Medicine & USMLE

Toxoplasma gondii

  1. Giardia lamblia
  2. Toxoplasma gondii
  3. Entamoeba histolytica
  4. Cryptosporidium
  5. Naegleria fowleri
  6. Trypanosoma brucei
  7. Plasmodium Overview
  8. Plasmodium Disease (Malaria)
  9. Babesia
  10. Trypanosoma cruzi
  11. Leishmania
  12. Trichomonas vaginalis
  13. Enterobius vermicularis (pinworm)
  14. Ascaris lumbricoides (giant roundworm)
  15. Strongyloides stercoralis (threadworm)
  16. Ancylostoma and Necator
  17. Trichinella spiralis
  18. Trichuris trichiura (whipworm)
  19. Toxocara canis
  20. Onchocerca volvulus
  21. Loa loa
  22. Wuchereria bancrofti
  23. Taenia solium
  24. Diphyllobothrium latum
  25. Echinococcus granulosus
  26. Schistosoma
  27. Clonorchis sinensis
  28. Sarcoptes scabiei (Scabies)
  29. Pediculus humanis and Phthirus pubis (Lice)


Toxoplasma gondii, or Toxoplasma for short, is a parasite transmitted mainly through fecal-oral route, by ingestion of oocysts from two major sources: cat feces and undercooked red meats. Vertical transmission from a pregnant mother to her fetus is also possible via the placenta.  In immunocompetent hosts, Toxoplasma infections are usually asymptomatic but may present with mild mono-like symptoms.

In immunocompromised individuals, Toxoplasma can cause severe disease. Specifically, two groups of immunocompromised patients are at especially high risk.

The first is AIDS patients with a CD4+ count less than 100. In these patients, Toxoplasmosis can cause severe meningoencephalitis with ring-enhancing brain abscesses  on imaging, as well as chorioretinitis manifesting as loss of visual acuity.

The other main population affected are the fetuses of pregnant women exposed to the parasite. Congenital toxoplasmosis presents as chorioretinitis, intracranial calcifications, and hydrocephalus in newborn infants.

Diagnosis is usually done by way of serologic testing or PCR assays, and treatment involves using pyrimethamine and sulfonamide antibiotics like sulfadiazine. A combination treatment of Trimethoprim-sulfamethoxazole is also used as a prophylactic agent to prevent infection in patients with AIDS.

Key Points

  • Toxoplasma gondii
    • Characteristics
      • Intracellular protozoan parasite
        • Cyst form is latent but infectious (consumed)
        • Tachyzoite form is invasive and reproductive
    • Transmission
      • Fecal-oral
        • Ingestion of oocysts in cat feces or undercooked beef/lamb
          • Reservoir is domestic cats
          • May also be seen in unpasteurized milk (e.g. goat milk)
      • Placental transmission
        • Pregnant women should reduce cat exposure (esp. feces)
    • Presentation: Toxoplasmosis
      • In immunocompetent host
        • Usually asymptomatic (>90%)
        • Mononucleosis-like symptoms (non-EBV)
          • Headache, fever, pharyngitis, hepatosplenomegaly
          • Heterophile antibody/Monospot negative
      • In severe immunocompromise or AIDS with CD4+ count <100
        • Meningoencephalitis
          • Brain abscesses may appear as ring-enhancing lesions on imaging
          • Presents with fever, headache and focal neurologic deficits 
        • Chorioretinitis
          • Presents with eye-pain and decreased visual acuity
          • Raised, yellow-white cottony lesions in fundoscopy
        • Pneumonia may be seen (rare)
      • Congenital toxoplasmosis
        • Presents in neonate with
          • Chorioretinitis
            • Seen in >85% of affected neonates (UTD)
          • Intracranial calcifications
            • Seen in 50-85% of affected neonates (UTD)
          • Hydrocephalus
            • Seen in 30-68% of affected neonates (UTD)
          • Jaundice/hepatosplenomegaly, thrombocytopenia, anemia, lymphadenopathy and fever are also seen
    • Diagnosis
      • Anti-Toxoplasma antibodies found on ELISA
    • Treatment
      • Pyrimethamine + Sulfadiazine
        • Leukovorin may be added in immunocompromised patients
      • TMP-SMX is used as a prophylactic agent in patients with AIDS
        • Also used as a second-line therapy for patients