Pediatric Toxoplasmosis Workup

Demonstration of T gondii in blood, body fluids, or tissues is evidence of toxoplasmosis infection (see the image below). Isolation by mouse inoculation of Toxoplasma from amniotic fluid or placental or fetal tissue is diagnostic of congenital infection. Lymphocyte transformation in response to Toxoplasma antigens indicates previous infection in adults. Detection of Toxoplasma antigens in blood or body fluids by means of enzyme-linked immunoassay (ELISA) or polymerase chain reaction indicates acute infection. A skin test showing delayed hypersensitivity to Toxoplasma antigens may be a useful screening test.

Laboratory tests include complete blood cell (CBC) count with differential, liver function tests, lumbar puncture, serum creatinine, urinalysis, urine viral culture for cytomegalovirus, and serum quantitative immunoglobulin testing.

The Sabin-Feldman dye test is a sensitive and specific neutralization test. It measures IgG antibody and is the standard reference test for toxoplasmosis; however, it requires live T gondii and thus is not available in most laboratories. High titers suggest acute disease.

The indirect fluorescent antibody (IFA) test measures the same antibodies as the dye test. Titers parallel dye test titers. The IgM fluorescent antibody test can be used to detect IgM antibodies within the first week of infection, but titers fall within a few months. The double-sandwich IgM ELISA test is more sensitive and specific than other IgM detection tests.

The indirect hemagglutination test measures a different antibody than does the dye test. Titers tend to be higher and remain elevated longer.

The IgG avidity test may be able to discriminate acute from chronic infection better than alternative assays, such as assays that measure IgM antibodies, can. As is true for IgM antibody tests, the avidity test is most useful when performed early in gestation, because a chronic pattern occurring late in pregnancy does not rule out the possibility that the acute infection may have occurred during the first months of gestation. A 2-fold rise in serum IgG obtained at 3-week intervals is diagnostic.

Perform an amniocentesis at 20-24 weeks' gestation in suspected cases of congenital disease. Performing polymerase chain reaction (PCR) assay testing on body fluids, including cerebrospinal fluid (CSF), amniotic fluid, bronchoalveolar lavage fluid, and blood, may be useful in establishing the diagnosis.^[6]

Antibody levels in aqueous humor or CSF obtained through a lumbar puncture may reflect local antibody production and infection at these sites.

Ultrasonography

Ultrasonography of the fetus to evaluate for evidence of congenital toxoplasmosis can be performed at 20-24 weeks' gestation.

CT scanning

Computed tomography (CT) scanning of the brain is useful in cerebral toxoplasmosis. In 70-80% of immunodeficient patients with Toxoplasma encephalitis, the CT scan reveals multiple bilateral, ring-enhancing cerebral lesions. Although multiple lesions are more common, finding a solitary lesion should not exclude Toxoplasma encephalitis. There is an approximately 80% likelihood that the disease is present in patients with AIDS who have detectable Toxoplasma IgG and multiple ring-enhancing lesions. Lesions are characteristically hypodense and tend to occur at the corticomedullary junction; they frequently involve the basal ganglia.

CT scanning frequently underestimates the number of lesions, although delayed imaging after a double dose of intravenous (IV) contrast material may improve the sensitivity of this imaging modality. An enlarging, hypodense lesion that does not enhance is a poor prognostic finding.

Improvement is seen in as many as 90% of patients with AIDS and Toxoplasma encephalitis after 2-3 weeks of treatment. Complete resolution lasts from 6 weeks to 6 months; peripheral lesions resolve more rapidly than do deeper ones. Radiographic response tends to lag behind clinical response.

MRI

Magnetic resonance imaging (MRI) is the preferred imaging modality to evaluate for lesions. MRI has superior sensitivity, particularly if gadolinium is used for contrast. It can often depict lesions or more extensive disease not apparent on CT scans. Hence, MRI should be used as the initial imaging procedure when feasible and should always follow CT demonstration of a single lesion.

MRI depicts Toxoplasma encephalitis lesions as high-signal abnormalities on T2-weighted studies and reveals a rim of enhancement surrounding the edema on T1-weighted, contrast-enhanced images.

Smaller lesions usually completely resolve on MRI studies within 3-5 weeks, but lesions with a mass effect tend to resolve more slowly and leave a small, residual lesion.

Even characteristic lesions on CT or MRI are not pathognomonic of Toxoplasma encephalitis. The major differential diagnosis in patients with AIDS is CNS lymphoma, which appears with multiple enhancing lesions in 40% of cases.

When single lesions are depicted on MRI, the probability of Toxoplasma encephalitis falls and that of lymphoma rises. Brain biopsy findings are generally required to obtain a definitive diagnosis.

Other modalities

To evaluate patients with AIDS and focal CNS lesions, various positron emission tomography (PET) and radionuclide scans have been used, generally with minimal benefit over the above modalities.

The histopathology of toxoplasmosis varies with the immune status of the host. In the healthy host with acquired toxoplasmosis, the characteristic histopathology of the lymph node is diagnostic, despite the relative paucity of organisms present. Typical findings include reactive follicular hyperplasia, irregular clusters of histiocytes encroaching on the margins of germinal centers, and focal distention of sinuses with monocytoid cells. Necrosis, granuloma formation, microabscesses, and vasculitis do not occur. At autopsy of normal hosts, tissue cysts are noted as incidental findings in skeletal muscle and myocardium, invoking little inflammatory response.

In contrast, in patients who are immunodeficient and in children with severe congenital toxoplasmosis, tachyzoite proliferation is accompanied by tissue necrosis and an intense, usually monocytic, inflammatory response. In patients with AIDS, toxoplasmosis typically produces brain abscesses that have a characteristic appearance. A central avascular area is surrounded by a region of necrosis and inflammatory cells that may also contain free and intracellular tachyzoites. Outside of the region of inflammation are cysts.

Demonstration of tachyzoites in a tissue specimen is required for definitive diagnosis of active infection. The presence of multiple cysts near an inflammatory lesion makes the diagnosis highly likely. Stains used to detect tachyzoites or cysts include hematoxylin and eosin, periodic acid-Schiff, and Gomori-methenamine silver. Immunoperoxidase and fluorescein-conjugated antibody stains can also be used.