Merck Manual

Please confirm that you are a health care professional

honeypot link



Chelsea Marie

, PhD, University of Virginia;

William A. Petri, Jr

, MD, PhD, University of Virginia School of Medicine

Reviewed/Revised Dec 2022
Topic Resources

Toxoplasmosis is infection with Toxoplasma gondii. Symptoms range from none to benign lymphadenopathy, a mononucleosis-like illness, to life-threatening central nervous system (CNS) disease or involvement of other organs in immunocompromised people. Encephalitis can develop in patients with AIDS and low CD4 counts. Retinochoroiditis, seizures, and intellectual disability occur in congenital infection. Diagnosis is by serologic tests, histopathology, or polymerase chain reaction (PCR). Treatment is most often with pyrimethamine plus either sulfadiazine or clindamycin. Corticosteroids are given concurrently for retinochoroiditis.

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 ). 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

Toxoplasma gondii life cycle

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

  • 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.

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

  • 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.

  • 3. Oocysts develop into tachyzoites shortly after ingestion.

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

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

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

  • 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.

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

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

  • 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.

After ingestion of oocysts or tissue cysts, tachyzoites are released and spread throughout the body. This acute infection is followed by the development of protective immune responses and the formation of tissue cysts in many organs. The cysts can reactivate causing disease, primarily in immunocompromised patients. Toxoplasmosis reactivates in 30 to 40% of AIDS patients who are not taking antibiotic prophylaxis, but the widespread use of trimethoprim/sulfamethoxazole for 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 ). 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 Histoplasmosis Histoplasmosis is a pulmonary and hematogenous disease caused by Histoplasma capsulatum; it is often chronic and usually follows an asymptomatic primary infection. Symptoms are those... read more Histoplasmosis , Trypanosoma cruzi Chagas Disease Chagas disease is infection with Trypanosoma cruzi, transmitted by Triatominae bug bites or, less commonly, via ingestion of sugar cane juice or foods contaminated with infected Triatominae... read more Chagas Disease , and Leishmania Leishmaniasis Leishmaniasis is caused by species of Leishmania. Manifestations include cutaneous, mucosal, and visceral syndromes. Cutaneous leishmaniasis causes painless chronic skin lesions ranging... read more Leishmaniasis . 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 Lumbar Puncture (Spinal Tap) Lumbar puncture is used to do the following: Evaluate intracranial pressure and cerebrospinal fluid (CSF) composition (see table ) Therapeutically reduce intracranial pressure (eg, idiopathic... read more 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

  • Pyrimethamine and sulfadiazine plus leucovorin (to prevent bone marrow suppression); alternatively, the fixed drug combination trimethoprim-sulfamethoxazole in some situations

  • Clindamycin or atovaquone plus pyrimethamine when the patient is allergic to sulfonamides or does not tolerate sulfadiazine

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

The most effective regimen in immunocompetent patients with visceral involvement or severe or persistent symptoms is pyrimethamine plus sulfadiazine, for 2 to 4 weeks. Dosage is

  • Pyrimethamine 50 mg twice a day for 2 days, then 25 to 50 mg once a day in adults (in children, 2 mg/kg orally on day 1, then 1 mg/kg once a day; maximum 25 mg/day) plus

  • Sulfadiazine 1 g orally 4 times a day in adults (in children, 50 mg/kg twice a day)

Folinic acid (leucovorin) is given concurrently to help protect against bone marrow suppression: adults 10 to 20 mg orally once a day (children 7.5 mg orally once a day)

In patients who have or develop sulfonamide hypersensitivity, clindamycin 600 to 800 mg orally 3 times a day is given with pyrimethamine and leucovorin instead of sulfonamides. Another option is atovaquone plus pyrimethamine and leucovorin. The fixed combination of trimethoprim with sulfamethoxazole has been used as an alternative as well as pyrimethamine and leucovorin plus clarithromycin, or dapsone, or azithromycin, but they have not been extensively studied.

Treatment of patients with AIDS or other immunocompromising conditions

Higher doses of pyrimethamine are used in immunocompromised patients, most of whom have AIDS with CNS toxoplasmosis or, uncommonly, involvement of other organs. A loading dose of pyrimethamine 200 mg orally is given the first day, then 50 mg once a day in patients < 60 kg and 75 mg once a day in those > 60 kg, plus sulfadiazine 1,000 mg orally 4 times a day in patients < 60 kg and 1,500 mg orally 4 times a day in patients > 60 kg for at least 6 weeks and 4 to 6 weeks after resolution of clinical signs and symptoms. Pyrimethamine bone marrow suppression can be minimized with leucovorin (also called folinic acid—not folate, which blocks the therapeutic effect). Dosage of leucovorin is 10 to 25 mg orally once a day (7.5 mg once a day in children). Even when leucovorin is given, the complete blood count should be monitored weekly.

If pyrimethamine is not available, trimethoprim-sulfamethoxazole, 5 mg/kg trimethoprim and 25 mg/kg sulfamethoxazole IV or orally twice a day is a potentially effective alternative, but pyrimethamine is more active than trimethoprim against the parasite's dihydrofolate reductase.

If patients cannot take sulfonamides, pyrimethamine and leucovorin plus clindamycin 600 mg 4 times a day can be used. Atovaquone 1,500 mg twice a day with or without pyrimethamine and leucovorin is another option.

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:

  • Sulfadiazine, pyrimethamine, and leucovorin

  • Clindamycin, pyrimethamine, and leucovorin

  • Atovaquone, pyrimethamine, and leucovorin

  • Atovaquone and sulfadiazine

  • Atovaquone

Sulfadiazine and pyrimethamine and leucovorin can be continued at lower doses than used for initial treatment: Sulfadiazine is given 1 g twice a day to 4 times a day with pyrimethamine 25 to 50 mg once a day and leucovorin 10 to 25 mg once a day. An alternative for patients who do not tolerate sulfonamides is clindamycin 600 mg 3 times a day plus pyrimethamine 25 to 50 mg once a day plus leucovorin 10 to 25 mg once a day, but an additional agent is needed to prevent pneumonia Prevention Pneumocystis jirovecii, an atypical fungus, is a common cause of pneumonia in immunosuppressed patients, especially in those infected with human immunodeficiency virus (HIV) and in those... read more Prevention . If pyrimethamine is not available or not tolerated, trimethoprim-sulfamethoxazole 1 double strength tablet twice a day can be used for maintenance. Other options for chronic maintenance therapy include atovaquone 750 to 1,500 mg twice a day with pyrimethamine 25 mg once a day plus leucovorin 10 mg once a day; atovaquone 750 to 1,500 mg plus sulfadiazine 1 g twice a day to 4 times a day; or atovaquone alone 750 to 1,500 mg twice a day. The relapse rate may be higher with the atovaquone-based alternatives.

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

  • Pyrimethamine 100 mg day 1 as a single loading dose, then 25 to 50 mg orally once a day in adults (in children, 2 mg/kg on day 1, then 1 mg/kg once a day) plus

  • Sulfadiazine 2 to 4 g orally day 1as a loading dose, then 500 mg to 1 g four times a day in adults (in children, 50 mg/kg twice a day) plus

  • Folinic acid (leucovorin) 5 to 25 mg orally once a day with each dose of pyrimethamine as long as pyrimethamine is given in adults (in children, 7.5 mg once a day)

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.

Spiramycin 1 g orally 3 or 4 times a day has been used safely to reduce transmission to the fetus in pregnant women with acute toxoplasmosis during the first 18 weeks of pregnancy, but spiramycin is less active than pyrimethamine plus sulfonamide and does not cross the placenta. Spiramycin is continued until fetal infection is documented or excluded at 18 weeks gestational age when amniotic fluid is obtained and tested using a polymerase chain reaction (PCR)–based assay. If no transmission has occurred, spiramycin can be continued to term. If the fetus is infected or a mother becomes infected after 18 weeks, pyrimethamine plus sulfadiazine plus leucovorin is used. Pyrimethamine is a potent teratogen and should not be used during the 1st and early 2nd trimester. NOTE: Spiramycin is not commercially available in the US, but is available through the U.S. Food and Drug Administration's Division of Anti-Infective Drug Products (phone: 301-796-1400).

Consultation with an infectious diseases expert is recommended.

Treatment of infants with congenital toxoplasmosis

Infants with congenital toxoplasmosis should be treated with pyrimethamine plus a sulfonamide for 1 year (1 Treatment reference Toxoplasmosis is infection with Toxoplasma gondii. Symptoms range from none to benign lymphadenopathy, a mononucleosis-like illness, to life-threatening central nervous system (CNS) disease... read more Treatment reference ). Infants should also receive leucovorin while receiving pyrimethamine and for 1 week after pyrimethamine is stopped to prevent bone marrow suppression. Recommendations from the National Reference Laboratory for Toxoplasmosis (PAMF-TSL) and the Toxoplasmosis Center at the University of Chicago for treatment of congenitally infected infants are

  • Pyrimethamine 1 mg/kg orally twice a day for the first 2 days; then from day 3 to 2 months (or 6 months if symptomatic) 1 mg/kg once a day, then 1 mg/kg 3 times per week to complete 12 months of therapyplus

  • Sulfadiazine 50 mg/kg twice a day plus

  • Folinic acid (leucovorin) 10 mg 3 times a week

Treatment reference

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 < 100/mcL. Trimethoprim/sulfamethoxazole one double-strength tablet once a day, which also is prophylactic against Pneumocystis jirovecii, is typically used. If this dosage is not tolerated, alternatives are one double-strength tablet 3 times a week or one single-strength tablet once a day. Alternatives for patients who cannot tolerate trimethoprim/sulfamethoxazole at all include dapsone 50 mg once a day plus pyrimethamine 50 mg once a week and leucovorin 25 mg once a week; or dapsone 200 mg once a week plus pyrimethamine 75 mg once a week plus leucovorin 25 mg once a week. Chemoprophylaxis is continued until the CD4 cell count is > 200/mcL.

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.

  • Use pyrimethamine and sulfadiazine plus leucovorin or, if the patient is allergic to sulfonamides or sulfadiazine is not tolerated, pyrimethamine and clindamycin.

  • Pyrimethamine is a potent teratogen and should not be used during the 1st and early 2nd trimester of pregnancy; spiramycin is recommended then for maternal infection.

  • 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.

Drugs Mentioned In This Article

Drug Name Select Trade
No brand name available
Cleocin, Cleocin Ovules, Cleocin Pediatric, Cleocin T, CLIN, Clindacin ETZ, Clindacin-P, Clinda-Derm , Clindagel, ClindaMax, ClindaReach, Clindesse, Clindets, Evoclin, PledgaClin, XACIATO
Primsol, Proloprim, TRIMPEX
No brand name available
Biaxin, Biaxin XL
Azasite, Zithromax, Zithromax Powder, Zithromax Single-Dose , Zithromax Tri-Pak, Zithromax Z-Pak, Zmax, Zmax Pediatric
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!