Step by Step
Host
Cats are the definitive host
Toxoplasma gondii completes its full life cycle only in cats, which shed infectious oocysts in their feces. Humans become infected either by accidental ingestion of these oocysts (from litter box exposure or contaminated soil) or by eating undercooked meat containing tissue cysts.
Preg
Congenital toxoplasmosis
If a woman is infected with Toxoplasma for the first time during pregnancy, the parasite can cross the placenta and cause serious harm to the fetus. The classic triad is remembered as "C-H-I-C": Chorioretinitis (eye inflammation), Hydrocephalus, and Intracranial Calcifications.
AIDS
Reactivation in AIDS
In someone with a healthy immune system, Toxoplasma infection is usually asymptomatic — the parasite is walled off in dormant tissue cysts. But when CD4 count drops below 100 (as in advanced, untreated HIV/AIDS), those dormant cysts can reactivate, most dangerously in the brain.
Dx/Rx
Diagnosis and treatment
Reactivation in the brain presents as ring-enhancing lesions on MRI, along with symptoms of encephalitis. Treatment is pyrimethamine plus sulfadiazine plus leucovorin (leucovorin protects against the bone marrow toxicity of pyrimethamine). Patients with CD4 <100 are given TMP-SMX prophylaxis specifically to prevent this reactivation.
Applied Walkthrough
1
A pregnant woman who owns a cat asks whether she should worry about toxoplasmosis. The concern is real: if she's infected for the first time during pregnancy, Toxoplasma can cross the placenta and cause the classic triad — chorioretinitis, hydrocephalus, and intracranial calcifications — in the newborn.
2
Separately, a patient with untreated HIV and a CD4 count of 80 develops confusion, headache, and focal neurologic symptoms. An MRI shows ring-enhancing brain lesions.
3
This pattern — CD4 well below 100 plus ring-enhancing lesions — points strongly to Toxoplasma reactivation causing encephalitis, since the dormant tissue cysts this patient has likely carried for years are no longer being suppressed by an intact immune system.
4
Treatment for this reactivation is pyrimethamine plus sulfadiazine plus leucovorin; going forward, this same patient would also need TMP-SMX prophylaxis to prevent this from recurring, since their CD4 count remains under the 100 threshold.
Exam Application
Exams test whether you know the two high-risk populations for Toxoplasma (pregnant women and AIDS patients with CD4 <100), the classic congenital triad, and the treatment regimen for CNS reactivation.
⚠ Common Trap
The most common trap is confusing Toxoplasma's ring-enhancing brain lesions with other causes of similar MRI findings. The specific combination of a CD4 count under 100 plus ring-enhancing lesions is what should point you toward Toxoplasma reactivation specifically, rather than a different opportunistic process.
✓ Quick Self-Check
1. What is the definitive host for Toxoplasma gondii?
Cats — they shed infectious oocysts in their feces.
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2. How can humans become infected with Toxoplasma?
By ingesting oocysts from cat feces/contaminated soil, or by eating undercooked meat containing tissue cysts.
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3. What is the classic congenital toxoplasmosis triad, and what mnemonic is it remembered by?
Chorioretinitis, Hydrocephalus, and Intracranial Calcifications — remembered as "C-H-I-C."
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4. At what CD4 count does Toxoplasma reactivation typically occur, and how does it present?
CD4 <100; it presents as encephalitis with ring-enhancing lesions on MRI.
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5. What is the treatment for Toxoplasma CNS reactivation?
Pyrimethamine plus sulfadiazine plus leucovorin.
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