Biomedical Engineering Reference
In-Depth Information
who have a physiologic status favoring the growth of the organisms (e.g. diabetes,
malnutrition). Sometimes, opportunistic infections occur in patients who are very old,
or very young. Most often, opportunistic infections occur in immune-compromised
patients. Specific diseases may increase susceptibility to specific types of organisms.
For example, diabetics are more likely to contract systemic fungal diseases than are
non-diabetic individuals. Some opportunistic infections arise from the population of
organisms that live within most humans, without causing disease under normal circum-
stances (i.e. commensals). The concept of an opportunistic organism is, at best, a gray
area of medicine, as virtually all of the organisms that arise in immune-compromised
patients will, occasionally, cause disease in immune-competent patients (e.g.
Cryptococcus neoformans
). Moreover, the so-called primary infectious organisms, that
produce disease in normal individuals, will tend to produce a more virulent version of
the disease in immunosuppressed individuals (e.g.
Coccidioides immitis
). Examples of
organisms that cause opportunistic infections are:
Acinetobacter baumanni
(Chapter 7),
Aspergillus
sp. (Chapter 36),
Candida
sp. (Chapter 36),
Clostridium diffi-
cile
(Chapter 12),
Cryptococcus neoformans
(Chapter 35),
Cryptosporidium parvum
(Chapter 19), cytomegalovirus (Chapter 39), herpes zoster (Chapter 39),
Histoplasma
capsulatum
(Chapter 36), human herpesvirus 8 (Chapter 39),
Pneumocystis jirovecii
(Chapter 36), polyomavirus JC (Chapter 39),
Proteus
sp. (Chapter 7),
Pseudomonas
aeruginosa
(Chapter 7),
Streptococcus pyogenes
(Chapter 12),
Toxoplasma gondii
(Chapter 19). See Commensals.
Parasite A parasite is an organism that lives and feeds in or on its host. In common usage,
the term “parasite” is often reserved for animals that are parasitic in humans and other
animals, and this has historically included the so-called one-celled animals. We now
recognize that the so-called one-celled animals are distributed among divergent taxo-
nomic classes. As we learn more and more about classes of organisms, the term “para-
site” seems to have diminishing biologic specificity. In this topic, the term “parasite”
refers to any infectious organism. See Protozoa.
Primary host Also called final host or definitive host, the primary host is infected with
the mature or reproductive stage of the parasite. In most cases, the mature stage of the
parasite is the stage that produces eggs, larvae, or cysts. See Intermediate host.
Protozoa Microbiologic nomenclature has many terms that have persisted long after they
have outlived their usefulness; “protozoa” is a perfect example. A commonly found
definition for protozoa is “one-celled animal,” but this is an oxymoron, as all animals
are multicellular. Still, it is reasonable to assume that multi-animals must have evolved
from unicellular organisms, and these one-celled prototypes animals could be called
protozoans. If we adhered to this line of logic, only Class Choanozoa (Class 23) could
lay claim as a “protozoan,” as the choanozoans are one-celled members of Class
Opisthokonta, to which Class Animalia belongs; most other so-called protozoans have
no ancestral relationship to animals. A better way of thinking about “protozoan” is as
a term that describes any one-celled heterotrophic eukaryotic organisms (i.e. members
of Class Eukaryota that lack chloroplasts). With luck, the term “protozoa” will cease
to appear in the scientific literature.
Sandfly Sandflies are small dipterans (flies), of several different genera, that live in sandy
areas. The sandfly of genus Phlebotomus transmits Leishmania species (leishmaniasis,
Chapter 17) and the phleboviruses that cause Pappataci fever (Chapter 43). The sand-
fly of genus Lutzomyia transmits
Bartonella bacilliformis
(bartonellosis, Chapter 5).
