrasound to evaluate secondary changes to the heart and lungs from chronic disease. 

Prevention of heartworm disease is usually accomplished with oral, topical or injectable medications that can range from daily, monthly to 6 month (only available for dogs) intervals. Prevention of heartworm disease prior to infection is much cheaper and safer than treating the disease after the animal is already infected. Preventatives need to be given on a regular basis to minimize the window of opportunity that the transmitted heartworm larvae have of maturing to adult heartworms. 

Treatment of heartworm disease in dogs consists of an injectable medication that kills the adult heartworms, preventing reinfection by young larval heartworms with preventatives, managing side effects from worm deaths using steroids and antibiotics, and limiting activity during the treatment period. Currently there is no approved medication to kill adult heartworms in cats and treatment is aimed at limiting clinical signs, although some cats may clear the infection naturally.