Fleas play a necessary role in the life cycle and reproduction of a certain type of tapeworm. Here is how:
The adult form of the tapeworm Dipylidium caninum lives in the small intestines of dogs and cats. The worm is made up of multiple segments. One at a time, the segments, full of eggs, are passed in the feces. While warm, the segments are active, but as they dry, they break open and liberate the eggs inside. A flea larva ingests the eggs. The egg develops into an immature form in the flea. When a dog or cat eats the flea (usually while the animal is grooming), the immature form of the tapeworm is released from the flea. This immature tapeworm then develops into an adult in the dog's or cat's intestine and the life cycle is completed.
The tapeworm D. caninum can not be passed directly from cat to cat or dog to dog. It must live part of its life in the flea. We need to treat an animal with tapeworms with a medication that will kill the tapeworms. But if we do not also control the flea problem, your animal has a good chance of becoming reinfected with tapeworms.
Pets may be infected with other types of tapeworms in which the larval form of the parasite can be found in rodents, pigs, or fish, instead of fleas.