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136 CHAPTER 6

INTRODUCTION
The term nematode means “roundworm” because
members of the phylum Nematoda can be viewed as a
cross-section and may range from a few millimeters to
more than 1 meter in length. A common characteristic
found in nematodes is that both male and female repre- Source: Centers for Disease Control and Prevention (CDC)
sentatives exist, unlike other parasites. As a general con-
clusion, the male roundworm is generally smaller than
the female. Body development is rather complex with
an exterior covering called a cuticle which covers several
layers of underlying musculature, a complex nerve cord
and a well-developed digestive system. Functional sexual
FIGURE 6-1 Eggs of the nematode, or roundworm
organs are present in the male and in addition the body
Enterobius vermicularis
of the male ends in either a curved or coiled tail.
Humans are considered the definitive host for the
species of roundworms that are considered medically most often confirmed by microscopic examination of the
important for the parasitic nematode infections. Where characteristic eggs where one side of the egg is flattened
fertilized eggs or larvae are the means for reproducing (Figure 6-1).
adult roundworms, three routes are generally involved All parasites require a host and some may require
by which the host becomes infected. Depending on the up to three different hosts for development and infec-
species of nematode, the adult female nematode is able to tion, but for pinworms, only the human serves as a host,
produce an infection in the following ways: where the organism develops into an adult and serves
■ Ingestion of eggs by a susceptible host can result in as a vessel for transmission to others. The life cycle of
immediate infection E. vermicularis is quite simple when compared with
many other parasitic organisms. Eggs are swallowed
■ After a required period of external development,
into the intestines, where they hatch and mature. Adult
usually in the soil, either eggs or larvae reach an
pinworms are a yellowish to white color and are slen-
infective stage before developing the capability to
der, resembling a filament or a thick thread. It should be
infect a host, often by skin penetration
mentioned here that some commonly call the pinworm
■ Eggs or larvae are transmitted to a new host by a “threadworm” but the term threadworm actually refers
insect vectors
to a particular hookworm. A month after infecting the
gut, the female worm moves down the intestinal system
PINWORMS and exits the body from the anus. She then deposits up
to several hundred eggs on the skin around the anus dur-
Pinworms are prevalent around the world, and although ing the night or early morning.
they are not a serious threat to the overall health of the After depositing her eggs, the female pinworm dies,
individual, they do cause some discomfort through anal as she has completed her life’s mission of propagating
irritation and nervousness mostly in small children. the species. Itching often occurs after the eggs are laid.
The most prevalent type of human worm infection in the Children continue the cycle, reinfecting themselves by
United States and perhaps around the world is by that of scratching their anus upon which eggs accumulate on
the pinworm. The term worms in children is a common the fingers and under the nails. The reproductive cycle
lay term for pinworms. This frequently used reference starts again when the eggs are swallowed and begin to
to pinworms is almost universal as a descriptive name hatch. The eggs may remain viable for up to several days,
for this parasite, which is an organism of the genus and if conditions are suitable. Treatment with medication is
species Enterobius vermicularis. Because many children available to kill the parasites, and to be completely effec-
put their dirty fingers into their mouths, they are more tive, all members of the family should be treated when
susceptible to this type of infection. The condition is a single family member is diagnosed with a pinworm