What is seroconversion in toxoplasmosis?

What is Seroconversion in Toxoplasmosis

Seroconversion in toxoplasmosis refers to the appearance of specific anti-Toxoplasma antibodies (IgM and/or IgG) in a previously seronegative individual, indicating acute primary infection with Toxoplasma gondii. 1

Definition and Serologic Pattern

Seroconversion represents the transition from a seronegative state to a seropositive state and is the hallmark of newly acquired toxoplasmosis infection. 1

Classic Seroconversion Pattern

• Typical sequence: IgM antibodies appear first, followed by IgG antibodies rising 1-3 weeks later 2
• IgM detection: Appears early in infection and can persist for months to over a year in some cases 3
• IgG development: Indicates established immune response and typically persists for life 3

Atypical Seroconversion Patterns

Important clinical caveat: Not all seroconversions follow the classic pattern, which can complicate diagnosis. 3, 2

• IgG seroconversion without detectable IgM: Occurs in approximately 15/26 cases (58%) in one French multicenter study, where IgG antibodies appeared but IgM was never detected 2
• Transient or equivocal IgM: Some cases show only brief or borderline IgM positivity (11/26 cases in the same study) 2
• Very early infection: Testing performed within 2 weeks of infection may show positive IgM with negative IgG, as IgG has not yet had time to develop 3

Clinical Significance in Pregnancy

Seroconversion during pregnancy or within 3 months before conception is the primary mechanism by which congenital toxoplasmosis occurs in immunocompetent women. 3

Timing and Transmission Risk

• Periconceptional infection (within 2 months before pregnancy): 0% transmission rate in one cohort 1
• First trimester seroconversion: 0% transmission rate (95% CI 0.0-11.9%) 1
• Second trimester seroconversion: 12.5% transmission rate (95% CI 5.6-19.4%) 1
• Third trimester seroconversion: 53.8% transmission rate (95% CI 41.7-66.0%) 1

Diagnostic Approach in Pregnancy

All pregnant women should be screened for both Toxoplasma IgG and IgM simultaneously to capture early seroconversions. 3

• Serial testing requirement: When initial IgM is positive but IgG is negative, repeat testing within 2-4 weeks differentiates very early infection (IgG will rise) from false-positive IgM 3
• Reference laboratory confirmation: Approximately 60% of positive IgM results from nonreference laboratories represent either false-positives (20%) or chronic infections with persistent IgM (40%), not acute seroconversion 3
• Comprehensive testing panels: Include IgG avidity, IgA, and IgE testing to estimate timing of infection, though low avidity alone cannot definitively diagnose acute infection as it can persist for years 3

Diagnostic Pitfalls

• False-positive IgM: Common in commercial assays, requiring reference laboratory confirmation before diagnosing acute infection 3
• Persistent IgM: Can remain positive in low titers beyond 1 year after primary infection, mimicking recent seroconversion 3
• Missed early seroconversion: Immunoblot testing (IB) detects seroconversion earlier than standard immunoassays, identifying 66.7% of cases when IgM was positive but IgG negative/equivocal by routine methods 4
• Atypical profiles: Rare cases of seroconversion without detectable IgM or with only transient IgM require clinical judgment and often presumptive treatment 3, 2

Management Implications

When seroconversion is confirmed or strongly suspected during pregnancy, spiramycin treatment should be initiated immediately and continued until delivery. 2, 5

• Treatment initiation: 22/26 pregnant women with atypical seroconversion patterns were treated with spiramycin from the time IgG appeared until delivery 2
• Pyrimethamine-sulfadiazine: Reserved for confirmed fetal infection (positive amniocentesis PCR) or third trimester infections with high transmission risk 5
• Amniocentesis timing: Can be performed after 18 weeks gestation to detect fetal infection via PCR, with no false positives or negatives reported in one cohort 1