Toxoplasma gondii

Indications for Testing

Pregnant female with suspected exposure; immunocompromised patient with undiagnosed flu-like illness (influenza/parainfluenza)

Laboratory Testing

Serology
- Primary diagnostic method; recommend paired acute and convalescent antibody testing to confirm presence of disease
- Pregnancy
  - IgG, IgM antibodies in pregnant women
    - Caution should be exercised in the use of IgM antibody levels due to lack of specificity in prenatal screening; positive results in pregnant patients must be confirmed positive by a reference laboratory that specializes in toxoplasmosis
      - CDC recommends the Toxoplasma Serology Laboratory at Palo Alto Medical Foundation
      - If positive at reference lab, testing should be evaluated by amniocentesis and PCR testing for T. gondii
  - See the Toxoplasmosis Serologic Testing Algorithm for test result interpretation or the CDC's toxoplasmosis information page for serology result interpretation
  - As suggested by the CDC, any equivocal or positive result should be retested using a different assay (eg, Sabin-Feldman IgG dye test, IgM ELISA, IgA ELISA, IgG ELISA, differential agglutination, avidity)
- Suspected congenital toxoplasmosis – IgG and IgA by EIA
  - IgA more sensitive than IgM in congenitally infected infants
  - No commercial assay in the U.S. is cleared by the FDA for in vitro diagnostic use in infants
  - Specimens from neonates suspected of having congenital toxoplasmosis should be sent for testing by the Toxoplasma Serology Laboratory
- Neonate – PCR on amniotic fluid
- Immunocompromised – PCR
  - Serological determination of active central nervous system toxoplasmosis in immunocompromised patients is not possible at this time
  - Toxoplasma-specific IgG antibody levels in AIDS patients are often low to moderate and occasionally undetectable
  - Tests for IgM antibodies are generally negative
Cerebrospinal fluid exam
- Not frequently performed because of increased central nervous system (CNS) pressure
- Elevated protein, variable glucose, mildly elevated white blood cell count with mononuclear predominance
- PCR for T. gondii may establish meningitis/encephalitis

Histology

Immunohistochemistry – T. gondii stain

Imaging Studies

CT/MRI of the brain in patients presenting with encephalitis

Differential Diagnosis

Flu-like illness
CNS disease
- Malignancy
  - Primary CNS lymphoma
  - Primary brain tumor
  - Metastasis
- Viral/bacterial meningitis
- Encephalitis
- Demyelinating disease
  - Multiple sclerosis
- Vasculitis
- Multifocal infarct
- Arteriovenous malformation

Toxoplasmosis is a zoonosis caused by the parasite Toxoplasma gondii, which infects both birds and mammals.

Epidemiology

Incidence – 15-29% of U.S. population is seropositive for toxoplasmosis
- Hot, arid climates have a low incidence of toxoplasmosis
Transmission
- Usually oral
- May occur via blood transfusion, organ transplant, or transplacentally from mother to infant

Organism

Obligate intracellular parasite
Cat is definitive host

Risk Factors

Predisposition to severe toxoplasmosis infection is common in persons with acquired immunodeficiency syndrome (AIDS) or in persons who are otherwise immunocompromised
Ingestion of raw or undercooked meat

Clinical Presentation

Usually mild or asymptomatic disease in adults
Constitutional – prolonged fever, headache, lymph node enlargement, myalgias
Gastrointestinal – hepatomegaly, hepatitis
Toxoplasmosis acquired in utero can result in blindness, encephalomyelitis, developmental delay, convulsions and death in infected neonates (TORCH syndrome – toxoplasmosis, rubella virus, cytomegalovirus, herpes simplex virus)
Immunocompromised
- Transplant patients – most common in heart transplant patients
- HIV patients
  - Most common presentation is encephalitis – altered mental status, headache, fever, focal neurologic deficits

Treatment

Toxoplasmosis can be treated
Early detection is crucial for effective therapy
Recommended treatment is antibiotics for immunocompromised patient or patient with congenital infection
Eradication of the parasite becomes difficult after long-term chronic infection

Prevention

Avoidance of cat feces in soil, water, and litter
Avoidance of undercooked meat that may have oocysts present

Tests generally appear in the order most useful for common clinical situations

Test name: Toxoplasma gondii Antibodies, IgG and IgM

ARUP #: 0050521

Methodology: Chemiluminescent Immunoassay

Use:

First-line test in endemic areas for identifying T. gondii infection in pregnant women

Diagnose opportunistic infections in immunocompromised hosts

CDC suggests equivocal or positive results should be retested using a different assay from another reference laboratory specializing in toxoplasmosis testing (IgG dye test, IgM ELISA, reflex to avidity and/or other tests)

Test name: Toxoplasma gondii by PCR

ARUP #: 0055591

Methodology: Qualitative Polymerase Chain Reaction

Use:

Confirm toxoplasmosis infection in immunocompromised hosts

Additional Tests Available

Test name: Toxoplasma gondii IgG Antibody, ELISA (CSF)

ARUP #: 0092534

Methodology: Enzyme-Linked Immunosorbent Assay

Comments:

Test name: Toxoplasma gondii Antibody, IgM

ARUP #: 0050557

Methodology: Chemiluminescent Immunoassay

Comments:

For male patients or nonpregnant female patients with equivocal or positive results, PCR may also be useful if a specimen can be collected from an affected body site

Any equivocal or positive T. gondii IgM result should be retested in parallel with a specimen collected 1-3 weeks later; further confirmation may be necessary using different assay from another reference lab specializing in toxoplasmosis testing

Test name: Toxoplasma gondii Antibody, IgG

ARUP #: 0050770

Methodology: Chemiluminescent Immunoassay

Comments:

Test name: TORCH Antibodies, IgG

ARUP #: 0050772

Methodology: Chemiluminescent Immunoassay

Comments:

Test name: TORCH Antibodies, IgM

ARUP #: 0050665

Methodology: Semi-Quantitative Chemiluminescent Immunoassay/Semi-Quantitative Enzyme-Linked Immunosorbent Assay

Comments: