What is toxoplasmosis?

Toxoplasmosis Overview

Toxoplasmosis is a parasitic infection caused by Toxoplasma gondii, an obligate intracellular parasite 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

The Causative Organism

• Toxoplasma gondii is an apicomplexan protozoan parasite capable of infecting any nucleated cell in warm-blooded animals 2
• The parasite exists in three main clonal lineages (types 1,2, and 3) in North America, with type 2 predominating in Western Europe and an additional fourth clonal lineage (type 12) recently identified in North America 1
• Atypical strains outside these clonal lineages are frequently reported in North and South America, with varying virulence 1

Epidemiology in the United States

• The seroprevalence among women of childbearing age (15-44 years) has declined dramatically: 15% in 1988-1994,11% in 1999-2004, and 9.1% in 2009-2010 1
• Among US-born women specifically, rates dropped from 13% to 6% over the same period, meaning approximately 91% of US women of childbearing age remain susceptible to infection 1
• People born outside the United States have significantly higher seroprevalence (25.1% vs 9.6%), as do Hispanic versus non-Hispanic white individuals (15.8% vs 10.2%) 1
• Toxoplasmosis is the second leading cause of death and fourth leading cause of hospitalizations from foodborne illnesses in the United States, with 789 deaths over 2000-2010 and $815 million in productivity losses 1

Routes of Transmission

Humans acquire toxoplasmosis primarily through three routes: 1

1. Contaminated Meat (Tissue Cysts)

• Eating undercooked or raw meat, especially pork, lamb, or venison containing tissue cysts 1
• Touching mouth after handling raw contaminated meat or using contaminated utensils 1
• Working with meat occupationally increases risk 1

2. Environmental Oocysts (Cat Feces)

• Ingesting oocysts from soil contaminated with cat feces through gardening, cleaning litter boxes, or eating unwashed produce 1, 3
• Oocysts are the predominant transmission route in the United States: 78% of pregnant women who delivered infants with congenital toxoplasmosis had serologic evidence of oocyst-acquired infection 1
• Drinking contaminated water (source of major community outbreaks in Canada and Brazil) 1

3. Other Routes

• Drinking unpasteurized raw milk (especially goat milk) 1
• Eating raw oysters, clams, or mussels (novel risk factor recently identified) 1
• Organ transplantation from infected donors 1
• Blood transfusion (rare) 1, 4
• Laboratory accidents (rare) 1

4. Vertical Transmission (Mother-to-Fetus)

• Transplacental transmission from acutely infected pregnant women to the fetus 1
• Reactivation in severely immunocompromised pregnant women with chronic infection 1, 5
• Reinfection with more virulent strains in previously immune pregnant women 1, 5

Clinical Manifestations by Population

Immunocompetent Adults

• Most infections (approximately 50%) are asymptomatic and do not exhibit conventional risk factors 1
• When symptomatic, may present with febrile illness resembling mononucleosis 1

Congenital Toxoplasmosis

• 85% of infants with congenital toxoplasmosis referred to US reference centers were severely affected: 92% had chorioretinitis, 80% had intracranial calcifications, 68% had hydrocephalus, and 62% had all three manifestations 1
• The incidence in the United States decreased from 0.91 per 10,000 live births (1988-1999) to 0.23 per 10,000 (2006-2014) 1
• Symptomatic disease rates increase with later maternal infection: 15% at 13 weeks gestation, 44% at 26 weeks, and 71% at 37 weeks 1, 5
• Transmission risk in untreated women ranges from 2-6% in first trimester to 81% in late pregnancy, but early infection causes more severe fetal damage 1, 5
• Most infected newborns are asymptomatic at birth but develop late sequelae including retinitis, visual impairment, and neurologic damage 5

Immunocompromised Patients

• In HIV-infected individuals, toxoplasmosis causes severe manifestations including encephalitis, pneumonitis, hepatitis, and cardiomyopathy 4
• CNS toxoplasmic encephalitis was reported in <1% of pediatric AIDS cases even before HAART 1
• Reactivation of latent infection is the primary concern in this population 1

Prevention Strategies

Food Safety Measures

• Cook meat to safe temperatures: 63°C (145°F) for whole cuts, 71°C (160°F) for ground meat, and 74°C (165°F) for poultry 1
• Freeze meat below -20°C (-4°F) for at least 48 hours to inactivate tissue cysts 1
• Microwave cooking is insufficient as it may not generate homogenous temperatures 1
• Peel or thoroughly wash fruits and vegetables before eating 3
• Clean cooking surfaces and utensils after contact with raw meat 3

Environmental Precautions

• Pregnant women should avoid changing cat litter or use gloves and wash hands thoroughly 3
• Do not feed raw or undercooked meat to cats and keep cats indoors 3
• Wear gloves when gardening and wash hands afterward 1

High-Risk Populations

• Having ≥3 kittens in the household increases risk 1
• Avoid eating raw oysters, clams, mussels, and unpasteurized dairy products 1
• Approximately 61% of women who delivered infants with congenital toxoplasmosis had no exposure to cat litter or raw meat, and 52% had no febrile illness during pregnancy, highlighting that conventional risk factors are absent in many cases 1

Treatment Considerations

• Early antibiotic treatment within 3 weeks of maternal seroconversion can reduce transmission risk by 52% 5
• Prompt initiation of prenatal treatment decreases mother-to-child transmission and ameliorates clinical manifestations 1
• Postnatal treatment for congenital toxoplasmosis has moderate quality evidence from observational studies 1
• In HIV-infected children, lifelong suppression is indicated following treatment for active disease 1

Diagnostic Approach

• Serologic testing includes IgG, IgM, IgA, IgE, IgG-avidity, and differential agglutination tests 1
• Samples should be sent to reference laboratories when clinical suspicion exists to avoid delays in diagnosis and treatment initiation 1
• Single-point testing is often insufficient; serial testing is needed to confirm acute versus chronic infection 5
• IgM positivity alone does not confirm recent infection as it can persist for months to years 5