Toxoplasma gondii

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Toxo Overview - The Cunning Cat Parasite

  • Obligate intracellular, zoonotic protozoan with a worldwide distribution.
  • Definitive host: Felines (cats) are the sole host for sexual reproduction.
  • Transmission forms & routes:
    • Oocysts: Ingested from soil or water contaminated with cat feces.
    • Bradyzoites: Tissue cysts consumed in undercooked/raw meat (esp. pork, lamb).
    • Tachyzoites: Active, replicating form; responsible for vertical (transplacental) transmission.

Toxoplasma gondii life cycle in humans and animals

⭐ In immunocompromised patients (e.g., HIV with CD4 < 100), reactivation is common, classically causing multiple ring-enhancing brain lesions.

Transmission & Lifecycle - From Cat to Human

Toxoplasma gondii life cycle with cat and human hosts

  • Definitive Host: Cat. Oocysts are shed in feces.
  • Intermediate Hosts: Humans, livestock, rodents.
  • Human Infection Routes:
    • Ingesting oocysts from contaminated water, soil, or cat litter.
    • Ingesting tissue cysts (bradyzoites) in undercooked meat.
    • Transplacental (congenital) transmission.

⭐ Risk of congenital transmission ↑ with gestational age (highest in 3rd trimester), but disease severity is greatest if infection occurs in the 1st trimester.

Clinical Syndromes - Brain, Eyes, & Baby

  • Immunocompromised (AIDS, CD4 < 100)

    • Toxoplasmic Encephalitis (TE): Headache, confusion, fever, focal deficits, seizures.
    • Imaging: Multiple ring-enhancing lesions on CT/MRI. Brain CT scans of toxoplasmosis and other conditions
  • Immunocompetent & Congenital

    • Chorioretinitis: Most common manifestation.
    • Eye pain, floaters, vision loss. Fundoscopy shows "headlights in the fog."
    • Often a reactivation of a congenital infection.
  • Congenital (Transplacental)

    • Classic Triad: Chorioretinitis, hydrocephalus, and intracranial calcifications.
    • Risk of transmission ↑ with gestational age, but severity ↓.

⭐ In AIDS patients, Toxoplasmic Encephalitis is the most common cause of a CNS mass lesion.

Diagnosis - Spotting the Intruder

  • Serology: Primary method. Detects anti-Toxoplasma IgG & IgM antibodies. IgM indicates recent infection.
  • Imaging (Immunocompromised): CT or MRI of the brain reveals multiple, ring-enhancing lesions, often with a predilection for the basal ganglia.
  • Definitive Diagnosis: Brain biopsy showing tachyzoites is the gold standard but reserved for unclear cases.
  • Congenital: PCR analysis of amniotic fluid.

Brain MRI showing ring-enhancing lesions in toxoplasmosis

⭐ In HIV patients, the presence of ring-enhancing lesions on brain imaging is most commonly caused by toxoplasmosis, especially with a CD4 count < 100 cells/μL.

Treatment & Prevention - The Toxo Takedown

  • Primary Regimen (Immunocompromised & Severe Disease):
    • Pyrimethamine + Sulfadiazine
    • Plus Leucovorin (folinic acid) to prevent hematologic toxicity.
  • Alternative: TMP-SMX (especially for prophylaxis), or regimens with Clindamycin/Atovaquone.
  • Prophylaxis (e.g., HIV):
    • Initiate when CD4 count < 100 cells/μL.
    • TMP-SMX is the preferred agent.
  • General Prevention:
    • Thoroughly cook meat to kill cysts.
    • Pregnant women should avoid contact with cat feces (litter boxes).

⭐ Leucovorin rescue is essential; it bypasses the dihydrofolate reductase (DHFR) block in human cells, preventing myelosuppression, but not in the parasite.

High‑Yield Points - ⚡ Biggest Takeaways

  • Key transmission routes: cat feces (oocysts), undercooked meat (cysts), or transplacental (tachyzoites).
  • Classic triad of congenital toxoplasmosis: chorioretinitis, hydrocephalus, and intracranial calcifications.
  • In immunocompromised patients (AIDS < 100 CD4), presents with multiple ring-enhancing brain lesions.
  • Diagnosis is confirmed with serology or PCR.
  • Prophylaxis and treatment rely on pyrimethamine and sulfadiazine (plus leucovorin).
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Practice Questions: Toxoplasma gondii

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A 26-year-old man comes to the physician for a follow-up examination. He was diagnosed with HIV infection 2 weeks ago. His CD4+ T-lymphocyte count is 162/mm3 (N ≥ 500). An interferon-gamma release assay is negative. Prophylactic treatment against which of the following pathogens is most appropriate at this time?

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Flashcards: Toxoplasma gondii

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Which helminth causes visceral and ocular larva migrans?_____

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Which helminth causes visceral and ocular larva migrans?_____

Toxocara canis

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