Parasitic

Flatworms

Imagine going to the doctor for a simple check up. Sure you\'ve had some minor
problems- indigestion, lack of energy, weight loss, and a bit of gas- but that\'s
not out of the ordinary....or is it? In most cases you would be correct...but
today is your unlucky day. The doctor has just informed you that you have a
tapeworm parasite. PARASITIC CHARACTERISTICS By definition, a parasite is an
organism that lives either in or on another organism. Infected organisms that
are carrying a parasite are called host organisms- or hosts. This parasitic
relationship can vary from benign to harmful- and sometimes even fatal. There
are two main types of parasites: endoparasites and exoparasites, however
endoparasites will be the focus of this paper, and flatworms in particular.

Endoparasites are parasites that live inside the host organism. Endoparasites
that inhabit vertebrates or invertebrates live off the nutrients in the food
host organisms eat as well as the tissue of the host. These parasites not only
live in the cavities of hollow organs but can also live within the tissue.

Endoparasites can range from microscopic in size to 25 feet or more in length.

Many worms are antiparasitic. Some live in the host\'s digestive tract feeding
off the host\'s blood. Others, such as trichinosis, enter the host through the
digestive tract and then migrate throughout the body tissue. Most microscopic
worms secrete toxins into the hosts blood stream which then circulates and often
causes damage to surrounding systems and tissue. The life cycle of endoparasites
is as varied as the parasites themselves. Some parasites are permanent fixtures
in a host\'s body, while others only live within the host for a limited amount of
time. For example, parasitic worms can live within a host for up to 30 years!

The host not even being aware of this fact because there are little or no
symptoms of the invasion. Not only are life cycles varied for parasites but the
number of hosts they live in are as well. Sometimes parasites live in only one
host for their entire life- known as autecious - while others change hosts-
known as heteroecious. In relation to the life cycle of parasitic worms, there
are also different reproductive methods. Many parasites do not reproduce within
their host, or reproduce to a limited degree. They are more likely to reproduce
eggs that enter another host before they develop in the final host. These
parasites just use their fist host as an intermediatory step in completing their
life cycle. The species schistosoma ( Refer to Figure 1 ) from the class
trematoda is an example of such a parasite. These parasites go through a life
cycle in which they use an invertebrate, usually a snail as an intermediatory
host. ( Refer to Figure 1a ) FLATWORM CHARACTERISTICS Flatworms from the phylum

Platyhelminthes, are parasites that live within the intermediatory host but
usually complete their sexual maturity within a vertebrate. They are broken into
three major classes: Turbellaria, the most primitive, free-living class that
resides either in or on a host, they generally live in a marine environment.

Trematoda which is the small parasitic flatworm ( most of which are called
flukes) has disk like suckers which attach to the outside or internal organs of
their host, and the class Cestoda which consist of the parasitic flatworm known
as the tapeworm. ( Refer to Figure 2 ) Tapeworms have no true digestive tract,
therefore they live inside the digestive tract of vertebrates and some
invertebrates, absorbing food through their body wall. They latch onto the walls
of their host\'s digestive tract with suckers and hooks, located at their head,
which is called a scolex. The phylum platyhelminthes are one of interest when
discussing parasitic flatworms that infect vertebrates and invertebrates.

INFECTION Humans and animals are in continuous contact with microorganisms,
because of this relationship there are numerous ways in which infection of
flatworms can occur. Organisms that transmit parasites are known as vectors.

Some vectors transmit parasites when they are eaten by the hosts. An example of
this would be a flea eaten by a dog or cat. When the animal eats the flea, the
immature form of the tapeworm emerges from the fleas body and later develops
into a mature tapeworm. Another way animals can become infected is by eating
feces of infected animals which carry the eggs of the parasites. Pigs and cattle
are known for this type of infection. Humans can become infected by larva
penetrating the skin, when walking barefoot on infected soil. An example of this
would be the species schistosoma which has a complex life cycle. One being