1. The adult dog tapeworm lives in the intestine of dogs and other canines (called the definitive hosts).
2. The adult tapeworms release eggs, which are passed in stool.
3. After the eggs are consumed by other animals (called intermediate hosts)—usually, sheep, goats, pigs, cattle, horses, camels, or people—the eggs hatch in the intestine and release spheres (called oncospheres) that contain tapeworm larvae. The spheres penetrate the wall of the intestine.
4. Then, the spheres travel through the bloodstream to various organs, such as the liver and lungs. In these organs, the spheres develop into cysts, which enlarge gradually and which, in people, can cause symptoms. Larvae (called protoscolices) and smaller cysts form within the cyst. Dogs and other canines (such as foxes or coyotes) become infected by consuming cysts in the organs of the infected intermediate host (such as a sheep, goat, or pig).
5–6. After a dog or other canine consumes the cysts, the cysts release the protoscolices, which attach to the wall of the intestine and develop into adults.
Image from the Centers for Disease Control and Prevention Image Library.