Roundworms are the most common type of worms found in dogs. They can be contracted from contaminated soil or feces. Hookworms attach to the intestinal wall and feed on the dog's blood, potentially causing anemia. Tapeworms can be contracted by ingesting fleas that carry tapeworm larvae. Whipworms live in the large intestine and can cause gastrointestinal issues. Heartworms are transmitted by mosquitoes and reside in the heart, which can be fatal if left untreated.