Toxoplasmosis

1. Clinical Overview

Summary

Toxoplasmosis is caused by the obligate intracellular protozoan parasite Toxoplasma gondii. This zoonotic infection affects approximately one-third of the global human population, with seroprevalence varying markedly by geographic region and dietary habits. [1] While infection is typically asymptomatic or self-limiting in immunocompetent individuals, toxoplasmosis causes severe disease in three key clinical scenarios: congenital infection following primary maternal infection during pregnancy, reactivation disease in profoundly immunocompromised patients (particularly HIV/AIDS with CD4 count less than 100 cells/μL), and ocular disease causing chorioretinitis. [2,3]

The parasite's complex life cycle involves cats as the definitive host, with humans and other warm-blooded animals serving as intermediate hosts. Transmission occurs through ingestion of oocysts from cat feces, consumption of tissue cysts in undercooked meat, or transplacental passage during acute maternal infection. [1]

Key Facts

Clinical Pearls

• Immunocompetent Adults: 80-90% asymptomatic; when symptomatic, presents as self-limiting cervical lymphadenopathy ("toxoplasma lymphadenitis"). [2]
• HIV/AIDS: Cerebral toxoplasmosis is the #1 cause of focal brain lesions in patients with CD4 less than 100 cells/μL; empiric treatment without biopsy is standard if classic imaging features are present. [4]
• Pregnancy: Earlier gestational age at maternal infection correlates with more severe fetal disease, but transmission rate increases with advancing gestation (10% at 13 weeks vs. 80% at 36 weeks). [5]
• Ocular Disease: Most common cause of infectious posterior uveitis worldwide; may present as congenital disease manifesting in childhood/adolescence or as acquired disease in adults. [6]
• Ring-Enhancing Lesions: In HIV patients, if no response to empiric toxoplasmosis therapy within 10-14 days, consider CNS lymphoma and pursue stereotactic brain biopsy. [4]

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2. Epidemiology

Global Seroprevalence

Toxoplasmosis exhibits marked geographic variation in seroprevalence, reflecting differences in dietary practices, climate, and cat exposure:

Seroprevalence increases with age, reflecting cumulative lifetime exposure risk. [1,7]

Transmission Routes

Importantly, chronic infection (IgG-positive, IgM-negative) does not transmit congenitally; only acute/primary maternal infection during pregnancy poses fetal risk. [5]

High-Risk Populations

Incidence of Congenital Toxoplasmosis

The incidence of congenital toxoplasmosis varies by screening and treatment practices:

• United States: 1 per 10,000 live births (no universal prenatal screening) [5]
• France: 2-3 per 10,000 live births (universal monthly screening and treatment) [5]
• Austria: Similar to France with screening programs
• United Kingdom: No routine screening; estimated 1-2 per 10,000 births

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3. Pathophysiology

The Parasite: Toxoplasma gondii

T. gondii is an obligate intracellular protozoan of the phylum Apicomplexa. It exists in three infectious forms: [1,2]

Life Cycle

The T. gondii life cycle involves both sexual (in cats only) and asexual reproduction (in all warm-blooded hosts): [1]

DEFINITIVE HOST (CATS)
    ↓
Cat ingests infected prey/tissue cysts
    ↓
Sexual reproduction in feline intestinal epithelium
    ↓
Oocysts shed in cat feces (millions per day for 1-3 weeks)
    ↓
Oocysts sporulate in environment (1-5 days)
    ↓
INFECTIOUS OOCYSTS contaminate soil, water, vegetables
    ↓
INTERMEDIATE HOSTS (humans, animals) ingest oocysts
    ↓
Sporozoites released in intestine → invade intestinal epithelium
    ↓
Transform to TACHYZOITES → disseminate via blood/lymph
    ↓
Tachyzoites invade nucleated cells (ALL tissues)
    ↓
Cell-mediated immune response develops (10-14 days)
    ↓
Tachyzoites convert to BRADYZOITES within tissue cysts
    ↓
LATENT INFECTION: Cysts persist lifelong (brain, muscle, retina)
    ↓
REACTIVATION if immunosuppression: Bradyzoites → Tachyzoites

Only felids (cats) can complete the sexual cycle and shed oocysts. Once infected, cats typically shed oocysts only once in their lifetime. [1]

Molecular Pathogenesis

Exam Detail: #### Host Cell Invasion

T. gondii employs an active invasion process distinct from phagocytosis:

1. Attachment: Parasite surface adhesins bind host cell receptors
2. Moving Junction Formation: Apical complex proteins (RON2, RON4, RON5, AMA1) form tight junction
3. Parasitophorous Vacuole (PV) Formation: Parasite invades within a non-fusogenic vacuole that resists lysosomal fusion
4. Intracellular Replication: Tachyzoites divide by endodyogeny (internal budding), forming 64-128 organisms before cell lysis

Immune Evasion Mechanisms

• PV Modification: Secretion of dense granule proteins (GRA) and rhoptry proteins (ROP) modifies the vacuole membrane, preventing phagolysosome fusion
• Interferon-γ Resistance: Parasites resist IFN-γ-mediated killing by degrading host immune effectors
• Bradyzoite Conversion: In response to immune pressure, tachyzoites convert to metabolically quiescent bradyzoites within cysts
• Immune Privilege Sites: Brain and eye provide relative immune sanctuary for cyst persistence [2,8]

Genetic Diversity and Virulence

T. gondii strains are classified into three main clonal lineages (Types I, II, III) with varying virulence:

• Type I: Highly virulent in mice; rare in humans
• Type II: Most common in Europe/North America; moderate virulence; predominant in congenital and AIDS-related infections
• Type III: Less virulent; associated with animal infections
• Atypical/Recombinant strains: Found in South America; associated with severe ocular disease [6]

Parasite genotype influences clinical severity of ocular toxoplasmosis, with atypical strains causing more aggressive disease. [6]

Immune Response

Protective Immunity

Cell-mediated immunity (CMI) is essential for control of toxoplasmosis:

1. Innate Response:

   • Toll-like receptors (TLR11, TLR12 in mice; TLR2, TLR4 in humans) recognize parasite profilin
   • NK cells and macrophages produce IL-12
   • IL-12 stimulates IFN-γ production by NK cells

2. Adaptive Response:

   • CD4+ Th1 cells produce IFN-γ (critical cytokine)
   • CD8+ T cells mediate cytotoxic killing of infected cells
   • IFN-γ activates antimicrobial mechanisms (IDO, iNOS, immunity-related GTPases)
   • Antibodies play a minor protective role but serve as serological markers [2,8]

Immunopathology

• CNS Disease in AIDS: Loss of CD4+ T cells → inability to maintain tachyzoite suppression → cyst rupture → uncontrolled tachyzoite proliferation → necrotizing encephalitis [4]
• Ocular Disease: Antigen release from retinal cysts → intense inflammatory response → retinal necrosis and scarring → vision loss [6]
• Congenital Infection: Vertical transmission of tachyzoites → fetal dissemination before immune system maturation → multi-organ damage [5]

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4. Clinical Presentation

Immunocompetent Adults (Acquired Toxoplasmosis)

The vast majority (80-90%) of infections in immunocompetent individuals are asymptomatic. [2] When symptomatic:

Key Point: Symptoms are self-limiting and resolve spontaneously. Treatment is not indicated for uncomplicated lymphadenitis in immunocompetent hosts. [2]

Cerebral Toxoplasmosis (HIV/AIDS)

Cerebral toxoplasmosis (toxoplasmic encephalitis, TE) is the most common cause of focal brain lesions in AIDS patients and occurs almost exclusively when CD4 count less than 100 cells/μL (typically less than 50). [4] It results from reactivation of latent brain cysts.

Clinical Features

Temporal Profile: Subacute onset over days to 1-2 weeks (contrast with stroke: hyperacute; CNS lymphoma: slower, weeks to months). [4]

Neuroimaging Characteristics

CT with Contrast: [4]

• Multiple (> 1 lesion in 70-80%) ring-enhancing lesions
• Predilection for basal ganglia, corticomedullary junction
• Surrounding edema
• Mass effect

MRI with Gadolinium (more sensitive):

• "Eccentric target sign": eccentric dot of enhancement within larger ring
• T2 hyperintense lesions
• Diffusion restriction at abscess rim
• Hemorrhage may be present

Differential Diagnosis of Ring-Enhancing Lesions in HIV:

Importantly, 3-10% of HIV patients with TE are Toxoplasma IgG-negative, so negative serology does not exclude the diagnosis. [4]

Congenital Toxoplasmosis

Congenital toxoplasmosis results from transplacental transmission of tachyzoites during primary maternal infection. Chronic (latent) maternal infection does NOT transmit. [5]

Transmission Risk by Trimester

Key Paradox: Transmission risk increases with gestational age, but severity decreases. [5]

Clinical Features at Birth

Classic Triad (occurs in less than 10% of congenital cases):

1. Chorioretinitis
2. Hydrocephalus
3. Intracranial calcifications (diffuse, scattered throughout cortex)

Generalized Infection (10-20% symptomatic at birth):

Subclinical Infection (70-90% asymptomatic at birth):

• Most infected infants appear normal at birth
• Sequelae develop later: Chorioretinitis (80%), learning disabilities (75%), hearing loss (15-26%) [5]
• Critical Point: Without treatment, 85% of subclinically infected infants develop sequelae, most commonly recurrent chorioretinitis in childhood/adolescence [5]

Ocular Toxoplasmosis (Chorioretinitis)

Toxoplasmosis is the most common cause of infectious posterior uveitis worldwide, accounting for 30-50% of cases. [6] It may result from congenital infection (more common) or postnatally acquired infection.

Clinical Features

Disease Course

• Acute Phase: Active inflammation lasts 1-4 months
• Recurrence Rate: 50-80% experience at least one recurrence over lifetime
• Visual Prognosis: Depends on macular involvement; macular lesions cause permanent vision loss [6]

Toxoplasmosis in Other Immunocompromised Hosts

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5. Clinical Examination

Immunocompetent Adults

HIV/AIDS with Cerebral Toxoplasmosis

Congenital Toxoplasmosis

Ocular Examination (Toxoplasmic Chorioretinitis)

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6. Investigations

Serological Testing (First-Line)

Serology is the primary diagnostic method for most toxoplasmosis scenarios:

IgM Caveats: [5,7]

• IgM can persist for > 12 months (even > 2 years) after acute infection
• False-positive IgM is common (rheumatoid factor, ANA, heterophile antibodies)
• IgM positivity alone is NOT diagnostic of acute infection

IgG Avidity Testing

Measures the binding strength of IgG antibodies; helps differentiate recent from remote infection: [5]

Critical Application: In a pregnant woman with IgG+/IgM+, high-avidity IgG in first trimester excludes gestational acquisition → fetus not at risk. [5]

PCR Testing

Note: Negative PCR does NOT exclude toxoplasmosis; sensitivity is moderate at best.

Neuroimaging (Cerebral Toxoplasmosis)

CT Head with Contrast

• First-line in resource-limited or emergent settings
• Findings: Multiple (or single) ring-enhancing lesions, predilection for basal ganglia, surrounding edema

MRI Brain with Gadolinium (Preferred)

• Higher sensitivity than CT
• Findings: "Eccentric target sign," T2 hyperintensity, diffusion restriction at rim
• Better visualization of brainstem/posterior fossa lesions [4]

Response to Empiric Therapy

• Clinical improvement: within 7-10 days
• Radiological improvement: within 10-14 days
• If no response by day 14: Consider alternative diagnosis (CNS lymphoma) → pursue brain biopsy [4]

Histopathology

Brain biopsy is rarely required but may be indicated if:

• No response to empiric therapy
• Atypical imaging
• Negative toxoplasma serology

Histological Findings: Tachyzoites, tissue cysts, necrotic foci [4]

Ophthalmological Investigations

Congenital Toxoplasmosis Investigations

Maternal Screening (if acute infection suspected)

Neonatal Investigations (if mother had acute infection)

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7. Differential Diagnosis

Lymphadenopathy in Immunocompetent Adults

Ring-Enhancing Brain Lesions in HIV/AIDS

Congenital Infections (TORCH)

Chorioretinitis

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8. Management

Management strategies vary by clinical scenario. The goals are to control active infection (tachyzoites), prevent complications, and in immunocompromised patients, prevent reactivation.

Immunocompetent Adults (Acquired Infection)

Indications for Treatment: [2]

• Severe or prolonged symptoms
• Visceral organ involvement
• Ocular involvement

No Treatment Required: Uncomplicated lymphadenopathy or mild flu-like illness (self-limiting)

If treatment indicated:

Alternative: TMP-SMX 160/800 mg (DS) BID [2]

Cerebral Toxoplasmosis in HIV/AIDS

Acute Treatment (Induction Therapy)

Standard regimen (6 weeks): [4,9]

Monitoring:

• Full blood count weekly (watch for leukopenia, thrombocytopenia)
• Clinical reassessment at days 7-10
• Repeat MRI at 2 weeks (expect ≥50% radiological improvement)

Alternative Regimens (if sulfa allergy or intolerance):

Corticosteroids

Indicated for significant mass effect/edema causing neurological compromise:

• Dexamethasone 4 mg IV/PO QID, taper once mass effect improves [4]
• Use sparingly; may mask response assessment

Anticonvulsants

Indicated for seizures:

• Levetiracetam (preferred; no drug interactions)
• Avoid enzyme inducers (phenytoin, carbamazepine) that may interact with ART

Antiretroviral Therapy (ART)

• When to start: Within 2 weeks of toxoplasmosis diagnosis (after acute stabilization) [9]
• Risk of IRIS: Low with TE (unlike TB or cryptococcal meningitis)
• Goal: Immune reconstitution allows discontinuation of secondary prophylaxis

Secondary Prophylaxis (Maintenance/Suppressive Therapy)

After successful induction (6 weeks), continue suppressive therapy to prevent relapse: [9]

Discontinuation Criteria: CD4 > 200 cells/μL for ≥6 months on ART, with undetectable viral load and complete/near-complete resolution of lesions. [9]

Primary Prophylaxis in HIV (Prevention of Toxoplasmosis)

Indicated for HIV patients with CD4 less than 100 cells/μL AND positive Toxoplasma IgG serology: [9]

Discontinuation: When CD4 > 200 cells/μL for ≥3 months on ART. [9]

Toxoplasmosis in Pregnancy

Maternal Treatment (Confirmed Acute Infection)

Goals: (1) Reduce vertical transmission; (2) Treat fetal infection if transmission has occurred [5]

Before 18 Weeks Gestation (fetal infection status unknown):

After 18 Weeks + Documented Fetal Infection (amniocentesis PCR-positive):

Switch to combination therapy:

Caution: Pyrimethamine is teratogenic and should NOT be used before 18 weeks. Sulfadiazine should be discontinued at 36 weeks (risk of neonatal kernicterus). [5]

Neonatal Treatment (Congenital Toxoplasmosis)

All infected infants (confirmed or suspected) should be treated for 12 months: [5]

Alternative: TMP-SMX if pyrimethamine not available

Corticosteroids: Add prednisolone 0.5 mg/kg BID if active chorioretinitis or CSF protein > 1 g/dL; taper once inflammation resolves. [5]

Monitoring: Monthly FBC, ophthalmic exams every 3 months, audiology annually, neurodevelopmental assessments

Ocular Toxoplasmosis

Indications for Treatment: [6]

• Vision-threatening lesions (near macula, optic nerve, large lesions, dense vitritis)
• Immunocompromised patients
• Persistent or worsening inflammation

Peripheral lesions in immunocompetent patients without vision threat may be observed.

Treatment Regimen:

Alternative: TMP-SMX 160/800 mg BID + prednisolone

Newer Agents: Intravitreal clindamycin/dexamethasone for severe cases (specialist use) [6]

Key Point: Corticosteroids should NEVER be given without concurrent antimicrobial therapy (risk of uncontrolled parasite proliferation). [6]

Toxoplasmosis in Transplant Recipients

Prophylaxis: TMP-SMX (as per PCP prophylaxis protocols) for 3-6 months post-transplant, especially for seronegative recipients receiving seropositive organs [10]

Treatment: Same as HIV patients; continue secondary prophylaxis while on significant immunosuppression

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9. Complications

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10. Prognosis

Immunocompetent Adults

• Excellent: Self-limiting disease; full recovery expected
• Latent Infection: Lifelong carriage of bradyzoite cysts (asymptomatic unless immunosuppression)

Cerebral Toxoplasmosis in HIV/AIDS

• Pre-HAART Era: 70% mortality despite treatment [4]
• HAART Era:
  • "Mortality: 10-20% if diagnosed early and treated appropriately [4]"
  • "Relapse rate: 50-80% without secondary prophylaxis; less than 5% with prophylaxis [9]"
  • "Functional recovery: Depends on extent of neurological damage; many regain independence"

Congenital Toxoplasmosis

• Untreated: 85% develop sequelae (chorioretinitis, hearing loss, cognitive impairment) [5]
• Treated in Infancy:
  • Reduced risk of new chorioretinitis lesions
  • Improved neurodevelopmental outcomes
  • Hearing loss still occurs in 15-26% [5]
• Long-term: Lifelong ophthalmology follow-up required (recurrent chorioretinitis common in adolescence/adulthood)

Ocular Toxoplasmosis

• Recurrence: 50-80% experience at least one recurrence [6]
• Visual Outcome: Depends on macular involvement; peripheral lesions have better prognosis
• Severe Vision Loss: 5-25% (more common with macular lesions, recurrent disease, or atypical parasite strains) [6]

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11. Prevention

Primary Prevention (Avoiding Infection)

Clinical Pearl: Pregnant Women and Immunocompromised Individuals should adhere strictly to the following: [5,7]

Food Safety

• Cook meat to safe temperatures:
  • "Beef/pork/lamb: 63°C (145°F) + 3-min rest"
  • "Ground meat: 71°C (160°F)"
  • "Poultry: 74°C (165°F)"
• Freeze meat at -12°C (10°F) for several days (kills cysts)
• Avoid tasting meat while cooking
• Wash cutting boards, utensils, hands after handling raw meat

Fruits and Vegetables

• Wash thoroughly, peel if possible
• Avoid unwashed salads when eating out (if immunocompromised/pregnant)

Cat Exposure

• Pregnant women should avoid changing cat litter (delegate to others)
• If unavoidable: wear gloves, change litter daily (before oocyst sporulation), wash hands thoroughly
• Keep cats indoors, feed commercial cat food (prevents cat infection)
• Avoid stray cats, especially kittens

Gardening

• Wear gloves when gardening or handling soil
• Wash hands thoroughly after outdoor activities

Water

• Drink filtered/boiled water in endemic areas

Secondary Prevention (Screening and Prophylaxis)

Prenatal Screening

Countries with Universal Screening (France, Austria, Italy):

• Monthly serological testing throughout pregnancy for seronegative women
• Immediate treatment if seroconversion detected

Countries without Routine Screening (USA, UK):

• Selective testing if exposure suspected or symptoms present
• Risk-benefit debate ongoing [5]

HIV/AIDS Patients

Primary Prophylaxis: CD4 less than 100 + Toxoplasma IgG-positive → TMP-SMX daily [9]

Screening: All newly diagnosed HIV patients should have Toxoplasma IgG serology

Transplant Recipients

Donor/Recipient Mismatch Testing: Screen donors and recipients

• Highest risk: D+/R- (donor seropositive, recipient seronegative), especially heart transplant [10]
• Prophylaxis: TMP-SMX for 3-6 months post-transplant

Tertiary Prevention (Preventing Complications)

• Congenital Toxoplasmosis: 12-month treatment to reduce sequelae [5]
• HIV/AIDS: Secondary prophylaxis until immune reconstitution (CD4 > 200 x 6 months) [9]
• Ocular Toxoplasmosis: Lifelong ophthalmology surveillance; prompt treatment of recurrences [6]

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12. Evidence & Guidelines

Key Clinical Guidelines

Landmark Evidence

1. Montoya & Liesenfeld (Lancet 2004): Comprehensive review of toxoplasmosis epidemiology, pathogenesis, and management [1]

2. SYROCOT Study (2007): Systematic review/meta-analysis of prenatal treatment; showed spiramycin reduces transmission, but combination therapy does not improve fetal outcomes if infection already established [5]

3. ACTG 077 Trial: Demonstrated efficacy of clindamycin + pyrimethamine for cerebral toxoplasmosis in AIDS patients [4]

4. HAART Impact Studies: Documented dramatic decline in cerebral toxoplasmosis incidence from 30-40% of AIDS patients (pre-HAART) to less than 5% (HAART era) [9]

5. Holland (Am J Ophthalmol 2004): Global reassessment of ocular toxoplasmosis; updated disease manifestations and management [6]

6. Elsheikha et al. (Clin Microbiol Rev 2020): Modern review of cerebral toxoplasmosis pathophysiology, diagnosis, and treatment [4]

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13. Examination Focus

MRCP/Infectious Diseases Viva Scenarios

Exam Detail: Scenario 1: HIV Patient with Ring-Enhancing Lesions

A 35-year-old man with newly diagnosed HIV (CD4 20 cells/μL) presents with 1-week history of headache, confusion, and left-sided weakness. MRI shows multiple ring-enhancing lesions in the right basal ganglia and left frontal lobe.

Viva Questions:

1. What is your differential diagnosis?

   • Answer: Cerebral toxoplasmosis (most likely), primary CNS lymphoma, tuberculoma, cryptococcoma, bacterial abscess, progressive multifocal leukoencephalopathy (though PML lesions don't enhance). Toxoplasmosis is #1 cause of ring-enhancing lesions in AIDS with CD4 less than 100.

2. What initial investigations would you perform?

   • Answer:
     • Toxoplasma IgG serology (positive in > 95% with cerebral toxo)
     • CT/MRI brain (already done; multiple lesions favor toxo)
     • Consider LP if safe (measure opening pressure, send CSF for PCR, cytology, TB/crypto testing)
     • Full blood count, liver function tests (baseline for treatment)
     • HIV viral load, CD4 count

3. The patient is Toxoplasma IgG-positive. What is your management plan?

   • Answer:
     • Empiric treatment for cerebral toxoplasmosis: Pyrimethamine (200 mg load, then 75 mg daily) + sulfadiazine (1.5 g QID) + folinic acid (10-25 mg daily)
     • Consider dexamethasone 4 mg QID if significant mass effect
     • Anticonvulsant if seizures (levetiracetam)
     • Monitor: Clinical reassessment days 7-10; repeat MRI day 14 (expect ≥50% improvement)
     • Start ART within 2 weeks (after clinical stabilization)
     • If no response by day 14: Consider brain biopsy for CNS lymphoma

4. How do you differentiate toxoplasmosis from primary CNS lymphoma?

   • Answer:

     Feature	Toxoplasmosis	Primary CNS Lymphoma
     Number of lesions	Multiple (70%)	Single (50-70%)
     Location	Basal ganglia	Periventricular, corpus callosum
     Toxo IgG	Positive (> 95%)	Variable
     Response to empiric toxo therapy	Yes (7-14 days)	No
     Thallium-201 SPECT	Cold	Hot
     CSF EBV PCR	Negative	Often positive

Scenario 2: Pregnant Woman with Positive Toxoplasma Serology

A 26-year-old woman at 12 weeks gestation has routine antenatal bloods showing Toxoplasma IgG-positive and IgM-positive. She is asymptomatic.

Viva Questions:

1. What is the significance of these results?

   • Answer: IgG+/IgM+ indicates possible recent infection, but IgM can persist for > 12 months, so this does NOT confirm acute gestational infection. Further testing is essential to date the infection.

2. What additional test would you order immediately?

   • Answer: IgG avidity testing. High avidity IgG indicates infection > 4 months ago, excluding acute infection in this pregnancy (fetus not at risk). Low avidity indicates infection within past 3-4 months (fetus potentially at risk).

3. The IgG avidity returns LOW. What are the implications and next steps?

   • Answer:
     • Low avidity confirms recent infection (within past 3-4 months), so acute gestational infection is possible
     • Risk to fetus: At 12 weeks, transmission risk ~10-15%, but if infected, severe disease likely
     • Management:
       • Start spiramycin 1 g TDS immediately (reduces transmission risk by ~60%)
       • Refer to maternal-fetal medicine specialist
       • Amniocentesis with PCR at ≥18 weeks gestation AND ≥4 weeks post-infection to confirm fetal infection
       • If amniocentesis positive: Switch to pyrimethamine + sulfadiazine + folinic acid (after 18 weeks; pyrimethamine teratogenic before)
       • Serial ultrasound monitoring for fetal abnormalities (hydrocephalus, intracranial calcifications)

4. If the baby is born to a mother with confirmed acute infection, what postnatal investigations and management are required?

   • Answer:
     • Investigations: Neonatal serology (IgM, IgG, IgA), PCR (blood, CSF, urine), cranial USS/CT, dilated ophthalmology exam, hearing assessment, FBC, LFTs, CSF analysis
     • Treatment (even if asymptomatic): Pyrimethamine + sulfadiazine + folinic acid for 12 months
     • Add corticosteroids if active chorioretinitis or CSF protein > 1 g/dL
     • Follow-up: Monthly FBC, ophthalmology every 3 months, audiology annually, neurodevelopmental assessments

Scenario 3: Recurrent Chorioretinitis

A 28-year-old woman presents with sudden-onset floaters and decreased vision in her left eye. Fundoscopy shows a focal area of white retinitis adjacent to an old pigmented scar. She had similar symptoms 5 years ago.

Viva Questions:

1. What is the most likely diagnosis?

   • Answer: Recurrent toxoplasmic chorioretinitis. The "satellite lesion" pattern (new active lesion adjacent to old scar) is classic for toxoplasmosis. It's the most common cause of infectious posterior uveitis.

2. How would you confirm the diagnosis?

   • Answer:
     • Clinical diagnosis based on characteristic fundoscopy findings + history
     • Toxoplasma IgG serology (usually positive, indicating past infection)
     • Aqueous/vitreous PCR if diagnosis uncertain (sens ~50%, spec > 95%)
     • Goldmann-Witmer coefficient (intraocular/serum antibody ratio > 3 supports ocular toxo)

3. What treatment would you recommend?

   • Answer:
     • This lesion is near the macula (vision-threatening) → TREAT
     • Regimen: Pyrimethamine (100 mg load, then 25-50 mg daily) + sulfadiazine (1 g QID) + folinic acid (10-25 mg daily) for 4-6 weeks
     • PLUS: Prednisolone 40-60 mg daily, starting 24-48 hours AFTER antimicrobials (to avoid uncontrolled parasite proliferation)
     • Alternative: TMP-SMX 160/800 mg BID + prednisolone
     • Monitor: FBC weekly, ophthalmology follow-up

4. What is the long-term prognosis?

   • Answer:
     • 50-80% of patients experience at least one recurrence over lifetime
     • Visual outcome depends on macular involvement; central lesions cause permanent vision loss
     • Requires lifelong ophthalmology surveillance
     • No effective prophylaxis to prevent recurrences in immunocompetent patients

High-Yield Facts for Written Exams

1. Definitive host: Cats (only felids shed oocysts)
2. Transmission: Oocysts (cat feces), tissue cysts (undercooked meat), congenital (transplacental)
3. Forms: Tachyzoites (acute), bradyzoites (latent cysts), oocysts (environmental)
4. Immunocompetent: 80-90% asymptomatic; if symptomatic, self-limiting lymphadenopathy
5. Cerebral toxo: #1 cause of ring-enhancing lesions in AIDS (CD4 less than 100); empiric treatment standard
6. Congenital: Earlier infection = worse severity; later infection = higher transmission rate
7. Classic triad (congenital): Chorioretinitis + hydrocephalus + intracranial calcifications
8. Treatment: Pyrimethamine + sulfadiazine + folinic acid (NOT folic acid)
9. Pregnancy: Spiramycin (prevents transmission); pyrimethamine-sulfadiazine (treats fetal infection, ≥18 weeks only)
10. Ocular: Most common infectious posterior uveitis; satellite lesions; corticosteroids ONLY with antimicrobials
11. IgM: Can persist > 12 months; use IgG avidity to date infection in pregnancy
12. HIV prophylaxis: TMP-SMX if CD4 less than 100 + Toxo IgG-positive
13. Secondary prophylaxis (HIV): Continue until CD4 > 200 for ≥6 months on ART
14. CNS lymphoma vs. toxo: Single vs. multiple lesions; high vs. low Thallium/PET uptake; no response vs. response to empiric therapy

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14. Patient / Layperson Explanation

What is toxoplasmosis?

Toxoplasmosis is an infection caused by a microscopic parasite called Toxoplasma gondii. This parasite is extremely common worldwide—about one in three people carry it—but most people never know they have it because it rarely causes symptoms in healthy individuals.

How do you catch it?

There are three main ways:

1. Eating undercooked or raw meat (especially lamb, pork, or game) that contains the parasite
2. Contact with cat feces, typically from:
   • Changing cat litter boxes
   • Gardening in soil where cats have been
   • Eating unwashed vegetables grown in contaminated soil
3. From mother to baby during pregnancy (ONLY if the mother catches the infection for the first time while pregnant)

Important: You CANNOT catch toxoplasmosis from petting a cat. The risk comes only from feces.

Who needs to be careful?

Most people don't need to worry, but two groups are at risk:

1. Pregnant women: If you catch toxoplasmosis for the first time during pregnancy, it can pass to your baby and cause serious problems (brain damage, eye damage). If you've already had the infection before pregnancy (shown by a blood test), your baby is safe.

2. People with very weak immune systems: Especially those with HIV/AIDS (with low immune counts) or those on strong immune-suppressing medications. In these cases, the parasite can reactivate and cause severe brain infection.

What are the symptoms?

• Healthy adults: Usually none! If symptoms occur, they're mild and flu-like (swollen glands in the neck, tiredness, mild fever) and go away on their own within weeks.

• Pregnant women: Usually no symptoms. That's why blood tests are important in some countries.

• People with weak immune systems: Severe headache, confusion, weakness on one side of the body, seizures (requires urgent medical care).

• Eye infection: Blurred vision, floaters, eye pain (can happen years after initial infection, even in healthy people).

How can you prevent it?

Everyone (especially pregnant women and immunocompromised individuals):

• Cook meat thoroughly: Use a meat thermometer (at least 160°F / 71°C for ground meat; 145°F / 63°C for whole cuts)
• Wash hands after handling raw meat, before eating
• Wash fruits and vegetables thoroughly; peel if possible
• Avoid raw/undercooked meat (steak tartare, rare burgers, cured meats in pregnancy)

If you're pregnant:

• Avoid changing cat litter if possible (ask someone else to do it). If you must, wear gloves and change it daily
• Wear gloves when gardening; wash hands thoroughly afterward
• Keep cats indoors and feed them commercial cat food (prevents them from getting infected)
• Avoid stray cats, especially kittens (more likely to shed the parasite)

If you have a weak immune system:

• Follow all the above precautions
• Your doctor may prescribe a daily antibiotic (trimethoprim-sulfamethoxazole) to prevent infection

Is it treatable?

Yes! Treatment involves antibiotics (usually pyrimethamine and sulfadiazine, plus a vitamin supplement called folinic acid). However:

• Healthy adults: Usually don't need treatment (the infection goes away on its own)
• Pregnant women: Treatment can reduce the risk of passing the infection to the baby
• People with weak immune systems: Treatment is essential and highly effective if started early
• Babies born with the infection: Treatment for one year can prevent long-term problems

Can you get rid of the infection completely?

Once infected, the parasite forms dormant "cysts" in your brain and muscles that stay there for life. In healthy people, these cysts cause no harm and your immune system keeps them under control. However, if your immune system becomes very weak later in life, the infection can reactivate.

Key Takeaway

Toxoplasmosis is very common but rarely dangerous for healthy people. If you're pregnant or have a weak immune system, simple precautions (cooking meat well, avoiding cat litter, washing vegetables) can greatly reduce your risk. If you're concerned, speak to your doctor about testing.

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