Toxoplasma gondii

Diagnosis

Indications for Testing

  • Pregnant female with suspected exposure
  • Immunocompromised patient with undiagnosed flu-like illness

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

Clinical Background

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

  • Nonimmunocompromised
    • Usually mild or asymptomatic disease in adults
    • Constitutional – prolonged fever, headache, lymph node enlargement, myalgias
    • Gastrointestinal – hepatomegaly, hepatitis
  • Pregnant females
    • Most women experience minimal symptoms
    • If immunocompromised, reactivation may occur – presents risk to fetus (rare vertical transmission)
  • Congenital
    • Risk of infection varies with gestational age
    • Symptoms range from asymptomatic to death
    • In infected neonates – visual impairment, encephalomyelitis, developmental delay, seizures, TORCH syndrome (toxoplasmosis, rubella virus, cytomegalovirus, herpes simplex virus)
    • Triad of chorioretinitis, hydrocephalus, brain calcification – highly suggestive of toxoplasmosis
  • Immunocompromised
    • Transplant patients – most common in heart transplant patients
    • HIV patients
      • Most common presentation is encephalitis – altered mental status, headache, fever, focal neurologic deficits

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Toxoplasma gondii Antibodies, IgG and IgM 0050521
Method: Quantitative Chemiluminescent Immunoassay/Semi-Quantitative Chemiluminescent Immunoassay

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

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)

   
Toxoplasma gondii by PCR 0055591
Method: Qualitative Polymerase Chain Reaction

Confirm toxoplasmosis infection in immunocompromised hosts

   
Additional Tests Available
 
Click the plus sign to expand the table of additional tests.
Test Name and NumberComments
Toxoplasma gondii IgG Antibody, ELISA (CSF) 0092534
Method: Qualitative Enzyme-Linked Immunosorbent Assay
Toxoplasma gondii Antibody, IgM 0050557
Method: Semi-Quantitative Chemiluminescent Immunoassay

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

Toxoplasma gondii Antibody, IgG 0050770
Method: Quantitative Chemiluminescent Immunoassay
TORCH Antibodies, IgG 0050772
Method: Semi-Quantitative Chemiluminescent Immunoassay/Quantitative Chemiluminescent Immunoassay
TORCH Antibodies, IgM 0050665
Method: Semi-Quantitative Chemiluminescent Immunoassay/Semi-Quantitative Enzyme-Linked Immunosorbent Assay
Toxoplasma gondii by Immunohistochemistry  2004157
Method: Immunohistochemistry