Evolution of syphilitic chancres with virulent Treponema pallidum in the rabbit.

In recent years there has been renewed interest in the immunology of syphilis and the possibility of developing a vaccine against this disease. The rabbit is the animal most frequently used for preliminary evaluation of vaccines. For this reason, we felt that a critical re-appraisal of the fate of Treponema pallidum when inoculated into the skin of normal rabbits was necessary. Early attempts to determine the lowest number of organisms necessary to produce infection gave varying results (Neisser, 1911; Chesney and Kemp, 1925; Wakerlin, 1926; Magnuson, Eagle, and Fleischman, 1948). The last group found that intracutaneous inoculation of 2 to 200,000 virulent organisms of the Nichols strain of T. pallidum produced darkfield positive lesions in rabbits. The incubation period varied greatly with the size of the inoculum. Using the same virulent strain, we attempted to determine the minimal infective dose of T. pallidum and to study, clinically and pathologically, the evolution of chancres resulting from injecting specified numbers of spirochaetes.