Medically reviewed by Edward Tavel, MD, Medical Director · Last reviewed 22 August 2026
Toxoplasmosis is an infection caused by Toxoplasma gondii, a single-celled parasite that is among the most widespread in the world. In the United States, national survey data put seroprevalence at roughly 12% of people aged six and older, meaning about one in eight Americans has been exposed at some point, a figure that has declined over recent decades. Transmission occurs through eating undercooked or contaminated meat, through contact with contaminated soil, water, or cat feces, through unwashed produce, and occasionally from mother to fetus during pregnancy or through organ transplantation.
For people with healthy immune systems, the story is usually uneventful. Roughly 90% of infections cause no symptoms at all, or produce a brief, mild illness with swollen lymph nodes and fatigue that resolves on its own. The parasite then persists in a dormant form, walled off in tissue cysts, without causing further trouble. The situations that require attention are infection acquired during pregnancy, which can affect the developing fetus, and reactivation in someone whose immune system has become significantly weakened.
Cerebral toxoplasmosis is the form that MRI is genuinely useful for, and it occurs almost exclusively in immunocompromised patients, classically in advanced HIV infection. On brain MRI it typically appears as multiple ring-enhancing lesions, most often at the junction between grey and white matter and within the basal ganglia, usually with surrounding swelling. Radiologists look for the eccentric target sign, a small enhancing nodule sitting off-center along the wall of the enhancing ring. That sign is highly specific when present, but it appears in fewer than a third of cases, so its absence does not rule anything out. The main alternative to consider is central nervous system lymphoma, which more often presents as a single lesion, so the presence of multiple lesions tends to favor toxoplasmosis. Because both are ring-enhancing masses, the imaging appearance overlaps with other lesions covered in our entry on brain tumors, and interpretation always depends on clinical context.
One point deserves clarity, because it is frequently confused. Ocular toxoplasmosis, the form that affects the retina, is a real and important manifestation, and it produces vascular changes including inflammation around retinal vessels, occasional blockage of small retinal vessels, and abnormal connections between retinal and choroidal circulation in old scars. Those findings are not seen on MRI. They are identified through dilated eye examination, fluorescein angiography, and optical coherence tomography, which resolve structures far smaller than any MRI can visualize. MRI does not diagnose ocular toxoplasmosis and is not part of its standard workup.
Most people who carry Toxoplasma gondii never know it and never need to. If you have questions about brain imaging or what a comprehensive scan can and cannot show, learn more about what we scan for.
Frequently asked questions
Can a whole-body or brain MRI diagnose toxoplasmosis?
MRI can show the brain lesions of cerebral toxoplasmosis, but it cannot confirm the diagnosis on its own. Diagnosis relies on the imaging appearance combined with blood testing, immune status, and sometimes response to treatment or tissue sampling.
Does toxoplasmosis show up in the eye on MRI?
No. Retinal findings in ocular toxoplasmosis are detected by dilated fundus examination, fluorescein angiography, and optical coherence tomography. MRI does not have the resolution to visualize retinal blood vessels.
Who is actually at risk for serious toxoplasmosis?
People with significantly weakened immune systems, such as those with advanced HIV, transplant recipients, or patients on strong immunosuppressive therapy, and pregnant women who acquire a new infection. Healthy adults rarely develop serious disease.
How do people catch it?
Most commonly through undercooked or contaminated meat, unwashed produce, contaminated soil or water, or contact with cat feces. It also passes from mother to fetus and, rarely, through organ transplantation.
Is toxoplasmosis treatable?
Yes. Active infection is treated with antiparasitic medication, typically a combination regimen. Dormant infection in a healthy person is not treated, because it causes no harm.
Sources
- Public Health Surveillance and Reporting for Human Toxoplasmosis – Six States, 2021
- Eccentric target sign in cerebral toxoplasmosis: neuropathological correlate to the imaging feature
- HIV-Related Cerebral Toxoplasmosis Revisited: Current Concepts and Controversies of an Old Disease
- Comparative evaluation of funduscopy, PCR and serology in the diagnosis of ocular toxoplasmosis
A whole-body MRI at MRI Wellness looks at the brain, spine, chest, abdomen and pelvis in a single session, without radiation or contrast dye. No referral is needed.
