Toxoplasmosis is caused by Toxoplasma gondii, a protozoan and obligate intracellular parasite capable of causing intrauterine infection (part of TORCH [toxoplasmosis, other, rubella, cytomegalovirus, herpes simplex virus] infections; see Chapter 148).
The incidence of congenital toxoplasmosis (CT) is 0.23 to 0.91 per 10,000 live births based on published data from the New England Newborn Screening Program; the true incidence might be higher, because the sensitivity of the newborn screening test (blot-spot immunoglobulin [Ig] M test) is approximately 50% to 75%, and fetal losses attributable to severe CT are not counted. Seroprevalence of T gondii among women of childbearing age (15–44 years) has declined over time (15%, 11%, and 9% in 1988–1994, 1999–2004, and 2009–2010, respectively).
T gondii is a coccidian parasite ubiquitous in nature. Members of the feline family are the definitive hosts. The organism exists in 3 forms: oocyst, tachyzoite, and tissue cyst (bradyzoites). Cats generally acquire the infection by feeding on infected animals such as mice or uncooked household meats. The parasite replicates sexually in the feline intestine. Cats may begin to excrete oocysts in their stool for 7 to 14 days after infection. During this phase, the cat can shed millions of oocysts daily for 2 weeks. After excretion, oocysts require a maturation phase (sporulation) of 24 to 48 hours before they become infective by the oral route. Intermediate hosts (sheep, cattle, and pigs) can have tissue cysts within organs and skeletal muscle. These cysts can remain viable for the lifetime of the host. The pregnant woman usually becomes infected by consumption of raw or undercooked meat that contains cysts or by the accidental ingestion of sporulated oocysts from soil or contaminated food. Ingestion of oocysts (and cysts) releases sporozoites that penetrate the gastrointestinal mucosa and later differentiate into tachyzoites. Tachyzoites are ovoid unicellular organisms characteristic of the acute infection. Tachyzoites spread throughout the body via the bloodstream and lymphatics. It is during this stage that vertical transmission from mother to the child (MTCT) occurs. In the immunocompetent host, the tachyzoites are sequestered in tissue cysts and form bradyzoites. Bradyzoites are indicative of the chronic stage of infection and can persist in the brain, liver, and skeletal tissue for the life of the individual. There are reports of transmission of toxoplasmosis through contaminated municipal water, blood transfusion, organ donation, and occasionally as a result of a laboratory accident.
Acute infection in the pregnant woman is often subclinical (90% of the cases). If symptoms are present, they are generally nonspecific: mononucleosis-like illness with fever, painless lymphadenopathy, fatigue, malaise, myalgia, fever, skin rash, and splenomegaly. Placental infection occurs and persists throughout pregnancy. The infection may or may not be transmitted to the fetus. The later in pregnancy that infection is acquired, the more likely is MTCT (first trimester, 17%; second trimester, 25%; ...