Cross-immunity between Hammondia and Toxoplasma infections in mice and hamsters

Cross-immunity between six strains of Hammondia hammondi and the M-7741 strain of Toxoplasma gondii was studied in mice and hamsters. Mice and hamsters were inoculated orally with 10(5) H. hammondi oocysts. All mice and hamsters survived. Four weeks later, they were challenged with 1 to 10(5) mean lethal doses (LD50) of the Toxoplasma oocysts. Animals that died were necropsied. Survivors were killed 30 days after challenge inoculation, and the number of cysts in brain and musculature was determined and compared with that of animals that were not immunized with H. hammondi. In one experiment, 36 mice were immunized with each of three H. hammondi strains. Four weeks later the mice were challenged with 1 to 10(5) LD50 doses of T. gondii oocysts. Of 108 mice immunized with H. hammondi, 103 survived challenge with Toxoplasma oocysts for 30 days, whereas only 3 of 36 unimmunized mice survived with similar doses of Toxoplasma oocysts. Fewer Toxoplasma cysts were found in mice immunized with H. hammondi than in unimmunized mice. In another experiment groups of six hamsters were each immunized with one of six H. hammondi strains and then challenged with 10(5) LD50 Toxoplasma oocysts. All unimmunized hamsters died between 9 and 13 days after inoculation. Percent protection in the various groups of immunized hamsters was: 100, 84, 66, 66, 50, and 33.