Definition and epidemiology
Toxoplasmosis is caused by Toxoplasma gondii, an obligate intracellular protozoan parasite. Cats are the definitive host; humans and other warm-blooded animals are intermediate hosts.
Seroprevalence varies widely by region, generally higher in areas with more undercooked meat consumption and warmer climates (roughly 10-30% in the US, higher in parts of Europe and South America). Most infections in immunocompetent people are subclinical and go undiagnosed.
Three clinically distinct presentations are tested:
- Acquired infection in immunocompetent hosts (usually asymptomatic or mild mononucleosis-like illness)
- Reactivation/opportunistic disease in immunocompromised hosts (especially AIDS, transplant recipients)
- Congenital toxoplasmosis (transplacental transmission during primary maternal infection)
- Cats are the definitive host; oocysts in cat feces and tissue cysts in undercooked meat are the main transmission routes
- Most common cause of multiple ring-enhancing brain lesions in AIDS patients with CD4 under 100
- Classic congenital triad: chorioretinitis, hydrocephalus, intracranial calcifications (diffuse, unlike periventricular CMV calcifications)
- First-line treatment for cerebral toxoplasmosis is pyrimethamine plus sulfadiazine plus leucovorin
- Leucovorin (folinic acid) is given to prevent pyrimethamine-induced bone marrow suppression
- Primary CNS lymphoma is the key differential; it is typically a single lesion with increased uptake on PET/SPECT
- Most common infectious cause of posterior uveitis/chorioretinitis worldwide
- TMP-SMX prophylaxis is started when CD4 drops below 100 in IgG-positive patients and stopped once CD4 exceeds 200 for 3+ months
Etiology and risk factors
Transmission routes:
- Ingestion of tissue cysts in undercooked or raw meat (pork, lamb, venison)
- Ingestion of oocysts from cat feces (contaminated soil, unwashed produce, litter box exposure)
- Transplacental transmission from an acutely infected mother to fetus
- Rarely, organ transplantation or blood transfusion
Risk factors for severe or reactivated disease:
- HIV/AIDS with CD4 count below 100 cells/microL (most important risk factor for cerebral toxoplasmosis)
- Other immunosuppression: chemotherapy, high-dose corticosteroids, solid organ or stem cell transplant
- Primary maternal infection during pregnancy, especially first trimester for severity, third trimester for transmission rate
- Occupational or household exposure to cat litter, gardening without gloves
Pathophysiology
After ingestion, Toxoplasma differentiates into tachyzoites that disseminate hematogenously and invade nucleated host cells. The immune system (particularly cell-mediated, interferon-gamma-driven responses) controls acute infection, and the parasite converts to a dormant bradyzoite form within tissue cysts, preferentially in the brain, retina, and skeletal/cardiac muscle.
Latent infection persists for life. If cell-mediated immunity declines (notably CD4 T-cell depletion in AIDS), cysts rupture and tachyzoites reactivate, causing necrotizing encephalitis with surrounding edema — the pathologic basis of ring-enhancing lesions seen on imaging.
In congenital infection, tachyzoites cross the placenta during maternal parasitemia. Earlier gestational infection is less likely to transmit but causes more severe fetal disease; later infection transmits more often but tends to be milder or subclinical at birth.
Clinical features
Immunocompetent acquired infection
- Usually asymptomatic
- When symptomatic: painless cervical lymphadenopathy, low-grade fever, malaise, mimicking mononucleosis (heterophile-negative)
Immunocompromised (cerebral toxoplasmosis)
- Headache, confusion, focal neurologic deficits, seizures, fever
- Most common cause of focal brain lesions in untreated AIDS patients
Congenital toxoplasmosis
- Classic triad: chorioretinitis, hydrocephalus, intracranial (diffuse) calcifications
- Many infected infants are asymptomatic at birth but develop sequelae later (chorioretinitis, learning disabilities, hearing loss)
- Can also cause fever, hepatosplenomegaly, jaundice, rash
Ocular toxoplasmosis
- Most common infectious cause of posterior uveitis/chorioretinitis worldwide
- Blurred vision, eye pain, floaters, scotomas; may be congenital reactivation presenting in adolescence/young adulthood
Diagnosis
| Scenario | Preferred test |
|---|---|
| Immunocompetent, suspected acute infection | IgM and IgG serology |
| Pregnant woman, screening | IgG/IgM serology, confirm with reference lab if positive |
| Suspected congenital infection | Maternal serology plus amniotic fluid PCR; neonatal IgM/IgA and PCR |
| Cerebral toxoplasmosis in AIDS | Clinical plus contrast MRI/CT showing ring-enhancing lesions, supported by positive IgG serology; empiric treatment response is often used as a diagnostic trial |
| Ocular disease | Clinical exam findings plus serology (aqueous humor PCR in select cases) |
| Tissue diagnosis | Brain biopsy if no response to empiric therapy or atypical imaging |
Serology: IgM rises early and may persist for months (false impression of recent infection), so IgG avidity testing helps distinguish recent versus remote infection, especially important in pregnancy.
Imaging in cerebral toxoplasmosis classically shows multiple bilateral ring-enhancing lesions with surrounding edema, often in the basal ganglia and at the gray-white junction.
Differential diagnosis
- Primary CNS lymphoma — key differential for ring-enhancing lesions in AIDS; typically single lesion, SPECT/PET shows increased uptake (vs. toxoplasmosis which does not), EBV-positive CSF favors lymphoma
- Progressive multifocal leukoencephalopathy (JC virus) — non-enhancing white matter lesions, no mass effect
- Bacterial brain abscess, tuberculoma, neurocysticercosis — other causes of ring-enhancing lesions
- Infectious mononucleosis (EBV) — for the lymphadenopathy/mono-like syndrome, but heterophile antibody positive
- Congenital infections (TORCH): CMV, rubella, herpes simplex, syphilis — differentiate congenital toxoplasmosis by calcification pattern (diffuse in toxo vs. periventricular in CMV) and specific serology
Management
Immunocompetent, non-pregnant
- Usually self-limited; treatment generally not required unless symptoms are severe or persistent, in which case pyrimethamine-sulfadiazine-leucovorin or alternative regimens may be used
Pregnancy
- Guideline recommendations vary by region; general approach involves spiramycin for suspected maternal infection to reduce transmission risk, switching to pyrimethamine-sulfadiazine-leucovorin if fetal infection is confirmed or in later gestation, per obstetric infectious disease consultation
Congenital infection
- Pyrimethamine, sulfadiazine, and leucovorin (folinic acid) for an extended course, with ophthalmologic and neurologic follow-up
Cerebral toxoplasmosis (AIDS/immunocompromised)
- First-line: pyrimethamine plus sulfadiazine plus leucovorin (leucovorin prevents bone marrow suppression)
- Alternative for sulfa allergy: pyrimethamine plus clindamycin plus leucovorin
- Trimethoprim-sulfamethoxazole is an acceptable alternative regimen in some guidelines
- Concurrent antiretroviral therapy optimization is essential
- Primary prophylaxis with trimethoprim-sulfamethoxazole recommended when CD4 falls below 100 cells/microL and IgG is positive; discontinued once CD4 rises above 200 for at least 3 months on ART
Ocular disease
- Pyrimethamine-sulfadiazine-leucovorin or clindamycin-based regimens, often with corticosteroids for significant inflammation, guided by ophthalmology
Complications and prognosis
- Immunocompetent infection: excellent prognosis, usually resolves without sequelae
- Cerebral toxoplasmosis: without treatment, progressive neurologic decline and death; with appropriate antiretroviral therapy and antiparasitic treatment, most patients improve substantially
- Congenital toxoplasmosis: risk of long-term visual impairment, intellectual disability, seizures, and sensorineural hearing loss even in infants asymptomatic at birth; lifelong ophthalmologic follow-up recommended
- Ocular toxoplasmosis: recurrent chorioretinitis can cause progressive vision loss over time
- Immunosuppressed patients remain at risk for reactivation as long as immune suppression persists; secondary prophylaxis continued until adequate immune reconstitution
PSL for cerebral toxo treatment: Pyrimethamine, Sulfadiazine, Leucovorin ("PSL" to Protect Spinal/bone marrow and treat the Lesions)
Practice questions
1. A 34-year-old man with HIV infection and a CD4 count of 45 cells/microL presents with headache, fever, and left-sided weakness over the past week. Contrast MRI of the brain shows multiple bilateral ring-enhancing lesions at the gray-white junction with surrounding edema. Toxoplasma IgG is positive. Which of the following is the most appropriate next step in management?
2. A pregnant woman at 10 weeks gestation is found to have a positive Toxoplasma IgM and IgG with low IgG avidity, consistent with recent infection. Which fetal finding, if confirmed by follow-up imaging and testing, would be most specifically associated with congenital toxoplasmosis rather than congenital CMV infection?
3. A 6-year-old child who emigrated from a region with high Toxoplasma seroprevalence presents with blurred vision and floaters in one eye. Fundoscopic exam reveals a focal area of necrotizing retinochoroiditis adjacent to an old pigmented scar. Which of the following best explains this presentation?
Frequently asked questions
What causes toxoplasmosis?
Toxoplasmosis is caused by the protozoan parasite Toxoplasma gondii, acquired by eating undercooked meat containing tissue cysts, ingesting oocysts from cat feces-contaminated soil or food, or transplacentally from mother to fetus. Cats are the only definitive host where the parasite completes its sexual reproductive cycle. Most human infections are incidental and asymptomatic.
What are the first symptoms of toxoplasmosis?
In healthy adults, most infections are asymptomatic; when symptoms occur, they resemble mononucleosis with mild fever, malaise, and painless swollen lymph nodes, usually in the neck. In immunocompromised patients, early symptoms of reactivation are headache, confusion, fever, and focal neurologic deficits from brain involvement. Congenitally infected infants may show fever, rash, or jaundice, or appear normal at birth.
How is toxoplasmosis diagnosed?
Diagnosis relies mainly on blood serology detecting IgG and IgM antibodies against Toxoplasma, with IgG avidity testing used to distinguish recent from past infection. In suspected cerebral disease, contrast MRI showing ring-enhancing lesions combined with positive serology supports the diagnosis, often confirmed by clinical improvement on empiric treatment. PCR on amniotic fluid, blood, or CSF is used for congenital or CNS disease.
Is toxoplasmosis curable?
Acute toxoplasmosis in healthy people usually resolves on its own without treatment, but the parasite forms dormant tissue cysts that remain in the body for life. Antiparasitic drugs can effectively treat active disease and reactivation episodes, but they do not eliminate the latent cysts. This is why immunocompromised patients need ongoing prophylaxis to prevent reactivation.
Toxoplasmosis vs CNS lymphoma — what is the difference?
Both cause brain lesions in AIDS patients, but toxoplasmosis typically presents with multiple ring-enhancing lesions that do not show increased uptake on PET/SPECT imaging, while primary CNS lymphoma usually appears as a single lesion with increased metabolic uptake. Toxoplasmosis is also associated with positive Toxoplasma IgG serology, while CNS lymphoma may show EBV DNA in the cerebrospinal fluid. Clinical response to empiric antiparasitic therapy within 1-2 weeks favors toxoplasmosis.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: B58
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.