Toxoplasma gondii: kinetics of the dissemination in the host tissues during the acute phase of infection of mice and rats.

Mice and rats differ in their susceptibility to Toxoplasma gondii infection. Here we have compared the parasitological parameters of acute infection in both mice and rats infected either orally with cysts of Prugniaud strain or intraperitoneally (ip) with tachyzoites of the RH strain. The animals were killed at regular interval during the acute phase, and the parasitic burden in mesenteric lymph nodes, spleen, liver, diaphragm, heart, lungs, brain, and blood was assessed by a tissue culture method in MRC5 fibroblast cells. Mice infected with the RH strain showed a drastic increase of the parasitic load in all organs (up to 10 (8) parasites / g of organs), and then died. When mice were infected with 50 cysts of Prugniaud strain, parasites were first observed in mesenteric lymph nodes, spleen, and lungs and then in other organs. In rats, infection with 1200 cysts of the same strain led to a similar pattern; however, the chronology of the infected organs changed when they were infected with RH strain tachyzoites. These results suggest that the parasites, present first in the peritoneal cavity in the case of RH ip infection, infect all the adjacent organs and then the blood which disseminates the tachyzoites all over the organism. In contrast, after an oral infection, the parasite crosses the intestinal barrier to reach the mesenteric lymph nodes and then the spleen and are finally distributed by the blood throughout the organism. We also showed that following infection with a lethal or a sublethal doses of the Prugniaud strain, the parasitic burden in the studied organs was similar and therefore does not directly correlate with the death of the mice.