Toxoplasmosis

1. Disease Information

Overview: Toxoplasmosis is a zoonotic parasitic infection caused by the obligate intracellular protozoan Toxoplasma gondii, an Apicomplexan parasite. Felids (cats) are the definitive host where the parasite undergoes sexual reproduction in the intestinal epithelium, producing oocysts shed in feces; virtually all warm-blooded animals, including humans, can serve as intermediate hosts. Infection is typically asymptomatic or mild in immunocompetent hosts but causes severe disease in immunocompromised individuals (encephalitis, particularly in AIDS patients) and in congenital transmission (fetal infection following primary maternal infection during pregnancy), producing the classic triad of chorioretinitis, hydrocephalus, and intracranial calcifications.

Key Identifiers: - MONDO: MONDO:0005108 (toxoplasmosis); congenital toxoplasmosis: MONDO:0018848 - OMIM: 314350 (Toxoplasmosis, Congenital) - Orphanet: ORPHA:857 (Congenital toxoplasmosis) - ICD-10: B58 (Toxoplasmosis); B58.0 (Toxoplasma oculopathy); B58.2 (Toxoplasma meningoencephalitis); P37.1 (Congenital toxoplasmosis) - ICD-11: 1F57 (Toxoplasmosis) - MeSH: D014123 (Toxoplasmosis); D014124 (Toxoplasmosis, Congenital); D014125 (Toxoplasmosis, Cerebral); D014126 (Toxoplasmosis, Ocular); D014128 (Toxoplasmosis, Animal) - NCBI Taxon (organism): NCBITaxon:5811 (Toxoplasma gondii)

Synonyms: Toxoplasma infection; congenital toxoplasmosis (TORCH infection); cerebral toxoplasmosis; ocular toxoplasmosis; toxoplasmic encephalitis (in AIDS context); "cat scratch fever" is a common lay misnomer/confusion (that is actually Bartonella henselae—distinct disease).

Data source type: Predominantly aggregated disease-level information from clinical case series, national/international birth cohorts (e.g., the European Multicentre Study on Congenital Toxoplasmosis - EMSCOT), CDC/WHO surveillance, and large epidemiological cohorts, supplemented by individual case reports for rare manifestations (e.g., PMID:15668997, PMID:11224510 for congenital case series).

2. Etiology

Disease Causal Factor: Infection with Toxoplasma gondii, an obligate intracellular apicomplexan protozoan parasite with three main infectious stages: tachyzoites (rapidly dividing, acute infection), bradyzoites (encysted in tissue cysts, chronic/latent infection, especially muscle and CNS), and sporozoites (within oocysts shed by cats).

Risk Factors:

Environmental/Behavioral: - Consumption of raw or undercooked meat (particularly pork, lamb, venison) containing tissue cysts (PMID:22218351 — Robert-Gangneux & Dardé review notes meat-borne transmission as a major route in industrialized countries) - Exposure to cat feces / handling litter boxes; soil contact (gardening) containing oocysts - Contaminated water supplies (waterborne oocyst outbreaks documented, e.g., Brazil, Canada) - Consumption of unwashed raw fruits/vegetables - Occupational exposure (farmers, abattoir workers, veterinarians) - Organ transplantation from seropositive donor to seronegative recipient - Blood transfusion (rare) - Geography: higher seroprevalence in regions with warm/humid climates (France, Brazil) vs. cold/dry (Scandinavia)

Genetic (host susceptibility): - HLA associations with severity of congenital and ocular toxoplasmosis — HLA-DQ3 and HLA-B associated with retinochoroiditis risk (PMID:16826765, Peyron et al., identified associations between HLA class II and mental retardation/hydrocephalus severity in congenital toxoplasmosis) - Polymorphisms in ABCA4, COL2A1 have been implicated in modifying ocular disease severity in some cohorts - Immunodeficiency (genetic or acquired) — CD4+ T-cell deficiency (AIDS, HIV) is the dominant host risk factor for reactivation of latent infection into toxoplasmic encephalitis

Parasite strain genotype: - Atypical/recombinant strains (particularly in South America) associated with more severe disease, including severe ocular disease in immunocompetent hosts (PMID:16880330 — Type I and atypical genotypes linked to more severe congenital and ocular disease compared to the milder Type II strains dominant in Europe/North America)

Protective Factors: - Pre-conceptional immunity (IgG seropositivity prior to pregnancy) is strongly protective against congenital transmission — established immunity essentially eliminates transmission risk except in profound immunosuppression or reinfection with a different/more virulent strain - Cooking meat to safe internal temperatures (destroys tissue cysts) - Freezing meat below -12°C for several days - Handwashing after soil/litter box contact - Antiretroviral therapy restoring CD4+ counts >200 cells/µL in HIV-infected individuals dramatically reduces reactivation risk (PMID:11815817 — HAART reduces incidence of toxoplasmic encephalitis)

Gene-Environment Interactions: Host genetic background (HLA, immune gene polymorphisms) interacts with environmental parasite exposure and infecting strain genotype to determine clinical outcome. For example, HLA-B*4901 and NALP1/COL2A1 have been implicated (in French and Brazilian cohorts respectively) in modulating risk for severe ocular disease following identical exposure (PMID:21966149 — Jamieson et al. review of host genetics in toxoplasmosis).

3. Phenotypes

Toxoplasmosis presentation differs markedly by host immune status and timing of infection (congenital vs. acquired). HPO terms suggested below.

A. Congenital Toxoplasmosis (classic triad + broader spectrum)

B. Acquired Toxoplasmosis in Immunocompetent Hosts

C. Ocular Toxoplasmosis

D. Reactivation/Immunocompromised (Toxoplasmic Encephalitis)

Onset: Congenital infection manifests in utero, at birth, or is delayed (asymptomatic at birth with sequelae — especially retinochoroiditis — emerging months to years later, sometimes into the third/fourth decade of life) (PMID:17825405, SYROCOT Study Group — pooled European cohort meta-analysis).

Severity/Progression: Highly variable — from entirely asymptomatic lifelong infection to fulminant, fatal disseminated disease in the severely immunocompromised. Ocular disease is characteristically recurrent/relapsing (episodic reactivation of quiescent retinal cysts), a hallmark distinguishing feature.

Quality of life impact: Ocular disease causes progressive visual impairment/blindness with recurrent flares impacting daily function; congenital neurological sequelae (intellectual disability, seizures) impose lifelong disability burden; toxoplasmic encephalitis in AIDS carries high mortality without treatment.

4. Genetic/Molecular Information

Toxoplasmosis is an infectious disease, not a classic monogenic disorder, so "causal genes" apply to the parasite genome and to host susceptibility modifier genes, not to a single Mendelian human locus.

Parasite Genetics: - T. gondii has three canonical clonal lineages: Type I, Type II, Type III, plus numerous atypical/recombinant strains especially in South America - Genotype correlates with virulence: Type I strains are highly virulent in mouse models (LD100 = 1 organism); Type II/III are less virulent - Key virulence factor genes: ROP18 (rhoptry kinase, polymorphic virulence determinant, PMID:16709124 — Taylor et al. showed ROP18 as a major virulence determinant via QTL mapping), ROP5 (pseudokinase, cooperates with ROP18 to inactivate host immunity-related GTPases/IRGs), GRA15, NLRP1/NLRP3 inflammasome interactions

Host Susceptibility/Modifier Genes: - HLA-DQ3, HLA-B*4901, HLA-Bw16 — associated with risk of retinochoroiditis and severity of neurological sequelae in congenital toxoplasmosis (PMID:16826765; Mack et al. earlier HLA studies) - NALP1 (NLRP1) — inflammasome gene, polymorphisms associated with congenital toxoplasmosis susceptibility in human cohorts, replicating findings from mouse Nalp1 studies (PMID:19468306 — Witola et al., NALP1 polymorphisms and human congenital toxoplasmosis) - P2X7 purinergic receptor gene polymorphisms — implicated in susceptibility to ocular toxoplasmosis in Brazilian cohorts - ABCA4, COL2A1 — implicated in modifying severity of retinal involvement in some studies

Pathogenic Variants: Not applicable in the classic ClinVar/ACMG sense (this is not a Mendelian disease); host susceptibility variants are typically common polymorphisms (SNPs) studied via candidate-gene and GWAS-style association studies rather than rare pathogenic variants.

Epigenetic Information: T. gondii infection has been shown to alter host cell epigenetics — the parasite secretes effectors (e.g., TgIST) that modulate host STAT1-dependent transcription and can affect histone modifications in infected cells to suppress interferon-gamma responses (PMID:27300474 — Gay et al., Toxoplasma gondii TgIST co-opts host chromatin repressors to block STAT1-dependent gene expression).

Chromosomal Abnormalities: Not applicable (infectious, not a chromosomal disorder).

5. Environmental Information

Environmental Factors: - Soil contaminated with oocysts (can remain infectious for over a year in moist soil) - Water supply contamination — waterborne outbreaks documented in Canada (Victoria, BC, 1995 — PMID:9366006, Bowie et al.) and Brazil - Environmental persistence of oocysts is a major reservoir independent of direct cat contact

Lifestyle Factors: - Dietary practices: consumption of raw/undercooked meat, unwashed produce, unpasteurized goat's milk - Cat ownership and litter box hygiene (though studies show meat consumption is often a larger risk factor than cat ownership in seroprevalence studies) - Gardening without gloves - Geographic/cultural dietary practices (e.g., high raw meat consumption in France correlates with higher seroprevalence)

Infectious Agent: - Toxoplasma gondii (NCBITaxon:5811), Phylum Apicomplexa, family Sarcocystidae - Definitive host: Felidae (domestic cats and wild felids) — sexual reproduction occurs in intestinal epithelium - Intermediate hosts: virtually all warm-blooded vertebrates (asexual reproduction; tissue cyst formation) - Transmission routes: (1) ingestion of oocysts from contaminated soil/water/produce, (2) ingestion of tissue cysts in undercooked meat, (3) congenital (transplacental) transmission, (4) organ transplantation, (5) blood transfusion (rare), (6) laboratory accident

6. Mechanism / Pathophysiology

Causal chain overview: Ingestion of oocysts or tissue cysts → excystation/release of sporozoites or bradyzoites in the gut → invasion of intestinal epithelium → conversion to tachyzoites → active replication and dissemination via blood/lymphatics → invasion of diverse cell types (especially neural, muscle, retinal, placental) → host immune response (IFN-γ-driven) controls acute infection → parasite converts to bradyzoite form and encysts, establishing lifelong latent infection → reactivation occurs upon loss of immune control (immunosuppression) or, in pregnancy, primary maternal infection allows transplacental passage of tachyzoites to the fetus.

Molecular Pathways: - Active host-cell invasion machinery: The parasite uses a unique glideosome (actin-myosin motor complex) for active invasion, independent of host phagocytosis; involves MIC (microneme), ROP (rhoptry), and GRA (dense granule) protein secretion (KEGG: Toxoplasmosis pathway hsa05145; Reactome) - Parasitophorous vacuole (PV) formation: Tachyzoites create a non-fusogenic PV that excludes host lysosomal fusion, evading destruction - Host IFN-γ/JAK-STAT1 pathway: Central to host control — IFN-γ activates macrophages and induces IRGs (immunity-related GTPases) and GBPs (guanylate-binding proteins) that disrupt the PV membrane - Parasite countermeasures: ROP18 phosphorylates and inactivates host IRGs (PMID:16709124); ROP5 pseudokinase cooperates with ROP18; GRA effectors (e.g., TgIST) block STAT1-dependent transcription (PMID:27300474) - NF-κB and inflammasome (NLRP1/NLRP3) signaling in host innate response

Cellular Processes: - Apoptosis modulation: T. gondii actively inhibits host cell apoptosis during acute infection to preserve its replicative niche (via effects on Bcl-2 family proteins and caspase inhibition) - Autophagy interactions: host autophagy machinery can be recruited to target the PV (IRG/GBP-mediated), and parasite effectors counteract this - Bradyzoite-tachyzoite interconversion: stress-induced (immune pressure, nitric oxide, alkaline pH) differentiation into slow-growing bradyzoites within tissue cysts, primarily in brain, retina, and skeletal/cardiac muscle — the biological basis of chronic latency

Immune System Involvement: - Innate immunity: dendritic cells, macrophages, NK cells produce early IL-12 → drives Th1 polarization - Adaptive immunity: CD4+ and CD8+ T cells, IFN-γ production is essential for control; CD8+ cytotoxic T cells particularly important for long-term control of cerebral cysts - Immunocompromise (HIV/AIDS with CD4+ <100-200 cells/µL, transplant immunosuppression, chemotherapy) permits reactivation of latent bradyzoite cysts → tachyzoite conversion → toxoplasmic encephalitis - In congenital infection, the developing fetal immune system is unable to mount an adequate Th1 response, permitting dissemination; placental infection precedes fetal transmission

Tissue Damage Mechanisms: - Direct cytolytic damage from tachyzoite replication and host cell rupture - Immune-mediated damage: local inflammatory response to reactivating cysts (particularly in retina) causes tissue destruction — ocular toxoplasmosis pathology is driven substantially by the host inflammatory response to ruptured cysts, not solely direct parasite cytotoxicity - CNS: necrotizing encephalitis with microglial nodules, perivascular cuffing; periventricular calcification and ependymitis leading to aqueductal stenosis/hydrocephalus in congenital disease

Biochemical Abnormalities: - Parasite salvages purines from host (lacks de novo purine synthesis) — pyrimethamine/sulfadiazine target parasite folate pathway (dihydrofolate reductase and dihydropteroate synthase, respectively), exploiting differences from host folate metabolism

Molecular Profiling: - Transcriptomic studies show marked host cell reprogramming during infection, including suppression of interferon-stimulated genes via TgIST-mediated STAT1 blockade (PMID:27300474) - Single-cell/organoid studies of placental and retinal models have illuminated tissue-specific tropism and barrier-crossing mechanisms

Suggested GO terms: GO:0044409 (entry into host), GO:0006955 (immune response), GO:0034341 (response to interferon-gamma), GO:0032491 (detection of molecule of fungal origin - N/A), GO:0140546 (defense response to symbiont), GO:0140367 (antibacterial innate immune response - use GO:0050832 defense response to fungus as analog term not applicable), GO:0006911 (phagocytosis, engulfment). Suggested CL terms: CL:0000235 (macrophage), CL:0000798 (gamma-delta T cell), CL:0000625 (CD8-positive T cell), CL:0000624 (CD4-positive T cell), CL:0000540 (neuron), CL:0000359 (vascular associated smooth muscle cell — for placental/vascular involvement), CL:0000669 (pericyte, retinal context).

7. Anatomical Structures Affected

Organ Level: - Primary: Brain (CNS), eye/retina, placenta (in congenital transmission), skeletal muscle, heart - Secondary: Liver, spleen (neonatal hepatosplenomegaly), lymph nodes (lymphadenitis form) - Body systems: Nervous system, ocular/visual system, reproductive system (placenta), immune system, musculoskeletal system

UBERON terms: - UBERON:0000955 (brain) - UBERON:0000966 (retina) - UBERON:0001987 (placenta) - UBERON:0001134 (skeletal muscle tissue) - UBERON:0000948 (heart) - UBERON:0002106 (spleen) - UBERON:0002107 (liver) - UBERON:0000029 (lymph node) - UBERON:0001769 (choroid) / UBERON:0001782 (retina/choroid complex for chorioretinitis)

Tissue and Cell Level: - Retinal pigment epithelium and neurosensory retina (chorioretinitis) - Neurons and glial cells (encephalitis, microglial nodules) - Trophoblast cells of the placenta (site of transplacental crossing) - Cardiac and skeletal myocytes (tissue cyst reservoir) - Cell Ontology: CL:0000540 (neuron), CL:0000127 (astrocyte), CL:0000129 (microglial cell), CL:0011026 (progenitor cell — placental cytotrophoblast: CL:0000351), CL:0000746 (cardiac muscle cell)

Subcellular Level: - Parasitophorous vacuole (a Toxoplasma-specific, non-host organelle) — GO Cellular Component: GO:0020005 (symbiont-containing vacuole) - Host mitochondria (recruited to PV membrane) - Host nucleus (STAT1 signaling interference) - GO:0005634 (nucleus), GO:0005739 (mitochondrion)

Localization: - CNS lesions: periventricular (congenital calcifications), diffuse in AIDS-related toxoplasmic encephalitis (often basal ganglia, corticomedullary junction) - Ocular lesions: posterior pole retina, often juxtapapillary or adjacent to old scars (classic "satellite lesion" recurrence pattern) - Bilateral involvement possible but ocular disease is often unilateral at any given episode

8. Temporal Development

Onset: - Congenital: infection occurs in utero; clinical manifestation may be present at birth or delayed by months to decades (especially chorioretinitis) - Acquired (immunocompetent): incubation ~1-3 weeks post-exposure before symptomatic mononucleosis-like illness (if symptomatic at all) - Reactivation (immunocompromised): can occur at any point following primary infection once immune control wanes (e.g., CD4+ count drop in AIDS)

Onset pattern: Acute (initial infection, encephalitis in immunocompromised) vs. insidious/chronic (latent cyst-forming stage, asymptomatic for life in most immunocompetent hosts)

Progression: - Acute tachyzoite stage → immune containment → chronic bradyzoite/cyst latency (lifelong) - In congenital disease: risk of transmission increases with gestational age at maternal infection (up to ~70-90% in third trimester) but severity of fetal disease is inversely related to gestational age — earlier infection (first trimester) is less frequently transmitted but produces more severe disease when it occurs (PMID:10535648, PMID:17825405 — SYROCOT meta-analysis established this gestational-age-dependent transmission/severity relationship) - Ocular disease: episodic, relapsing-remitting pattern with recurrent retinochoroiditis flares from reactivation at the margin of pre-existing scars

Progression rate: Variable — congenital sequelae can be rapidly apparent (severe neonatal disease) or slowly evolving (chorioretinitis appearing in the second or third decade of life); toxoplasmic encephalitis in untreated AIDS progresses over days to weeks and is fatal without treatment

Disease course pattern: Latent chronic infection punctuated by episodic reactivation (ocular disease, encephalitis) — classic relapsing-remitting pattern for ocular toxoplasmosis

Critical periods: Pregnancy (maternal seroconversion timing determines both transmission risk and fetal disease severity) is the single most important critical window; immunosuppression onset/degree is the critical window for reactivation disease.

9. Inheritance and Population

Epidemiology: - Seroprevalence: highly variable globally, ranging from ~10-30% in the US and UK to >60-80% in parts of France, Brazil, and other regions with high raw meat consumption or warm/humid climates (PMID:19257814 — Pappas et al., global toxoplasmosis seroprevalence review) - Congenital toxoplasmosis incidence: estimated ~1-10 per 10,000 live births globally, varies by region and screening program (France has historically had among the highest rates with mandatory prenatal screening) - US estimate: CDC estimates >40 million people in the US may be infected with Toxoplasma, most asymptomatic

Inheritance pattern: Not applicable (infectious disease, not genetic); however, host susceptibility modifier alleles show typical complex/polygenic association patterns (not Mendelian)

Penetrance/Expressivity: N/A for classic Mendelian sense; clinical "penetrance" of symptomatic disease following infection is low in immunocompetent hosts (~10-20% develop symptoms) but essentially complete for reactivation disease in profound immunosuppression if untreated

Population Demographics: - Affected populations: universal susceptibility; seroprevalence increases with age (cumulative lifetime exposure) - Geographic distribution: higher in tropical/subtropical, humid climates (Brazil, France) vs. cold/dry (Scandinavia, parts of North America); notably higher and more severe (including in immunocompetent hosts) in South America due to atypical/more virulent parasite genotypes (PMID:16880330) - Sex ratio: no strong intrinsic sex predilection for acquisition, though congenital transmission obviously depends on maternal infection - Age distribution: seroprevalence rises steadily with age due to cumulative exposure

10. Diagnostics

Clinical/Laboratory Tests: - Serology (primary diagnostic tool): IgG and IgM antibody detection via ELISA, indirect fluorescent antibody (IFA), or the Sabin-Feldman dye test (historic gold standard) - IgG avidity testing: low avidity suggests infection within the last ~4 months; high avidity suggests infection >4 months prior — critical for dating infection relative to conception in pregnant women (PMID:11114023 — Montoya, diagnosis review) - PCR: detection of T. gondii DNA in amniotic fluid (for congenital diagnosis), blood, CSF, or vitreous/aqueous humor (ocular disease), or bronchoalveolar lavage (immunocompromised pulmonary disease) - Histopathology: tissue biopsy showing tachyzoites or cysts with characteristic staining (immunohistochemistry using anti-Toxoplasma antibodies)

Imaging: - CT/MRI brain: ring-enhancing lesions (typically multiple, basal ganglia/corticomedullary junction) in toxoplasmic encephalitis; periventricular calcifications in congenital disease - Ophthalmologic exam/fundoscopy: focal necrotizing retinochoroiditis, often adjacent to pigmented scar ("satellite lesion") - Prenatal ultrasound: ventriculomegaly, intracranial calcifications, hepatosplenomegaly, placental thickening

Genetic Testing: Not applicable in the traditional sense (not a heritable Mendelian disease); parasite genotyping (PCR-RFLP or multilocus sequence typing of T. gondii isolates) is used for epidemiological/virulence characterization, not clinical diagnosis of the patient.

Clinical Criteria: - Congenital toxoplasmosis diagnosis relies on combination of maternal seroconversion timing, amniotic fluid PCR, neonatal IgM/IgA serology (since maternal IgG crosses placenta), and clinical/imaging findings - Differential diagnosis for congenital: other TORCH infections (CMV, rubella, herpes, syphilis); for cerebral toxoplasmosis in AIDS: primary CNS lymphoma, progressive multifocal leukoencephalopathy (PML), CNS tuberculosis - Differential for ocular disease: other infectious retinitis (CMV, herpetic), sarcoidosis, other causes of posterior uveitis

Screening: - Prenatal maternal serologic screening (mandatory in France, recommended/variable elsewhere) with monthly retesting in seronegative women - Newborn screening programs in some regions (US state programs vary; not universal)

11. Outcome/Prognosis

Survival and Mortality: - Immunocompetent acute infection: essentially 0% mortality, self-limited - Untreated toxoplasmic encephalitis in AIDS: historically high mortality without antiretroviral therapy and specific treatment; with HAART and treatment, prognosis markedly improved (PMID:11815817) - Congenital toxoplasmosis: mortality is low with modern treatment but can be substantial in severe untreated cases (hydrocephalus, disseminated neonatal disease)

Morbidity: - Long-term neurological and visual sequelae from congenital infection are the major morbidity driver — chorioretinitis recurrence can occur throughout life, cumulative visual field loss with repeated episodes - SYROCOT meta-analysis (PMID:17825405) found that treatment during pregnancy reduces but does not eliminate transmission/sequelae risk, and the relationship between treatment timing and outcome remains debated

Complications: - Congenital: hydrocephalus requiring shunting, epilepsy, cognitive impairment, blindness from recurrent chorioretinitis - Ocular: recurrent inflammation, macular scarring, retinal detachment, cataract, glaucoma (secondary) - CNS reactivation: seizures, focal deficits, coma if untreated

Prognostic factors: Immune status (CD4 count in HIV patients is the dominant prognostic factor for reactivation disease and response to treatment); gestational timing of maternal infection; prompt initiation of treatment; parasite strain virulence (atypical strains → worse ocular prognosis).

12. Treatment

Pharmacotherapy: - Pyrimethamine + sulfadiazine + folinic acid (leucovorin) — first-line combination for active disease (encephalitis, severe congenital disease, ocular disease); targets parasite folate pathway (DHFR inhibition by pyrimethamine, DHPS inhibition by sulfadiazine) — NCIT term candidates: NCIT:C500 (Pyrimethamine), NCIT:C568 (Sulfadiazine) - This maps directly to the dismech bacterial_folate_synthesis_inhibition module pattern (antifolate mechanism), though here applied to a protozoan rather than bacterial target - Spiramycin — used in pregnancy for maternal infection to reduce transplacental transmission (does not cross placenta well, so used before confirmed fetal infection) — NCIT:C29014 (macrolide-class) - Trimethoprim-sulfamethoxazole (TMP-SMX) — alternative regimen, also prophylaxis in HIV - Clindamycin — alternative to sulfadiazine in sulfa-allergic patients, combined with pyrimethamine - Atovaquone — alternative agent for treatment/prophylaxis in sulfa-intolerant patients

Treatment term (NCIT): NCIT:C15986 (Pharmacotherapy) as the general treatment_term, with therapeutic_agent entries for pyrimethamine (CHEBI:8460), sulfadiazine (CHEBI:9328), spiramycin (CHEBI:9216), clindamycin (CHEBI:3745).

Surgical/Interventional: - Ventriculoperitoneal shunt placement for hydrocephalus secondary to congenital toxoplasmosis (NCIT:C15329, Surgical Procedure)

Supportive Care: - Corticosteroids as adjunctive therapy for ocular toxoplasmosis when inflammation threatens the macula or optic nerve, and for CNS disease with significant edema/mass effect (NCIT:C2144, Corticosteroid) - Anticonvulsants for seizure management

Prophylaxis: - Secondary prophylaxis (lower-dose pyrimethamine-sulfadiazine) in AIDS patients following treatment of acute toxoplasmic encephalitis, until CD4+ count recovers >200 cells/µL for ≥6 months on ART - Primary prophylaxis (TMP-SMX) recommended for HIV-positive, Toxoplasma-seropositive patients with CD4+ <100 cells/µL

Treatment Outcomes: - Response rates to pyrimethamine-sulfadiazine for toxoplasmic encephalitis are generally high (>80%) with appropriate ART co-management - Adverse effects: sulfadiazine — crystalluria, hypersensitivity, bone marrow suppression; pyrimethamine — bone marrow suppression (mitigated by folinic acid co-administration, not folic acid which can reduce efficacy)

Experimental/Investigational: - Newer agents in development targeting apicoplast biology and novel parasite enzymes are in preclinical/early trial stages; no major approved gene/cell/RNA therapies exist for this infectious disease (not applicable in the way it would be for a genetic disorder)

13. Prevention

Primary Prevention: - Dietary: cook meat to safe internal temperatures (≥63-74°C depending on meat type), freeze meat before consumption, wash fruits/vegetables, avoid unpasteurized dairy - Avoid changing cat litter during pregnancy (or use gloves and wash hands; litter boxes should be cleaned daily since oocysts require 1-5 days to become infectious) - Wear gloves gardening; wash hands after soil contact - Avoid drinking untreated water in endemic areas

Secondary Prevention: - Prenatal serologic screening programs (monthly in France for seronegative women) enabling prompt initiation of spiramycin/pyrimethamine-sulfadiazine upon seroconversion to reduce transplacental transmission - Regular CD4+ monitoring and prophylaxis initiation in HIV-positive patients

Tertiary Prevention: - Secondary chemoprophylaxis after treated toxoplasmic encephalitis until immune reconstitution - Regular ophthalmologic follow-up for patients with known ocular toxoplasmosis to catch recurrences early

Immunization: No approved human vaccine exists. A live-attenuated vaccine (Toxovax/S48 strain) is licensed for veterinary use in sheep to prevent ovine congenital toxoplasmosis (abortion), but no human vaccine has reached approval — an active area of research (PMID:24581229, review of vaccine development efforts).

Screening: - Prenatal maternal serology (universal in France; risk-based or not routine in the US, UK) - HIV patients: baseline Toxoplasma IgG serology at HIV diagnosis to identify those at risk for reactivation

Counseling: Genetic counseling is not applicable in the traditional sense; however, prenatal counseling regarding transmission risk, treatment options, and prognosis is standard of care once maternal seroconversion is documented.

Public Health: Health education campaigns regarding food safety and cat litter hygiene for pregnant women; water treatment infrastructure to prevent oocyst-contaminated water supplies (relevant post major outbreaks, e.g., Victoria BC 1995, PMID:9366006).

14. Other Species / Natural Disease

Taxonomy: Toxoplasma gondii infects essentially all warm-blooded vertebrates. - Definitive host: domestic and wild Felidae (NCBITaxon:9685, Felis catus) - Intermediate hosts include: sheep (NCBITaxon:9940), pigs (NCBITaxon:9823), cattle (NCBITaxon:9913), rodents (mouse NCBITaxon:10090), birds, marine mammals

Natural Disease: - Ovine toxoplasmosis: major cause of abortion and stillbirth in sheep flocks worldwide — significant agricultural/economic impact; this is the target of the licensed veterinary vaccine (Toxovax) - Feline toxoplasmosis: typically subclinical in cats; occasional clinical disease in kittens or immunocompromised cats (systemic disease with pneumonia, hepatitis, encephalitis) - Marine mammal toxoplasmosis: significant cause of mortality in California sea otters (Enhydra lutris) — linked to land-based oocyst runoff into marine environments, an important One Health/environmental sentinel finding (well documented in veterinary/wildlife literature, e.g., Miller et al. studies on sea otter toxoplasmosis) - Marsupial toxoplasmosis: particularly severe/fatal disease in Australian marsupials (which lack coevolutionary exposure), an important conservation concern

Comparative Biology: - Mouse models are the dominant experimental system and recapitulate acute (tachyzoite-driven) and chronic (cyst-forming, CNS) infection stages effectively, forming the basis for most virulence factor discovery (ROP18, ROP5, IRG/GBP biology) - Disease severity is highly species-dependent — mice are relatively susceptible, while natural definitive/intermediate hosts co-evolved with the parasite show milder disease; naive species (marsupials, some marine mammals) show disproportionate severity

Zoonotic potential: Yes — this is a fundamentally zoonotic parasite; humans are dead-end intermediate hosts (no onward transmission from human to human except transplacentally, transfusion, or transplant).

15. Model Organisms

Mouse Models (dominant system): - Standard laboratory mice (various inbred strains — C57BL/6, BALB/c) are highly susceptible and used extensively to study acute virulence, chronic cyst formation in brain, and reactivation upon immunosuppression - Genetically modified mice: IFN-γ knockout, IRG (immunity-related GTPase) knockout, and GBP knockout mice have been central to dissecting host innate resistance pathways (PMID:16709124 context) - MGI (Mouse Genome Informatics) catalogs relevant knockout lines for Ifng, Irgm1, Irgm3, and related immune genes used in toxoplasmosis research

Cellular/In Vitro Models: - Human foreskin fibroblasts (HFF) — standard cell line for T. gondii in vitro culture and invasion assays - Retinal pigment epithelial cell lines and organoids — used to model ocular tropism and blood-retinal barrier crossing - Placental trophoblast/organoid models and placental explants — used to study transplacental transmission mechanisms - Human iPSC-derived neurons and brain organoids — emerging models for CNS tropism and neuroinflammation studies

Applications: - Mouse models recapitulate the acute-to-chronic transition and cyst formation in brain very well, making them the primary tool for testing anti-parasitic drugs and vaccine candidates - Reactivation models (immunosuppressing chronically infected mice) model AIDS-associated toxoplasmic encephalitis

Limitations: - Mouse models do not fully recapitulate human congenital transmission dynamics (placental structure differs substantially between mice and humans — hemochorial similarities exist but timing/susceptibility windows differ) - Human ocular disease natural history (chronic recurrent decades-long relapsing pattern) is difficult to model in short-lived rodents

Resources: MGI (Mouse Genome Informatics) for knockout strain catalogs; ATCC and BEI Resources for T. gondii strains (RH, Pru, ME49, VEG representing Type I/II/III reference strains) and host cell lines; ToxoDB (a dedicated Toxoplasma genomics database, part of VEuPathDB) for parasite genomic/genetic resources.

Summary of Key PMID Citations

Note on evidence gaps: Precise, universally-agreed quantitative frequencies for individual congenital phenotypes (chorioretinitis %, hydrocephalus %) vary substantially by cohort, screening intensity, and follow-up duration — curators should pull exact frequency figures directly from specific cohort studies (e.g., SYROCOT, EMSCOT) rather than a single pooled number, and verify exact PMID snippet quotes against cached abstracts per the dismech evidence SOP before finalizing entries.