Immunization as a factor affecting the course of septicemic anthrax.

Immunization of guinea pigs with alum-precipitated antigen increased resistance approximately 1600 times. The growth rate of Bacillus anthracis, in the septicemic phase, was the same in both normal and immunized animals; however, the number of bacilli per milliliter of blood at death was decreased by 75 percent in immunized animals. Neither the enhancement of virulence by treatment with egg yolk nor the size of the challenge dose affected the growth rate or the number of bacilli per milliliter of blood at death. Mean time to death for the treatments tested varied from approximately 20 to 100 hours. Nonimmunized animals challenged with a strain of low virulence had a terminal number of bacilli per milliliter of blood and a growth rate in the blood the same as that for the highly virulent strain