Ocular Toxoplasmosis: Diagnosis, When to Treat and Regimens
Last updated: October 2026
Toxoplasmosis is the commonest cause of infectious posterior uveitis worldwide. Most cases are recurrences at the edge of an old scar, and the key decisions are whether to treat and which regimen to use.
Recognising it
- Typical lesion: a fluffy white focal retinochoroiditis beside a pigmented scar, with overlying vitritis: the "headlight in the fog".
- Often unilateral, with floaters and blurred vision; anterior uveitis and raised IOP can occur.
- Atypical forms: punctate outer retinal toxoplasmosis, large or multifocal lesions in the elderly and immunocompromised, neuroretinitis, and retinal vasculitis (Kyrieleis plaques).
- Congenital toxoplasmosis: bilateral macular scars, with hydrocephalus and intracranial calcification in severe cases.
Diagnosis
- Mainly clinical. Positive IgG serology supports it (a negative result makes it unlikely); IgM suggests recent infection.
- In atypical cases, PCR of aqueous or vitreous for Toxoplasma DNA, or the Goldmann–Witmer coefficient, confirms the diagnosis.
- Test for HIV in atypical, extensive or bilateral disease.
When to treat
- Lesions within the vascular arcades or near the disc or fovea, large lesions (over one disc diameter), marked vitritis, reduced vision, immunocompromise, pregnancy and neonates.
- Small peripheral lesions in immunocompetent adults are self-limiting and may be observed.
Regimens (adults)
| Regimen | Dose | Notes |
|---|---|---|
| Classic triple therapy | Pyrimethamine 100 mg loading, then 25–50 mg daily; sulfadiazine 1 g four times daily; folinic acid 15 mg (daily or three times weekly) | Weekly blood counts; folinic acid (not folic acid) protects the marrow |
| Trimethoprim–sulfamethoxazole | 160/800 mg twice daily | Cheap and well tolerated; similar efficacy in trials |
| Intravitreal clindamycin | 1 mg/0.1 mL, often with dexamethasone 400 µg | Useful for macular lesions, sulfa allergy or pregnancy; may be repeated |
| Azithromycin | 500 mg daily (often with pyrimethamine) | Alternative in sulfa allergy |
| Corticosteroid | Oral prednisolone 0.5 mg/kg/day started 24–48 hours after antimicrobials, tapered | Never give steroids alone or as a periocular depot |
- Treat for 4–6 weeks, until the lesion scars.
- Prophylaxis: trimethoprim–sulfamethoxazole every 2–3 days for frequent recurrences or sight-threatening lesions.
- Pregnancy: spiramycin in the first trimester (avoid pyrimethamine); involve the obstetrician.
Pitfall: starting steroids without antimicrobials can cause fulminant necrotising retinochoroiditis, especially in the elderly or immunocompromised.
Related: uveitis work-up, masquerade syndromes, intravitreal doses.
Key sources
- Kim SJ et al. Interventions for toxoplasma retinochoroiditis: a report by the American Academy of Ophthalmology. Ophthalmology 2013;120:371–8
- Soheilian M et al. Prospective randomized trial of trimethoprim/sulfamethoxazole versus pyrimethamine and sulfadiazine in the treatment of ocular toxoplasmosis. Ophthalmology 2005;112:1876–82
- Felix JP et al. Trimethoprim-sulfamethoxazole versus placebo to reduce the risk of recurrences of Toxoplasma gondii retinochoroiditis. Am J Ophthalmol 2014;157:762–6
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