Heartworm disease is a serious and potentially fatal disease in pets in the United States that is transmitted by mosquitoes. Heartworms live in the heart, lungs and associated blood vessels of affected pets, causing severe lung disease, heart failure and damage to other organs in the body. Heartworm disease affects dogs, cats and ferrets, but heartworms also live in other mammal species, including wolves, coyotes, and foxes. This disease is easily preventable with a monthly heartworm medication for your pet. Dogs should be tested annually, and puppies can be started on preventative as soon as 8 weeks. Most heartworm medications also protect your pet against other parasites, such as roundworms, hookworms, whipworms, and ear mites.