Dipylidium caninum

Dipylidium caninum is a world-wide parasite of dogs and cats that requires a flea intermediate host to develop. Therefore its range is dependent upon the availability of both flea and vertebrate hosts, as well as the ability to survive outside of the host until ingested by a flea. (Roberts and Janovy, 1996)

The first habitat for these organisms is feces of the definitive hosts, where they are still eggs. Next they live in fleas, which accquire the parasites by eating the feces. If the flea is eaten by a dog, the larval worm finds itself in the intestine where it becomes an adult and remains the rest of its life. Humans can also host the worms. We accquire them by accidentally ingesting fleas from a pet that has fleas containing juvenile worms. (Reddy, 1982)

A Dipylidium caninum adult is a long flat worm, around 40 to 50 cm. The body is made up of the head or scolex, the neck, and a segmented section called the strobilus. The scolex has hooks for attachment. Each segment contains two proglottids. A proglottid is one set of reproductive organs. Dipylidium caninum is often identified by examing segments passed in feces. Dipylidium caninum has two genital pores located laterally on each segment, with two proglottids per segment. Segments are often described as resembling cucumber seeds, and are quite active when seen outside their hosts in fecal material. Larvae are called oncospheres generally, but specifically for D. caninum. Larvae of Eucestoda are termed "hexacanth" because of the six hooks on the posterior end. (Chappell, et al., 1990; Roberts and Janovy, 1996)

Dipylidium caninum is monoecious, or hermaphroditic. Each segment of its strobilus has two sets of male and female reproductive organs (proglottids). Each proglottid will eventually contain about a dozen eggs. Segments break free from the body of the worm and exit the host via fecal excretion, where they are then available for ingestion by the intermediate host, fleas. Fleas feeding on dog feces ingest the segment. Inside the flea the egg develops into the larval form called the cysticercoid, which is not sexually mature. When it finally reaches its definitive host it can reach sexual maturity. (Roberts and Janovy, 1996)

When Dipylidium caninum finally reaches its definitive host it can reach sexual maturity. Only one individual is necessary for reproduction since these worms are monecious, or hermaphroditic. Each segment of its strobilus has two sets of male and female reproductive organs (proglottids). Each proglottid will eventually contain about a dozen eggs. Segments break free from the body of the worm and exit via feces. Because of the complexity of this process it is necessary for these worms to have high reproductive potentials. In fact, tapeworms are known to produce anywhere from a few to millions of eggs in their lifetimes. (Roberts and Janovy, 1996)

Dipylidium caninum, known as the dog tapeworm, is a parasite of dogs, and sometimes cats and humans. Human infections occur most commonly in children. Its life cycle requires the use of two hosts, a flea and a vertebrate. The tapeworm eggs are ingested by flea larvae, where development reaches the larval cysticercoid stage. The vertebrate host is reached when an infected flea is ingested by the vertebrate. The worm reaches adulthood in the vertebrate and proglottids containing eggs (gravid proglottids) are excreted with feces to renew the cycle. The hexacanth larva uses its hooks to transport itself from the gut of the flea into the hemocoel. In the hemocoel, larval development continues into further juvenile stages. When detached segments are excreted from the vertebrate host, they are mobile and can easily be seen crawling around. (Chappell, et al., 1990; Roberts and Janovy, 1996)

Cestodes in general have sensory organs in the scolex, which are attached to longitudinal nerves extending down the body. The nerves are attached to organs and the cestodes can detect tactile stimulation. (Brusca and Brusca, 2003)

Dipylidium caninum attaches itself to the intestinal lumen of its definitive host (dogs, cats, or sometimes humans) as an adult. Its hooked scolex is specialized to hold it in place in the intestine. Dipylidium caninum, and all cestodes lack digestive tracts. It feeds by absorption through its body covering, or tegument. Because of this absorption method of feeding it is logical that the worms have evolved to locate themselves in the intestines of their hosts, where the partially digested food is of maximum benefit. (Neafie and Marty, 1993; Roberts and Janovy, 1996)

The tapeworms are probably not intentially eaten. However, mortality is high at the egg and larval stages because the tapeworms do not reach a suitable host.

Infection with D. caninum is often asymptomatic in humans, though there are some reports of abdominal pain, diarrhea, irritability, and anal pruritis (Reddy, 1982). There is no discussion of pathogenicity in the dog or cat hosts, however Chappell states that infections in humans are usually limited to one worm. If the same is true for dogs and cats, then the effects of infestation should be similar. Other reports show contrary evidence, that up to 25% of infections involve multiple worms in human cases, though no difference in pathogenicity was mentioned. (Currier 1973)

Almost all infections in humans are found in children, even infants (Reid et. al, 1992). The most likely cause of this pattern of infection is the proximity and duration of play between children and canine or feline pets. Behavior that is particulary advantageous from the tapeworm's point of view is mouth to mouth contacts between human and animal, because a recently nipped flea can still be on the mouth of the pet and then be passed into the human. A doctor from Delaware advised that "the habit of kissing canines should not be encouraged". (Reddy 1982) (Currier, et al., 1973; Reddy, 1982; Reid, et al., 1992)