Dr Leigh Martens

General Practitioner

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Blood-tests Explained


Disclaimer:

This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion

Toxoplasmosis


At a Glance

Why Get Tested?

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 to Get Tested?

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 health practitioner suspects are due to toxoplasmosis

Sample Required?

A blood sample drawn from a vein in your arm; sometimes a cerebrospinal fluid (CSF) or amniotic fluid sample

Test Preparation Needed?

None

The Test Sample

What is being tested?

Toxoplasmosisis an infection caused by the parasite Toxoplasma gondii. Testing detects antibodies in the blood that are produced in response to an infection or detects the genetic material (DNA) of the parasite in the blood. Testing is performed in order to detect and diagnose a current or past 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 very common. It is found throughout the world and in some countries has infected up to 95% of the population. In the United States, the Centers for Disease Control and Prevention estimates that about 23% of those 12 years of age and older have had toxoplasmosis. The infection can be acquired by ingesting contaminated food or water, especially eating raw or undercooked meat, by handling contaminated cat litter or playing in contaminated sandboxes, by transmission from mother-to-child, and rarely may be transmitted during an organ transplantation or blood transfusion.

The definitive host for T. gondii is 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 oocysts may be released for several weeks into the cat's stool. The oocysts 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 sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm. Rarely, a cerebrospinal fluid (CSF) sample is collected from the lower back using a procedure called a lumbar puncture or spinal tap, or a sample of amniotic fluid is collected from a pregnant woman using amniocentesis.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it 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.

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.

When is it ordered?

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.

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.

Antibody detection

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.

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.

Is there anything else I should know?

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.