Response of immune and susceptible ewes to infection with Toxoplasma gondii.

Inocula containing 75, 250 or 1000 Toxoplasma gondii tissue cysts were used to infect seronegative gimmers and seropositive ewes in the fourth month of pregnancy. The seronegative gimmers developed typical toxoplasma infections at all dose levels. Four of them aborted and the surviving lambs showed rising indirect haemagglutination titres in the first two to three months of life indicating congenital infection. The seropositive ewes showed no response to challenge, all their lambs survived and there was no serological evidence of congenital infection. Indirect haemagglutination titres in the seropositive ewes remained unchanged throughout the experiment, titres in the gimmers rose sharply from the 10th day after infection and by three months were the same as those in the ewes.