What is a tapeworm? Most people know that tapeworms live in the intestine and that they come from food that was eaten raw or undercooked — but that’s where the knowledge ends. Beyond that, folklore, superstition, and a general cultural revulsion make tapeworms a mystery to most.
There are many different species of tapeworms (cestodes, belonging to the taxonomic class Cestoda), almost all living in the intestines of vertebrates (animals with backbones) in their adult stage. Many of them have not been thoroughly studied and new species are still being discovered, but science has learned a lot about this group of animals nonetheless.
Physical Description of Tapeworms
The typical adult tapeworm is a long slender worm comprising a head, or scolex; a neck; and a long slender strip of segments (proglottids) called a strobila. The size of the adult worm varies from tiny, with a strobila of just one or two proglottids to many meters long with thousands of proglottids. The anatomy of a tapeworm allows it to attach to the intestinal wall, obtain food from intestinal contents, and reproduce.
- The scolex has hooks, suckers, grooves, tentacles or similar structures, depending on the species, all of which allow firm attachment to the intestinal wall. Though it’s often referred to as a head, the scolex does not have eyes, ears, nose, a mouth, or teeth, and functions only as an organ of attachment.
- The neck produces the proglottids that become the strobila, adding them one by one so that the worm grows where the strobila attaches to the scolex — the proglottids at the tail end of the worm are the oldest.
- Proglottids contain the reproductive organs. In almost all species, each proglottid is hermaphroditic (has both male and female organs), and produces both eggs and sperm; however, in general, the sperm of one proglottid fertilize the eggs of another, rather than each proglottid fertilizing itself.
How Do Tapeworms Eat?
If a tapeworm has no mouth, how does it eat? Tapeworms absorb nutrients from the intestinal contents around them through their outer covering, or tegument. Though the tegument of a tapeworm appears relatively smooth to the naked eye, it is actually a surface composed of many projections, rather like a shag rug, providing enormous surface area for absorption. The tegument also protects the tapeworm from being digested by its host.
Tapeworm Young
Only the adult tapeworm lives in the host’s intestine. Young — larval forms — live in other animals, usually in the tissues or body cavities. The complete life cycles of tapeworms are complicated and generally require specific conditions; only a few known species can complete their entire life cycle in a single host.
For example, the beef tapeworm, Taenia saginata, lives as an adult in the human intestine, and as a larva, or cysticercus, in a cow’s muscle tissue. The fresh water fish tapeworm, Diphyllobothrium latum, lives as an adult in the human intestine, as a first stage larva, or procercoid, in a copepod, and as a second stage larva, or plerocercoid, in a fish.
Tapeworm eggs are released into the environment in feces, and either hatch after being eaten by the next host, or hatch in water with the free swimming larva being eaten by the next host.
Fascinating Facts About Tapeworms
Though there’s no shortage of myth and folklore about tapeworms, the truth is strange enough:
- Though adults live attached firmly to the intestinal wall, at least some species are capable of moving to a new attachment site, and some do so regularly.
- Proglottids eventually detach from the tail end of the worm and are passed in feces or digested by the host. In some species proglottids can contract and move on their own, and may crawl out of the host through the anus. In other cases, long pale strips of proglottids are passed with a bowel movement.
- Though most intestinal tapeworms cause few symptoms or health problems, D. latum can compete with the host for dietary vitamin B12 resulting in anemia.
- In the few tapeworm species that have male and female individuals, it appears that each worm has the potential to become either one, depending on what’s needed. Infections involving these species typically involve one male and one female worm.
Sources:
Foundations of Parasitology 6th ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
Clinical Parasitology 9th ed. Beaver, Paul Chester, Rodney Clifton Jung, and Eddie Wayne Cupp. Philadelphia: Lea and Febiger, 1984.
Diagnostic Medical Parasitology 3rd ed. Garcia, Lynn S. and David A. Bruckner. Washington: ASM Press, 1997.
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