What is being tested?
Toxoplasmosis testing is performed in order to detect and diagnose a current or past Toxoplasma gondii infection. T. gondii is a microscopic parasite that infects cats and can be found in the soil. 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 becomes infected, the microorganism can cause severe complications.
T. gondii infection is relatively common and the vast majority of people do not have symptoms when they acquire it. In Australia, 20-40 per cent of women of child bearing age have evidence of previous infection. The infection can be acquired by ingesting contaminated food or water, especially eating raw or undercooked meat, by handling contaminated cat litter, by transmission from mother-to-child, and rarely may be transmitted during an organ transplantation or blood transfusion.
A definitive host is an animal in which all stages of a parasites lifecycle occur. 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 those 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 per cent chance that the infection will be passed to the 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 learning difficulties, 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. In Australia, however, the risk of acquiring toxoplasmosis whilst pregnant is extremely low.
How is it used?
There are several methods of testing for T. gondii. The choice of tests and samples collected depends on the patient, their symptoms, and on the doctor's clinical findings.
When someone is exposed to T. gondii, their immune system responds by producing antibodies to the parasite. Two types 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 (titre) of Toxoplasma IgM antibody falls below detectable levels in most people. Additional IgM may be produced when dormant T. gondii is reactivated and/or when a person has a chronic infection. IgM antibodies are the only type produced by the fetus. When they are present in a newborn, they indicate a congenital infection.
IgG antibodies are produced by the body several weeks after the initial infection to provide long-term protection. Levels of IgG rise during the active infection, then stabilise as the Toxoplasma infection resolves and the parasite becomes inactive. Once a person has been exposed to T. gondii, they will have some measurable amount of IgG antibody in their blood for the rest of their life. T. gondii IgG antibody testing can be used, along with IgM testing, to help confirm the presence of a recent or previous Toxoplasma infection.
Molecular testing may be performed to detect and measure T. gondii DNA in a blood, CSF, vitreal, or amniotic fluid sample
When is it requested?
T. gondii antibody testing may be ordered when investigating a patient who has signs and symptoms of glandular fever but no evidence of glandular fever on routine blood tests. It is also routinely ordered in patients with advanced Humman Immunodeficiency Virus (HIV) and in those people who are to undergo liver, lung, heart or kidney transplantation. Routine screening of pregnant women to detect infection whilst pregnant is not recommended as the risk of acquisition of Toxoplasma whilst pregnant in low.
Molecular testing on body fluids may be performed when a toxoplasmosis infection is suspected in an immunocompromised person or to determine if a foetus has been infected following confirmation of acute maternal acquisition of toxoplasma.
What does the 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.
|Negative||Negative||No infection or very early infection; no previous exposure|
|Positive||Negative||Early infection; in a newborn, indicates congenital infection|
|Positive||Positive||Current infections; chronic infection; could indicate re-activation; IgM may be positive for several months after the infections 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 specialises in toxoplasmosis testing.
If the molecular test is positive for T. gondii DNA, then the person tested has an active case of 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 but can also come from unwashed fruits and vegetables that are grown in contaminated soil, from the soil itself, from contaminated water, and from unpasteurised 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.
Ocular toxoplasmosis (image): severe, reactive retinochoroiditis
Most healthy people do not require treatment, but there are treatments available for those with compromised immune systems, chorioretinitis, for pregnant women to help lessen the risk that the infection will be passed to the foetus, and for newborns with congenital toxoplasmosis.
Actions that can be taken include:
It is not directly passed from person to person, except from mother-to-child. Almost all cases of infections come from eating, drinking, or handling something that is contaminated.
No. In the vast majority of circumstances infection is asymptomatic and no treatment is required.
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