The course of babesiosis in humans is characterized by various intensity levels − from a subclinical level to the severe one − associated with multiple organ failure, which leads to death. The aim of this study was to evaluate the effect of 21-day and 6-month invasion of B. microti on Wistar rats spleen. Histological changes in the rats’ spleen were characterized by swelling of splenic tissue, especially the tissue adjacent to the capsule. In the structure of the white pulp in some rats, high concentrations of lymphocytes occurred. The boundary between the white pulp and red pulp was blurred. In the red pulp structure of rats, a lot of macrophages and extracellular deposits of bilirubin were present. The submicroscopic studies showed that the nuclear matrix was slightly shrunken. In the red pulp fragments of the damaged cells were located in the intercellular spaces. Near these areas, many thrombocytes were visible. The ultrastructural observation also revealed thickened endoplasmic reticulum membranes, local cellular swelling filled with amorphous substance, and digested erythrocytes. B. microti invasion affects the splenic morphology and ultrastructure in rats. The immunological hyperactivity and signs of inflammation indicate an important role of spleen in a fight against parasites.
