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Toxoplasmosis

BASICS

  • Causative agent: Toxoplasma gondii, an obligate intracellular protozoan
  • Forms of infection:
  • Acute: Self-limiting in immunocompetent
  • Reactivated: Dangerous in immunocompromised
  • Congenital: Transplacental transmission during acute maternal infection
  • Ocular: Chorioretinitis

Most common latent protozoan infection worldwide

EPIDEMIOLOGY

Characteristic Detail
Global seroprevalence >90% in some areas; ~11% in U.S. (CDC)
Congenital incidence 10–100 per 100,000 live births (U.S.)
Gender bias Slight male predominance
Risk increases With age (IgG seropositivity accumulates over time)
Targeted by CDC As a neglected parasitic infection

TRANSMISSION & PATHOPHYSIOLOGY

  • Reservoir: Domestic & wild cats (sexual cycle in feline gut)
  • Transmission to humans:
  • Ingestion of raw/undercooked meat, contaminated food/water
  • Oocyst exposure (e.g., from cat feces in soil/litter)
  • Transplacental (maternal primary infection)

  • Blood transfusion or organ transplantation

  • Pathogenesis:

  • Oocysts β†’ GI invasion β†’ tachyzoites β†’ hematogenous spread β†’ encystment (bradyzoites) in tissues

RISK FACTORS

  • Immunocompromised: AIDS (CD4 <100/ΞΌL), chemotherapy, transplant
  • Pregnancy:
  • Infection risk increases with gestational age
  • Severity inversely proportional to gestational age
  • HLA-DQ3: Genetic susceptibility in HIV/AIDS
  • Environmental: Undercooked meat, cat exposure, gardening, poor hygiene

PREVENTION

  • Cook meat to β‰₯66Β°C (152Β°F) or freeze at ≀-12Β°C
  • Wash fruits and vegetables thoroughly
  • Wear gloves while gardening or cleaning litter boxes
  • Avoid handling raw meat with bare hands
  • No raw shellfish or unpasteurized dairy
  • Pregnant women: avoid cat litter, undercooked meat

CLINICAL MANIFESTATIONS

Congenital Toxoplasmosis

Feature Description
Triad (uncommon) Chorioretinitis, hydrocephalus, intracranial calcifications
Other findings IUGR, prematurity, jaundice, seizures, visual defects, microcephaly
Long-term sequelae Hearing loss, mental retardation, spasticity

Acute (Immunocompetent)

  • 90% asymptomatic
  • Most common: bilateral, nontender cervical lymphadenopathy
  • Mild systemic symptoms: fever, myalgia, fatigue, headache, hepatosplenomegaly

Ocular Toxoplasmosis

  • Chorioretinitis: necrotizing retinitis, blurred vision, photophobia, pain
  • Congenital = bilateral; acquired = unilateral

CNS Toxoplasmosis (Immunocompromised)

  • Encephalitis: headache, seizures, focal deficits
  • MRI: Ring-enhancing lesions
  • May involve lungs, liver, GI, heart, bone marrow

DIFFERENTIAL DIAGNOSIS

  • TORCH infections (esp. CMV, rubella)
  • Lymphoma, leukemia
  • Cryptococcosis, PML (HIV)
  • Syphilis, tuberculosis
  • EBV, CMV, tularemia
  • Erythroblastosis fetalis

DIAGNOSTIC EVALUATION

Serology

Marker Interpretation
IgM Appears within 1st week of acute infection
IgG Appears after ~2 weeks, persists for life
Avidity testing High avidity IgG in 1st trimester rules out recent infection
  • Tests: ELISA (most common), IFA, ISAGA, Sabin-Feldman dye test (gold standard)

Imaging

  • MRI brain: Ring-enhancing lesions in toxoplasmic encephalitis
  • Fetal U/S: Hydrocephalus, calcifications
  • CT scan: Less sensitive than MRI for CNS

Other Investigations

  • PCR on amniotic fluid (if fetal infection suspected)
  • Lymph node biopsy
  • Cord/peripheral blood serology in neonates
  • Placental histology (rare)

TREATMENT

General Principles

  • Immunocompetent patients: often no treatment
  • Tachyzoites are treatment targets β€” tissue cysts not eliminated

First-Line Therapy (Non-pregnant)

Pyrimethamine + Sulfadiazine + Leucovorin

Component Dose & Notes
Pyrimethamine 200 mg LD, then 50 mg/day PO
Sulfadiazine 4–6 g/day in 4 divided doses
Leucovorin 10–25 mg/day PO to prevent bone marrow suppression

Alternatives: - Pyrimethamine + Clindamycin 600–1200 mg IV or 450 mg PO QID - TMP-SMX (10/50 mg/kg/day PO/IV) Γ— 30 days - Atovaquone Β± Pyrimethamine

Prophylaxis (HIV CD4 <100 + IgG+)

Prophylaxis Type Regimen
Primary TMP-SMX DS 1 tab daily OR Dapsone + Pyrimethamine + Leucovorin
Secondary Pyrimethamine 25–50 mg + Sulfadiazine 2–4 g/day + Leucovorin

Pregnancy Management

Gestation Age Drug & Comments
<18 weeks Spiramycin 1 g PO q8h (if amniotic PCR negative)
>18 weeks + PCR+ Pyrimethamine + Sulfadiazine + Leucovorin only if fetal infection confirmed

Neonatal Treatment

  • Pyrimethamine: 2 mg/kg/day Γ— 2d, then 1 mg/kg/day Γ— 2–6 months, then M/W/F
  • Sulfadiazine: 100 mg/kg/day in 2 divided doses
  • Leucovorin: 10 mg 3Γ—/week during and 1 week after pyrimethamine

Ocular Disease

  • Clindamycin 900–1200 mg TID Β± Pyrimethamine Β± corticosteroids (prednisone 1–2 mg/kg/day)
  • Trimethoprim-sulfamethoxazole DS BID Γ— 45 days for prevention

MONITORING & PRECAUTIONS

  • Bone marrow suppression, especially with pyrimethamine
  • Ensure adequate hydration with sulfadiazine (risk of crystalluria)
  • Monitor renal/liver function
  • Beware of drug interactions: sulfonamides ↑ phenytoin, warfarin levels

PATIENT EDUCATION

ICD-10 CODES

Code Description
B58.9 Toxoplasmosis, unspecified
B58.2 Toxoplasma meningoencephalitis
P37.1 Congenital toxoplasmosis

CLINICAL PEARLS

  • Most immunocompetent patients are asymptomatic
  • Bilateral nontender cervical lymphadenopathy = hallmark in immunocompetent hosts
  • Primary prevention crucial in pregnancy & immunocompromised
  • Universal screening in pregnancy not recommended
  • Diagnosis of congenital toxoplasmosis requires serology, imaging, and PCR