Originally published 16 October 2020. Revised October 2026.
Update note: This post has been reorganised and checked against current sources. The main changes: the “fatal” wording is softened, transmission rates are now given for each trimester, the treatment paragraph now says how uncertain the evidence is, and a short prevention section has been added.
Toxoplasmosis is usually asymptomatic or a mild, non-specific illness. A primary infection during pregnancy, however, can seriously harm the fetus, and the damage is sometimes severe. Treatment exists and may reduce transmission or the severity of disease, although the evidence for how much is limited.
Who is at risk?
- There is no clearly defined group of women at increased risk. Undercooked meat and food or soil contaminated with cat faeces are the usual sources, so cat ownership is only one possible exposure.
- Most women are susceptible. In Australian antenatal testing, seroprevalence has been about 23% to 35%, which leaves roughly 65% to 75% of women without immunity.
- Toxoplasma remains latent for life (as CMV does). Clinical reactivation is confined to the severely immunosuppressed.
- Infants of mothers who were seropositive before conception are generally not at risk. The usual exception is a mother who is immunosuppressed.
Interpreting serology
- A false-positive IgM is not uncommon, so a positive IgM should be confirmed at a reference laboratory.
- Low levels of IgM may persist for months or years after a primary infection, so a positive IgM alone does not date the infection.
- IgG positive with IgM negative suggests past infection. A fourfold rise in IgG, or a new positive IgG after a negative result, points to recent infection.
- Dating the infection relative to conception matters most. Discuss borderline results with a microbiologist or infectious diseases physician before counselling the patient.
Considerations in acute toxoplasmosis
- Treatment: In a confirmed case, or one that cannot be excluded, antibiotic treatment appears to reduce the risk of fetal infection and sequelae. This is more likely to help when treatment starts early. Evidence for the size of the benefit is limited and not all studies agree.
- First trimester: Fetal infection is relatively uncommon (about 15%), but the damage is more severe when it occurs.
- Later in pregnancy: Fetal infection becomes more likely (about 25% in the second trimester and about 65% in the third), but damage is less likely and, if it occurs, generally less severe.
| Trimester of maternal infection | Chance of fetal infection (approx.) | Severity if infected |
|---|---|---|
| First | 15% | Greatest |
| Second | 25% | Intermediate |
| Third | 65% | Usually milder |
Treatment in brief
- Spiramycin is generally used to reduce transmission when infection is acquired earlier in pregnancy and fetal infection is not documented. It works best when started soon after seroconversion.
- Pyrimethamine, sulfadiazine and folinic acid are generally used when fetal infection is documented or suspected, or when infection is acquired later in pregnancy. Pyrimethamine is usually avoided in the first trimester, and the blood count needs monitoring.
- Amniotic fluid PCR is the usual way to check whether the fetus is infected. Regimens vary by centre, so involve maternal-fetal medicine and infectious diseases early.
Prevention (advice for pregnant women)
- Cook meat thoroughly and wash hands, utensils and surfaces after handling raw meat.
- Wash or peel fruit and vegetables, and avoid unpasteurised milk and untreated water.
- Avoid changing cat litter. If you must, wear gloves and wash your hands. Keep cats on cooked or commercial food.
- Wear gloves when gardening and cover sandpits.
References
- Congenital toxoplasmosis over 10 years in a low-incidence population. Med J Aust. 2012;196(7):443-444. mja.com.au
- Toxoplasmosis. Pregnancy, Birth and Baby (Australian Government). pregnancybirthbaby.org.au
- Toxoplasmosis: clinical care. CDC. cdc.gov
- Congenital toxoplasmosis. GPnotebook. gpnotebook.com
