Toxoplasma gondii is a
protozoan parasite that infects most species of warm blooded animals, including humans,
causing the disease toxoplasmosis.
Members of the cat family
(Felidae) are the only known definitive hosts for the sexual stages of T. gondii
and thus are the main reservoirs of infection. Cats become infected with T.
gondii by carnivorism. After tissue cysts or oocysts are ingested by the cat,
viable organisms are released and invade epithelial cells of the small intestine where
they undergo an asexual followed by a sexual cycle and then form oocysts, which are then
excreted. The unsporulated oocyst takes 1 to 5 days after excretion to sporulate
(become infective). Although cats shed oocysts for only 1 to 2 weeks, large numbers
may be shed. Oocysts can survive in the environment for several months and are
remarkably resistant to disinfectants, freezing, and drying, but are killed by heating to
70oC for 10 minutes.
Human infection may be acquired in several ways: 1) ingestion of undercooked infected meat
containing Toxoplasma cysts; 2) ingestion of the oocyst from fecally contaminated
hands or food; 3) organ transplantation or blood transfusion; 4) transplacental
transmission; 5) accidental inoculation of tachyzoites. The parasites form tissue
cysts, most commonly in skeletal muscle, myocardium, and brain; these cysts may remain
throughout the life of the host.
data indicate that toxoplasmosis is one of the most common of humans infections throughout
the world. Infection is more common in warm climates and at lower altitudes than in
cold climates and mountainous regions. High prevalence of infection in France (85%)
has been related to a preference for eating raw or undercooked meat, while high prevalence
in Central America has been related to the frequency of stray cats in a climate favoring
survival of oocysts. The overall seroprevalence in the United States as determined
with specimens collected by the third National Health and Nutritional Assessment Survey
(NHANES III) between 1988 and 1994 was found to be 22%, with seroprevalence among women of
childbearing age (15 to 45 years) of 10% to 15%.
Acquired infection with Toxoplasma
in immunocompetent individuals is generally an asymptomatic infection. However,
10% to 20% of patients with acute infection may develop cervical lymphadenopathy and/or a
flu-like illness. The clinical course is benign and self-limited; symptoms usually
resolve within a few months to a year.
Immunodeficient patients often have central nervous system (CNS) disease but may have
myocarditis, or pneumonitis. In patients with AIDS, toxoplasmic encephalitis is the
most common cause of intracerebral mass lesions and is thought to be due to reactivation
of chronic infection. Toxoplasmosis in patients being treated with immunosuppressive
drugs may be due to either newly acquired or reactivated latent infection.
Congenital toxoplasmosis results from an acute primary infection acquired by the mother
during pregnancy. The incidence and severity of congenital toxoplasmosis vary with
the trimester during which infection was acquired. Because treatment of the mother
reduces the incidence of congenital infection, prompt and accurate diagnosis is extremely
important. Most infants with subclinical infection at birth will subsequently
develop signs or symptoms of congenital toxoplasmosis unless the infection is treated.
Ocular Toxoplasma infection, an important cause of chorioretinitis in the United
States, is usually a result of congenital infection. Patients are often asymptomatic
until the second or third decade of life, when lesions develop in the eye.
The diagnosis of
toxoplasmosis may be documented by:
- Observation of parasites in
patient specimens, such as bronchoalveolar lavage material from immunocompromised
patients, or lymph node biopsy.
- Isolation of parasites from
blood or other body fluids, by intraperitoneal inoculation into mice or tissue culture.
The mice should be tested for the presence of Toxoplasma organisms in the
peritoneal fluid 6 to 10 days post inoculation; if no organisms are found, serology can be
performed on the animals 4 to 6 weeks post inoculation.
- Detection of parasite genetic
material by PCR, especially in detecting congenital infections in utero.
- Serologic testing is the
routine method of diagnosis, because the techniques described above are technically
complex and generally not rewarding.
A: Toxoplasma gondii
in the bronchoalveolar lavage (BAL) material from an HIV infected patient. Numerous
trophozoites (tachyzoites) can be seen, which are typically crescent shaped with a
prominent, centrally placed nucleus. Most of the tachyzoites are free, some are
still associated with bronchopulmonary cells.
gondii in tissue from a cat.
Serologic examination is used
initially to indicate the presence of the infection by detecting Toxoplasma-specific
antibodies. Serum, plasma, cerebrospinal fluid (CSF), and eye fluid specimens may all be
tested for antibodies. If determination of immune status is the reason for testing, a
single specimen is satisfactory. If determining the time of infection is important, two
specimens drawn at least 3 weeks apart may or may not be useful. In most cases, detection
of a rising IgG or IgM titer is not possible because the titers have already reached a
plateau by the time the initial sample is drawn. The following algorithm may be used to
determine the appropriate sequence of serological testing.
Several commercial kits for Toxoplasma
serologic testing are available. However, the sensitivity and specificity of these
kits may vary widely from one commercial brand to another. This is of concern
because serology results can influence decisions on continuation or termination of
asymptomatic healthy, but infected, person does not require treatment. For pregnant women
or persons who have weakened immune systems, spiramycin or pyrimethamine plus sulfadiazine
may be used in specific cases.