Page 449 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART VI Parasitology
438
Prevention
occurs from eating undercooked meat (e.g., lamb and pork)
Chemoprophylaxis of malaria for travelers to areas where
from animals that grazed in soil contaminated with infected
chloroquine-resistant P. falciparum is endemic consists of
cat feces.
mefloquine or doxycycline. A combination of atovaquone
and proguanil (Malarone), in a fixed dose, can also be used.
Chloroquine should be used in areas where P. falciparum is
sensitive to that drug. Travelers to areas where the other
Toxoplasma gondii is usually acquired by ingestion of cysts
three plasmodia are found should take chloroquine starting
in uncooked meat or in food accidentally contaminated by
2 weeks before arrival in the endemic area and continuing Pathogenesis & Epidemiology
cat feces.
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for 4 weeks after leaving the endemic area. This should be
Transplacental transmission from an infected mother
followed by a 2-week course of primaquine if exposure was
to the fetus occurs also. Human-to-human transmission,
high. Primaquine will kill the hypnozoites of P. vivax and
P. ovale.
After infection of the intestinal epithelium, the organisms
Other preventive measures include the use of mosquito
spread to other organs, especially the brain, lungs, liver, and
netting, window screens, protective clothing, and insect
eyes. Progression of the infection is usually limited by a
repellents. The mosquitoes feed from dusk to dawn, so
competent immune system. Cell-mediated immunity
protection is particularly important during the night.
plays the major role, but circulating antibody enhances kill-
Communal preventive measures are directed against
ing of the organism. Most initial infections are asymptom-
reducing the mosquito population. Many insecticide
sprays, such as DDT, are no longer effective because the
tissues. There is no inflammation, and the individual
mosquitoes have developed resistance. Drainage of stag-
remains well unless immunosuppression allows activation
nant water in swamps and ditches reduces the breeding atic. When contained, the organisms persist as cysts within
of organisms in the cysts.
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areas. There is no vaccine.
Congenital infection of the fetus occurs only when
the mother is infected during pregnancy. If she is infected
TOXOPLASMA
form and there will be no trophozoites to pass through
Disease
the placenta. The mother who is reinfected during preg-
nancy but who has immunity from a previous infection
Toxoplasma gondii causes toxoplasmosis, including con-
will not transmit the organism to her child. Roughly one-
genital toxoplasmosis.
third of mothers infected during pregnancy give birth to
infected infants, but only 10% of these infants are
Important Properties
Infection by T. gondii occurs worldwide. Serologic sur-
The life cycle of T. gondii is shown in Figure 52–5. The
veys reveal that in the United States antibodies are found in
definitive host is the domestic cat and other felines;
humans and other mammals are intermediate hosts. Infec- symptomatic.
5% to 50% of people in various regions. Infection is usually
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sporadic, but outbreaks associated with ingestion of raw
tion of humans begins with the ingestion of cysts in under-
cooked meat or from accidental contact with cysts in cat
meat or contaminated water occur. Approximately 1% of
feces. In the small intestine, the cysts rupture and release
domestic cats in the United States shed Toxoplasma cysts.
forms that invade the gut wall, where they are ingested by
macrophages and differentiate into rapidly multiplying
trophozoites (tachyzoites), which kill the cells and infect
Most primary infections in immunocompetent adults are
other cells (Figures 52–2G and 52–6). Cell-mediated
asymptomatic, but some resemble infectious mononucleo-
immunity usually limits the spread of tachyzoites, and the
sis, except that the heterophil antibody test is negative.
parasites enter host cells in the brain, muscle, and other
tissues, where they develop into cysts in which the parasites
neonatal disease with encephalitis, chorioretinitis, and
multiply slowly. These forms are called bradyzoites. These
hepatosplenomegaly. Fever, jaundice, and intracranial cal-
tissue cysts are both an important diagnostic feature and a
cifications are also seen. Most infected newborns are
source of organisms when the tissue cyst breaks in an Congenital infection can result in abortion, stillbirth, or
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asymptomatic, but chorioretinitis or mental retardation
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immunocompromised patient.
will develop in some children months or years later. Con-
The cycle within the cat begins with the ingestion of
genital infection with Toxoplasma is one of the leading
cysts in raw meat (e.g., mice). Bradyzoites are released from
the cysts in the small intestine, infect the mucosal cells, and
cell-mediated immunity (e.g., patients with acquired
differentiate into male and female gametocytes, whose
immunodeficiency syndrome [AIDS]), life-threatening
gametes fuse to form oocysts that are excreted in cat feces.
disseminated disease, primarily encephalitis, occurs.
The cycle is completed when soil contaminated with cat
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