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Toxoplasmosis

Toxoplasmosis is a parasitic infection caused by Toxoplasma gondii, an obligate intracellular protozoan. Most immunocompetent people are asymptomatic or develop only a mild flu-like illness, but infection can cause severe disease in fetuses, newborns, and immunocompromised patients

Also called

Toxoplasma gondii infection

ICD-10

B58.9

Specialty

Infectious

Onset

Acute

Reviewed

August 2026
On This Page

Overview

–Toxoplasmosis is acquired mainly by eating undercooked meat containing tissue cysts or ingesting oocysts from contaminated soil, water, food, or cat feces. Congenital transmission, organ transplantation, and blood transfusion are less common routes.

 

The major clinical forms are:

  • Acquired infection: usually asymptomatic or mild.

 

  • Congenital infection: occurs following maternal infection during or just before pregnancy.

 

  • Ocular infection: causes retinochoroiditis and visual symptoms.

 

  • Cerebral infection: mainly occurs in severely immunocompromised patients.

Etiology & Risk Factors

-Etiology

–Toxoplasmosis is caused by Toxoplasma gondii. Cats and other felids are the definitive hosts and shed environmentally resistant oocysts. Humans become infected through contaminated food, soil, water, or contact with infected cat feces.

The parasite has three important forms:

  • Tachyzoite: rapidly multiplying acute form
  • Bradyzoite: slowly multiplying form within tissue cysts
  • Oocyst: environmentally resistant form shed by cats

 

-Risk Factors

  • Eating undercooked meat
  • Exposure to contaminated soil or cat feces
  • Pregnancy
  • Advanced HIV infection or severe immunosuppression
  • Organ transplantation

Pathophysiology

Ingestion of tissue cysts or oocysts → transformation into tachyzoites → dissemination through blood and lymphatics → immune response → conversion into bradyzoites → tissue cyst formation → lifelong latent infection → reactivation when cellular immunity is severely impaired

Clinical Presentation

-Symptoms:

  • Toxoplasmosis is frequently asymptomatic in immunocompetent patients. When symptomatic, it may cause:

    • Fever
    • Fatigue
    • Headache
    • Myalgia
    • Malaise
    • Tender cervical lymphadenopathy
    • Sore throat

    Symptoms may persist for several weeks and usually resolve without specific treatment in healthy individuals.

 

-Signs:

  • Tender cervical lymphadenopathy
  • Fever
  • Mild pharyngitis
  • Hepatosplenomegaly in some patients

 

Ocular Disease

  • Blurred vision
  • Reduced visual acuity
  • Eye pain
  • Photophobia
  • Retinochoroiditis

 

Cerebral Disease

  • Headache
  • Confusion
  • Seizures
  • Focal neurological deficits
  • Poor coordination
Toxoplasmosis Overview
Toxoplasmosis Overview

History Taking

-Ask about:

  • Undercooked meat consumption
  • Exposure to cats or cat litter
  • Gardening or soil exposure
  • Residence or travel in endemic areas
  • Pregnancy
  • HIV infection or other immunosuppression
  • Organ transplantation
  • Previous Toxoplasma infection
  • Visual symptoms
  • Headache or neurological symptoms

Physical Examination

-General Examination

  • Temperature
  • General condition
  • Lymphadenopathy
  • Nutritional and immune status

 

 

-System-Specific Examination:

  • Neurological examination when CNS disease is suspected
  • Ophthalmological examination when visual symptoms are present
  • Assessment for hepatosplenomegaly

Investigations

-Complete Blood Count

May show:

  • Mild lymphocytosis

  • Atypical lymphocytes

Not diagnostic.

 

-Biochemistry / Specific Tests

  • Toxoplasma IgG

  • Toxoplasma IgM

  • Liver function tests when systemic disease is suspected

  • HIV testing when clinically appropriate

  • Polymerase Chain Reaction (PCR) in selected cases

IgG generally indicates previous exposure, while IgM may suggest recent infection but can persist for a prolonged period, so positive IgM often requires careful interpretation or confirmatory testing.

 

-Imaging

Brain Computed Tomography (CT) or Magnetic Resonance Imaging (MRI)

In cerebral disease, imaging commonly demonstrates multiple ring-enhancing lesions, frequently involving the basal ganglia.

 

-Special / Confirmatory Tests

  • Ophthalmological examination for ocular disease

  • Amniotic fluid PCR when congenital infection is suspected

  • Tissue examination or biopsy in selected diagnostically uncertain cases

Diagnosis

-The diagnosis of Toxoplasmosis depends on the clinical presentation and appropriate laboratory or imaging findings.

Exposure or risk factors → compatible clinical syndrome → serology/PCR when indicated → imaging for CNS disease → assess for ocular or congenital involvement.

-In cerebral disease, a compatible clinical picture, characteristic brain lesions, and serological evidence often support the diagnosis. Brain biopsy is reserved for selected cases when the diagnosis remains uncertain.

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Management

Toxoplasmosis · Management

Management — Exam Focus
Clinical situationTreatment
Immunocompetent
Mild / asymptomatic		Usually no treatment required.
Severe / symptomatic diseasePyrimethamine + sulfadiazine + folinic acid (leucovorin)
PregnancySpiramycin when fetal infection is not established. If fetal infection is confirmed/suspected: pyrimethamine + sulfadiazine + folinic acid after the first trimester.
HIV / immunocompromisedPyrimethamine + sulfadiazine + folinic acid, followed by maintenance therapy until immune recovery.
Ocular toxoplasmosisPyrimethamine + sulfadiazine + folinic acid ± corticosteroids for significant inflammation.
High-Yield Exam Facts
Classic regimen
• Pyrimethamine + sulfadiazine + leucovorin → classic treatment
• Leucovorin reduces bone-marrow toxicity
Pregnancy
• Spiramycin → when fetal infection is not established
• Confirmed/suspected fetal infection → pyrimethamine + sulfadiazine + leucovorin after first trimester
Most tested: Mild/asymptomatic immunocompetent → no treatment · Classic severe disease → pyrimethamine + sulfadiazine + leucovorin · Pregnancy → spiramycin when fetal infection is not established.

Complications

  • Encephalitis
  • Seizures
  • Chorioretinitis
  • Visual impairment
  • Pneumonitis
  • Myocarditis
  • Congenital neurological disease
  • Hydrocephalus
  • Fetal loss
  • Death in severe untreated disease

Prognosis

The prognosis of Toxoplasmosis is generally excellent in immunocompetent patients with mild disease, with most recovering without specific treatment. Congenital, ocular, and cerebral disease can cause permanent neurological or visual complications

Key Points / Clinical Pearls

  • Toxoplasmosis is caused by Toxoplasma gondii.
  • Cats are the definitive hosts.
  • Undercooked meat is an important source of infection.
  • Most immunocompetent patients are asymptomatic.
  • Cervical lymphadenopathy is a common symptomatic finding.
  • Reactivation is especially important in advanced HIV infection.
  • Cerebral disease is the major severe opportunistic manifestation.
  • Brain MRI or CT may show multiple ring-enhancing lesions.
  • IgG mainly indicates previous exposure.
  • IgM may persist and requires careful interpretation.
  • Ocular disease causes retinochoroiditis and visual symptoms.
  • Congenital infection can cause neurological and ocular disease.
  • Pyrimethamine, sulfadiazine, and leucovorin are a standard treatment combination.
  • TMP-SMX is an important alternative.
  • Centers for Disease Control and Prevention (CDC). Toxoplasmosis .
  • Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet. 2004;363(9425):1965-1976. PubMed .
  • Robert-Gangneux F, Dardé ML. Epidemiology of and Diagnostic Strategies for Toxoplasmosis. Clin Microbiol Rev. 2012;25(2):264-296. PubMed .
  • Dunay IR, Gajurel K, Dhakal R, Liesenfeld O, Montoya JG. Treatment of Toxoplasmosis: Historical Perspective, Animal Models, and Current Clinical Practice. Clin Microbiol Rev. 2018;31(4):e00057-17. PubMed .