Cestoda (tapeworms)

Class: Cestoda (tapeworms)

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Lophotrochozoa > Platyhelminthes

Adult cestodes are known as tapeworms. (The name cestode is derived from the Greek kestos, meaning a strap). Several thousand species have been described and all are parasitic. Some are of great economic and medical importance. Depending on species they vary from a few millimetres to about 30 metres in lebgth. They are perhaps the most specialised of the worm parasites, as indicated by the lack of a digestive system. (Who needs one when nutrients can be absorbed through the body surface?) A further consequence of a parasitic life style is that much body space is given over to reproductive organs.

The adult tapeworm usually lives in the digestive tract of a vertebrate host and it is essentially an egg-producing machine with an anchor, the scolex, armed with suckers and a rostellum of hooks. The egg-producing part, the strobilus, is a chain of flattened, and usually rectangular, segments called proglottids (sing. proglottis). New proglottids develop from the neck between scolex and strobilus. As older proglottids are pushed away by growth of new ones at the neck, a flat tape-like chain is produced. Each proglottis has male and female reproductive organs but the male system matures first. In the older proglottides the female system is predominant. By folding of the strobilus in the intestine of the host the active male proglottides are brought into contact with the older proglottides containing mature female organs and fertilization of the eggs can occur. The eggs gow as they mature so that the proglottis eventually becomes just a bag of eggs. Eventually it detaches from the end of the tape and may disintegrate to release the eggs, or it may pass out in the faeces. A number of larval stages may occur in vertebrates or invertebrates according to the species of worm.

Pork (Taenia solium) and beef (Taenia saginata) tapeworms are so similar that they can be described together. Taenia solium grows to a length of about 4 metres and Taenia saginata to about 6. Both have two vertebrates hosts in the life cycle, the adult worm always being found in humans. Symptoms of infection in humans include diarrhea and weight loss. The adult worm releases eggs that pass out with the faeces. When these are ingested by the second host, they hatch in the gut as hook-bearing hexacanth larvae or oncospheres (Greek hex = six + akantha = a prickle; onkos = a tumour because the oncosphere swells tumour-like into a cysticercus). The hexacanth penetrates the gut wall and travels through the blood to muscle and nervous tissue of the second host (normally a pig or cow ). Here it develops into a cysticercus, 10-12 mm in diameter, with a scolex inverted like the finger of an inside-out glove. When a human ingests the cysticercus, the scolex everts and fixes the larva to the gut wall. Proglottides develop from the neck and a new tapeworm adult completes the cycle.

Eggs of Taenia solium can also infect humans, developing into cysticerci in the tissues just as they do in a pig. Cysticerci can become lodged in human muscle tissue, heart, brain, spinal cord or eye. This stage is clearly more harmful than is infection with the adult worm. A significant proportion of human hosts with adult Taenia solium also harbour cysticerci in their tissues. It may be that regurgitation of proglottids from the small intestine to the stomach induces ripe eggs to hatch, releasing infective hexacanth larvae that give rise to cysticerci, which penetrate the gut wall and become lodged in the tissues of the human host.

Fleas are the intermediate hosts of the dog tapeworm, Dipylydium caninum. Flea larvae become infected when they ingest tapeworm eggs. In the larval flea's gut a hexacanth larva hatches from the egg and migrates into the body tissues, where it develops into a cysticercoid larva and remains in the adult flea after metamorphosis. The final host (dog or cat) becomes infected if it eats a flea containing a cysticercoid during grooming. The cysticercoid develops into an adult worm in the final host's gut. The life cycle is completed when mature proglottides are shed and escape from the anus to later burst and liberate eggs that flea larvae can ingest. Humans may become infected with the adult worm if a flea containing a cysticercoid is accidentally ingested.

Publications (by date)

Verster AJM. 1965. Review of Echinococcus species in South Africa. Onderstepoort J.Vet. Res. 32 (1965) 7-118.
Alexander S, McLaughlin JD. 1993. Microsomacanthus macrotesticulata n. sp. (Cestoda: Hymenolepididae) from South African waterfowl. Journal of Parasitology 79(6): 843-846.
Khalil LF, Jones A, Bray RA (eds.). 1994. Keys to the cestode parasites of vertebrates. CAB International, Wallingford.
Retief NR, Avenant-Oldewage A, Du Preez H. 2007. Ecological aspects of the occurrence of Asian tapeworm, Bothriocephalus acheilognathi Yamaguti, 1934 infection in the largemouth yellowfish, Labeobarbus kimberleyensis (Gilchrist and Thompson, 1913) in the Vaal Dam, South Africa. Physics and Chemistry of the Earth 32: 1384–1390.
Madanire-Moyo G, Barson M. 2010. Diversity of metazoan parasites of the African catfish Clarias gariepinus as indicators of pollution in a subtropical African river system. Journal of Helminthology 84: 216–227. doi: 10.1017/S0022149X09990563