ByChelsea Marie, PhD, University of Virginia;
William A. Petri, Jr, MD, PhD, University of Virginia School of Medicine
Reviewed/Revised Dec 2022
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Toxoplasmosis is infection with Toxoplasma gondii

Human exposure to toxoplasmosis is common wherever cats are found. An estimated 11% of residents ≥ 6 years in the US are seropositive, which indicates that they have been infected, and more than 60% of some populations in other regions have been infected (see Centers for Disease Control and Prevention: Epidemiology & Risk Factors). The risk of developing serious disease is very low except for a fetus infected in utero and people who are or become severely immunocompromised with AIDS or other diseases.

Pathophysiology of Toxoplasmosis

T. gondii is ubiquitous in birds and mammals. This obligate intracellular parasite invades and multiplies asexually as tachyzoites within the cytoplasm of any nucleated cell (see figure Toxoplasma gondii life cycle). When host immunity develops, multiplication of tachyzoites ceases and tissue cysts form; cysts persist in a dormant state for years, especially in brain, eyes, and muscle. The dormant Toxoplasma forms within the cysts are called bradyzoites.

Sexual reproduction of T. gondii occurs only in the intestinal tract of cats; the resultant oocysts passed in the feces remain infectious in moist soil for months.

Toxoplasma gondii life cycle

The only known definitive hosts for T. gondii are members of family Felidae (domestic cats and their relatives).

  1. 1a. Oocysts are shed in the cat’s feces. Large numbers are shed, but usually only for 1–2 weeks. Oocysts take 1–5 days to sporulate and become infective.

  2. 1b. Cats become reinfected by ingesting sporulated oocysts.

  3. 2. Soil, water, plant material, or cat litter becomes contaminated with oocysts. Intermediate hosts in nature (eg, birds, rodents, wild game, animals bred for human consumption) become infected after ingesting infective materials.

  4. 3. Oocysts develop into tachyzoites shortly after ingestion.

  5. 4. Tachyzoites spread throughout the body and form tissue cysts in neural, eye, and muscle tissue.

  6. 5. Cats become infected after consuming intermediate hosts containing tissue cysts.

  7. 6a. Humans can become infected by ingesting undercooked meat containing tissue cysts.

  8. 6b. Humans can become infected by ingesting food or water contaminated with cat feces or other feces-contaminated materials (eg, soil) or contact with a pet cat’s litter.

  9. 7. Rarely, human infection results from blood transfusion or organ transplantation.

  10. 8. Rarely, transplacental transmission from mother to fetus occurs.

  11. 9. In the human host, parasites form tissue cysts, most commonly in skeletal muscle, myocardium, the brain, and the eyes; these cysts may remain throughout the life of the host and can reactivate if the host becomes immunocompromised.

Infection can occur by

  • Ingestion of oocysts

  • Ingestion of tissue cysts

  • Transplacental transmission

  • Blood transfusion or organ transplantation

Ingestion of oocysts in food or water contaminated with cat feces is the most common mode of oral infection. Infection can also occur by eating raw or undercooked meat containing tissue cysts, most commonly lamb, pork, or rarely beef.

Pneumocystis prophylaxis has dramatically reduced the incidence.

Toxoplasmosis can be transmitted transplacentally if the mother becomes infected during pregnancy or if immunosuppression reactivates a prior infection. Transmission of Toxoplasma to a fetus is extraordinarily rare in immunocompetent mothers who were infected with Toxoplasma and developed immunity prior to pregnancy.

Transmission may occur via transfusion of whole blood or white blood cells or via transplantation of an organ from a seropositive donor.

In otherwise healthy people, congenital or acquired infection can reactivate in the eyes. Nonocular reactivation is very rare in healthy people. Past infection confers resistance to reinfection.

Symptoms and Signs of Toxoplasmosis

Infections may manifest in several ways:

  • Acute toxoplasmosis

  • Central nervous system (CNS) toxoplasmosis

  • Congenital toxoplasmosis

  • Ocular toxoplasmosis

  • Disseminated or non-CNS disease in immunocompromised patients

Acute toxoplasmosis

Acute infection is usually asymptomatic, but 10 to 20% of patients develop bilateral, nontender cervical or axillary lymphadenopathy. A few of these also have a mild flu-like syndrome of fever, malaise, myalgia, hepatosplenomegaly, and less commonly, pharyngitis, which can mimic infectious mononucleosis and include lymphadenitis. Atypical lymphocytosis, mild anemia, leukopenia, and slightly elevated liver enzymes are common. The syndrome may persist for weeks but is almost always self-limited.

CNS toxoplasmosis

Most patients with AIDS or other immunocompromised patients who develop toxoplasmosis present with encephalitis and ring-enhancing intracranial mass lesions seen on CT or MRI scans, both with contrast. Risk is greatest among those with CD4 counts of < 50/mcL; toxoplasmic encephalitis is rare when CD4 counts are > 200/mcL. These patients typically have headache, altered mental status, seizures, coma, fever, and sometimes focal neurologic deficits, such as motor or sensory loss, cranial nerve palsies, visual abnormalities, and focal seizures.

Congenital toxoplasmosis

Congenital toxoplasmosis results from a primary, often asymptomatic infection acquired by the mother during pregnancy. Women infected before conception ordinarily do not transmit toxoplasmosis to the fetus unless the infection is reactivated during pregnancy by immunosuppression. Spontaneous abortion, stillbirth, or birth defects may occur. The percentage of surviving fetuses born with toxoplasmosis depends on when maternal infection is acquired; it increases from 15% during the 1st trimester to 30% during the 2nd to 60% during the 3rd. The severity of congenital diseases decreases if the mother becomes infected later in pregnancy.

Disease in neonates may be severe, particularly if acquired early in pregnancy; symptoms include jaundice, rash, hepatosplenomegaly, and the characteristic tetrad of abnormalities:

  • Bilateral retinochoroiditis

  • Cerebral calcifications

  • Hydrocephalus or microcephaly

  • Psychomotor retardation

Prognosis is poor.

Many children with less severe infections and most infants born to mothers infected during the 3rd trimester appear healthy at birth but are at high risk of seizures, intellectual disability, retinochoroiditis, or other symptoms developing months or even years later.

Ocular toxoplasmosis

This type usually results from congenital infection that is reactivated, often during the teens and 20s, but rarely, it occurs with acquired infections. Focal necrotizing retinitis and a secondary granulomatous inflammation of the choroid occur and may cause ocular pain, blurred vision, and sometimes blindness. Relapses are common.

Disseminated infection and non-CNS involvement

Disease outside the eye and CNS is much less common and occurs primarily in severely immunocompromised patients. They may present with pneumonitis, myocarditis, polymyositis, diffuse maculopapular rash, high fevers, chills, and prostration.

In toxoplasmic pneumonitis, diffuse interstitial infiltrates may progress rapidly to consolidation and cause respiratory failure, whereas endarteritis may lead to infarction of small lung segments. Myocarditis, in which conduction defects are common but often asymptomatic, may rapidly lead to heart failure.

Untreated disseminated infections are usually fatal.

Diagnosis of Toxoplasmosis

  • Serologic testing

  • For central nervous system involvement, CT or MRI and lumbar puncture

  • Histopathologic evaluation of biopsies

  • Polymerase chain reaction–based assays of blood, cerebrospinal fluid, tissue, or, during pregnancy, amniotic fluid

Toxoplasmosis is usually diagnosed serologically using an indirect fluorescent antibody (IFA) test or enzyme immunoassay (EIA) for IgG and IgM antibodies (see table Interpretation of Toxoplasma Serologic Testing). Specific IgM antibodies appear during the first 2 weeks of acute illness, peak within 4 to 8 weeks, and eventually become undetectable, but they may be present for as long as 18 months after acute infection. IgG antibodies arise more slowly, peak in 1 to 2 months, and may remain high and stable for months to years. Assays for Toxoplasma IgM lack specificity.


The diagnosis of acute toxoplasmosis during pregnancy and in the fetus or neonate can be difficult, and consultation with an expert is recommended. If the patient is pregnant and IgG and IgM are positive, an IgG avidity test should be done. High avidity antibodies in the first 12 to 16 weeks of pregnancy essentially rules out an infection acquired during gestation. But a low IgG avidity result cannot be interpreted as indicating recent infection because some patients have persistent low IgG avidity for many months after infection. Suspected recent infection in a pregnant woman should be confirmed before intervention by having samples tested at a toxoplasmosis reference laboratory. If the patient has clinical illness compatible with toxoplasmosis but the IgG titer is low, a follow-up titer 2 to 3 weeks later should show an increase in antibody titer if the illness is due to acute toxoplasmosis, unless the host is severely immunocompromised.

In general, detection of specific IgM antibody in neonates suggests congenital infection. Maternal IgG crosses the placenta, but IgM does not. Detection of Toxoplasma-specific IgA antibodies is more sensitive than IgM in congenitally infected infants, but it is available only at special reference facilities (eg, Toxoplasma Serology Laboratory, Palo Alto Research Institute, Palo Alto, CA). An expert should be consulted when fetal or congenital infection is suspected.

Toxoplasma are occasionally demonstrated histologically. Tachyzoites, which are present during acute infection, take up Giemsa or Wright stain but may be difficult to find in routine tissue sections. Tissue cysts do not distinguish acute from chronic infection. Toxoplasma must be distinguished from other intracellular organisms, such as Histoplasma, Trypanosoma cruzi, and Leishmania. Polymerase chain reaction (PCR) tests for parasite DNA in blood, cerebrospinal fluid (CSF), or amniotic fluid are available at several reference laboratories. PCR-based analysis of amniotic fluid is the preferred method to diagnose toxoplasmosis during pregnancy.

If central nervous system (CNS) toxoplasmosis is suspected, patients should have head CT with contrast or MRI with contrast, or both plus a lumbar puncture if there are no signs of increased intracranial pressure. MRI is more sensitive than CT. MRI and CT with contrast typically show single or multiple rounded, ring-enhancing lesions. Although these lesions are not pathognomonic, their presence in patients with AIDS and CNS symptoms warrants a trial of chemotherapy for T. gondii. CSF may be positive for lymphocytic pleocytosis, and the protein level may be elevated.

Acute infection should be suspected in immunocompromised patients if the IgG is positive. However, IgG antibody levels in AIDS patients with Toxoplasma encephalitis are usually low to moderate, and IgG antibodies are sometimes absent; IgM antibodies are not present.

If the suspected diagnosis of CNS toxoplasmosis is correct, clinical and radiographic improvement should become evident within 7 to 14 days. If symptoms worsen over the 1st week or do not lessen by the end of the 2nd week, a brain biopsy should be considered.

Ocular disease is diagnosed based on the appearance of the lesions in the eye, symptoms, course of disease, and results of serologic testing.

Treatment of Toxoplasmosis

Treatment of toxoplasmosis is not indicated for immunocompetent patients who are asymptomatic or have mild, uncomplicated acute infection; treatment is required only when visceral disease is present or symptoms are severe or persist.

However, specific treatment is indicated for acute toxoplasmosis in the following:

  • Neonates

  • Pregnant women with acute toxoplasmosis

  • Immunocompromised patients

Pearls & Pitfalls

  • No treatment is required for immunocompetent patients who are asymptomatic or have mild, uncomplicated acute toxoplasmosis.

Treatment of immunocompetent patients

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Treatment of patients with AIDS or other immunocompromising conditions

It is important to optimize antiretroviral therapy.

Chronic maintenance therapy is used after successful treatment of acute disease to prevent relapses in patients who remain immunocompromised. Relapses are particularly common in AIDS patients with CD4 counts < 200/mcL. Maintenance therapy is continued until the CD4 counts remain > 200/mcL for > 6 months on antiretroviral therapy.

There are several options for maintenance therapy:

prevent pneumonia

Treatment of ocular toxoplasmosis

Treatment of ocular toxoplasmosis is based on results of a complete ophthalmologic evaluation (degree of inflammation; visual acuity; size, location, and persistence of lesion). Doses are

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The Centers for Disease Control and Prevention (CDC) recommends that therapy for ocular toxoplasmosis be continued for 4 to 6 weeks, followed by reevaluation of the patient's condition (see also CDC: Toxoplasmosis: Resources for Health Professionals).

Patients with ocular toxoplasmosis are also frequently given corticosteroids to reduce inflammation.

Treatment of pregnant patients

Treatment of pregnant women with acute toxoplasmosis can decrease the incidence of fetal infection.

Consultation with an infectious diseases expert is recommended.

Treatment of infants with congenital toxoplasmosis


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Treatment reference

  1. 1. Maldonado YA, Read JS, Committee on Infectious Diseases: Diagnosis, treatment, and prevention of congenital toxoplasmosis in the United States. Pediatrics 139(2):e20163860, 2017. doi:10.1542/peds.2016-3860

Prevention of Toxoplasmosis

Washing hands thoroughly after handling raw meat, soil, or cat litter is essential to help prevent toxoplasmosis. Food possibly contaminated with cat feces should be avoided. Meat should be cooked to 165 to 170° F (73.9 to 76.7° C).

Pregnant women are advised to avoid contact with cats. If contact is unavoidable, pregnant women should at least avoid cleaning cat litter boxes or wear gloves when doing so. Gloves should also be worn while gardening to avoid contact with soil.

Primary chemoprophylaxis is recommended for patients with HIV and a positive IgG T. gondii serologic test once CD4 cell counts are <Pneumocystis jirovecii

Key Points

  • T. gondii reproduces sexually in the intestinal tract of cats; most human infections result from direct or indirect contact with cat feces but can be acquired transplacentally or by ingestion of poorly cooked meat that contains cysts.

  • About 11% of the US population have been infected with T. gondii, but symptomatic disease is rare and occurs mainly in fetuses who are infected when the mother acquires acute infection during pregnancy and transmits the infection transplacentally or in people who are immunocompromised by HIV or other causes.

  • Acute infection is usually asymptomatic in immunocompetent patients, but 10 to 20% have manifestations, similar to those of mononucleosis, including lymphadenopathy.

  • Immunocompromised patients typically present with encephalitis and have ring-enhancing intracranial mass lesions, seen on MRI or CT with contrast.

  • To diagnose, use serologic tests (for IgG and IgM antibodies), histopathology, or polymerase chain reaction.

  • Treatment is indicated mainly for congenitally infected neonates, pregnant women with acute infection, and immunocompromised patients.

  • Antiretroviral therapy should be optimized in patients with AIDS; suppressive treatment is continued until patients are asymptomatic and CD4 cell counts are > 200/mcL for > 6 months.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  1. Centers for Disease Control and Prevention (CDC): Resources for Health Professionals: Toxoplasmosis

  2. AIDSinfo: Guidelines for Prevention and Treatment of Opportunistic Infections in HIV-Infected Adults and Adolescents

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