The eye: ocular toxoplasmosis
Of all the things this parasite does to healthy people, damage to the retina is the most common and the least talked about. If you have new floaters or blurred vision, this page is worth five minutes.
The short answer
The parasite can settle in the retina and cause an inflamed patch that heals into a scar. Symptoms are floaters, blurred vision and sometimes pain or light sensitivity. It can affect otherwise healthy people, it usually follows an infection acquired after birth rather than before, and it tends to come back. Treatment exists, and a low-dose antibiotic taken long term reduces recurrences. New eye symptoms mean seeing an ophthalmologist promptly.
What happens in the eye
Toxoplasma reaches the retina through the bloodstream and can persist there as dormant cysts, just as it does in the brain and muscle. When the parasite reactivates, it destroys a patch of retina and provokes inflammation, a condition called retinochoroiditis. The patch heals into a scar that no longer sees. If the scar sits at the edge of the retina, vision may be barely affected; if it sits at the center, the loss can be severe1.
Symptoms
The classic complaints are floaters, blurred vision and a hazy area in the field of view. Pain, redness and sensitivity to light occur in some people. In the Canadian waterborne outbreak of 1995, nineteen of the hundred recognized cases had eye disease, and those cases were how the outbreak was first noticed2. Sudden new floaters or a blurred patch are reasons to see an ophthalmologist within days, not weeks. Active lesions are treatable, and the point of treatment is to limit the size of the scar.
Who gets it
Anyone who carries the parasite can, and most people who develop eye disease are otherwise healthy. In the United States, one estimate is that about 2 percent of infected people have had an episode1, and that roughly 4,800 people develop symptomatic ocular toxoplasmosis each year3. Traditional teaching held that ocular disease was a late consequence of infection before birth. The evidence now points the other way: most people with ocular toxoplasmosis were infected after birth14, and lesions can first appear many years after the infection.
The numbers are entirely different in parts of South America. In one town in southern Brazil, 17.7 percent of the population examined had eye lesions from toxoplasmosis, rising to more than 21 percent of teenagers and adults, a prevalence more than thirty times higher than elsewhere5. Congenitally infected children in Brazil developed eye lesions five times as often as their European counterparts, and worse ones6. The reason is the parasite strains that circulate there, which is the subject of the strains page.
Recurrences
Ocular toxoplasmosis tends to come back, typically as a new lesion next to an old scar, and the risk is highest in the first year after an episode1. This is the part of the disease where prevention has real evidence behind it. In a randomized trial in Brazil, patients with a history of recurrences who took one tablet of trimethoprim-sulfamethoxazole every three days had recurrences in 6.6 percent of cases over twenty months, against 23.8 percent of those who took nothing7. Whether to use such prophylaxis is a decision for the ophthalmologist, based on how often and how badly a person’s disease has recurred.
Treatment
Not every lesion needs treatment. Small lesions far from the center of the retina in a healthy person are sometimes observed rather than treated, while lesions near the center, large lesions and any lesion in an immunocompromised person are treated. The choice depends on the size and location of the lesion and whether it is active8. The classic regimen combines pyrimethamine and sulfadiazine with folinic acid, often with a corticosteroid to limit the inflammation once the antiparasitic drugs have started, and trimethoprim-sulfamethoxazole and other alternatives are widely used9. No drug clears the dormant cysts, which is why recurrences remain possible after successful treatment.
What to do
- New floaters, a blurred patch, or light sensitivity: see an ophthalmologist promptly. Say that you have had toxoplasmosis if you know it.
- A history of ocular toxoplasmosis: ask about long-term prophylaxis if recurrences have been frequent or sight-threatening, and have your eyes checked during pregnancy and any period of immunosuppression.
- Travel or residence in South America: the precautions on the food and water page carry more weight, including for people infected before.
Sources
- Holland GN. Ocular toxoplasmosis: a global reassessment. Part I: epidemiology and course of disease. Am J Ophthalmol 2003;136:973-88. PubMed · DOI
- Bowie WR, King AS, Werker DH et al. Outbreak of toxoplasmosis associated with municipal drinking water. The BC Toxoplasma Investigation Team. Lancet 1997;350:173-7. PubMed · DOI
- Jones JL, Holland GN. Annual burden of ocular toxoplasmosis in the US. Am J Trop Med Hyg 2010;82:464-5. PubMed · DOI
- Gilbert RE, Stanford MR. Is ocular toxoplasmosis caused by prenatal or postnatal infection? Br J Ophthalmol 2000;84:224-6. PubMed · DOI
- Glasner PD, Silveira C, Kruszon-Moran D et al. An unusually high prevalence of ocular toxoplasmosis in southern Brazil. Am J Ophthalmol 1992;114:136-44. PubMed · DOI
- Gilbert RE, Freeman K, Lago EG et al. Ocular sequelae of congenital toxoplasmosis in Brazil compared with Europe. PLoS Negl Trop Dis 2008;2:e277. PubMed · DOI
- Silveira C, Belfort R, Muccioli C et al. The effect of long-term intermittent trimethoprim/sulfamethoxazole treatment on recurrences of toxoplasmic retinochoroiditis. Am J Ophthalmol 2002;134:41-6. PubMed · DOI
- Centers for Disease Control and Prevention. Treatment of Toxoplasmosis. Accessed 17 September 2026.
- Dunay IR, Gajurel K, Dhakal R et al. Treatment of Toxoplasmosis: Historical Perspective, Animal Models, and Current Clinical Practice. Clin Microbiol Rev 2018;31. PubMed · DOI