For the past few tutorials we have been examining various clades within phylum Platyhelminthes.
We will finish up this phylum by looking at the class Cestoda. The cestodes are a clade
of parasitic flatworms, the most infamous of which are those of subclass Eucestoda,
commonly referred to as tapeworms. These have long flattened bodies that vary dramatically in length.
The smallest sexually mature specimens are about half a millimeter in length,
while the largest can grow to over thirty meters, or almost 100 feet, and inhabit sperm whales.
Their bodies begin with a headlike scolex, or holdfast, with two attachments called
bothria, or four suckers that it uses to attach to its host.
These scolexes are often also armed with hooks or spiny tentacles. Behind the scolex
is the strobila, the tapeworm’s main body. The strobila begins with a germinative zone,
sometimes referred to as the “neck” that forms new proglottids through mitotic division.
This body plan is unique among the flatworms, not only for the apparent proglotization, but also
because of the lack of any head region. Though the scolex superficially resembles a head, it
is actually derived from the posterior end of the larva. These larval forms are known as oncospheres
and have six hooks on their anterior end. However, some research suggests that as the larva grows
these hooks are detached, and on the opposite end, the posterior end, the scolex develops.
This would mean that tapeworms completely change the orientation of their bodies as they grow.
In addition, tapeworms do not have intestines, and all nutrients are absorbed by a specialized
surface layer of cells called neodermis, covered by unique appendages called microtriches, that are
found only in tapeworms and no other animals. Almost all tapeworms are hermaphrodites,
meaning they are both male and female at the same time, and each proglottid contains an
independent reproductive complex as well as a chain of excretory canals. In addition,
two nerve cords extend from the nerve ring in the scolex and run back through each proglottid.
Like the flukes, the tapeworm reproductive system takes up the majority of their bodies and includes
mostly numerous testes, seminal vesicles, sperm ducts, a copulatory organ called a cirrus, a
single lobed or unlobed ovary, vagina, and uterus with eggs and unique vitellarium or multiple
vitelline follicles, which produce vitelline cells that are included in the formation of eggs.
Both male and female parts share a common external opening known as the genital pore,
which is situated at the surface opening of the cup-shaped atrium. The uterus, meanwhile,
is separated by the uterine pore on the ventral side of the proglottid. These genital complexes
can also multiply within a single proglottid to two or even more sets of complexes,
maximizing the reproduction potential of the tapeworm. Often individual tapeworms will
cross-fertilize with other tapeworms in mutual exchange of sperm, however, if no other tapeworms
are present, individual proglottids can instead fertilize other proglottids or self-fertilize.
Since these worms are unable to synthesize lipids, which they need for reproduction, they are
entirely dependent upon the diet of their host, whose food they absorb through their body walls.
As the individual proglottids mature after fertilization they become gravid with eggs
and continue to grow as new proglottids are added from the tapeworm neck.
Eventually, they will break off or even crawl out of the anal cavity. An infected host can expel
numerous gravid proglottids, and some individual tapeworms, such as Dibothriocephalus latus, also
known as the broad fish tapeworm, can produce up to one million eggs per day
The most common human tapeworm is often called the “beef tapeworm”, Taenia saginata,
even though cows are their intermediate host and humans are their final host.
Shelled whole proglottids with eggs in human feces are consumed by cattle. The cow’s digestive system
dissolves their shells, and they burrow through the intestinal wall, eventually reaching the
muscle where they encrust themselves. Here they remain dormant in this stage called cysticercosis
unless the undercooked meat, such as rare burgers, steak, or barbeque, is consumed by a human.
The beef tapeworm is found around the world and is especially common in countries with a heavy
beef diet, such as the United States, where a voluntary study that represented over 28.6 million
cows found the infection rate to be between 10.8 and 18.4%. However, these parasites are
easily identified by routine veterinary control in Europe and the United States.
Other human tapeworms include the pork tapeworm, Taenia solium, and the broadfish tapeworm,
Dibothriocephalus latus. Both of these make their way into their final hosts through the
consumption of undercooked pork or fish. The pork tapeworm in particular can cause
cysticercosis. In addition, there are some cases where pork tapeworms migrate to other
parts of the body and cause damage to the liver, eyes, heart, and brain, or even induce epilepsy.
If you’re feeling a little worried right now, we should note that it is always more dangerous to be
an intermediate host of the tapeworm, rather than the final host. In total, less than 1,000 people
are treated for tapeworm infections annually in the United States, but the total number
of infections is probably much higher since they cause almost no discernable damage to their final
host. Tapeworms normally cause considerably less damage to their final hosts than the trematodes,
and many humans, especially those that regularly consume beef, may already house a tapeworm without
knowing it. Usually, the only symptom in adults is the presence of proglottids in bowel movements,
but unless the host is unhealthy, or paranoid, such proglottids are difficult to identify.
For intermediate hosts, however, it’s a different story. Intermediate hosts generally suffer quite
a bit more than the final hosts because the purpose of a secondary host, from the tapeworm’s
perspective, is to help get the tapeworm into its final host. They generally cause health
problems in their intermediate host so that they are more easily consumed by their final host.
Once in their final host, like a human that eats a lot of beef, they generally cause
little damage since that final host will be their home for the rest of their lives.
Globally, there are more than 6,000 tapeworm species
that as adults infect all groups of vertebrates, with the highest number of species in birds.
However, the most diverse are those that infect elasmobranchs, namely, the sharks.
Collectively, tapeworms are capable of infecting almost all vertebrate species on the planet, and
each one, including humans, has its own specific fauna of tapeworms. And with that we conclude our
study of phylum Platyhelminthes. Let’s move forward and check out some other phyla next.