What is Toxoplasma gondii?

What is Toxoplasma gondii?

Toxoplasma gondii is an obligate intracellular parasitic protozoan with worldwide distribution that infects approximately one-third of the human population and can cause devastating consequences for fetuses, immunocompromised patients, and occasionally immunocompetent individuals. 1

Organism Classification and Biology

• T. gondii is an obligate intracellular parasite, meaning it can only survive and replicate inside host cells, not in extracellular environments. 1, 2

• The parasite exists in multiple forms: rapidly dividing tachyzoites (acute infection stage), dormant bradyzoites within tissue cysts (chronic/latent stage), and oocysts (environmental stage shed by cats). 2

• T. gondii infects a remarkably wide range of warm-blooded animals and birds, making it one of the most successful parasites globally. 1, 3

Transmission Routes

Humans acquire T. gondii primarily through three routes: ingestion of oocysts from cat feces-contaminated sources (accounting for ~78% of infections in pregnant women), consumption of tissue cysts in undercooked meat, or transplacental transmission from acutely infected mothers to fetuses. 1, 4

Primary transmission mechanisms include:

• Oocyst ingestion from contaminated soil, water, unwashed produce, or contact with cat litter—this is the predominant route in the United States. 1

• Tissue cyst consumption in undercooked or raw meat, particularly pork, lamb, venison, and locally produced cured/dried/smoked meats. 1

• Congenital transmission occurs when pregnant women acquire acute primary infection, with transmission risk ranging from 2-6% in first trimester to 81% in late pregnancy (though early infection causes more severe fetal disease). 1

• Rare routes include organ transplantation, blood transfusion, and laboratory accidents. 1

Critical food safety temperatures to kill tissue cysts:

• Whole cut meat (excluding poultry): 63°C (145°F) 1, 4
• Ground meat (excluding poultry): 71°C (160°F) 1, 4
• All poultry: 74°C (165°F) 1, 4
• Freezing below -20°C (-4°F) for at least 48 hours also inactivates cysts 1

Microwave cooking is insufficient because it does not generate homogenous temperatures. 1

Clinical Significance and Disease Manifestations

In immunocompetent individuals:

• 70-90% of acquired infections remain asymptomatic. 1, 5

• When symptomatic, the most common presentation is cervical lymphadenopathy with a mononucleosis-like syndrome (fever, malaise, sore throat, myalgia). 1, 5

• The infection is typically self-limited and benign in this population. 5

In congenitally infected infants:

• 70-90% are asymptomatic at birth, but the majority develop late sequelae including retinitis, visual impairment, intellectual disability, and neurologic impairment over months to years if untreated. 1

• Without maternal treatment, 85% develop vision impairment, 36% have recurrent eye disease, 27% have abnormal cognition, and 16% experience IQ decrease >15 points. 4

• Symptomatic newborns may present with generalized disease (rash, lymphadenopathy, hepatosplenomegaly, jaundice, cytopenias) or predominantly neurologic disease (hydrocephalus, intracranial calcifications, microcephaly, chorioretinitis, seizures). 1

In immunocompromised patients:

• Toxoplasma encephalitis is the primary concern, presenting with focal neurologic deficits, fever, altered consciousness, or seizures. 1, 5

• This occurs from reactivation of latent tissue cysts when immunity becomes impaired (particularly CD4 <200 cells/mm³ in HIV patients). 1, 5

• Less common manifestations include pneumonitis, myocarditis, and disseminated disease. 1

Epidemiology

• The incidence of congenital toxoplasmosis in the United States is estimated at 1 per 1,000-12,000 live births and has decreased substantially over the past 20 years. 1, 5

• Toxoplasma seropositivity among U.S. women is approximately 15%, with no difference between HIV-infected and uninfected women. 1

• Approximately 50% of infected individuals have no conventional risk factors (no cat litter or raw meat exposure) and report no symptoms at the time of primary infection. 1, 4

Genetic Strains

• Three main clonal lineages exist (Types 1,2, and 3) in North America and Europe, with Type 2 predominating in Western Europe. 1

• All three types plus a fourth lineage (Type 12) are found in North America. 1

• Atypical strains outside these lineages are frequently reported in the Americas and may have different virulence profiles. 1

Key Clinical Pitfalls

• Do not rely on selective screening based on self-reported risk factors alone—this approach misses >50% of women who give birth to infants with congenital toxoplasmosis. 4

• Cats are not the only or even primary source of infection—oocyst contamination of food, water, and soil accounts for most U.S. infections. 1, 4

• Having ≥3 kittens in the household is a specific risk factor because young cats shed more oocysts. 1

• Novel risk factors include eating raw oysters, clams, or mussels and drinking unpasteurized goat milk. 1