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132 CHAPTER 5

lateral spine near their posterior end. The anterior end is
Source: Centers for Disease Control and Prevention (CDC) Life Cycle of Pinworms
tapered, as well as slightly curved.

All parasites, including those termed as worms, require
a host in order to survive. In the case of pinworms, the
human serves as the host. The life cycle of a pinworm is
quite simple in comparison with other parasitic organ-
isms. Infections originate with the soiled fingers being
placed in the mouth or when pinworm eggs are eaten,
usually directly by contaminated hands or indirectly
through contaminated food, bedding, clothing, or other
FIGURE 5-15 Schistosomal cercaria, which is the
personal items. The eggs are swallowed into the intes-
larval stage of a parasite that causes swimmers’ itch
tines where they hatch and mature. An adult pinworm is
yellowish to white, somewhat slender, and is slightly less
than one-half inch in length. The worms are readily vis-
Source: Centers for Disease Control and Prevention (CDC) the gut, the adult female will move down the intestinal
ible in the feces of children with heavy infections.
Approximately one month following infestation of

system and will exit the body through the anus. There
she lays a batch of several hundred eggs on the skin
surrounding the anus, usually during the night or early
in the morning before light enters the bedroom. Upon
depositing her eggs, the female pinworm has fulfilled
her purpose in life so she then dies. Severe itching often
accompanies the laying of the eggs, especially at night.
In this way children are able to reinfect themselves by
FIGURE 5-16 Egg from Schistosoma mansoni soiling their fingers and also accumulating eggs on the
fingers and under the nails from the bed linens. The cy-
S. hematobium and S. japonicum and measure 115 μm cle of reproduction by transfer of the eggs to the mouth
to 175 μm as an average in length for these species. But where they are again swallowed ensures a steady source
other than the size, the greatest differentiating char- of infection is present. Also, the eggs may remain viable
acteristic is that of its shape, with its very prominent for up to several days if the conditions are suitable.

SUMMARY

The organisms commonly called “worms” actually almost exclusively in northern Africa and India, whereas
include three different morphological group, called the Necator americanus species is the main species seen
nematodes, cestodes, and trematodes. Besides amoe- in North and South America. The A. braziliense and
bal infections of the gastrointestinal tract, worms are a A. tubaeforme species infect only cats but not humans,
major contributor also. Nematodes are roundworms whereas A. caninum infects mainly dogs but sometimes
that include the much smaller threadworms with the humans, especially children.
major roundworm groups being the hookworms. The Illnesses resulting from nematodal infections with
hookworm species of Ancylostoma duodenale are found a number of organisms called hookworms often include