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What causes toxoplasmosis?
Last reviewed: 06.07.2025
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Causes of toxoplasmosis
The causative agent of the disease, Toxoplasma gondii, belongs to the class of sporozoans, the order of coccidia, the genus of toxoplasma - obligate intracellular parasites.
Toxoplasma resembles an orange slice or a crescent in shape. They are curved, one end is pointed, the other is more round, measuring (4-7) x (2-5) µm. When stained according to Romanovsky-Giemsa, the cytoplasm of the parasite is stained blue, and the nucleus is ruby-red.
Toxoplasmas are intracellular parasites (endozoites) capable of reproducing asexually (schizogony) in the cells of various tissues (liver, placenta, central nervous system, etc.) of many species of warm-blooded animals, including humans.
During the process of reproduction, clusters of toxoplasms are formed inside the cells, which are called pseudocysts at this stage of development, since, unlike cysts, they do not have their own membrane. When the disease becomes chronic, true cysts (cystozoites or bradyzoites) are formed from pseudocysts.
The sexual reproduction cycle of toxoplasma occurs in the intestinal epithelium of the definitive host, which is the domestic cat and some other members of the feline family.
Pathogenesis of toxoplasmosis
From the entry gate (gastrointestinal tract), toxoplasmas enter the regional lymph nodes with the lymph flow, where they multiply, causing inflammatory changes with the formation of an infectious granuloma. Clinically, this can be manifested by mesadenitis. Having reached a certain concentration, the parasites penetrate the blood in large quantities and spread throughout the body, affecting the liver, spleen, nervous system, myocardium, skeletal muscles, eye membranes and other organs and tissues. Active proliferation of toxoplasmas is accompanied by the release of various toxins and allergens, leading to delayed-type hypersensitivity. As immunity develops, the reproduction of toxoplasmas slows down, eventually vegetative forms (endocytes) disappear from the blood and internal organs and cysts begin to form, which can persist in the body throughout life.
In most cases (95-99%), toxoplasma infection does not lead to the development of manifest forms of the disease, but a latent infection with delayed-type hypersensitization to toxoplasmin and the production of humoral antibodies is immediately formed. Clinically, such forms do not manifest themselves in any way, the person remains healthy, although in the initial period of the latent disease, toxoplasma dissemination occurs throughout the body. If this period coincides with pregnancy, there is a high probability of infection of the fetus.