C h a p t e r 1 4
Mechanisms of Infectious Disease
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the characteristic microscopic appearance of the asexual
fruiting structures and spores.
Like the bacterial pathogens of humans, fungi can
produce disease in the human host only if they can grow
at the temperature of the infected body site. For example,
a number of fungal pathogens called
dermatophytes
are
incapable of growing at core body temperature (37°C),
and the infection is limited to the cooler cutaneous sur-
faces. Diseases caused by these organisms, including
ringworm, athlete’s foot, and jock itch, are collectively
called
superficial mycoses.
Systemic mycoses are seri-
ous fungal infections of deep tissues and, by definition,
are caused by organisms capable of growth at 37°C.
Yeasts such as
Candida albicans
are commensal flora of
the skin, mucous membranes, and gastrointestinal tract
and are capable of growth at a wider range of tempera-
tures. Intact immune mechanisms and competition for
nutrients provided by the bacterial flora normally keep
colonizing fungi in check. Alterations in either of these
components by disease states or antibiotic therapy can
upset the balance, permitting fungal overgrowth and
setting the stage for opportunistic infections.
Parasites
In a strict sense, any organism that derives benefits
from its biologic relationship with another organism is
a parasite. In the study of clinical microbiology, how-
ever, the term
parasite
has evolved to designate mem-
bers of the animal kingdom that infect and cause disease
in other animals and includes protozoa, helminths, and
arthropods.
The protozoa are unicellular animals with a complete
complement of eukaryotic cellular machinery, includ-
ing a well-defined nucleus and organelles. Reproduction
may be sexual or asexual, and life cycles may be simple
or complicated, with several maturation stages requir-
ing more than one host for completion. Most are sapro-
phytes, but a few have adapted to the accommodations
of the human environment and produce a variety of
diseases, including malaria, amebic dysentery, and giar-
diasis. Protozoan infections can be passed directly from
host to host through sexual contact, indirectly through
contaminated water or food, or by way of an arthropod
vector. Direct or indirect transmission results from the
ingestion of highly resistant cysts or spores that are shed
in the feces of an infected host. When the cysts reach the
intestine, they mature into vegetative forms called
tro-
phozoites
, which are capable of asexual reproduction or
cyst formation. Most trophozoites are motile by means
of flagella, cilia, or ameboid motion.
The helminths are a collection of wormlike parasites
that include the nematodes or roundworms, cestodes or
tapeworms, and trematodes or flukes. The helminths
reproduce sexually within the definitive host, and some
require an intermediate host for the development and
maturation of offspring. Humans can serve as the defini-
tive or intermediate host or, in certain diseases such as
trichinosis, as both. Transmission of helminth diseases
occurs primarily through the ingestion of fertilized
eggs (ova) or the penetration of infectious larval stages
through the skin—directly or with the aid of an arthro-
pod vector. Helminth infections can involve many organ
systems and sites, including the liver and lung, urinary
and intestinal tracts, circulatory and central nervous
systems, and muscle. Although most helminth diseases
have been eradicated from the United States, they are
still a major health concern of developing nations.
The parasitic arthropods of humans and animals
include the vectors of infectious diseases (e.g., ticks,
mosquitoes, biting flies) and the ectoparasites. The ecto-
parasites infest external body surfaces and cause local-
ized tissue damage or inflammation secondary to the bite
or burrowing action of the arthropod. The most promi-
nent human ectoparasites are mites (scabies), chiggers,
lice (head, body, and pubic), and fleas. Transmission
of ectoparasites occurs directly by contact with imma-
ture or mature forms of the arthropod or its eggs found
on the infested host or the host’s clothing, bedding, or
grooming articles such as combs and brushes. Many of
the ectoparasites are vectors of other infectious diseases,
including endemic typhus and bubonic plague.
SUMMARY CONCEPTS
■■
Throughout life, humans are continuously
and harmlessly exposed to and colonized by
a multitude of microscopic organisms.This
relationship is kept in check by the intact defense
mechanisms of the host (e.g., mucosal and
cutaneous barriers, normal immune function)
and the innocuous nature of most environmental
microorganisms.
■■
The agents of infectious disease represent a
diversity of microorganisms that are usually
not visible to the human eye.The term
infection describes the presence and injurious
multiplication of an infectious agent within a
human host, whereas colonization describes the
act of establishing a presence, a step required in
the multifaceted process of infection.
■■
Microorganisms can be separated into eukaryotes
(fungi and parasites), organisms containing a
membrane-bound nucleus; and prokaryotes
(bacteria), organisms in which the nucleus is
not separated. Both eukaryotes and prokaryotes
are organisms because they contain all the
biologic equipment necessary for replication and
metabolism.
■■
Viruses, which are the smallest pathogens, have
no organized cellular structure, but consist of a
protein coat surrounding a nucleic acid core of
DNA or RNA. Unlike eukaryotes and prokaryotes,
viruses are incapable of replication outside of a
living cell.
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