Toxoplasma gondii
Obligate intracellular apicomplexan; definitive hosts are felids (cats) where sexual reproduction produces environmentally hardy oocysts. Three infectious forms in humans: oocysts (environmental; cat feces → soil/water), tachyzoites (acute infection; rapidly dividing, disseminating), and bradyzoites (chronic; slow-metabolising, tissue-cyst-enclosed in brain and muscle). Invasion uses gliding motility (glideosome) and AMA1/RON2-mediated tight junction formation to enter virtually any nucleated cell; the resulting non-fusogenic parasitophorous vacuole (PV) actively excludes lysosomal fusion markers (Rab5, Rab7, LAMP). ROP18 kinase phosphorylates and inactivates host IRG/GBP immunity-related GTPases, preventing vacuole disruption. Dormant bradyzoite tissue cysts in neurons persist lifelong; reactivation in CD4+ T-cell deficiency produces necrotising focal encephalitis (ring-enhancing lesions). Treatment: pyrimethamine + sulfadiazine + leucovorin; prophylaxis: TMP-SMX.
Classification & Structure
| Taxonomy | Apicomplexa; Coccidia; Sarcocystidae; Toxoplasma gondii — 3 major clonal lineages (Type I, II, III) in Europe/North America; greater diversity in South America and Africa |
| Tachyzoite | 4–8 µm, banana-shaped; rapidly dividing (endodyogeny every 6–8 h); fills host cell PV as rosette of 8–32 parasites before lysis; responsible for acute dissemination |
| Bradyzoite | 5–9 µm, slower metabolism; enclosed in tissue cysts (5–200 µm diameter) in neurons and muscle; cyst wall (CST1/MAG1 matrix proteins); amylopectin granules (energy storage); stage conversion triggered by stress |
| Oocyst | 10–12 µm; produced by cats (only felid definitives); unsporulated (non-infectious); sporulates in 1–5 days → 2 sporocysts × 4 sporozoites; extremely hardy (months in soil); resistant to most disinfectants |
| MIC proteins | Microneme proteins (MIC2, MIC3, MIC8) secreted upon host cell contact; mediate initial adhesion (MIC2 binds heparan sulfate/ICAM-1); bridging molecules between parasite and host |
| RON proteins | RON2/RON4/RON5/RON8 complex injected into host cell membrane; form the moving junction — the constricting ring through which the parasite threads itself during active invasion |
| ROP kinases | ROP18 (active kinase): phosphorylates IRGAs (IRGA6, IRGB6) at inhibitory Thr/Ser → prevents vacuole disruption; ROP5 (pseudokinase): stabilises ROP18, co-factor; ROP16: activates STAT3/STAT6 (anti-inflammatory); injected into host cell cytoplasm at invasion |
| GRA proteins | Dense granule proteins (GRA7, GRA15, GRA24) secreted into PV and host cytosol; GRA15 activates NF-κB; GRA24 activates p38 MAPK → IL-12 from host; modulate the immune response toward a controlled Th1 that contains but does not eliminate infection |
Infection Mechanism & Pathogenesis
1 · Acquisition and gut invasion
Three transmission routes: (a) ingestion of oocysts from cat-feces-contaminated soil/water/produce (sporulated oocysts viable months); (b) ingestion of tissue cysts in undercooked/raw meat (most common in seropositive adults); (c) congenital transmission — primary maternal infection → tachyzoitaemia → transplacental passage. Risk increases with gestational age (1st trimester: low risk, severe disease; 3rd trimester: high risk, milder disease). Sporozoites/bradyzoites are released by gastric acid and trypsin; tachyzoites emerge in intestinal epithelium and begin active invasion using gliding motility (glideosome: MyoA/MLC1/GAP45 actin-myosin corkscrew motion at ~1–2 µm/s).
2 · Parasitophorous vacuole (PV) formation and non-fusogenicity
During active invasion, AMA1 on the parasite surface engages RON2 embedded in the host cell membrane, forming the moving junction. Cholesterol is selectively excluded from the nascent PV membrane — preventing Rab5/Rab7 endosomal fusion markers from accumulating. The sealed PV does not fuse with lysosomes; it is maintained as an isolated compartment in the host cell cytoplasm. GRA proteins are secreted to remodel the PV membrane and establish nutrient-scavenging contacts with host mitochondria and ER. This non-fusogenic PV is the central immune evasion mechanism of T. gondii.
3 · ROP kinase-mediated immune evasion
ROP18 and ROP5 are injected into the host cell cytoplasm at the moment of invasion. ROP18 phosphorylates IRGAs (immunity-related GTPases: IRGA6, IRGB6) at regulatory Thr/Ser residues → prevents their oligomerisation on the PV membrane → IRGAs cannot disrupt the vacuole. ROP5 co-localises with GBP (guanylate-binding proteins) and blocks their coating of the PV. Host autophagy of the PV is blocked by ROP18-mediated ATG13 phosphorylation. ROP16 directly phosphorylates STAT3 and STAT6 → IL-4/IL-13 anti-inflammatory signalling → Th2-skewed environment reduces IFN-γ. Net result: the PV persists indefinitely despite IFN-γ stimulation.
4 · Bradyzoite cyst formation and latency
Stress signals (alkaline pH, nutrient stress, immune pressure, nitric oxide) activate AP2IV transcription factors → tachyzoite-to-bradyzoite differentiation. Bradyzoites upregulate CST1 (cyst wall glycoprotein), BAG1 (small HSP), and amylopectin granules; downregulate TgSAG1 (tachyzoite surface antigen). Tissue cysts are immunologically "invisible" — cyst wall is poorly immunogenic; bradyzoites express stage-specific surface antigens. Neurons and skeletal/cardiac muscle are preferentially colonised. Neurotropism involves lower STAT1 expression in neurons (less IFN-γ responsiveness) and possible dopaminergic neuron preference (bradyzoites express tyrosine hydroxylase).
5 · Reactivation in immunosuppression
CD8+ T-cell depletion (AIDS: CD4 <100/µL; haematological malignancy; transplant immunosuppression) removes the primary effector controlling cyst stability. Bradyzoites convert back to tachyzoites, lyse cyst walls, and establish rapidly expanding necrotic foci in grey matter — predominantly at grey-white junction and basal ganglia. MRI: multiple ring-enhancing lesions with mass effect (must be distinguished from primary CNS lymphoma by EBV CSF PCR and empirical treatment response at 2 weeks). Without treatment: uniformly fatal progression to herniation.
Host Immune Response
Disease Spectrum
| Context | Manifestation | Key Details |
|---|---|---|
| Immunocompetent adult | Asymptomatic (90%) or cervical lymphadenopathy (10%) | Mild self-limited; heterophile-negative mononucleosis-like; seropositivity may be only evidence of infection; no treatment required |
| Congenital (1st trimester) | Severe; low transmission risk (~15%) | Chorioretinitis, hydrocephalus, intracranial calcifications, psychomotor retardation; "Sabin tetrad"; many appear normal at birth but develop late sequelae |
| Congenital (3rd trimester) | High transmission risk (~60%); milder disease | Majority subclinical at birth; chorioretinitis may develop years later; treatment for 12 months reduces sequelae |
| AIDS (CD4 <100) | Cerebral toxoplasmosis — most common focal brain lesion | Multiple ring-enhancing lesions at grey-white junction/basal ganglia on CT/MRI; empirical pyrimethamine/sulfadiazine → radiological improvement at 2 weeks confirms diagnosis; maintain secondary prophylaxis until CD4 >200 on ART |
| Transplant / haematological malignancy | Primary or reactivation encephalitis; pneumonitis; myocarditis | Seronegative donor + seropositive recipient highest risk; PCR monitoring of blood; prophylaxis with TMP-SMX |
| Ocular toxoplasmosis | Chorioretinitis (most common cause of infectious posterior uveitis globally) | Reactivation of congenital or acquired cysts in retina; white-yellow retinal lesion adjacent to pigmented scar; pyrimethamine/sulfadiazine + systemic steroids for macular involvement |
Treatment & Prophylaxis
Pyrimethamine + Sulfadiazine + Leucovorin — First-line (active infection)
Pyrimethamine (DHFR inhibitor) + sulfadiazine (dihydropteroate synthase inhibitor) synergistically block folate synthesis in T. gondii. Leucovorin (folinic acid) prevents pyrimethamine-induced bone marrow toxicity. Duration: 6 weeks for immunocompromised; 12 months for congenital neonatal treatment; followed by lifelong secondary prophylaxis in AIDS until CD4 >200 on ART.
TMP-SMX — Primary Prophylaxis (AIDS, CD4 <100)
Trimethoprim-sulfamethoxazole 1 DS tablet daily; prevents toxoplasma encephalitis in seropositive AIDS patients with CD4 <100; also prevents Pneumocystis jirovecii pneumonia (dual indication); discontinue when CD4 >200 on sustained ART.
Alternative: Pyrimethamine + Clindamycin
For sulfadiazine allergy or intolerance; atovaquone can substitute for sulfadiazine in some regimens; spiramycin used in pregnancy (after 1st trimester) to prevent maternal-fetal transmission before confirmed fetal infection.
Congenital: Neonatal Treatment & Prenatal Screening
Pyrimethamine + sulfadiazine × 12 months for all congenitally infected neonates (even asymptomatic); reduces late chorioretinitis and neurodevelopmental sequelae. Prenatal IgG avidity test determines timing; universal maternal serological screening (France); targeted screening in USA/UK.
Cross-Atlas Connections
References
- Blader IJ, Coleman BI, Chen CT, Bhatt D. Lytic Cycle of Toxoplasma gondii: 15 Years Later. Annu Rev Microbiol. 2015;69:463-85. doi:10.1146/annurev-micro-091014-104100 · PubMed 26332089
- Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet. 2004;363(9425):1965-76. doi:10.1016/S0140-6736(04)16412-X · PubMed 15194258
- Bennett JE, Dolin R, Blaser MJ. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Elsevier; 2020.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
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This entry covers Toxoplasma gondii biology, PV biology, and clinical disease. ROP kinase structural biology, congenital screening protocols by country, and novel anti-toxoplasma drug targets (TgCDPK1) are planned expansions.