Interferon in rabbit sera after inoculation with Treponema pallidum suspensions contaminated with PED virus

Paired rabbits sera were examined for the presence of interferon and pathogenicity for rabbits. The sera were obtained before and 48 hours after inoculation with Treponema pallidum suspensions of rabbit origin in 12 selected laboratories. Classical interferon, detectable in dilutions from 1/9 to 1/81, were found in 27 out of 39 postinoculation sera from which the pleural effusion disease (PED) virus was isolated. Serum interferon was not detectable in dilutions greater than or equal to 1/9 in 16 virus-negative postinoculation sera or in any of the 55 preinoculation sera. Interferon was found more often in sera from which highly virulent strains of PED virus were isolated than in sera from which strains of low virulence were isolated. The serum interferon assay provides useful presumptive evidence of contamination of rabbit-passaged treponemes with PED virus, but the assay is least useful when PED virus is present subclinically.