To detect a Toxoplasma gondii infection in a pregnant woman, unborn baby, or in a person with a weakened immune system (immunocompromised) who has flu-like symptoms; sometimes to determine if a person has been previously infected or to help determine if complications are due to an active Toxoplasma infection
When a pregnant woman or a person with a weakened immune system has been exposed to T. gondii and/or has flu-like symptoms or a severe eye or brain infection that a healthcare practitioner suspects are due to toxoplasmosis
Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. Diagnosis can be made by serologic testing or by molecular testing. Serologic testing detects antibodies in the blood that are produced in response to an infection and, depending on the type of antibodies present (IgG or IgM), a current or past infection can be determined. Molecular testing such as PCR detects the genetic material (DNA) of the parasite in the blood and indicates an acute infection.
T. gondii is a parasite that cannot be seen with the naked eye (microscopic). In most healthy humans, the infection either causes no symptoms or results in a mild flu-like illness. When a pregnant woman becomes infected, however, and passes it to her unborn child, or a person with a weakened immune system (immunocompromised) becomes infected, the parasite can cause severe complications.
T. gondii is widely distributed. It is found throughout the world and in some countries has infected up to 60% of the population. In the United States, the Centers for Disease Control and Prevention estimates that about 11% of those 6 years of age and older have had toxoplasmosis and that more than 40 million people carry it. The infection can be acquired by:
- Eating contaminated food, especially eating raw or undercooked meat, such as pork, lamb, and venison
- Eating unwashed fruits and vegetables that are grown in contaminated soil
- Having contact with contaminated soil
- Drinking contaminated water
- Drinking unpasteurized milk
- Handling contaminated cat litter
- Playing in contaminated sandboxes
- Transmission from mother-to-child
- Rarely, transmission during an organ transplantation or blood transfusion
The main hosts for T. gondii are wild and domestic cats. When cats become infected by eating infected birds, rodents, or contaminated raw meat, T. gondii replicates and forms eggs encased in protective coverings (oocysts). During an active infection, millions of microscopic eggs may be released for several weeks into the cat's stool. The eggs become infective within a couple of days and can remain viable for several months. In all other hosts, including humans, T. gondii only goes through a limited portion of its lifecycle and then forms inactive cysts in the muscles, brain, and eyes. The host's immune system keeps these cysts dormant and protects the body against further infection. The dormant stage can persist throughout the host's life, unless the immune system becomes compromised.
An initial or re-activated Toxoplasma gondii infection can cause significant symptoms and complications in people with weakened immune systems, such as those who have HIV/AIDS, are undergoing chemotherapy, have had a recent organ transplant, or are on immunosuppressant medications. It can affect the nervous system and eyes, causing headaches, seizures, confusion, fever, encephalitis, loss of coordination, and blurred vision.
When a woman becomes infected during pregnancy, there is a 30-40% chance that the infection will be passed to her unborn child. If this congenital infection occurs early in the pregnancy, it can cause miscarriages or stillbirths or can lead to severe complications in the newborn, including mental retardation, seizures, blindness, and an enlarged liver or spleen. Many infected babies, especially those exposed later in the pregnancy, will appear normal at birth but may develop symptoms, such as severe eye infections, hearing loss, and learning disabilities, years later.
How is the test used?
A toxoplasmosis test is used to detect a current or past infection with the microscopic parasite Toxoplasma gondii. Most often it may be performed for:
- A woman prior to or during a pregnancy to determine if she has been previously exposed to Toxoplasma gondii and during a pregnancy if exposure is suspected
- An individual who has a weakened immune system (immunocompromised) and has flu-like symptoms
- A person who has signs or symptoms of toxoplasmosis
- An unborn baby, in which case, amniotic fluid may be tested
There are several methods of testing for T. gondii. The choice of tests and samples collected depends on the person, their symptoms, and on the healthcare practitioner's clinical findings.
Serologic (Antibody) testing
When someone is exposed to T. gondii, their immune system responds by producing antibodies to the parasite. Two classes of Toxoplasma antibodies may be found in the blood: IgM and IgG.
IgM antibodies are the first to be produced by the body in response to a Toxoplasma infection. They are present in most individuals within a week or two after the initial exposure. IgM antibody production rises for a short time period and declines. Eventually, sometimes months after the initial infection, the level (titer) of IgM antibody falls below a detectable level in most people. Additional IgM may be produced when dormant T. gondii is reactivated and/or when a person has a chronic infection.
IgG antibodies are produced by the body several weeks after the initial infection and provide long-term protection. Levels of IgG rise during the active infection, then stabilize as the Toxoplasma infection resolves and the parasite becomes inactive. Once a person has been exposed to T. gondii, that person will have some measurable amount of IgG antibody in their blood for the rest of their life and is considered immune (protected) from re-infection.
Antibody testing may sometimes be performed as part of a TORCH panel. TORCH is an acronym for several infections that can affect an unborn child and typically includes testing for: toxoplasmosis, rubella, cytomegalovirus, and herpes simplex virus.
When is it ordered?
T. gondii antibody testing is not routinely ordered on pregnant women but may be ordered when a healthcare practitioner wants to determine if a woman has previously had toxoplasmosis and when she has been exposed to cat feces (stool), raw or undercooked meat, or contaminated food or water. When the antibody test is negative but suspicion is still high, the test may be repeated one or more times during the pregnancy.
Antibody testing may be ordered for a person with a weakened immune system (immunocompromised) when the person has symptoms of a flu-like illness and for anyone who has symptoms or complications that suggest toxoplasmosis. Signs and symptoms of toxoplasmosis may include:
- Swollen lymph nodes
- Fever, night sweats
- Weakness, fatigue
- Body aches
- Sometimes a sore throat
A more serious infection that affects the central nervous system, including the brain, may cause visual and mental changes or seizures, for example.
Molecular (DNA) testing may be performed when an acute toxoplasmosis infection is suspected in an immunocompromised person or to determine if an unborn baby is infected. Amniotic fluid may be tested during pregnancy when a mother has evidence of an acute infection as indicated by a positive Toxoplasma IgM antibody test result.
What does the test result mean?
Care must be taken when interpreting the results of toxoplasmosis testing. An immunocompromised person may not have a strong antibody response to the T. gondii infection – their IgM and IgG levels may be lower than expected even though they have an active case of toxoplasmosis.
IgM IgG Possible Interpretation Negative Positive Past infection Negative Negative No infection or very early infection; no previous exposure Positive Negative Early infection; in a newborn, indicates congenital infection Positive Positive Current infection; chronic infection; could indicate re-activation; IgM may be positive for several months after the infection resolves
False positives can occur. In most cases, positive IgM antibody tests should be confirmed by another test method, typically by sending the sample to a reference laboratory that specializes in toxoplasmosis testing.
IgM antibodies are the only class produced by an unborn baby. When Toxoplasma IgM antibodies are present in a newborn, they indicate a congenital infection.
Molecular (DNA) testing
If the molecular test is positive for T. gondii DNA, then the person tested has an active toxoplasmosis infection. A negative test result means that it is less likely that the person has toxoplasmosis but does not rule out infection – Toxoplasma may not be present in sufficient numbers in the blood or fluid sample to be detected.
Is there anything else I should know?
Is toxoplasmosis contagious?
Should everyone be tested for toxoplasmosis?
Once I have had a Toxoplasma gondii infection, can I get infected again?
In general, once you have been infected and the primary infection becomes dormant, you have immunity against re-infection from an outside source if you remain healthy. However, if your immune system is weakened for some reason, a dormant infection may become reactivated. It is thought that most infections in people with weakened immune systems, such as those with HIV/AIDS, are due to reactivations rather than new infections.
Why is the T. gondii infection rate higher in some countries?
Is toxoplasmosis treatable?
Most healthy people do not require treatment, but there are treatments available for those with compromised immune systems, for pregnant women to help lessen the risk that the infection will be passed to the fetus, and for newborns with congenital toxoplasmosis. For more on treatment, see the Mayo Clinic or CDC web sites.
What steps should a pregnant woman (or anyone else) take to prevent toxoplasmosis infection?
Actions that can be taken include:
- Have someone else clean the cat's litter box. If this is not possible, then wear gloves and change the litter box every day to prevent any eggs from becoming infective.
- If possible, keep your cat indoors to prevent it from hunting or possibly eating contaminated soil, and do not feed it raw meat.
- Don't get a new cat or handle stray cats during pregnancy.
- Cover a child's sandbox when not in use (to prevent it from being used as a litter box).
- Wear gloves when gardening.
- Don't eat raw or undercooked meat.
- Wash cutting boards, hands, and any utensils used to prepare raw meat in hot soapy water and avoid cross-contamination.
- Don't drink unpasteurized milk.
- Wash and/or peel fruits and vegetables.
Besides antibody and molecular tests, are there other ways of testing for toxoplasmosis?
Other tests, such as an IgG avidity test, may be performed by a reference laboratory to help confirm a T. gondii infection. Rarely, a sample of tissue may be removed (biopsy) from an area of the body that is suspected of being infected with the Toxoplasma parasite. The sample may be cultured and/or stained and examined under a microscope to detect the parasite. These procedures are usually reserved for cases that are difficult to diagnose.