What is Toxoplasmosis?
Also Known As:
Toxoplasma gondii, T. gondii
Toxoplasma gondii Antibodies, IgG, IgM, Toxoplasma gondii Molecular Detection by PCR
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.
Molecular (DNA) testing
Molecular testing may be performed to detect and measure T. gondii DNA in a blood, cerebrospinal fluid (CSF), or amniotic fluid sample.