The Food-Borne Risk of Toxoplasmosis

Toxoplasma gondii is an intracellular, parasitic protozoan found in cats, which causes toxoplasmosis. T. gondii is found almost everywhere worldwide, to varying degrees, and can infect many different types of animals, including humans.

T. gondii often appears on our radar in relation to a pregnancy, during which women are warned not to clear out cat litter trays or eat uncooked meats. Toxoplasmosis during pregnancy can have tragic consequences including stillbirth, premature delivery, physical deformities, neurological damage and long-term visual impairment. Gestational age at the time of maternal infection influences greatly mother-to-fetus transmission rates (transmission during late gestational ages are associated with >50% transmission rates) and probability of severe disease in the fetus/newborn (transmission during early gestational ages are associated with and increased likelihood of severe disease).

What a lot of people are unaware of is that toxoplasmosis is considered to be a leading cause of death attributed to food-borne illness in the United States (1). Although it may appear relatively easy to protect yourself from toxoplasmosis, T. gondii can be transmitted not only through contaminated cat litter and undercooked meat, but also by untreated drinking water, cured or dry meat, contaminated vegetables, gardening in areas where infected cats have been spreading oocysts in soil and plants, raw shellfish (e.g. oysters, mussels and clams). Awareness of transmission routes is essential, especially for those most at risk including pregnant women and immunocompromised individuals. However, it must be taken into account that up to 50% of patients cannot identify the specific risk factor that led them to become infected. For this reason, each pregnant women and immunocompromised individual should be tested for Toxoplasma IgG and IgM, regardless of the presence or not of risk factors for T. gondii infection, in order to determine their risk for toxoplasmosis.

Undercooked meat
Parasitic cysts can be transmitted in a variety of food including undercooked meat (particularly venison, lamb and pork), and shellfish such as raw oysters; the accidental ingestion of undercooked contaminated meat by handling it and not washing hands; and eating food contaminated by kitchen utensils that have been in contact with raw, contaminated meat.

Non-food-borne related transmission can occur by zoonosis, from from cats to humans; transmission can also occur from a mother to her child when the mother acquires the parasite for the first time during gestation.

Both molecular and serologic methods are useful in the diagnosis of T. gondii infection. Polymerase chain reaction (PCR) can be used to detect T. gondii DNA in body fluids and tissues PCR testing of amniotic fluid has been extremely useful in rapidly identifying fetal infection and instituting early anti-toxoplasma treatment. Introduction of amniotic fluid PCR for earlier diagnosis and treatment of the fetus has been associated with lower rates of clinical manifestations in infected children. Although there is a potential for false negative test results when the amniotic fluid test is performed too early (before 18 weeks of gestation) or in women infected late in gestation, positive PCR test results performed in reference laboratories always mean that the fetus is infected (100% specificity and 100% negative predictive value).

Antibody investigation
Antibody serology is an effective way of identifying infection. In adults, Toxoplasma IgM, IgE, IgA become detectable within 7-15 days following infection; Toxoplasma IgG is usually detected after 15 days following infection. Since establishing the gestational age is critical to determine risk of mother-to-fetus transmission rates and likelihood of severe disease, results of antibody investigation are used by experienced investigators as an indication of when the infection might have occurred.

In countries (such as France, Austria, Uruguay) where each mother is tested for Toxoplasmosis early in gestation, those who are found to be negative IgG and negative IgM are tested serially during pregnancy in order to diagnose primary infection when negative IgG/IgM test results become positive (seroconversion). These women are immediately started on treatment and become eligible for amniotic fluid PCR to investigate whether the fetus was infected as well.

In countries where antibody serologies are not performed routinely during pregnancy, only a single serum sample is available for testing Toxoplasma IgG and IgM. In these patients positive Toxoplasma IgM test results should be followed by an additional battery of confirmatory tests that can establish whether the positive IgM test result is indicative of an acute infection acquired during pregnancy (risk of transmission or the fetus) or chronic infection acquired before pregnancy (no risk of transmission to the fetus unless mother is immunocompromised).

Improved methods
Serology methods that can be more specific and sensitive for the diagnosis of toxoplasmosis include the complex Sabin-Feldman Dye Test, which is considered to be the ‘gold standard’ for Toxoplasma IgG detection, other methods include double-sandwich ELISA capture method (IgM, IgA, IgE), immunosorbent agglutination assay (IgM-ISAGA), differential IgG agglutination (AC/HS) and IgG avidity. These additional tests are available in reference laboratories (e.g. in the United States, the Palo Alto Medical Foundation Toxoplasmosis Serology Laboratory) and are used in various combinations according to each clinical scenario.

Patients are usually tested for Toxoplasma-specific IgG and IgM antibodies initially. If negative IgG/IgM, patient has never been infected with T. gondii and should made every effort possible not to become infected during pregnancy. In some countries (such as France, Austria, Uruguay and some obstetric practices in the United States and other countries), negative IgG/IgM pregnant women are serially tested during pregnancy in order to diagnose and treat early those who acquired toxoplasma infection during gestation since most women do not exhibit symptoms when infected and are not aware of how they got infected. If the result is positive IgG but negative IgM early in gestation, this gives an indication that the patient has been infected with the parasite at some time in his or her life and that the infection was acquired before gestation; the risk of transmission to the fetus is essentially zero unless the woman is immunocompromised. If the IgG and IgM are positive, an additional battery of tests (avidity, AC/HS, IgA, and/or IgE) is required in order to determine when the infection was acquired and the risk of transmission to the fetus and likelihood of clinical manifestations in the fetus and child. Determining the time of infection is also relevant in immunocompromised individuals, or in cases where Ocular Toxoplasmosis is suspected.

Screening tests
Many laboratories in the US are capable of carrying out tests for T. gondii. This test might be included in a TORCH screen (TORCH is an acronym for a group of tests which can cause significant birth defects (Toxoplasmosis, Other (Special ID), Rubella, Cytomegalovirus (CMV), Herpes simplex virus (HSV)). Commercially available TORCH screens include Siemens TorCH Assay (automated EIA for determination of IgM or IgG antibodies), Bio-Rad ToRCH Kit (manual EIA for determination of IgM or IgG antibodies). Randox also runs a global RIQAS (Randox International Quality Assessment Scheme) ToRCH programme.

While screening tests can be carried out in laboratories across the US, confirmation and diagnosis of T. gondii is usually carried out in specialist serology labs such as the Toxoplasma Serology Laboratory of the Palo Alto Medical Foundation Research Institute (TSL-PAMFRI), which offers confirmatory serological and PCR testing and methods for isolation of the organism. Labs like this are necessary because toxoplasma serology tests are extremely complex to decipher, and rapid, accurate interpretation is necessary in certain situations.

The TSL-PAMFRI is a not-for-profit institution that is solely dedicated to the laboratory diagnosis of T. gondii. The laboratory offers an array of diagnostic tests, almost all of which have been developed in-house and are therefore not available commercially.

This specialist serology laboratory has a wealth of knowledge on toxoplasma and toxoplasmosis and offers laboratories a full serology work-up for patients identified to be at risk. The tests included in the work-up depend on the clinical profile of the patient; certain patient profiles will indicate specific panels of tests. Most of the labs clients are hospital labs, physicians and other large reference laboratories. Samples are received from across the US, as well as other countries around the world.

Unique tests
The tests carried out in the laboratory are all developed in-house; the only commercially available system and kit in use is the Biomerieux VIDAS used for determining IgG avidity. The other tests have been developed and perfected over years of experience. Eventually, the laboratory hopes to consolidate all of its available tests into one single T. gondii test.

Dr Montoya, Director of the National Reference Laboratory for the Diagnosis and Management of Toxoplasmosis in the United States at the Palo Alto Medical Foundation Research Institute explained that early diagnosis is vital in successfully dealing with acute cases. The laboratory is working with hospitals to ensure that samples are sent for analysis as quickly as possible. Dr Montoya highlights that some further education is required in helping doctors to identify patients at risk of toxoplasmosis. Many hospitals assess individual patient risk by asking if a patient has had contact with cats, or if they have eaten raw meat. In fact 50% of the toxoplasmosis cases diagnosed by the TSL-PAMFRI serology laboratory are not associated with these two risk factors. In hospitals all around the world, individuals die as a result of toxoplasmosis, before this disease has even been suspected.

Prenatal screening
Another emotive area of concern is that of prenatal testing. There is no mandatory screening programme in place for pregnant women in the US or in the UK (a mandatory testing programme is in place in a minority of other countries including France, Austria, Portugal, Uruguay and Italy). Dr Montoya is concerned that far too many babies are born with severe toxoplasmosis, and probably more than are officially accounted for. These babies may suffer from fetal death, hydrocephalus, intracranial calcifications and chorioretinitis. They might also present at birth with hepatosplenomegaly, thrombocytopenia, convulsions and fever. Neonates are often asymptomatic at birth, with symptoms such as deafness, mental retardation, visual impairment and learning difficultieswhich become apparent later in life. Despite these risks, mandatory screening is not in place because it is deemed to be too costly. A recent US study* concluded that systematic screening for toxoplasmosis in the United States could be made cost-effective assuming less expensive screening tests are developed and specialized confirmatory tests are performed as currently indicated by the PAMF-TSL lab. Dr Montoya and the TSL-PAMFRI continue to campaign for the introductory screening of toxoplasma of all pregnant women.

While many of us have heard of toxoplasmosis, the food-borne risk of this disease is not widely appreciated. Toxoplasmosis can be contracted by infection with T. gondii directly through the handling of cat litter, but it can also be transmitted indirectly by eating raw meat (especially lamb, venison and pork), raw oysters and seafood that contain oocysts originating from infected cats; parasites can also be accidentally ingested after handling infected raw meat and not washing hands, or by eating food contaminated with knives and kitchen utensils which have been in contact with infected meat. Compulsory screening of T. gondii in all pregnant women would help to identify those who are at risk of this disease, which is so devastating to their unborn children. As yet, the cost of such a screening programme apparently continues to make it a prohibitive measure for many countries.

1 http://www.cdc.gov/parasites/toxoplasmosis/

*Stillwaggon E, Carrier CS, Sautter M, McLeod R. Maternal serologic screening to prevent congenital toxoplasmosis: a decision-analytic economic model. PLoS neglected tropical diseases 2011; 5(9): e1333.