Toxoplasmosis
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p424-425
2025-11-25
12
Toxoplasmosis is a zoonosis caused by the intracellular parasitic protozoan Toxoplasma gondii. The cat is the definitive host of Toxoplasma, and humans, and other mammals and various birds, are intermediate hosts. The parasite multiplies in the intestine of felines and produces oocysts that are excreted with feces; accidentally, other animals (cattle or sheep) may ingest the oocysts and become infested. Therefore, the meat and viscera of these intermediate hosts, which contain Toxoplasma, can be ingested by other animals, where the cycle begins again. The disease can be contracted by coming into contact with oocysts contained in cat feces, raw or undercooked infected meat, vegetables, and fruit that have not been well washed. The acute infection is asymptomatic in 90% of cases, while the remaining 10% develop only mild lateral and retrocervical adenopathies. In Italy, 60% of the population is susceptible to infection. Maternal–fetal transmission occurs when the infection is contracted during pregnancy, that is, in a previously seronegative woman. The overall risk of transmission to the fetus and the clinical picture and severity of the congenital infection depend on the gestational age. In contrast, the risk of fetal infection increases with increasing gestational age because the placenta progressively thins out. Vice versa, the earlier the infection, the more serious the damage to the fetus. The major sequelae are chorioretinitis, blindness, hydrocephalus, neurological damage, and mental retardation.
Prenatal screening is based on a serological test aimed at establishing the immunological status of the woman through the detection of specific anti-toxoplasma IgG and IgM anti bodies. In this way, it will be possible to determine if the woman is seronegative and, therefore, susceptible to infection, if she had a previous or ongoing infection (Table 1).
Table1. Serological toxoplasmosis screening
IgM must be considered an alarm signal, but it is not a specific indicator of the acute phase of the infection. IgMs appear early, 10–12 days after infection, and reach a plateau after 2–4 weeks; in 72% of cases, they are still detectable after 9–10 months and can remain low titers for more than a year. Serum IgM does not necessarily indicate an ongoing or recent infection. However, it could be due to long-lasting IgM or nonspecific “natural” IgM, produced independently from the presence of Toxoplasma infection, recent or past, and often detectable in the first trimester of pregnancy. In order to confirm and date toxoplasmosis, level II investigations are performed, such as the IgA assay and the assessment of IgG avidity. IgA is generally produced after IgM and before IgG; initially, there is a rapid increase and subsequent decrease by 6–9 months, earlier than IgM; they may reappear after reactivation in the absence of IgM. The simultaneous presence of IgA and IgM allows a fairly reliable diagnosis of recent toxoplasmosis acquired in the last 6 months. The IgG avidity test is the most reliable method to date the infection. Avidity represents the strength of binding between an antibody and its antigen. The IgG avidity test, in general, is based on the principle that IgG undergoes an increase in antigen-binding affinity during the evolution of the immune response, which diagnostically translates into an antigen- antibody bond that is less sensitive to the action of a denaturing agent. Consequently, the closer the production of IgG in time, the lower the strength of antigen binding and the lower the avidity; this allows reliable dating of the infection. In other words, IgG with low avidity is usually present during the acute phase (Table 2).
Table2. Immunoglobulin G avidity test
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