Dipylidium caninum (Linnaeus, 1758)
The Double-pored Dog Tapeworm (Dipylidium caninum) mainly infects dogs and cats, but is occasionally found in humans. The distribution of this tapeworm is worldwide and human infections have been reported from Europe, the Philippines, China, Japan, Argentina, and the United States.
Gravid proglottids (bisexual reproductive segments) are passed intact in the feces from the definitive host (i.e., the host in which adult parasites occur) or emerge from the perianal region of the host. Subsequently, these proglottids release typical egg packets. On rare occasions, proglottids rupture and egg packets are seen in stool samples. Following ingestion of an egg by an intermediate host (larval stages of the Dog or Cat Flea, Ctenocephalides spp.), an oncosphere is released into the intermediate host's intestine. The oncosphere penetrates the intestinal wall, invades the insect's hemocoel (body cavity), and develops into a cysticercoid larva. The larva develops into an adult and the adult flea harbors the infective cysticercoid. The vertebrate host becomes infected by ingesting an adult flea containing a cysticercoid. The domestic dog is the principal definitive host of D. caninum. Other potential hosts include cats, foxes, and humans (mainly children). Humans acquire infection by ingesting a cysticercoid-contaminated flea. This can result from close contact between children and their infected pets. In the small intestine of the vertebrate host, the cysticercoid develops into an adult tapeworm, reaching maturity around 1 month post-infection. The adult tapeworms (which reach up to around 60 cm in length and 3 mm in width) reside in the small intestine of the host, where each attaches by its scolex (the anterior part of the tapeworm that is specialized for attachment to the gut wall of the host). Each worm produces proglottids with two genital pores. The proglottids mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool, bringing the life cycle full circle.