Chimpanzees recovered from typhoid fever induced by ingestion of large numbers of phage-type T S. typhosa were rechallenged approximately a year and a half later with a like number of a phage-type E1 strain. Control animals exhibited febrile responses, bacteriemia, and other significant laboratory and clinical findings, including increases in C-reactive protein levels and sedimentation rates. All of the previously infected and recovered chimpanzees were negative with respect to the aforementioned observations, and appeared to have resisted significant re-infection. On the other hand, recovery from typhoid fever induced by smaller numbers of S. typhosa failed to protect completely against rechallenge with larger numbers of these organisms.

These findings indicate that chimpanzees recovered from typhoid fever may be protected against re-infection, even though the re-infecting organism is of a different phage type. Differences in the magnitude of the original and subsequent infecting inocula appear to influence the response to rechallenge.

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