It appears to be generally accepted that repeated passages of Spirochaeta pallida are usually required for the development of pathogenic properties for the experimental animal, when first transferred from the human host. This view was widely expressed in the early literature on experimental syphilis, together with an explanation that it was a question of adaptation to a new environment. In some of the more recent work in this field, freshly isolated strains are also reported to be of low pathogenicity for the rabbit. Thus, Ecker1 found that freshly isolated strains, as compared with older ones, produced a significantly smaller number of "takes," with a longer incubation period. There are instances, however, in which high virulence or unusual pathogenic properties have attracted attention even in the first animal passages. The best known example of this kind is the strain V of Nichols' series which was isolated in 19122 and
HU CK. THE HIGH PATHOGENICITY OF A RECENTLY ISOLATED STRAIN OF SPIROCHAETA PALLIDA. Arch Derm Syphilol. 1934;30(6):847–853. doi:10.1001/archderm.1934.01460180089015
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