Dipylidiasis is a common tapeworm infection of dogs and cats caused by Dipylidium caninum. Linnaeus first described dipylidiasis in humans in 1758. Dipylidiasis has a worldwide distribution. Humans become infected by accidental ingestion of dog or cat fleas that contain D caninum cysticercoids (larva). The risk of infection to humans is low. Dipylidiasis in humans occurs through accidental ingestion of the dog or cat flea or the dog louse infected with cysticercoids (the larval form of D caninum). These fleas and lice are the intermediate host for D caninum. Cysticercoids develop into adult worms in the small intestine of the host in about 20 days. The adult worm may attain a length of 10-70 cm and is 2-3 mm in diameter. The worms have a lifespan of less than 1 year.