Toxoplasmosis

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.

T. gondii is a microscopic parasite that infects cats and can be found in the soil. The infection can be acquired by ingesting contaminated food or water, by handling contaminated cat litter, or by transmission from mother-to-child. In most healthy people, toxoplasmosis either causes no symptoms or results in a mild flu-like illness. A person's immune system keeps the infection dormant and protects the body against further infection, unless the immune system becomes weakened, in which case the infection can re-activate.

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 health practitioner's clinical findings.

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.

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 detection
Molecular testing may be performed to detect and measure T. gondii DNA in a blood, cerebrospinal fluid (CSF), or amniotic fluid sample.

T. gondii antibody testing is not routinely ordered on pregnant women but may be ordered when a health 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
• Headache
• 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 testing may be performed when a toxoplasmosis infection is suspected in an immunocompromised person or to determine if an unborn baby has been 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.

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.

Antibody detection

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.

DNA detection
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.

Exposure to Toxoplasma gondii comes primarily from eating undercooked or raw meat, especially pork, lamb, and venison, but can also come from unwashed fruits and vegetables that are grown in contaminated soil, from the soil itself, from contaminated water, and from unpasteurized milk.

T. gondii infection is the most common cause of intraocular inflammation in the world. In those with complications, it can scar the retina and cause severe eye infections.

Other tests, such as an IgG avidity test, may be performed by a reference laboratory to help confirm a T. gondii infection. Tests such as a tissue culture or biopsy stain are rarely done.