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For further information, see CMDT Part 35-06: Toxoplasmosis

Key Features

Essentials of Diagnosis

  • Infection confirmed by isolation of Toxoplasma gondii or identification of tachyzoites in tissue or body fluids

Primary infection

  • Fever, malaise, headache, sore throat

  • Lymphadenopathy

  • Positive IgG and IgM serologic tests

Congenital infection

  • Follows acute infection of seronegative mothers and leads to CNS abnormalities and retinochoroiditis

Infection in immunocompromised persons

  • Reactivation leads to encephalitis, retinochoroiditis, pneumonitis, myocarditis

  • Positive IgG but negative IgM serologic tests

General Considerations

  • T gondii, an obligate intracellular protozoan, is found worldwide in humans and in many species of mammals and birds

  • Definitive hosts are cats

  • Humans are infected after ingestion of cysts in raw or undercooked meat, ingestion of oocysts in food or water contaminated by cats, transplacental transmission of trophozoites or, rarely, direct inoculation of trophozoites via blood transfusion or organ transplantation


  • Toxoplasma seroprevalence

    • Varies widely

    • Has decreased in the United States to < 20%

    • However, it is much higher in other countries in both the developed and developing worlds, where it may exceed 80%

  • In the United States, T gondii is estimated to infect 1.1 million persons each year, with resultant chorioretinitis developing in 21,000 and vision loss in 4800

  • In the United States, an estimated 400 to 4000 congenital infections occur yearly

Clinical Findings

Primary infection

  • After ingestion, T gondii infection progresses from the gastrointestinal tract to lymphatics, and then dissemination

  • Most acute infections are asymptomatic

  • About 10–20% are symptomatic after an incubation period of 1–2 weeks

  • Typically present as mild, febrile illnesses that resemble infectious mononucleosis

  • Nontender cervical or diffuse lymphadenopathy may persist for weeks to months

  • Systemic findings may include

    • Fever

    • Malaise

    • Headache

    • Sore throat

    • Rash

    • Myalgias

    • Hepatosplenomegaly

    • Atypical lymphocytosis

  • Severe manifestations are rare and include

    • Pneumonitis

    • Meningoencephalitis

    • Hepatitis

    • Myocarditis

    • Polymyositis

    • Retinochoroiditis

Congenital infection

  • Fetal infection follows maternal infection in 30–50% of cases, but this risk varies by trimester:

    • 10–25% during the first trimester

    • 30–50% during the second trimester

    • 60% or higher during the third trimester

  • Early fetal infections commonly lead to

    • Spontaneous abortion

    • Stillbirths

    • Severe neonatal disease, including neurologic manifestations

  • Systemic findings include

    • Fever or hypothermia

    • Jaundice

    • Vomiting

    • Diarrhea

    • Hepatosplenomegaly

    • Pneumonitis

    • Myocarditis

    • Rash

  • Retinochoroiditis and other sight-threatening eye lesions may develop

    • Retinochoroiditis presents weeks to years after congenital infection, commonly in teenagers or young adults

    • Uveitis is also seen

    • Disease presents with pain, photophobia, and visual changes, usually without systemic symptoms

    • Signs and symptoms eventually improve, but visual defects may persist

    • Rarely, progression may result in glaucoma and blindness

Infection in the immunocompromised person


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