Heartgard Plus chewables contain the active ingredients Ivermectin and Pyrantel and are given once a month for the prevention of heartworm disease caused by the parasitic worm Dirofilaria Immitis in dogs and to control roundworms and hookworms. Heartgard Plus eliminates the heartworm larvae that infect the dog during the previous month. Heartworm is spread by infected mosquitoes. Heartworm disease is very serious and can cause death. Treatment is very involved, complex and expensive, therefore prevention is preferable to the cure.