Early kinetics of Toxoplasma gondii infection in mice infected orally with cysts of an avirulent strain.

We determined the early kinetics of Toxoplasma gondii infection in Swiss Webster mice inoculated with the avirulent C strain by counting parasites in the blood, spleen, Peyer's patches, liver, lungs, and brain. Animals were orally inoculated with cysts on day zero (D0), and parasites were counted using a tissue culture method at 12, 24, and 36 hr and 2, 3, 7, 10, 14, 21, 30, 50, and 72 days postinfection. The spleen and Peyer's patches were the first organs found parasitized, on D2 and D3, respectively, followed by the lungs and liver on D7 and the brain on D10. No parasitemia was detected. This suggests that early dissemination of this avirulent strain from the intestine into the general circulation occurs essentially via the lymphatic system. Parasites persisted at a high level in the brain during the chronic phase. In the lungs, parasites were no longer detected by D72, while parasite numbers initially declined in the spleen and Peyer's patches but then showed a second peak, possibly due to recirculation of T. gondii. These results suggest that lymphoid organs play a key role in T. gondii dissemination during the acute phase and may also constitute a persistent source of parasite resurgence.