Congenital and neonatal syphilis in guinea-pigs show a different pattern of immune response.

C4-deficient (C4D) and Albany strains of guinea-pigs transplacentally and neonatally infected with Treponema pallidum showed distinctive patterns of humoral immune responses. Congenitally infected progeny of both strains originated from dams intradermally (i.d.) infected at mid-pregnancy with virulent T. pallidum. In the neonatal groups families of C4D and Albany strains consisting of 1-3-day-old offspring and their mothers were i.d. infected with a similar dose of T. pallidum. Regardless of the strain, asymptomatic congenitally infected guinea-pigs (n = 16) responded from the first day of life with high levels of IgM [T. pallidum (TP) ELISA] antitreponemal antibodies and up to 85% presented with IgM CIC (circulating immune complexes) and IgM RF (rheumatoid factor). Although relatively high levels of IgM antitreponemal antibodies persisted in these animals throughout the 4-month experimental period, significant levels of host IgG antitreponemal antibodies were detectable after 2-3 months of age. Neonatally infected guinea-pigs of both strains (n = 27) responded similar to the infected sow but with relatively lower levels of IgM and IgG antitreponemal antibodies at 1 and 4 weeks, respectively, both of which increased with the time of infection. Antibodies were also detected in these animals by fluorescent treponemal antibody adsorption test (FTA-ABS). Unlike congenital syphilis, neonatally infected animals developed IgG-CIC after 2-3 months of infection and none of them showed any RF. In neonatal syphilis, FTA-ABS antibody levels were closely associated with the onset of lesions, whereas those of TP ELISA were not. The distinctive immune responses observed in these experimental models have the potential to differentiate between congenitally and neonatally infected human infants, even though the current clinical management is the same.