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About one in three people worldwide carry a parasite called Toxoplasma gondii and have no idea. The infection usually causes no symptoms, but once the parasite enters your body, it forms dormant cysts in your brain, muscles, and other tissues that persist for life. A single blood draw measuring your Toxoplasma gondii IgG (immunoglobulin G, a type of antibody your immune system produces in response to infection) tells you whether you have ever been exposed.
Knowing your status matters more than most people realize. If you are planning a pregnancy, a positive result before conception means your body already has protective antibodies, while a negative result means you are vulnerable to a first infection that could harm a developing baby. If you are living with a weakened immune system, a positive result flags the risk that dormant cysts could reactivate into serious disease. And emerging research links chronic infection to inflammation, cardiovascular risk in men, and changes in brain health.
This test is fundamentally different from most blood markers. It answers a yes or no question: has your immune system ever encountered Toxoplasma gondii? A positive IgG means you were infected at some point, the parasite is almost certainly still present as dormant cysts, and your immune system maintains a lasting antibody response against it. A negative IgG means there is no evidence of prior infection.
One thing IgG cannot tell you is when you were infected. IgG antibodies typically appear one to three weeks after an initial immune response marker called IgM (immunoglobulin M, the first antibody your body makes during a new infection), and IgG then remains detectable for life. Because both recent and decades-old infections produce a positive IgG, additional testing such as IgG avidity (a measure of how tightly the antibody binds to the parasite, which increases over time after infection) is needed to distinguish new infections from old ones.
Seroprevalence (the percentage of a population with detectable IgG antibodies) varies widely by geography, diet, and exposure patterns. Major risk factors include eating undercooked or raw meat, contact with unwashed vegetables or fruits, exposure to cat feces (cats are the parasite's primary host), soil contact, and drinking untreated water.
| Region or Population | IgG Seroprevalence |
|---|---|
| Europe (general population) | 32.1% |
| Iran (general population) | 32.9% |
| Indonesia (general population) | 60.1% |
| Pregnant women globally | ~33% |
| Romania, women aged 15 to 45 | 46.1% |
| Germany (nationally representative) | Increases sharply with age |
Seroprevalence rises with age across virtually every population studied, simply because the longer you live, the more opportunities you have for exposure. Genetic factors and individual immune makeup also influence how strongly your body produces IgG after infection.
This is where the test has its most established and actionable clinical role. If a woman contracts Toxoplasma for the first time during pregnancy, the parasite can cross the placenta and cause congenital toxoplasmosis in the baby. Consequences can include brain calcifications, hydrocephalus (fluid buildup in the brain), eye damage, and long-term neurodevelopmental problems. A meta-analysis of 53 studies estimated the vertical transmission rate at approximately 20% when a mother is infected during pregnancy.
If you already test IgG positive before becoming pregnant, that is generally reassuring. Your existing antibodies provide substantial protection against a new primary infection reaching the fetus. If you test negative, you are at risk of a first infection during pregnancy, which makes food safety precautions and periodic retesting during pregnancy especially important.
When a pregnant woman tests positive for both IgG and IgM, IgG avidity testing becomes essential. High avidity (strong antibody binding) indicates the infection occurred months ago and the fetus is likely protected. Low avidity raises concern for recent infection and may prompt further investigation, including testing amniotic fluid for parasite DNA. A meta-analysis of studies in pregnant women found that combining IgG avidity with IgM and IgA (immunoglobulin A, another antibody type) testing significantly improves accuracy for distinguishing recent from past infection.
Toxoplasma gondii has a well-documented affinity for brain tissue, and multiple studies connect chronic infection to psychiatric and neurocognitive outcomes. A large study of over 11,500 Danish blood donors found that Toxoplasma IgG positivity was significantly associated with schizophrenia. A separate meta-analysis covering multiple psychiatric conditions confirmed the association with schizophrenia and found evidence that reactivation of latent infection may play a role.
In a study of 263 adults living with HIV (human immunodeficiency virus), those who were Toxoplasma IgG positive showed worse performance on neurocognitive testing compared to those who were IgG negative. The association was strongest in people with higher CD4 T-cell counts (a measure of immune strength), suggesting the effect is not simply a marker of advanced immune suppression. Among 96 young people at ultra-high risk for psychosis, higher IgG levels were associated with increased long-term risk of actually transitioning to a psychotic disorder.
These associations do not mean that Toxoplasma infection causes mental illness in every carrier. Most infected people never develop psychiatric symptoms. But the data suggest that chronic infection may be one contributing factor among many, particularly in people with other vulnerabilities.
A U.S. population-based study of 10,237 adults found that Toxoplasma IgG positivity was associated with increased cardiovascular disease mortality in men, but not in women. This sex-specific pattern held after adjusting for age, body mass, smoking, cholesterol, blood pressure, diabetes, and other standard risk factors.
A separate study of 694 adults found that people with latent Toxoplasma infection (IgG positive) had elevated blood markers of chronic inflammation and vascular injury. In older adults, a study of 601 people found that higher IgG levels (not just positive versus negative, but the intensity of the antibody response) were associated with greater frailty, and that markers of chronic low-grade inflammation partially explained this link.
Toxoplasma is one of the leading causes of posterior uveitis (inflammation in the back of the eye). A study of 344 patients with ocular toxoplasmosis in Brazil found that 20% of affected eyes progressed to legal blindness. Risk factors for vision loss included lesions located at the macula (the central part of the retina responsible for sharp vision) and atypical disease patterns. Ocular disease can occur in people with chronic infection, sometimes years after the initial exposure.
Most laboratories report Toxoplasma IgG as negative, equivocal (borderline), or positive, with specific cutpoints measured in international units per milliliter (IU/mL). The exact thresholds vary by assay platform and manufacturer. A systematic review of commercial IgG assays found that most have sensitivity (the ability to correctly identify infected individuals) between 90% and 100%, and specificity (the ability to correctly identify uninfected individuals) of 91% to 99%. A rapid point-of-care test achieved 99.3% sensitivity and 100% specificity when compared to reference laboratory methods.
Because cutpoints differ between assay platforms, you should always compare results from the same lab using the same method. Switching labs between tests can produce apparent changes that reflect assay differences rather than real changes in your immune status.
Unlike most biomarkers where serial tracking reveals a trend, Toxoplasma IgG is essentially a binary status check for most people. Once positive, it stays positive for life. Retesting a clearly positive result adds no new information. The value of this test lies in establishing your status at the right time: ideally before pregnancy, before starting immunosuppressive therapy, or as part of a thorough baseline health assessment.
There are two situations where follow-up testing matters. First, if your initial result is equivocal or borderline, repeat the test in two to three weeks to see if the result clarifies. Second, if you are IgG negative and entering a high-risk period (pregnancy, organ transplant, starting biologic medications), periodic retesting during that period can catch a new infection early, when intervention is most effective.
If your result is negative and you are not in a high-risk group, no immediate action is needed. You now know you have not been exposed, which is useful information to have on file. Practice standard food safety: cook meat thoroughly, wash produce well, wear gloves when gardening, and clean cat litter boxes daily (or have someone else do it) since the parasite's eggs become infectious after one to five days in the environment.
If your result is negative and you are pregnant or planning a pregnancy, take food and cat hygiene precautions seriously and ask about periodic retesting during pregnancy. Many countries with routine prenatal screening programs retest seronegative women monthly or each trimester.
If your result is positive and you are healthy, this means you carry dormant cysts but your immune system is keeping them in check. No treatment is needed. Be aware that if you ever face significant immunosuppression (cancer treatment, organ transplant, HIV with falling immune cell counts), your medical team should know your Toxoplasma status so they can consider preventive medication.
If your result is positive and you are pregnant, follow-up testing is essential. Your doctor should check whether you also have IgM antibodies and, if so, order IgG avidity testing to determine whether infection is recent or old. High avidity is reassuring. Low avidity during early pregnancy may warrant specialist referral to an infectious disease or maternal-fetal medicine physician and possibly amniocentesis to test for fetal infection.
If your result is positive and you are immunocompromised, discuss prophylaxis with your treatment team. The standard preventive regimen in transplant and advanced HIV settings is trimethoprim-sulfamethoxazole (a combination antibiotic), which suppresses parasite replication and prevents reactivation.
Toxoplasma gondii Ab IgG is best interpreted alongside these tests.