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Toxoplasmosis is
caused by the obligate intracellular protozoan Toxoplasma gondii.
The parasite
was originally identified in the small North African rodent
Ctenodactylus gundi, hence its name. "Toxo"- refers to the curved or
arcuate shape of the organism, not to a toxin.
It is widely
distributed among domestic animals and humans throughout the world.
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Life Cycle-Diagram-
Toxoplasma is a tissue coccidium. Both coccidian stages, schizogonic
and gametogonic, are found in the epithelium of the small intestine of
cats.
The toxoplasma
oocyst, similar in morphology to that of
Isospora belli
, is excreted in cat faeces. The oocyst , millions of which may
be shed in a single stool of an infected cat, survive for months in
moist environments, water or soil.
In human
tissues proliferating tachyzoites (trophozoites) and cysts may be
identified.
The presence
of tachyzoites is diagnostic of acute infection.
Cysts
containing hundreds and sometimes thousands of bradyzoites make their
appearance in brain, skeletal muscles, and other tissues with the
development of immunity. Rupture of these cysts has been proposed as a
pathogenetic mechanisms for the development of inflammatory lesions.
Toxoplasma
tachyzoites enter the cells by both phagocytosis and active invasion.
The latter
mode of entry depends on specialized organelles, called rhoptries, and
on "penetration-enhancing factor." During this infectious
process an oxidative respiratory burst is not stimulated ; thus no
oxygen metabolites are formed to kill the protozoan. T. gondii also
alters the membrane of the parasitophorous vacuole so that lysosomal
fusion does not take place.
Mode of
infection:
- Oocysts and
cysts are the principal infective forms. Infection occurs when a
person eats undercooked or raw meat of animals with chronic
toxoplasmosis or ingests oocysts from the faeces of cats.
- Recently an
outbreak of acute toxoplasmosis associated with ingestion of
contaminated water was reported. Most of these primary infections are
asymptomatic and result in a chronic carrier stage. These cases are
mostly symptomless.
- Transplacental
transmission occurs rarely but accounts for the majority of patients.
Infection of the fetus is seen in 30% to 40% of cases of acquired
toxoplasmosis during pregnancy.
Relatively few
cases of human disease have been reported in immuno-competent hosts,
but results of the skin test for toxoplasma have been positive in 10%
to 50% of the adults tested, indicating that although disease is rare
infection is common.
The prevalence
of the disease can be less than 10% in dry areas (eg Arizona) and
close to 100% in moist lowland tropical areas (eg. Costa Rica and
Guatemala). Prevalence increases with age.
The majority
of immunocompetent individuals infected with Toxoplasma have minimal
or no symptoms. However, in immunocompromised patients
disseminated disease can occur and it is often fatal. Central nervous
system toxoplasmosis is the major form of involvement in these
patients. Patients with the
acquired immunodeficiency syndrome (AIDS)
frequently develop neurologic manifestations resulting from
toxoplasmic encephalitis or brain abscess, or both.
Toxoplasmosis in Pregnancy:
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Pregnant women who acquire infection from Toxoplasma gondii usually
remain asymptomatic, although they can still transmit the infection to
their fetuses with severe consequences. Given the asymptomatic nature
of most Toxoplasma infections, primary prevention in pregnant women
may lower the risk of congenital toxoplasmosis. Both consumption of
undercooked meat and unprotected contact with soil are independent
risk factors for T. gondii seroconversion during pregnancy, while
contact with cat litter may pose a risk in certain situations.
The
clinicopathologic presentation in toxoplasmosis varies according to
the age and the immune status of the patient.
The three
distinct presentations are as follows:
(1)
CONGENITAL TOXOPLASMOSIS IN NEONATES:
Congenital or
neonatal toxoplasmosis should be strongly suspected when the
characteristic ocular lesions and the presence of cerebral
calcifications in radiological examinations are associated with
hydrocephalus and pleocytosis of the cerebrospinal fluid.
The pathologic
lesions are those of hydrocephalus caused by necrotic foci in the
brain, usually located in the periventricular areas. These lesions are
microglial nodules surrounded by areas of vasculitis and necrosis.
Calcium
deposits in these focal lesions and bilateral chorioretinitis are
evident.
Focal areas of
necrosis may be seen in viscera, leading to myocarditis, pneumonitis
and rarely hepatitis.
(2)
TOXOPLASMA LYMPHADENITIS IN IMMUNOCOMPETENT ADULTS:
The clinical
picture of adult toxoplasmosis is uncommon. It consists in
lymphadenitis (usually posterior cervical), fever and malaise.
A biopsy of
the firm, rubbery nodes is commonly done to rule out
lymphoproliferative diseases, and it is the pathologist who first
suggests the diagnosis of toxoplasmosis.
The
architecture of the lymph node is preserved.
There is
pronounced follicular hyperplasia, sinus histiocytosis and clusters of
epithelioid histiocytes inside and surrounding the follicles.
Specificity of the histopathological triad for
the diagnosis of toxoplasmic lymphadenitis: polymerase chain reaction
study.Pathol
Int. 2001 Aug;51(8):619-23
Organisms are
rarely seen.
Pathologic
diagnosis can be confirmed serologic methods such as indirect
immunofluorescence, indirect hemagglutination, complement fixation,
and enzyme-linked immunosorbent assay.
Some of these
serologic tests have been suggested as screening methods for
toxoplasmosis in pregnancy. Acute infection is diagnosed on the
basis of a fourfold rise in antibody titre or detection of IgM
antibodies by the indirect immunofluorescence method. A Paul-Bunell
test for heterophil antibodies is usually done to rule out infectious
mononucleosis.
(3)
TOXOPLASMA ENCEPHALITIS IN IMMUNOCOMPROMISED HOST:
Well
circumscribed areas of hemorrhage and necrosis are frequently
identified by CT scan and on gross inspection.
Microscopic
examination usually reveals necrosis, vascular thrombosis, and the
presence of tachyzoites.
Immunoperoxidase and immunofluorescence methods have recently been
developed for the diagnosis of toxoplasmosis in tissue sections.
Other, less
common manifestations of toxoplasmosis are interstitial pneumonitis (Toxoplasmosis
of Lungs: Image Link
),
myocarditis and hepatitis. Chorioretinitis and uveitis sometimes
occur.
The most
effective treatment for toxoplasmosis consists of sulfadiazine and
pyrimethamine.
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