Effect of cortisone on Toxoplasma gondii infection studied by electron microscopy.

The ultrastructural features of cortisone effect on the early development and formation of Toxoplasma gondii tissue cysts were examined in the brains of experimental mice on days 8-47 post inoculation. The cortisone was injected subcutaneously into mice twice a week: 1) at the early acute phase, 5 days post infection, and 2) at the transition phase of development, 12 days post infection. The disturbed regulation of the host-parasite relationship due to the effect of cortisone showed itself in an increase of T. gondii tachyzoites, cystozoites, and cysts not only by intracellular but also by as yet not described extracellular division. The persisting parasitaemia induced by tachyzoites and the earlier development and differentiation of cysts resulted in the cyst rupture releasing a highly resistant antigen of cystozoite type. The toxoplasmic encephalitis had a lethal effect in all mice. The tachyzoites penetrated into many types of cells, but a predilection of development into cysts was detected in neurons and astrocytes. According to the character of host cells, two types of cysts arose: less resistant electron-transparent cysts in neurons often rupturing or budding and more resistant electron-dense cysts in astrocytes.