Syphilitic rabbits, whether untreated or treated after the 90th day of infection, were found to be more refractory to subsequent inoculation with the homologous strain of Treponema pallidum than to inoculation with heterologous strains of the same organism, when clinical criteria alone were employed in judging the outcome of reinoculation. The incidence of second infection with homologous strains was 5.4 per cent, as against 50 per cent with heterologous strains.2 The resistance which develops in rabbits during the course of a syphilitic infection appears therefore to be strain-specific rather than species-specific. The protection afforded against homologous strains was found to persist for at least as long as 6 months after treatment was discontinued.
A given strain may afford a higher degree of protection against some strains than against others, but whether this is to be explained upon the basis of biologic relationship or of differences in virulence, or possibly as the result of both of these factors was not disclosed by the experiments. Rabbits infected with a strain (Nichols) which had been adapted to this species for over a decade could be infected with strains which had been recovered recently from the human body. The previous existence of a syphilitic lesion in the testis which was used as the site for reinoculation did not seem to exert any influence upon the incidence of successful second infections obtained with heterologous strains of Treponema pallidum. Sometimes the course of the second infection produced by inoculation with heterologous strains was less pronounced than that observed in the controls, but in most instances no significant alteration was observed.
In syphilitic rabbits treated late in the course of the disease and reinoculated with heterologous strains of Treponema pallidum no lesion may develop at the site of reinoculation but nevertheless the Wassermann reaction may become positive and remain so for weeks thereafter. It is suggested that such animals may be examples of asymptomatic reinfection.