Toxoplasmosis is a worldwide zoonosis caused by Toxoplasma gondii, an obligate intracellular protozoan parasite. Toxoplasma gondii exists in 3 morphologic forms: oocyst, tissue cyst (contains bradyzoites), and tachyzoite (the proliferative form). Felines are the definitive host. Mammals (herbivores and carnivores), birds, and reptiles can also be infected. Sexual reproduction takes place only in the feline intestinal tract, resulting in oocysts that are shed in stools.

Human infection may occur from the following:

• Ingestion of oocysts in contaminated food or water
• Ingestion of tissue cysts contained in undercooked meat (particularly pork, lamb, bear, and deer) or from tissue cysts in transplanted organs (kidney and heart)
• Intake of tachyzoites (congenital infection and rarely from blood transfusions or laboratory accidents)

Congenital toxoplasmosis results from acute maternal infection during pregnancy. The incidence of fetal transmission is highest during second- and third-trimester maternal infection. Though less frequent, fetal transmission during first-trimester maternal infection more commonly results in severe illness in the newborn.

Early (during the first and second trimester) congenitally acquired toxoplasmosis can result in abortion, stillbirth, neonatal illness, or death. Most infants infected during the third trimester are born without obvious evidence of toxoplasmosis.

Congenital toxoplasmosis can present with any of the following manifestations: hydrocephalus, chorioretinitis, intracerebral calcifications, microcephalus, intellectual disability, seizures, blindness, hepatosplenomegaly, hemolytic anemia, and jaundice. Infected but apparently normal infants at birth may go on to develop chorioretinitis, blindness, intellectual disability, or seizures if left untreated.

Chorioretinitis is the most common manifestation of congenital toxoplasmosis and may recur despite therapy. Treatment during pregnancy may modify the risk of transmission and severity of infection in the fetus.

Immunocompromised Patient Considerations:
HIV-infected infants with congenital toxoplasmosis may have multi-organ involvement including central nervous system (CNS), pulmonary, and cardiac disease.