The Class Cestoda contains about 4,000 species of tapeworms, all of which are highly modified endoparasites that live in just about every known vertebrate species. The long, flattened body of a tapeworm (which is referred to as the strobila) is divided into segments called proglottids. Most forms have an organ called a scolex at the anterior end with suckers, hooks, etc. that attach to the wall of the gut and prevent them from being swept away.

Tapeworms lack a digestive system and feed by absorbing nutrients directly from the host. The entire body surface is covered with minute projections called microtriches that greatly increase the absorptive surface area of the tapeworm. Tapeworms also secrete substances that inhibit the digestive enzymes of their host as well as lowering the pH around them to a level that they but not the digestive enzymes of their host can function. In tapeworms, much of the strobila is given over to reproduction. Each proglottid is monoecious, and cross-fertilization or even self-fertilization is common. Proglottids can be filled with up to 100,000 eggs!

The Tapeworm Life Cycle

With few exceptions, all cestodes require at least two hosts, and the adult is the parasite in the digestive tract of vertebrates. Often one of the intermediate hosts is an invertebrate (most often an arthropod such as a flea, louse or copepod) that is eaten by the final host. The eggs within the proglottids are shed daily in the feces into the soil where they may lie dormant for quite some time. Sometimes the egg-bearing proglottids crawl out of the anus by themselves and can be found wriggling about on an infected dog, cat or child or on infected clothing and bedding. Once the eggs are released, they must be ingested by an intermediate host in order to hatch into hooked larvae called oncospheres, which bore through the intestinal wall and picked up by the circulatory system where they are transported to skeletal muscle, heart or even some other organ where they encyst as cysticerci (bladder worms). Each cysticercus is essentially an inside-out scolex that everts after the infected tissue (so-called “measly meat”) of the intermediate host is eaten by the final host. The scolex then attaches to the lining of the intestine by means of suckers and/or hooks.

Tapeworm Infection in Humans

Humans can become infected with tapeworms by eating poorly cooked meat containing the cysticerci of the tapeworm. The most important tapeworms that infect humans are the beef tapeworm (Taenia saginata) and the pork tapeworm (Taenia solium).

Another species of cestode that can infect humans is the broad fish tapeworm (Diphyllobothrium latum), which is common in fish inhabiting the Great Lakes. Again, infection occurs by ingesting cysticerci in raw or poorly cooked fish. In most cases, tapeworms found in the gut do not cause much damage to their human hosts, but occasionally they migrate to other organs such as the eyes or even the brain, where they can cause serious neurological problems and even death from cerebral cysticercosis!

The dog tapeworm (Diplydium caninum) is common in dogs but can be picked up by humans (usually kids) who ingest infected fleas that serve as intermediate hosts of the parasite.