SUSCEPTIBILITY TO CHRONIC INFECTION WITH TOXOPLASMA-GONDII DOES NOT CORRELATE WITH SUSCEPTIBILITY TO ACUTE INFECTION IN MICE

Resistance against acute and chronic infection with Toxoplasma gondii in BALB/c and CBA/Ca mice was compared. Intraperitoneal inoculation of either 20, 40, or 80 cysts of the ME49 strain resulted in mortality r ates in BALB/c mice of 12% (2 of 17), 50% (6 of 12), and 75% (9 of 12) , respectively, within 3 weeks after infection (acute stage). There wa s no mortality in the CBA/Ca mice for any of the doses. In marked cont rast, CBA/Ca mice were highly sensitive to chronic infection with deve loping toxoplasmic encephalitis; they began dying 2 months after infec tion with 10 cysts of the ME49 strain, and mortality reached 53% (16 o f 30) by the sixth month postinfection. No mortality (0 of 20) was obs erved in the chronically infected BALB/c mice. CBA/Ca mice had markedl y more cysts in their brains than BALB/c mice in the chronic stage. Se vere inflammatory changes were observed only in the brains of CBA/Ca m ice. Interestingly, in the acute stage (the first 3 weeks), numbers of cysts in the brains were significantly greater in CBA/Ca than BALB/c mice, whereas only BALB/c mice showed mortality in that time period. N o inflammatory changes were observed in brains of BALB/c mice during t he acute stage of the infection. Thus, resistance against chronic infe ction appears to be regulated by a mechanism(s) that is different from those conferring resistance against acute infection. There was no dif ference in gamma interferon levels in sera between CBA/Ca and BALB/c m ice during the acute stage. However, during the chronic stage, only BA LB/c mice had detectable levels of gamma interferon in their sera.