What is the principle of Toxoplasma testing?

Toxoplasma gondii is a parasite that lives inside cells. It follows the flow of blood to every part of the body. It can reduce a person's immunity and cause various diseases. Routine examinations for Toxoplasma gondii are generally divided into pathological examinations, immunological examinations, intradermal tests and other examination methods.

Pathogen examination

1. Direct microscopic examination: Take smears of the patient's blood, bone marrow or cerebrospinal fluid, pleural effusion, sputum, bronchoalveolar lavage fluid, aqueous humor, amniotic fluid, etc., or biopsy of lymph nodes, muscles, liver, placenta, etc., and perform Wright or Gibbs staining microscopy to find trophozoites or cysts, but the positive rate is not high. Direct immunofluorescence can also be used to detect Toxoplasma gondii in tissues.

2. Animal inoculation or tissue culture: Take the body fluid or tissue suspension to be tested and inoculate it into the abdominal cavity of mice. Infection can occur and pathogens can be found. When the first generation inoculation is negative, it should be blindly subcultured three times. Or use tissue (monkey kidney or pig kidney cells) culture to isolate and identify Toxoplasma.

3. DNA hybridization technology: Domestic scholars used 32P-labeled probes containing Toxoplasma-specific DNA sequences for the first time to perform molecular hybridization with the cells or tissue DNA in the patient's peripheral blood. Specific hybridization bands or spots were detected as positive reactions. Both specificity and sensitivity are high. In addition, the polymerase chain reaction method for diagnosing this disease has been established in China, and compared with probe hybridization, animal inoculation and immunological examination methods, it has been shown to have the advantages of high specificity, sensitivity and rapidity. [4]

Immunological tests

1. The antigens used to detect antibodies mainly include tachyzoite soluble antigens (cytoplasmic antigens) and cell membrane antigens. The antibodies of the former appear earlier (detected by staining test, indirect immunofluorescence test), while the antibodies of the latter appear later (detected by indirect hemagglutination test, etc.). Using multiple detection methods at the same time can complement each other and improve the detection rate. Because Toxoplasma can exist in human cells for a long time, it is generally difficult to distinguish current infection from past infection by detecting antibodies. It can be judged based on the level of antibody titer and its kinetic changes.

2. Antigen detection uses immunological methods to detect pathogens (tachyzoites or cysts) in host cells and their metabolic or cleavage products (circulating antigens) in serum and body fluids. It is a reliable method for early diagnosis and confirmation. Scholars at home and abroad have established McAb-ELISA and McAb and polyclonal antibody sandwich ELISA methods to detect circulating antigens in the serum of acute patients, with a sensitivity of 0.4 μg/ml of antigens in the serum. [4]

Intradermal test

The peritoneal fluid of infected mice or chicken embryo fluid is used as the antigen. Delayed, tuberculin reactions are common. Can be used for epidemiological surveys. There are not many applications at present.