The term "monthly heartworm" is a common misconception. Heartworms are actually parasites that live in the bloodstream and lungs of dogs, cats, and other mammals. The correct way to say it would be "preventive monthly medication for heartworm." This refers to giving your pet a preventative medication once a month to help protect them from getting heartworms. Heartworm disease is caused by the bite of an infected mosquito and can cause severe health problems in pets, including death if left untreated. Preventive medications work by killing the larvae before they mature into adult worms that can damage a pet's heart and lungs.