Mother-to-Child Transmission of Toxoplasmosis: Risk Estimates for Clinical Counselling

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Most women who develop toxoplasmosis while pregnant are detected by serological screening, and they should be counseled about the risk of congenital infection. With the goal of obtaining reliable risk estimates, data were obtained from women referred to a toxoplasmosis reference laboratory in Lyon, France, in the years 1987 to 1995. Obstetrical and pediatric information was available on 603 confirmed cases of maternal toxoplasmosis. At least 564 of these women received antiparasitic drugs. A total of 554 offspring were followed up for congenital infection, and those infected were observed for a median of 4.5 years. Only 36 infected women were symptomatic. The minimal rate of maternofetal transmission was 29 percent, excluding indeterminate cases. Infection was demonstrated in three of six fetal losses. All five planned terminations followed the diagnosis of fetal infection. The risk was relatively low when women were infected in early pregnancy but reached 40 percent at 26 weeks’ gestation and 72 percent by 36 weeks’ gestation. Of 153 congenitally infected live-born children, 27 percent had at least one specific sign of toxoplasmosis: chorioretinal lesions, intracranial calcification, or hydrocephaly. Chorioretinitis was the most common clinical sign of toxoplasmosis, followed by intracranial calcification. Most diagnoses were made in the first year of life, although chorioretinitis often was detected at a later time. The chance of clinical signs being detected declined as the duration of pregnancy before maternal IgM seroconversion occurred lengthened. Clinical signs of toxoplasmosis in the young infant do not necessarily mean functional impairment. Birth cohort studies and randomized treatment trials will be needed to assess the long-term risk of disability and to learn whether it can be prevented by early treatment. Lancet 1999;353:1829–1833