Page 451 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART VI Parasitology
440

mebooksfree.com mebooksfree.com mebooksfree.com FIGURE 52–7 Pneumocystis jiroveci—arrow points to a cyst of mebooksfree.com
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FIGURE 52–6
Toxoplasma gondii—tachyzoite. Arrow points to
P. jiroveci in lung tissue. (Source: Dr. E. Ewing, Jr., Public Health Image Library,
a tachyzoite of T. gondii in cardiac muscle. (Source: Dr. E. Ewing, Jr., Public
Centers for Disease Control and Prevention.)
Health Image Library, Centers for Disease Control and Prevention.)
infections. Cysts may be seen in the tissue. The organism
of rRNA sequences published in 1988 indicates that
can be grown in cell culture. Inoculation into mice can
confirm the diagnosis. but it also has several attributes of a protozoan. An analysis
Pneumocystis should be classified as a fungus related to
yeasts such as Saccharomyces cerevisiae. Subsequent analysis
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mebooksfree.com mebooksfree.com mebooksfree.com terol in its membranes as do the fungi. It has cholesterol. mebooksfree.com
Treatment
of mitochondrial DNA and of various enzymes supports
the idea that it is a fungus. However, it does not have ergos-
Congenital toxoplasmosis, whether symptomatic or asymp-
tomatic, should be treated with a combination of sulfadia-
Medically, it is still thought of as a protozoan. In tissue,
zine and pyrimethamine. These drugs also constitute the
it appears as a cyst that resembles the cysts of protozoa
treatment of choice for disseminated disease in immuno-
(Figures 52–2H and 52–7). The findings that it does not
compromised patients. Acute toxoplasmosis in an immu-
grow on fungal media and that antifungal drugs are ineffec-
nocompetent individual is usually self-limited, but any
tive have delayed acceptance of its classification as a
patient with chorioretinitis should be treated.
fungus.
Pneumocystis species are found in domestic animals
Prevention
The most effective means of preventing toxoplasmosis is to
is thought that these animals are not a reservoir for human
cook meat thoroughly to kill the cysts. Pregnant women such as horses and sheep and in a variety of rodents, but it
infection. Each mammalian species is thought to have its
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should be especially careful to avoid undercooked meat and
own species of Pneumocystis.
contact with cat feces. They should refrain from emptying
Pneumocystis species have a major surface glycoprotein
cat litter boxes. Cats should not be fed raw meat. Trime-
thoprim-sulfamethoxazole is used to prevent Toxoplasma
similar to that of Trypanosoma brucei. Pneumocystis species
encephalitis in patients infected with human immunodefi-
have multiple genes encoding these surface proteins, but
ciency virus (HIV).
only one is expressed at a time. This process of pro-
grammed rearrangements was first observed in T. brucei.
PNEUMOCYSTIS
Pathogenesis & Epidemiology
Di sease
Pneumocystis jiroveci is an important cause of pneumonia
dominantly in the lungs. The presence of cysts in the alveoli
in immunocompromised individuals. In 2002, taxonomists Transmission occurs by inhalation, and infection is pre-
induces an inflammatory response consisting primarily of
renamed the human species of Pneumocystis as P. jiroveci
mebooksfree.com mebooksfree.com mebooksfree.com invade the lung tissue. mebooksfree.com mebooksfree.com
plasma cells, resulting in a frothy exudate that blocks oxy-
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and recommended that Pneumocystis carinii be used only
gen exchange. (The presence of plasma cells has led to the
to describe the rat species of Pneumocystis.
name “plasma cell pneumonia”.) The organism does not
Important Properties
Pneumonia occurs when host defenses (e.g., the number
of CD4-positive [helper] T cells) are reduced. This accounts
The classification and life cycle of Pneumocystis are unclear.
for the prominence of Pneumocystis pneumonia in patients
Many aspects of its biochemistry indicate that it is a yeast,
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