Toxoplasmosis is a common infection caused by a parasite called Toxoplasma gondii. A person may become infected by eating undercooked lamb or pork or by handling an infected cat or its stool (feces). However, most people who own cats have developed immunity to toxoplasmosis.

After the initial infection, the toxoplasmosis parasite usually remains in an inactive form in the person's body for the remainder of his or her life. The infection usually does not cause any symptoms in a healthy person. Occasionally, a person may experience muscle aches or swollen lymph glands that last for a few days to several weeks.

A woman who becomes infected with Toxoplasma gondii during pregnancy may pass the infection to her fetus. Infection during pregnancy can cause miscarriage, stillbirth, or severe birth defects in the fetus. Most infants who become infected during pregnancy have mild symptoms, such as a rash or an eye infection. Rarely, severe defects (such as hydrocephalus, mental retardation, and anemia) develop. Many infants exposed during pregnancy have no symptoms at all.

Toxoplasmosis may also cause problems in people with severely weakened immune systems (such as a person who has AIDS or is receiving cancer chemotherapy).

Antibodies are produced by the immune system to fight a toxoplasmosis infection. The test for toxoplasmosis detects antibodies to the parasite in a sample of blood. The toxoplasmosis antibodies usually form within two weeks of infection and persist for life. The amount and type of antibodies in a blood sample may indicate whether a person was infected recently or in the past.

This test is usually done on a blood sample taken from a vein. More than one blood sample may be taken over a period of several weeks. This test can also be done on blood from the umbilical cord to determine whether a fetus or newborn baby is infected. The test may also be done on a sample of amniotic fluid collected during amniocentesis.

Toxoplasmosis Test
Results

The results of a toxoplasmosis test that detects antibodies are usually given in titers. A titer is a measure of how much the blood sample can be diluted before the antibodies against the toxoplasmosis parasite can no longer be detected.

A titer of 1 to 8 (1:8) means that antibodies can be detected when 1 part of the blood sample is diluted by up to 8 parts of a salt solution (saline). A larger second number means there are more antibodies in the blood. Therefore, a titer of 1 to 128 indicates more toxoplasmosis antibodies in the blood than a titer of 1 to 32.

Toxoplasmosis antibodies usually form within 2 weeks after a person becomes infected. They reach their highest levels 1 to 2 months after infection.

• When a person first becomes infected with an organism, the immune system usually forms the IgM type of antibody. Later, after infection has occurred, the immune system makes the IgG type of antibody. Therefore, it is sometimes possible to know if an infection is new by finding out what type of antibody is present in the bloodstream.
• Several blood samples may be taken over a period of several weeks to see whether the numbers of antibodies are rising. Increasing titers mean a person has been recently infected with the parasite that causes toxoplasmosis.
• Low levels of titers that do not increase usually mean a person has been infected with toxoplasmosis in the past and is now immune from getting the disease again.