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 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.