What is toxoplasmosis?

What is Toxoplasmosis

Toxoplasmosis is a parasitic infection caused by the obligate intracellular protozoan Toxoplasma gondii that infects approximately one-third of the world's population and can cause devastating consequences for fetuses, immunocompromised patients, and occasionally immunocompetent individuals. 1

The Causative Organism

• Toxoplasma gondii is an obligate intracellular parasite with worldwide distribution that infects any nucleated cell in any warm-blooded animal. 1
• The parasite has a complex life cycle with cats (Felidae family) serving as the definitive hosts that shed infectious oocysts in their feces. 1
• Humans and other animals serve as intermediate hosts and become infected incidentally through various routes. 1

Routes of Transmission

The primary routes of human infection are oral ingestion of oocysts from cat feces-contaminated soil, food, or water (accounting for approximately 78% of infections in pregnant women), and consumption of undercooked meat containing tissue cysts. 2

Additional transmission routes include:

• Transplacental transmission from an acutely infected mother to the fetus during pregnancy or within 3 months before conception. 1
• Organ transplantation from an infected donor. 1
• Blood transfusion (rare). 1
• Laboratory accidents (rare). 1
• Reactivation in severely immunocompromised pregnant women who were previously immune. 1
• Reinfection with a more virulent strain in previously immune individuals. 1

Clinical Manifestations

In Immunocompetent Individuals

• 70-90% of acquired infections in children and adults are asymptomatic. 3
• When symptomatic, cervical lymphadenopathy is the most frequent clinical finding, often accompanied by a mononucleosis-like syndrome with malaise, fever, sore throat, myalgia, and occasionally a maculopapular rash and hepatosplenomegaly. 3
• The disease is usually benign and self-limited in immunocompetent individuals. 3

In Congenitally Infected Infants

Congenital toxoplasmosis can cause significant fetal and neonatal harm, with devastating long-term consequences. 1

• 70-90% of infected infants are asymptomatic at birth, but the majority develop late sequelae ranging from months to years later. 4
• Among severely affected infants, 92% develop chorioretinitis, 80% have intracranial calcifications, 68% have hydrocephalus, and 62% have all three manifestations. 1
• Without maternal treatment, 85% of infected children develop vision impairment, 36% have recurrent eye disease, 27% have abnormal cognition, and 16% experience IQ decrease >15 points. 2
• Late sequelae include progressive retinitis, seizures (which may develop de novo between 3-5 years of age), developmental delays, intellectual disability, and neurologic deterioration. 4

In Immunocompromised Patients

• Toxoplasmic encephalitis should be considered in all HIV-infected patients presenting with new neurologic findings (fever, altered consciousness, seizures, focal deficits), regardless of whether symptoms are focal or diffuse. 3
• The disease can be fatal in immunosuppressed individuals, causing disseminated disease affecting virtually any organ system. 5

Epidemiology

• In the United States, approximately 91% of women of childbearing age are susceptible to Toxoplasma infection, with seroprevalence declining from 15% in 1988-1994 to 9% in 2009-2010. 1
• The incidence of congenital toxoplasmosis is estimated at 1 per 1,000-12,000 live births in the United States. 3
• Approximately half of infected individuals have no conventional risk factors (no exposure to cat litter or raw meat) and report no symptoms at the time of primary infection. 1, 2

Important Clinical Pitfalls

• Do not assume a normal newborn screen or asymptomatic presentation at birth excludes congenital toxoplasmosis—most affected infants develop problems months to years later. 4
• Do not rely on selective screening based on self-reported risk factors alone, as this approach misses >50% of women who give birth to infants with congenital toxoplasmosis. 2
• In HIV-infected children, do not exclude toxoplasmic encephalitis based on negative serology alone—cases have been reported without detectable IgG antibodies. 4