Class: Cestoda (tapeworms)
Metazoa (animals) > Bilateria > Lophotrochozoa >
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
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,
- 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.