Tapeworms (Dipylidium caninum and Taenia spp.) are the largest of the intestinal parasites that infect dogs and cats. An adult tapeworm can reach 8 inches in length, although the part of the worm that is observed in stool is only a fraction of an inch. They get their name from being thin and flat. They are well tolerated by the host, so an animal may have tapeworms for years with little or no symptoms.
Tapeworms have a strange but very effective method of finding and infecting a host animal. The adult worm attaches to the intestinal wall and grows a long tail. Tail segments of the worm filled with eggs are passed intermittently in the stool. The segments dry in the environment and rupture. Flea larvae consume the eggs, and the tapeworm begins its larval stage inside the developing flea. Once an adult, the flea finds a host in order to take blood meals. The pestered pet licks and grooms itself and accidentally ingests the flea. Then, inside the animal, the tapeworm emerges and reaches full maturity in the intestine. Tapeworms can also form cysts in the liver of rodents and rabbits that ingest the eggs. A dog or cat that consumes the prey can become infected with the tapeworm cyst that will continue its life inside the primary host.
Tapeworms feed on nutrients that pass along the digestive tract, but not enough to cause malnutrition or weight loss. Occasionally, a pet may vomit the entire tapeworm, but usually only the segments are seen passing from the rectum. In fact, because the eggs are encapsulated in a segment, they are usually not seen on a fecal flotation exam performed by the veterinarian. Sometimes an owner may not be aware that a pet has had fleas until tapeworm segments are seen.
De-worming causes the tapeworm to lose its natural protection from digestive enzymes, and so it is digested and not passed when the pet is de-wormed. The medication does not prevent re-infection; it only removes the existing tapeworms. Flea prevention is the only way to prevent subsequent infection.