Most people with toxoplasmosis never know they have it. The parasite, which infects roughly 11% of the U.S. population over age six, causes no symptoms at all in the majority of healthy adults. When symptoms do appear, they tend to mimic a mild flu, making it nearly impossible to identify without a blood test. Knowing your risk factors, recognizing the subtle signs, and understanding what testing involves are the main ways to figure out whether you’ve been infected.
Most Infections Produce No Symptoms
This is the frustrating reality of toxoplasmosis: it’s often invisible. Healthy people who pick up the parasite typically feel completely fine. Their immune system keeps it in check, and the parasite settles into a dormant state in muscle and brain tissue where it can persist for years or even a lifetime without causing problems.
When symptoms do show up, they usually begin one to three weeks after exposure. The most common signs are swollen lymph nodes (especially in the neck), muscle aches, and general flu-like fatigue. These symptoms resolve on their own within a few weeks to months. Because they look identical to dozens of other mild infections, most people who experience them never connect them to toxoplasmosis.
Risk Factors That Should Raise Suspicion
If you’re wondering whether you might have toxoplasmosis, your exposure history matters more than your symptoms. The parasite spreads through a few well-known routes:
- Cat litter: Cats shed the parasite in their feces after hunting and eating infected prey. Cleaning a litter box, or gardening in soil where cats have been, can expose you.
- Undercooked meat: Pork, lamb, and venison are the most common sources. The parasite forms cysts in animal muscle tissue that survive if the meat isn’t cooked to a safe internal temperature.
- Contaminated water or produce: Unwashed fruits and vegetables, or untreated water in areas with outdoor cats, can carry the parasite.
If you’ve had recent contact with any of these sources and are experiencing swollen glands or unexplained fatigue, testing is reasonable. If you’re pregnant or have a weakened immune system from HIV, cancer treatment, or an organ transplant, testing becomes especially important even without symptoms.
How Blood Tests Detect the Infection
The only reliable way to confirm toxoplasmosis is through blood testing. Your doctor will order tests that look for two types of antibodies your immune system produces in response to the parasite.
The first antibody, IgM, appears within the first couple of weeks of infection and signals a recent or active exposure. The second, IgG, develops later and remains in your blood permanently, indicating that you were infected at some point in your life. A positive IgG with a negative IgM generally means an older infection that’s no longer active. A positive IgM with a rising IgG suggests a new or recent infection.
The interpretation isn’t always straightforward. IgM antibodies can sometimes linger for months after the initial infection, making it hard to pinpoint exactly when you were exposed. When timing matters, particularly during pregnancy, additional tests can help narrow the window. One of these, called an avidity test, measures how tightly IgG antibodies bind to the parasite. Loosely binding antibodies suggest a recent infection, while tightly binding ones point to an infection that happened months ago.
Testing During Pregnancy
Toxoplasmosis carries its most serious risks when a person gets infected for the first time during pregnancy or shortly before conception. The parasite can cross the placenta and infect the developing fetus, potentially causing miscarriage, stillbirth, or congenital toxoplasmosis, which can involve brain calcifications, abnormal head size, and vision problems.
If blood tests suggest a new maternal infection, doctors can test the amniotic fluid directly using a DNA-based test to determine whether the parasite has reached the fetus. This testing is most accurate when performed at least four weeks after the initial maternal infection and no earlier than 18 weeks of gestation. Ultrasound can also reveal signs of fetal infection, including enlarged brain ventricles, calcifications in the brain or liver, and growth restriction, though a normal ultrasound doesn’t rule out infection. According to a pooled analysis of studies published by the American Academy of Pediatrics, fetal ultrasound findings have about 49% sensitivity for detecting congenital toxoplasmosis, meaning roughly half of affected pregnancies will show no visible abnormalities on imaging.
Symptoms That Signal Serious Infection
For people with weakened immune systems, toxoplasmosis looks very different from the mild version healthy adults experience. The parasite can reactivate from its dormant state and attack the brain, causing a condition called toxoplasmic encephalitis. Symptoms include fever, confusion, headache, seizures, nausea, and poor coordination. Brain imaging in these cases typically shows multiple lesions concentrated in deep brain structures and at the boundary between gray and white matter, often with surrounding swelling.
Toxoplasmosis can also affect the eyes. Ocular toxoplasmosis causes blurred or reduced vision, eye pain (particularly in bright light), redness, and tearing. An ophthalmologist examining the retina will look for characteristic patches of inflamed, whitish tissue, sometimes adjacent to older scarring from previous episodes. In severe cases, inflammation inside the eye can be dense enough to obscure the retinal view entirely. Immunocompromised patients may develop atypical eye involvement, including larger lesions, infection in both eyes, or inflammation without the usual scarring pattern.
When Testing Makes Sense
Routine screening for toxoplasmosis isn’t standard practice in the United States for most people. Testing is most useful in specific situations: if you’re pregnant or planning to become pregnant, if you have HIV or another condition that suppresses your immune system, if you’re experiencing unexplained swollen lymph nodes lasting more than a few weeks, or if you develop vision changes that your eye doctor can’t explain with a routine exam.
For otherwise healthy adults who suspect a past exposure, a simple IgG blood test can confirm whether you’ve ever been infected. A positive result with no symptoms means your body successfully controlled the infection and you now carry some degree of immunity. For most people, that’s all the information they need, and no treatment is required.