Toxoplasma Infections¶
Chapter 235 | Harrison's 22e · Part 5 – Infectious Diseases: Parasitic · Chapter 235
Key Clinical Points¶
- Toxoplasmosis is caused by the obligate intracellular parasite Toxoplasma gondii.
- Clinical presentation varies by host status: congenital, immunocompromised reactivation (e.g., HIV/AIDS), or acute acquired disease.
- Tachyzoites are responsible for rapid replication and tissue damage; Bradyzoites form slow-growing cysts in muscle and CNS tissues.
- Transmission occurs via oocysts (fecal contamination) or bradyzoites (undercooked meat, particularly lamb and pork).
- Immunocompromised patients often present with toxoplasmic encephalitis characterized by ring-enhancing lesions on MRI.
- Congenital infection typically presents with chorioretinitis, strabismus, and developmental delays.
- Strains from South/Central America are more virulent and associated with higher rates of ocular disease.
- Treatment is tailored to the clinical stage: Spiramycin for pregnancy, Pyrimethamine/Sulfadiazine for immunocompromised patients, and Clindamycin/Azithromycin for retinitis.
1. DEFINITION & OVERVIEW¶
• Pathogen: Toxoplasma gondii (obligate intracellular parasite) • Clinical Forms: ◦ Congenital ◦ Acquired immunocompromised reactivation ◦ Acute acquired disease • Parasite Stages: ◦ Tachyzoites: Infect nucleated cells → cause tissue damage ◦ Bradyzoites: Form cysts in muscle and CNS tissues
1.1 Life Cycle Stages¶
• Tachyzoites: Infect all nucleated cells, replicate, and cause tissue damage • Bradyzoites: Develop in tissue cysts 7–10 days post-infection; resistant to gastric acid • Oocysts: Form in feline intestines (definitive host) → contain eight sporozoites after 2–3 days of environmental exposure
2. EPIDEMIOLOGY¶
• Seroprevalence: ◦ Global: >30% ◦ Brazil: Up to 78% ◦ USA (>6-year-olds): 11% (2011–2014) • Risk Factors: ◦ Foreign-born populations → higher seroprevalence ◦ South/Central American strains → more virulent and associated with ocular disease • Transmission Routes: ◦ Foodborne: Undercooked meat (lamb, pork) containing bradyzoites ◦ Waterborne: Contaminated water with sporulated oocysts ◦ Congenital: Transplacental passage from infected mothers ◦ Zoonotic: Contact with cat feces containing unsporulated oocysts
3. PATHOGENESIS¶
• Mechanism of Infection: ◦ Bradyzoites in meat → resistant to gastric acid → invade intestinal epithelium → transform into tachyzoites → disseminate via bloodstream/lymphatics • Host Response: ◦ Immunocompetent hosts: Clear tachyzoites → form tissue cysts ◦ Immunosuppression: Allows bradyzoite reactivation → tachyzoite proliferation in CNS
3.1 Clinical Manifestations¶
• Immunocompetent: Asymptomatic or ocular disease (chorioretinitis) • Congenital: Chorioretinitis, strabismus, epilepsy, developmental delay • Immunocompromised: Encephalitis, pneumonitis, myocarditis, lymphadenopathy
4. DIAGNOSTIC APPROACH¶
- Serology: Test for IgG/IgM to identify acute or latent infection
- PCR: Perform on CSF or tissue samples in immunocompromised patients
- Ophthalmologic Exam: Evaluate for chorioretinitis
- Imaging: MRI of the brain; look for ring-enhancing lesions (microabscesses) in AIDS patients
4.1 Differential Diagnosis (Table 235-1)¶
• Mononucleosis syndrome: ◦ EBV, CMV, HIV, Bartonella (cat-scratch disease), Lymphoma, HSV, Rubella, Syphilis, Listeriosis • Chorioretinitis in immunocompetent individual: ◦ Tuberculosis, Syphilis, Histoplasmosis, CMV, HSV, VZV, Fungal infection • CNS lesions in AIDS patient: ◦ Lymphoma or metastatic tumor, Brain abscess, Progressive multifocal leukoencephalopathy (JC virus), Fungal infection, Mycobacterial infection
5. CLINICAL MANAGEMENT¶
- Congenital Infection: ◦ First trimester → Spiramycin ◦ Second/third trimester → Pyrimethamine + Sulfadiazine
- Immunocompromised Patients: ◦ Pyrimethamine + Sulfadiazine + Leucovorin
- Ocular Disease (Retinitis): ◦ Clindamycin or Azithromycin
- Prophylaxis: ◦ Trimethoprim-sulfamethoxazole for HIV patients with CD4 <100
5.1 Drug Regimens & Side Effects¶
• Pyrimethamine: ◦ Dose: 75–100 mg/day, PO, Daily, 2–6 weeks ◦ Side effects: Bone marrow suppression, folate deficiency, hypersensitivity • Sulfadiazine: ◦ Dose: 1 g every 6 h (QID), PO/IV, 2–6 weeks ◦ Side effects: Stevens-Johnson syndrome, Sulfonamide allergy
Reference Tables¶
TABLE 235-1 Differential Laboratory Diagnosis of Toxoplasmosis¶
Harrison's 22e, p.1801
| CLINICAL SETTING | ALTERNATIVE DIAGNOSIS | DISTINGUISHING CHARACTERISTICS |
|---|---|---|
| Mononucleosis syndrome |
Epstein-Barr virus infection | Serology/PCR |
| Cytomegalovirus infection | PCR/viral load/serology | |
| HIV infection | Serology/antigen/viral load |
|
| Bartonella infection (cat- scratch disease) |
Biopsy (PCR or culture)/ serology |
|
| Lymphoma | Biopsy | |
| Cytomegalovirus infection | ||
| Herpes simplex virus infection |
||
| Rubella virus infection | ||
| Syphilis | ||
| Listeriosis | ||
| Chorioretinitis in immunocompetent individual |
Tuberculosis | Bacterial culture/PCR |
| Syphilis | Serology | |
| Histoplasmosis | Serology/culture/antigen | |
| Cytomegalovirus infection | ||
| Syphilis | ||
| Herpes simplex virus infection |
||
| Varicella-zoster virus infection |
||
| Fungal infection | ||
| CNS lesions in AIDS patient |
Lymphoma or metastatic tumor |
Tissue biopsy |
| Brain abscess | Culture/biopsy | |
| Progressive multifocal leukoencephalopathy |
PCR for JC virus | |
| Fungal infection | Antigen/PCR/biopsy/ culture |
|
| Mycobacterial infection | PCR/biopsy/culture |