Transplacental transmission and fetal parasitosis of Trypanosoma cruzi in outbred white Swiss mice.

Two strains of Trypanosoma cruzi, isolated from humans and assayed for their biological capacity to kill outbred white Swiss mice (HaM/CR-CD) following reticuloendothelial system blockade with thorium dioxide, were used in these experiments: the Maria Cristina strain, which killed all blocked mice at a rate following a rectangular dose-response curve, and the José Cardoso strain, which did not kill blocked mice at comparable dosages. When inoculated into pregnant HaM/CR-CD mice, the non-pathogenic José Cardoso strain did not cross the placental barrier, in either blocked or unblocked mice, to cause fetal parasitosis. The pathogenic Maria Cristina strain did not cross the barrier in non-blocked mice, but in thorium-dioxide blocked mice it produced an incidence of fetal parasitosis of 8.9% (7 of 79 fetuses). These results indicate that the transplacental transmission of T. cruzi was dependent on two restrictions: pathogenicity of the strain of T. cruzi, and blockade of phagocytic activity by thorium dioxide, suggesting that transplacental transmission of T. cruzi is related to interference with the phagocytic activity of the placenta.