[Natural adaptation of the white rat to Toxoplasma gondii]

White rats were found to be resistant to 3 x 10(7) Toxoplasma gondii organisms (RH strain) inoculated either sc or ip. That resistance was age-dependent since survival time of 1- or 5-day-old rats inoculated with 10(4) or 10(6) tachyzoites was lower as compared with those 10 or 15 days old. Organisms kept in contact with a lysate of white rat complete peritoneal exudate showed reduced capacity to infect mice. Peritoneal exudate of 5-day-old rats showed a stronger effect than that of 1, 2, 3, or 4 day-infected animals. After 10 days the anti-toxoplasma effect diminished and it almost disappeared after 15 days of infection. Apparently age and macrophage activity are very important factors in the natural adaptability of the white rat to Toxoplasma gondii.