Page 234 - parasitology for medical and clinical laboratoryprofessionals

214 CHAPTER 10

INTRODUCTION
A large number of ectoparasites feed upon the skin cells
of the human body. Some survive by embedding them-
selves into the skin of the body and feeding upon the
body’s blood supply, whereas others actually feed upon
the bacteria and oils of the body, in at least one case.
Ectoparasites are differentiated from endoparasites in
that they thrive outside of the human body. Therefore,
those known as endoparasites live inside the body, some
of which have already been described and that include Source: Centers for Disease Control and Prevention (CDC)
amoebae, worms, and flukes. Three major ectoparasites
plague human beings and these have apparently been in
existence for possibly as long as humanity has inhabited
the world. Lice and mites are the two major groups that
include four common genera, three of which are lice and
one of which is classified as a mite. Besides lice and mites, FIGURE 10-1 Life cycle of a “sucking louse,”
ticks are also classified as ectoparasites, and these three Pediculus humanus var. corporis, and the morphologic
are commonly encountered in human diseases. Several changes that take place during development
other groups that may cause considerable discomfort
will also be discussed in this chapter. The term used for threat to humans but are of agricultural and horticultural
those on whom ectoparasites are living is called an infes- importance. Lice are apterygotes, which means that they
tation rather than the term infection used for those who are wingless and they are dorsoventrally (backs and ab-
have parasites dwelling within. domens) flattened.
Lice, the singular form of which is louse, belong
LICE to the suborder Anoplura for those that infect humans.
They are usually found in crowded areas where sanita-
Infestations of lice, of which Pediculus humanus and tion and hygiene are poor. The control of these infections
Pthirus pubis are the best-known parasites known as lice, will require a significant change in hygienic practices and
have infested entire armies, villages, and cities. A com- control of living conditions for those infested, to avoid
mon name for P. pubis is that of crabs, a term based on infestations of those who come in contact with them.
the physical appearance of the organism with its stout Infestations are called pediculosis and the bites of these
crablike appearance. These organisms are transmitted organisms allow obtaining of blood meals by piercing the
sexually as well as through common contact with oth- skin and sucking the blood through their mouth parts.
ers that are infested, or by use of clothing and bedding The areas where bites have occurred become itchy and
in which the organisms are found. Lice must spend their inflamed and frequently become infected from scratch-
entire lives on their host, undergoing metabolic and mor- ing the areas with fingernails that may also contain other
phologic changes, and gaining all of their nourishment organisms, such as bacteria, resulting in a secondary
from their host (Figure 10-1). They have developed ad- infection. Another important role of some lice is one in
aptations which enable them to stay in close contact with which they play the role of a vector that is able to trans-
a host and the various species may occupy different re- mit diseases through their bites. Typhus is one such dis-
gions of the body depending on their physical needs. ease that is spread through the bites of lice.
Lice are extremely small, some less than 1 mm long. Some species of lice have no eyes but do possess
Both P. humanus and P. pubis are species possess stout short sensory antennae as a substitute for eyes and the
legs with claws. These claws allow them to cling tightly bodies are not well defined between the thoracic and
to hair, fur, and feathers because some species also af- the abdominal portions of their bodies. Most lice have
flict animals and birds as well as humans. Some species relatively simple chewing mouthparts, but in some these
are equipped to live on plant leaves and pose no medical organs may be highly adapted for piercing and sucking.