Experimental neonatal syphilis. I. Evidence of resistance to symptomatic infection in neonatal rabbits following intradermal inoculation with Treponema pallidum (Nichols strain).

Resistance of 5- to 8-day-old neonatal rabbits to dermal lesion development after intradermal inoculation of Treponema pallidum was demonstrated. Clinical evidence of infection following inoculation of 1 X 10(6) Treponema pallidum at each of two sites was either minimal or absent. Atypical, nonprogressive, nonulcerative lesions occurred in 59% of the inoculated neonates and at 45% of inoculated sites. Differences in incubation periods, duration, and maximum diameters of lesions among adult controls versus neonatal rabbits were significant. The age of waning resistance was determined by inoculating groups of neonates ranging from 1 to 7 weeks of age. Five-week-old (31-36 days) neonates demonstrated waning resistance by the appearance of typically ulcerative, progressive lesions, though their parameters (duration, size) were not yet those of adult control lesions. The resistance demonstrated by neonates may be due in part to group housing (nesting) which could create unfavorable temperatures for T. pallidum survival; comparison of lesion development between nesting and individually housed neonates, 31 to 46 days of age, revealed a greater percentage of typical lesions developing among those individually housed (95 versus 52%). However, these differences may reflect the variability of typical lesion development found among animals of this age when resistance begins to wane. In both groups, the duration of typical lesions was significantly shorter than for adult controls. A heat-stable serum factor(s) was demonstrated in 19 of 20 basal sera from neonates 4 to 6 days of age; this presented another possible mechanism of resistance. The neutralizing serum factor(s) was not demonstrable in the sera of does either before mating, during gestation, or shortly after kindling.(ABSTRACT TRUNCATED AT 250 WORDS)