Toxoplasma gondii is a protozoan parasite firstly described in 1909 by Charles Nicolle and Louis Manceaux. T. gondii is an intracellular parasite, which has a complex life cycle consisting of three stages: a) tachyzoite - this form of the parasite invades and replicates within cells during the acute stage of infection; b) bradyzoite - present in tissue cysts; and c) sporozoite - is found in environmentally resistant oocysts excreted by the members of the family Felidae (including domestic and feral cats), which are the only definitive hosts of T. gondii.
Incubation period of primary toxoplasmosis is 5–23 days. Central nervous system (CNS), retina and cardiac/skeletal muscles are the main and ultimate target organs for tachyzoites. Symptoms, if they arise, in otherwise healthy people include: fever, mild flu-like symptoms, enlarged lymph nodes in the head and neck, muscle pain, pneumonia, and disturbances of CNS. Tissue cysts in immunocompetent patients usually remain latent for life. In immunosuppressed individuals (patients with AIDS or lymphoproliferative disorders, organ transplant patients) the course of toxoplasmosis is less favourable, often resulting in meningoencephalitis. In this group of patients toxoplasmosis is mostly a result of reactivation of pre-existing latent T. gondii infections. Another group at risk are pregnant females. Damage to the unborn child is sometimes severe and pregnancy can result in miscarriage, stillbirth, or a child born with signs of toxoplasmosis. The classic triad of signs suggestive of congenital toxoplasmosis (CT) include chorioretinitis, intracranial calcifications, and hydrocephalus. Although most severe cases are diagnosed during the first month of life, severe disease can sometimes only become obvious in the second or third month of life. Visual impairment is the most common long-term sequelae, and can greatly impact the quality of life of congenitally infected children.
Women infected with T. gondii before conception, with rare exceptions, do not transmit the infection to their fetuses. Women infected with T. gondii less than 6 months before conception or after it can transmit the infection across the placenta to their fetuses. The overall risk of transmission of CT from mother acutely ill with toxoplasmosis to fetus could be as high as 50-60% in untreated cases. The frequency of vertical transmission increases with the gestational age. In contrast, more severe clinical signs in the infected infant are more commonly observed in offspring of women whose infection was acquired early in gestation.
Toxoplasma can be transmitted to humans via three principal routes: a) foodborne-by ingestion of raw or inadequately cooked infected meat; b)catborne- due to ingestion of oocysts that cats pass in their feces through exposure to cat litter or soil; and c) vertical-an infected pregnant woman passing the infection to her fetus. The infection leaves a long-lasting immunity except for cases of infection with more virulent strain.
It is estimated that more than a third of the world’s population has been infected with the parasite, but infection is unevenly distributed across countries, being mostly in the range of 10-80% .
The incidence of CT per 10,000 live births also varies significantly across countries. In the USA, it is estimated in one case per 10,000 live births. In France, it was reported in 2.9 per 10,000 live births. In certain countries, such as Brazil, rates as high as nine per 10,000 live births have been reported, while in other areas, such as the UK, CT was estimated in 0.33-1 per 10,000.
A well-orchestrated cellular, humoral and innate immune response must be triggered upon parasite invasion, in order to prevent the uncontrolled proliferation of tachyzoites. IgM are considered as the earliest immunoglobulines in acute infection, since they are produced during the first week after infection, and become undetectable after several months. IgA antibodies show kinetics similar to IgM. IgG can usually be detected 2–3 weeks after IgM, then decreases to reach a plateau in 2–3 months, leading to persistent detectability.
Acute toxoplasmosis in immunocompetent patients is rarely diagnosed by directly detecting the parasite in body fluids, tissue, or secretions; the clinical symptoms are also not specific enough. Therefore, the most common methods of diagnosis are based on antibody detection. A number of serological methods are used to detect antibodies to T. gondii (dye test, indirect immunofluorescence assays, agglutination assays, such as the immunosorbent agglutination assay (ISAGA), immunoenzyme assays). IgG and IgM ELISA are the methods most often used in non-specialized laboratories, whereas many other serological methods should be used by trained personnel of reference laboratories.
In immunosuppressed patients, antibody production can be affected. Therefore, serological tests may not be as efficient as in immunocompetent people. Hence, direct evidence of the parasites must be sought by PCR or parasite isolation.
Serological methods are mostly used in following clinical situations:
1) In pregnant women-to determine serological status before conception/in the first trimester of pregnancy
2) In pregnant seronegative women-to carry out systematic screening for possible CT during pregnancy
3) In newborns and infants-to diagnose possible CT (IgG at 12 months is a ‘gold standard’ for ultimate and definite CT diagnosis)
4) In organ donors and recipients-to determine serological status before transplantation
In the case of CT the early diagnosis and treatment has proved to be efficient. In countries where serological screening and prenatal treatment is systematically offered to pregnant women, such as France, the majority of the cases of congenital T. gondii infection do not have overt clinical disease during gestation or the neonatal period. In Austria, nearly all women who become pregnant are serologically screened early in pregnancy and, if found to be negative initially, are tested again during the second and third trimesters. Women with T. gondii infections are treated as soon as infection is detected. These measures helped reduce the incidence of CT from 50-70 cases per 10,000 births before the program to 1 per 10,000 births thereafter.
It needs to be emphasized that a positive IgM antibody test result does not necessarily mean a recently acquired infection. IgM antibodies may persist for 1 year following acute infection, and most positive IgM antibody test results ( 60%) are obtained in pregnant women with past infection. Therefore, IgM results should not be used by alone; the greatest value of a positive IgM antibody test result is that it raises the question of a recently acquired infection, thereby necessitating confirmatory testing, for example, IgG avidity test.
High-avidity IgG antibodies develop at least 12–16 weeks after infection. Therefore, the presence of high-avidity antibodies in the serum taken during the first trimester indicates that infection was acquired before pregnancy, and the risk for fetus is negligible. Low-avidity IgG results indicate the necessity for further tests, since the peculiarity of T. gondii infection is the persistence of low-avidity antibodies in some individuals for many months-year or more after the primary infection.
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