Overview
Toxoplasmosis
1. Clinical Overview
Summary
Toxoplasmosis is caused by the intracellular protozoan parasite Toxoplasma gondii. Infection is usually asymptomatic or mild in immunocompetent individuals, but causes severe disease in the immunocompromised (especially HIV with low CD4) and can damage the fetus if acquired during pregnancy.
Key Facts
| Aspect | Detail |
|---|---|
| Organism | Toxoplasma gondii (protozoan) |
| Definitive Host | Cats (oocysts in faeces) |
| Transmission | Cat faeces, undercooked meat, unwashed vegetables |
| Major Concerns | Congenital toxo, cerebral toxo in AIDS |
| Classic CT Finding | Ring-enhancing lesions (cerebral toxo) |
Clinical Pearls
- Immunocompetent: Usually asymptomatic or causes self-limiting lymphadenopathy
- HIV (CD4 <100): #1 cause of ring-enhancing brain lesions - treat empirically
- Pregnancy: Earlier infection = worse fetal outcome
- Cat litter: Pregnant women should avoid changing it
2. Epidemiology
Prevalence
| Population | Prevalence |
|---|---|
| Global seropositivity | 30-50% (varies by region) |
| Higher in France/Latin America | Dietary habits (raw meat) |
| AIDS-defining illness | Common in advanced HIV |
Transmission Routes
| Route | Source |
|---|---|
| Oocysts | Cat faeces, contaminated soil/water |
| Tissue cysts | Undercooked meat (pork, lamb) |
| Transplacental | Congenital infection |
| Transplant/transfusion | Rare |
3. Pathophysiology
Life Cycle
Cat ingests infected prey
↓
Oocysts shed in faeces (millions/day for 1-2 weeks)
↓
Oocysts contaminate soil, water, vegetables
↓
Intermediate hosts (humans, animals) ingest oocysts
↓
Tachyzoites disseminate (acute infection)
↓
Bradyzoites encyst in tissues (latent infection)
↓
Reactivation if immunosuppressed
Forms
| Form | Phase |
|---|---|
| Tachyzoites | Rapidly dividing (acute infection) |
| Bradyzoites | Slow, encysted (latent in brain, muscle) |
| Oocysts | Environmental form (from cats) |
4. Clinical Presentation
Immunocompetent Adults
| Presentation | Features |
|---|---|
| Asymptomatic | Most cases |
| Lymphadenopathy | Cervical, non-tender, weeks-months |
| Flu-like illness | Fever, myalgia, fatigue |
| Chorioretinitis | Posterior uveitis |
Immunocompromised (HIV CD4 <100)
| Manifestation | Features |
|---|---|
| Cerebral toxoplasmosis | Headache, fever, confusion, focal neurology |
| CT/MRI | Ring-enhancing lesions (multiple, basal ganglia) |
| Encephalitis | Altered consciousness |
| Chorioretinitis | Ocular involvement |
Congenital Toxoplasmosis
| Feature | Notes |
|---|---|
| Classic Triad | Chorioretinitis, hydrocephalus, intracranial calcifications |
| Other | Hepatosplenomegaly, jaundice, seizures |
| Risk | Higher transmission later in pregnancy, but earlier = more severe |
5. Clinical Examination
Immunocompetent
| Finding | Notes |
|---|---|
| Lymphadenopathy | Usually cervical |
| Fever | Low-grade |
| Hepatosplenomegaly | Rare |
HIV/Cerebral Toxo
| Finding | Notes |
|---|---|
| Focal neurology | Hemiparesis, ataxia |
| Altered mental status | Confusion |
| Seizures | May occur |
| Cranial nerve palsies |
Congenital
| Finding | Notes |
|---|---|
| Microcephaly or hydrocephalus | |
| Chorioretinitis | Fundoscopy |
| Hepatosplenomegaly |
6. Investigations
Serological Tests
| Test | Interpretation |
|---|---|
| IgG positive, IgM negative | Past infection |
| IgG positive, IgM positive | Recent or acute infection |
| IgG negative | No prior exposure (at risk if pregnant) |
| IgG avidity | Low avidity = recent infection |
Imaging (Cerebral Toxo)
| Modality | Findings |
|---|---|
| CT with contrast | Ring-enhancing lesions |
| MRI | More sensitive, "eccentric target sign" |
Differential of Ring-Enhancing Lesions in HIV
| Diagnosis | Features |
|---|---|
| Toxoplasmosis | Multiple lesions, basal ganglia |
| CNS lymphoma | Single, periventricular, crosses midline |
| Brain abscess | Variable |
| PML | No enhancement, white matter |
Congenital Toxo
| Test | Timing |
|---|---|
| Maternal serology | IgM/IgG, avidity |
| Amniocentesis PCR | If maternal seroconversion |
| Neonatal serology | IgM, IgA (baby's own), persistent IgG |
| Cranial USS/MRI | Calcifications, hydrocephalus |
7. Management
Immunocompetent (Most Cases)
- Usually self-limiting
- No treatment required unless severe/prolonged/ocular
Cerebral Toxoplasmosis (HIV)
| Drug | Dose | Duration |
|---|---|---|
| Pyrimethamine | Loading then 50-75mg/day | 6 weeks |
| Sulfadiazine | 1-1.5g QDS | 6 weeks |
| Folinic acid | 10-25mg/day | Prevents bone marrow toxicity |
Alternative: TMP-SMX (cotrimoxazole)
Secondary Prophylaxis (HIV)
- Continue until CD4 >200 for ≥6 months on ART
- TMP-SMX is preferred
Pregnancy
| Scenario | Treatment |
|---|---|
| Maternal seroconversion | Spiramycin (reduce fetal transmission) |
| Confirmed fetal infection | Pyrimethamine + sulfadiazine + folinic acid |
Congenital Toxo (Neonate)
- Pyrimethamine + sulfadiazine + folinic acid for 12 months
- Monitor FBC, LFTs
8. Complications
| Complication | Population |
|---|---|
| Chorioretinitis | All - may cause visual loss |
| Brain abscess | Immunocompromised |
| Encephalitis | HIV |
| Fetal loss/stillbirth | Congenital |
| Developmental delay | Congenital |
9. Prognosis & Outcomes
| Scenario | Outcome |
|---|---|
| Immunocompetent | Excellent, self-limiting |
| HIV on treatment | Good if early treatment + ART |
| Congenital (treated) | Improved outcomes, but long-term sequelae possible |
10. Evidence & Guidelines
| Organisation | Key Points |
|---|---|
| CDC | Prevention in pregnancy |
| BHIVA | HIV OI guidelines |
| NICE | Antenatal screening (not routine in UK) |
11. Patient / Layperson Explanation
What is toxoplasmosis? It's an infection caused by a tiny parasite called Toxoplasma gondii. Most people don't even know they have it because it usually causes no symptoms.
How do you catch it?
- Eating undercooked or raw meat
- Contact with cat faeces (e.g., changing litter trays)
- Eating unwashed fruit or vegetables
Who needs to be careful?
- Pregnant women: Can pass infection to baby
- People with weak immune systems: Especially those with HIV
How to prevent it:
- Cook meat thoroughly
- Wash vegetables and fruit
- Wear gloves when gardening
- If pregnant, avoid changing cat litter or wear gloves
- Wash hands after handling raw meat
Is it treatable? Yes - antibiotics work well, especially if started early. Most healthy people don't need treatment at all.
12. References
- Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet. 2004.
- CDC Toxoplasmosis Guidelines. www.cdc.gov
- BHIVA Guidelines for HIV-associated OIs. 2019.