Diagnosis

Most pregnant women in the United States aren't routinely screened for toxoplasmosis, and most states don't screen infants for the infection. Without specific screening, toxoplasmosis is often difficult to diagnose because signs and symptoms, when they occur, are similar to those of more common illnesses, such as the flu and mononucleosis.

Testing during pregnancy

If your doctor suspects you have the infection, you may have blood tests that check for antibodies to the parasite. Antibodies are proteins produced by your immune system in response to the presence of foreign substances, such as parasites. Because these antibody tests can be difficult to interpret, the Centers for Disease Control and Prevention recommends that all positive results be confirmed by a laboratory that specializes in diagnosing toxoplasmosis.

What test results mean

Sometimes you may be tested early in the course of the disease before your body has a chance to produce antibodies. In that case, you may have a negative result, even though you're infected. To be certain, your doctor may recommend retesting several weeks later.

In most cases, a negative toxoplasmosis test result means you've never been infected and therefore aren't immune to the disease. If you're at high risk, you can take certain precautions so that you don't become infected.

A positive result could mean that you have an active infection, or it could mean that you were once infected and are immune to the disease. Additional tests can pinpoint when the infection occurred, based on the types of antibodies in your blood. This is especially important if you're pregnant or you have HIV/AIDS.

Testing your baby

If you are pregnant and have a current toxoplasmosis infection, the next step is to determine whether your baby also is infected. Tests your doctor may recommend include:

• Amniocentesis. In this procedure, which may be done safely after 15 weeks of pregnancy, your doctor uses a fine needle to remove a small amount of fluid from the fluid-filled sac that surrounds the fetus (amniotic sac). Tests are then performed on the fluid to check for evidence of toxoplasmosis. Amniocentesis carries a slight risk of miscarriage and minor complications, such as cramping, leaking fluid or irritation where the needle was inserted.
• Ultrasound scan. This test uses sound waves to produce images of your baby in the womb. A detailed ultrasound can't diagnose toxoplasmosis. It can however, show whether your baby has certain signs, such as fluid buildup in the brain (hydrocephalus). However, a negative ultrasound doesn't rule out the possibility of infection. For that reason, your newborn will need an examination and follow-up blood tests during the first year of life.

Testing in severe cases

If you've developed a life-threatening illness such as encephalitis, you may need one or more imaging tests to check for lesions or cysts in your brain. These include:

• Magnetic resonance imaging (MRI). This test uses a magnetic field and radio (electromagnetic) waves to create cross-sectional images of your head and brain. During the procedure, you lie inside a large, doughnut-shaped machine that contains a magnet surrounded by coils that send and receive radio waves.

  In response to the radio waves, your body produces faint signals that are picked up by the coils and processed into images by a computer. MRI is noninvasive and poses minimal risks to your health.

• Brain biopsy. In rare cases, especially if you don't respond to treatment, a neurosurgeon may take a small sample of brain tissue. The sample is then analyzed in a laboratory to check for toxoplasmosis cysts.