Toxoplasmosis: determination of IgM and IgG antibodies to toxoplasm in blood

IgM antibodies to toxoplasma are normally absent in the blood serum.

Toxoplasmosis is a disease caused by the obligate intracellular protozoan Toxoplasma gondii, which has a complex development cycle. The final host of toxoplasma can be a domestic cat, as well as wild representatives of the cat family. When a cat is infected by the alimentary route, the parasites penetrate the epithelial cells of the intestine, where, after several asexual generations, macro- and microgametes are formed. The sexual process ends with the formation of oocysts, which are excreted into the external environment. Humans are intermediate hosts of the parasite, but do not excrete the pathogen into the external environment and do not pose an epidemic danger to others. In the human body, toxoplasmas reproduce only asexually and go through two stages of development:

• endozoite - a rapidly multiplying intracellular form that causes cell destruction and an inflammatory reaction; the presence of endozoites is characteristic of the acute stage of toxoplasmosis;
• Cysts are a spherical form of the parasite, surrounded by a dense shell and adapted to long-term existence in the human body; they are localized in the brain, retina, muscles and do not cause an inflammatory reaction; the presence of cysts is characteristic of the chronic stage of toxoplasmosis; cysts continue to grow slowly, their rupture and destruction leads to a relapse of organ damage.

The main route of infection with toxoplasmosis is oral (eating raw meat, vegetables and berries contaminated with soil, through dirty hands when in contact with cats). However, for clinical practice, the congenital route of infection is no less important - intrauterine infection of the fetus from a pregnant woman through the placenta. Fetal infection has been proven only from women with a primary infection acquired during this pregnancy. When a woman is infected in the first trimester of pregnancy, congenital toxoplasmosis in a child is recorded in 15-20% of cases, it is severe. When infected in the third trimester of pregnancy, 65% of newborns are infected. In women with chronic or latent toxoplasmosis, transmission of the pathogen to the fetus has not been proven.

It is necessary to distinguish between toxoplasma infection (carriage) and toxoplasmosis itself (disease), therefore the main thing in laboratory diagnostics is not the fact of detection of a positive immune response (antibodies), but the clarification of the nature of the process - carriage or disease. Complex determination of IgM and IgG antibodies makes it possible to quickly confirm or refute the diagnosis. The main method at present is ELISA, which allows detection of IgM and IgG antibodies.

IgM antibodies to toxoplasma appear in the acute period of infection (in the first week in a titer of 1:10), reach a peak within a month (in the 2nd-3rd week after infection) and disappear after 2-3 months (at the earliest - after 1 month). They are detected in 75% of congenitally infected newborns and in 97% of infected adults. Negative results of IgM antibody determination allow to exclude acute infection lasting less than 3 weeks, but do not exclude infection of a longer period. In case of reinfection, the IgM antibody titer increases again (in the presence of immunodeficiency it does not increase, in such cases, computed tomography or magnetic resonance imaging of the brain is indicated for diagnosis, revealing multiple dense round foci). The presence of rheumatoid factor and/or antinuclear antibodies in the blood of patients can lead to false-positive test results. In individuals with immunodeficiency, IgM antibodies are usually absent during the acute period of infection.

Early diagnosis of toxoplasmosis is especially important for pregnant women due to the risk of intrauterine infection of the fetus, which can lead to fetal death (spontaneous abortion) or the birth of a child with serious lesions. Specific treatment of women in the early stages of the infectious process reduces the risk of fetal damage by 60%. Since IgM antibodies do not penetrate the placenta, their detection in the blood of a newborn indicates a congenital infection.

IgG antibodies to toxoplasma appear during the convalescence period and persist in those who have recovered for up to 10 years. Determination of IgG antibodies is used to diagnose the convalescence period of toxoplasmosis and to assess the intensity of post-vaccination immunity. False positive test results can be obtained in patients with systemic lupus erythematosus and rheumatoid arthritis.

Persons with positive antibody titers for toxoplasmosis are recommended to undergo repeated serological tests in 10-14 days to establish the dynamics of the disease development. The absence of an increase in antibody titers indicates chronic toxoplasmosis. An increase in titers by 3-4 serum dilutions indicates an active course of the invasion.

Indications for serological testing for toxoplasmosis:

• pregnant women according to indications, with seroconversion;
• patients with toxoplasmosis receiving specific treatment;
• children born to mothers with a history of toxoplasmosis;
• Epidemiologically significant contingents: veterinarians and other specialists involved in working with cats and dogs;
• patients with clinical manifestations characteristic of toxoplasmosis.