Ocular Toxoplasmosis

Ocular toxoplasmosis is necrotizing retinochoroiditis caused by Toxoplasma gondii. It is the most common infectious cause of posterior uveitis worldwide. Seroprevalence is 30 to 50% globally, and ocular disease is highest in southern Brazil, where up to about 18% of seropositive people have lesions.

Infection is congenital or acquired (from undercooked meat, contaminated water or vegetables, or oocysts from cat feces). Ocular disease follows reactivation of bradyzoite cysts in a retinal scar, producing a fresh lesion adjacent to old pigmentation (a satellite lesion). Recurrence is common (about 50% within 3 years). Congenital disease is bilateral in 40 to 60%. Immunocompromised patients show extensive, multifocal necrosis without a scar.

Without treatment active lesions heal over 6 to 8 weeks. Complications: macular scarring, choroidal neovascularization, retinal detachment, optic neuropathy, vascular occlusion and glaucoma.

• Floaters, blurring, mild pain and photophobia
• A focal white retinal lesion with dense overlying vitritis ("headlight in the fog"), often next to a pigmented scar
• Granulomatous anterior uveitis with raised pressure
• Retinal arteritis, optic disc swelling, punctate outer retinal variant
• Atypical in immunocompromised and elderly patients: large, multifocal, hemorrhagic lesions
• Red flags: lesions near the macula or optic disc, size over 1 disc diameter, HIV infection, pregnancy

• Clinical diagnosis by dilated fundus examination, in most patients.
• Serology: toxoplasma IgG (a negative result nearly excludes disease) and IgM for recent infection.
• Aqueous humor antibody coefficient over 3 and PCR (sensitivity 30 to 70%) when the picture is atypical.
• Optical coherence tomography shows full thickness hyperreflectivity with overlying vitreous cells. Fluorescein angiography shows early blockage and late staining.
• HIV testing in atypical disease.

Differentials: cytomegalovirus retinitis, acute retinal necrosis, syphilitic retinitis, tuberculosis, toxocariasis, sarcoidosis, Behçet disease, fungal retinitis, lymphoma.

Treatment is indicated for lesions near the macula or optic disc, size over 1 disc diameter, severe vitritis, reduced vision, immunocompromise, and congenital or pregnancy associated disease. Small peripheral lesions in immunocompetent adults can be observed.

Adults:

• Trimethoprim-sulfamethoxazole 160/800 mg orally twice daily for 6 weeks.
• Classic regimen: pyrimethamine 100 mg loading dose, then 25 to 50 mg daily, plus sulfadiazine 2 g loading dose, then 1 g four times daily, plus folinic acid 10 to 25 mg daily, for 4 to 6 weeks. Check full blood count weekly. Adverse effects: marrow suppression, crystalluria (maintain fluid intake of 2 to 3 litres daily), rash and Stevens-Johnson syndrome.
• Alternatives: clindamycin 300 mg orally four times daily, or intravitreal clindamycin 1 mg with dexamethasone 0.4 mg in 0.1 mL.
• Prednisolone 0.5 to 1 mg/kg daily (usually 40 to 60 mg) tapering over 2 to 4 weeks, started 24 to 48 hours after antiparasitic therapy. Never use alone, especially in immunocompromised patients.
• Atropine 1% twice daily for anterior inflammation. Timolol 0.5% twice daily and dorzolamide 2% three times daily for raised pressure.

Recurrent disease prophylaxis: trimethoprim-sulfamethoxazole 160/800 mg orally every second day for 6 to 12 months.

Pregnancy: spiramycin 1 g (3 million units) orally three times daily. Pyrimethamine is avoided in the first trimester, and specialist care is needed for sulfadiazine, pyrimethamine and folinic acid in later trimesters.

Children: trimethoprim-sulfamethoxazole 8 mg/kg daily of trimethoprim in 2 divided doses for 6 weeks. Alternatively pyrimethamine 2 mg/kg daily for 2 days then 1 mg/kg daily (maximum 25 mg), sulfadiazine 100 mg/kg daily in 2 divided doses, and folinic acid 10 mg 3 times weekly.

Surgery: vitrectomy for tractional detachment, dense opacities or retinal detachment. Intravitreal aflibercept 2 mg for choroidal neovascularization.

Review at 1 to 2 weeks, then monthly until the lesion is scarred. Counsel on cooking meat to 67 degrees Celsius or above, washing vegetables and hands, gloves for cat litter, and a high recurrence risk.