The prevalence of chronic or latent toxoplasmosis infection varies widely and is dependent upon age, socioeconomic conditions, cat exposure, travel, and eating customs. IgG anti-Toxoplasma seroprevalence rates among women of childbearing age in Europe range from approximately 50% in Belgium and France to 10% in Norway. Seroprevalence rates may change over time, because the prevalence in pregnant French women has dropped from 87% in the 1960s to 70% in the mid-1980s. In the United States, the seroprevalence among women of childbearing age has been relatively constant at approximately 15%, with a high of 30% in Birmingham, Alabama and a low of 13% in Minneapolis, Minnesota. The incidence of congenital Toxoplasma infection mirrors local or national seroprevalence rates. The estimated incidence of congenital infection in Belgium is 2 per 1,000 and in France 1 per 1,000 live births compared to 1 per 10,000 live births in the United States.

Congenital T. gondii infection occurs during maternal parasitemia associated almost exclusively with primary maternal infection. Transmission occurs following asymptomatic (the vast majority) and symptomatic infection. Occasionally, transmission may occur following reactivation of chronic infection among severely immunocompromised women. Very rare case reports have documented congenital infection when primary maternal infection was documented before conception or following maternal reinfection during pregnancy. Generally, the risk of fetal infection increases while the severity of clinical manifestation decreases with advancing gestational age at the time of maternal parasitemia. Overall, the risk of transmission of Toxoplasma from mother to fetus in untreated maternal–fetal diads is 30% to 50%. Historically, the transmission rate in the first trimester has been reported to be 10% to 25%, compared with 35% to 60% or more during the third trimester. The risk of severe clinical manifestations apparent at birth decreases from 75% in the first trimester to essentially 0% in the third. The placenta plays a pivotal role in congenital infection, because monozygotic twins have nearly identical congenital toxoplasmosis infection rates but dizygotic twins do not.

More than 90% of infants with congenital toxoplasmosis are asymptomatic or have no abnormal findings on routine examination at birth. Infants symptomatic at birth are more likely to have been infected early in gestation. First-trimester infection is associated with in utero death and severe neurologic and ophthalmic disease. Although no single finding is pathognomic of congenital T. gondii infection, concomitant findings of hydrocephalus, chorioretinitis, and intracranial calcifications are highly suggestive of congenital toxoplasmosis infection. Ocular lesions are seen in or develop in almost 90% of symptomatic infants (see Figure 222.1). The most common ocular findings are chorioretinal scars, which frequently involve the macula. Intracranial calcifications are distributed diffusely throughout the brain, in contrast to the periventricular pattern associated with symptomatic congenital cytomegalovirus infection (Figs. 77.2 and 222.2). Other findings include hepatosplenomegaly, lymphadenopathy, fever, and rashes. Abnormal laboratory findings consist of cerebrospinal fluid pleocytosis and elevated protein concentrations, hyperbilirubinemia, and anemia.

Most newborn infants with congenital T. gondii infection appear normal at birth. However, careful examination may demonstrate ocular disease or central nervous system (CNS) findings in these apparently normal infants. Fifty of 52 infants born with IgM antibodies to T. gondii detected by a New England screening program had normal routine newborn examinations. After the results of the serologic testing were known, 40% of those undergoing additional testing had evidence of CNS or eye abnormalities. Most often, ocular lesions consisted of unilateral macular retinal scars, and CNS lesions were characterized by small, focal cerebral calcifications and mild to moderate elevations of CSF protein. Long-term follow-up studies have shown that as many as 85% of untreated asymptomatic infants go on to develop chorioretinitis, while other significant neurologic sequelae may develop in 10% to 20%. Chorioretinitis or new eye lesions secondary to T. gondii infection can occur as late as in the third decade of life and lead to significantly impaired vision.

Confirming maternal or congenital T. gondii infection is challenging, despite the plethora of diagnostic tests available.
Methodologies currently available include culture, polymerase chain reaction (PCR), histologic examination of tissues, antigen detection, and serologic tests. However, few laboratories culture for Toxoplasma, and results make take weeks to get back. PCR can be used to detect T. gondii DNA in amniotic fluid. Examination of the placenta may show tachyzoites and tissue cysts. Antigen tests are insensitive and rarely used. In the United States, the diagnosis of toxoplasmosis most often is made serologically. Given the surfeit of tests with different sensitivities and specificities looking for anti-Toxoplasma IgM, IgG, IgA, and IgE antibody (see Table 222.1), practitioners should consult with an infectious disease specialist and rely on a reference laboratory to confirm maternal or congenital infection.