Morphology: Obligate intracellular protozoan (apicomplexan). Tachyzoite (acute, rapidly dividing) and bradyzoite (latent, in tissue cysts — brain, muscle, heart) forms. Definitive host: cats. Humans = intermediate host.
Typical drugs
- #1Trimethoprim-sulfamethoxazole— **Preferred for both prophylaxis (HIV CD4 <100) and acute treatment** of cerebral toxoplasmosis — more practical than pyrimethamine + sulfadiazine + leucovorin combo (drug availability + cost).
- #2Dapsone— Alternative PCP + toxo prophylaxis in TMP-SMX intolerance (with pyrimethamine + leucovorin for toxo).
- #3Atovaquone— Alternative for both prophylaxis and treatment when sulfa-intolerant. Less effective than TMP-SMX.
Empiric therapy when resistant
Cerebral toxoplasmosis: TMP-SMX 5 mg/kg IV/PO BID × 6 weeks (acute), then chronic suppression until CD4 >200 × 6 months on ART. Alternative regimen: pyrimethamine 200 mg load → 50 mg daily + sulfadiazine 1.5 g QID + leucovorin 10–25 mg daily.
Resistance notes
No clinically significant drug resistance in T. gondii.
Pearls
Most common cause of mass lesion in HIV/AIDS with CD4 <100 — empiric treatment first, biopsy only if no improvement in 1–2 weeks. Distinguish from CNS lymphoma: SPECT thallium-201 uptake favors lymphoma; PCR for EBV in CSF positive in PCNSL. Congenital toxoplasmosis — chorioretinitis + hydrocephalus + intracranial calcifications (TORCH). Pregnant + acute infection — spiramycin (reduces transmission to fetus) until fetal status confirmed, then pyrimethamine + sulfadiazine if fetal infection documented. Reactivates with CD4 <100 — TMP-SMX prophylaxis covers both PCP + toxo, perfect single-agent option.