Worms in the Class Cestoda are generally flat, segmented and ribbon-like, and are popularly known as tapeworms. They have no gut and absorb nutrients across the body wall. The adults of most tapeworms of vertebrates live in the alimentary tract. Cestode life cycles are often complex and involve intermediate hosts.
Although adult tapeworms can cause disease, it is the cystic forms in intermediate hosts that are of greatest overall veterinary and public health importance. Thus, some species live in the intestines of dogs and cause no detectable harm to them, but can grow into football-sized cysts in the liver and lungs of humans unfortunate enough to ingest the eggs. A closely related tapeworm forms cysts in the brains of sheep. Ingestion of cysts in meat and fish can also cause disease in humans, ranging from the benign to the life-threatening depending on the tapeworm species involved, and they are kept out of the food chain by careful meat inspection. Adult tapeworms in the intestine have also been associated with intestinal upsets and anaemia in humans, and with colic in horses. The complex life cycles of the tapeworms of veterinary significance present a challenge to control, and mathematical modelling is an increasingly useful tool in the development of effective and efficient control strategies.